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fix(watcher): drop unsolicited TCP SYNs to closed ports - #1001
Network probes, external port scanners, or misconfigured cluster traffic frequently send unsolicited TCP SYN packets to ports where no listening socket exists inside a container. Because eBPF captures these packets on ingress before socket demultiplexing, they previously flowed into the container profiling pipeline and rule engine as legitimate incoming connections, polluting learned profiles with spurious ingress ports or triggering false positive alerts.
This PR introduces listenerCache in pkg/containerwatcher/v2:
Reads /proc/<pid>/net/tcp and /proc/<pid>/net/tcp6 to identify active LISTEN sockets (state 0A).
Snapshots listening ports per container PID with a 5-second TTL cache to minimize procfs reads.
In EventHandlerFactory.ProcessEvent, drops un-attributed incoming host-packet TCP SYNs directed to ports where no process is listening.
Outgoing traffic, UDP, and SYNs attributed to known processes remain completely unaffected.
Extracted as part of the upstreaming roadmap from entlein's work.
Testing
Unit tests TestParseListeningPorts, TestListenerCache_TTLAndUnknown, and the full matrix in TestUnsolicitedIngress_TruthTable pass cleanly.
Summary by CodeRabbit
New Features
Host-originated TCP events without an attributed process are filtered when their destination port has no container listener. Known listening ports and cases where listener information is unavailable are not filtered.
Bug Fixes
Dropped-event counts are reported even when an event is filtered, keeping profile reporting accurate.
Listener information is cleared after container removal to avoid retaining stale entries.
The container watcher now detects TCP listener ports and filters unsolicited host-to-container ingress events. Dropped-event reporting occurs before this filter. Listener cache entries are removed after the container removal grace period.
Changes
Unsolicited ingress filtering
Layer / File(s)
Summary
Listener detection and ingress predicate pkg/containerwatcher/v2/listeners.go, pkg/containerwatcher/v2/listeners_test.go, pkg/containerwatcher/v2/unsolicited_ingress_test.go
The listener cache stores successful per-PID TCP listener snapshots for five seconds. Listener discovery reads TCP and TCP6 procfs tables. The predicate filters eligible host-originated TCP events when the destination port is not a known listener. Tests cover parsing, cache behavior, and filtering decisions.
Event handling and cache lifecycle pkg/containerwatcher/v2/event_handler_factory.go, pkg/containerwatcher/v2/unsolicited_ingress_test.go
The event handler initializes the listener cache, reports dropped events before ingress filtering, and forgets a container’s cached listener entry after the removal grace period. Tests cover dropped-event reporting and cache eviction.
sequenceDiagram
participant EventHandlerFactory
participant ProfileManager
participant unsolicitedIngress
participant ListenerCache
participant procfs
EventHandlerFactory->>ProfileManager: report dropped events
EventHandlerFactory->>unsolicitedIngress: evaluate event and container
unsolicitedIngress->>ListenerCache: look up destination port for container PID
ListenerCache->>procfs: read TCP and TCP6 listener tables on cache miss or expiry
ListenerCache-->>unsolicitedIngress: return listener status
unsolicitedIngress-->>EventHandlerFactory: return unsolicited-ingress verdict
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The title clearly and concisely describes the main change: dropping unsolicited TCP SYNs sent to closed container ports.
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- Move dropped-event accounting before unsolicited ingress filtering to prevent missed drop reporting
- Revalidate cached misses against procfs before dropping SYNs so newly opened listening ports are not dropped
- Evict listener cache entries on container removal after grace period
- Improve procfs open tracking across IPv4 and IPv6 listener tables
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Actionable comments posted: 3
🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @pkg/containerwatcher/v2/listeners.go:
- Line 46: Update the cache-miss handling around `c.read(pid)` in the listener
code to avoid rescanning procfs and holding the cache mutex on every closed-port
event. Reuse a bounded five-second snapshot or another bounded refresh strategy,
while ensuring newly opened listeners are detected before their events are
suppressed.
- Line 78: Update listeningTCPPorts and parseListeningPorts so failures opening
either procfs table and Scanner.Err() are propagated as an unknown verdict,
keeping the event when the destination’s lack of a listener cannot be
established; handle an unavailable IPv6 table separately when IPv6 is disabled.
- Line 99: Update the listener filtering that populates `ports` to retain each
procfs socket’s local address alongside its port, then match both against the
event destination, treating wildcard binds as matching any address. Include
entries from the process network namespace’s tcp and tcp6 tables rather than
filtering for sockets owned only by that PID.
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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win
Match the listener’s local address as well as its port.
ports discards the local address from /proc/<pid>/net/tcp and tcp6. A socket bound to loopback or another interface therefore makes the same port appear open for an incoming packet addressed elsewhere. The predicate keeps that packet even when no socket can receive it. Preserve the bind address and compare it with the event destination, including wildcard binds. The procfs table reports local addresses, and its entries cover the process’s network namespace rather than only sockets owned by that PID. (docs.kernel.org)
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Review comment at @pkg/containerwatcher/v2/listeners.go at line 99:
Update the listener filtering that populates `ports` to retain each procfs
socket’s local address alongside its port, then match both against the event
destination, treating wildcard binds as matching any address. Include entries
from the process network namespace’s tcp and tcp6 tables rather than filtering
for sockets owned only by that PID.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
- Read procfs outside cache mutex and coalesce concurrent per-PID reads with singleflight
- Bound negative verdict caching with negativeSnapshotTTL to avoid unbounded rescans under scan load
- Propagate Scanner.Err and handle absent tcp6 when IPv6 is disabled
In-flight refresh can recreate entries after forget
pkg/containerwatcher/v2/listeners.go:80
forget cannot invalidate a refresh already running in sf.Do, so this unconditional write can recreate an entry after the removal timer has deleted it. A delayed reader (or waiter) can therefore leave stale listener state retained for an exited container, defeating the new cleanup path. Associate each refresh with a generation/token and publish it only if no forget occurred since the refresh began.
Test releases gate before all callers join singleflight
pkg/containerwatcher/v2/listeners_test.go:160
This does not guarantee that the other four goroutines have joined the in-flight singleflight call before the gate is released. The first reader may return immediately after started is received, after which late-scheduled goroutines perform additional reads and make this assertion flaky. Keep the read blocked until the test can establish that every caller has reached the coalescing point (for example via a test hook/barrier).
…currency test deterministic (5396219974)
- Track container generations in listenerCache so in-flight procfs reads cannot recreate entries after forget
- Synchronize all concurrent callers with an entry barrier before unblocking singleflight read
Read failures are not cached, so every subsequent unattributed SYN immediately retries both procfs files. When /proc/<pid>/net is persistently unavailable (for example during the removal grace window or because of permissions), scan traffic can drive unbounded procfs opens despite singleflight only coalescing overlapping calls. Cache an unknown/error result for a short TTL so events remain allowed without retrying per packet.
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Description
Network probes, external port scanners, or misconfigured cluster traffic frequently send unsolicited TCP SYN packets to ports where no listening socket exists inside a container. Because eBPF captures these packets on ingress before socket demultiplexing, they previously flowed into the container profiling pipeline and rule engine as legitimate incoming connections, polluting learned profiles with spurious ingress ports or triggering false positive alerts.
This PR introduces
listenerCacheinpkg/containerwatcher/v2:/proc/<pid>/net/tcpand/proc/<pid>/net/tcp6to identify activeLISTENsockets (state0A).EventHandlerFactory.ProcessEvent, drops un-attributed incoming host-packet TCP SYNs directed to ports where no process is listening.Extracted as part of the upstreaming roadmap from entlein's work.
Testing
TestParseListeningPorts,TestListenerCache_TTLAndUnknown, and the full matrix inTestUnsolicitedIngress_TruthTablepass cleanly.Summary by CodeRabbit
New Features
Bug Fixes