[mrsabath]

Closes #1483.

What

A MOCA sandbox may reach only the destinations its operator lists. listener.egress_allow_hosts (exact host:port entries) arms a deny-by-default boundary on the forward proxy: an off-list CONNECT target, absolute-form http:// host, or plugin-redirected dial target is refused by name — 403, X-Moca-Egress-Error: destination_denied, Content-Type: text/plain, body destination_denied: <host>\n — before DNS resolution and before any dial, so the request is never left to time out and the upstream never hears it.

When the config field is empty, MOCA_EGRESS_ALLOW (comma-separated) is the fallback — that env var is the channel the MOCA conformance harness drives into the target. Neither set ⇒ feature off, today's behavior byte for byte.

Why a listener feature and not an opa policy in the MOCA profile

• A pipeline-level gate is bypassed by skip_hosts, which skips the whole pipeline (and session recording) for matched hosts — an egress boundary that an infrastructure-bypass list can switch off is not a boundary. The allowlist is therefore checked before the skip match, and a skip pattern cannot exempt an off-list host.
• Profile composition decides which plugins run; an egress boundary should not depend on a profile happening to include a policy.
• The conformance harness drives the target only via env, not via policy bundles.

Shape of the list

Exact host:port only — deliberately narrower than every other host list in the tree: skip_hosts and authproxy-routes glob; an allowlist must not. Entries that would widen or silently empty the boundary are startup errors: portless (example.com — a guessed port widens), wildcard hosts, empty/invalid ports. Hosts compare lowercased, trailing-dot-stripped, IPv6 canonicalized; a destination naming no port is judged at its scheme's default (80 for http, 443 otherwise), so http://host/x means host:80. Unparseable destinations are denied, not guessed at.

Enforcement points

1. CONNECT, at the top — before the skip match, before noteTunnel. Opaque and bridged tunnels both pass through here.
2. Absolute-form URLs in serveOutbound, same placement. Bridged requests skip this one: their destination was gated at CONNECT and is pinned to the tunnel authority.
3. Redirect re-check before the dial — a redirecting plugin (inference-router) must not be able to steer bytes somewhere the client never asked for. Deny-by-default means the dial target is what counts, so the check is unconditional at the dial, not only when a redirect happened.

Each refusal records a denied-phase session row with an egress-allowlist deny invocation (details.host names the host; the row's host is the full authority, like every tunnel row), and is written with plain WriteRejection — the MCP JSON-RPC 200 rewrite would hide the named 403.

pipeline.MocaRefusal is the shared constructor for this wire shape. #1481's plaintext_credential gate (open in #1537, not yet on main) is the intended second user; adopting it there is a follow-up on that PR rather than part of this one, since the gate it would change isn't on main yet.

Scope and follow-ups

• Audit events: the denied session row is today's audit surface. The moca JSON audit channel and vm_id are [MOCA SBX] Exactly one audit event per injection, in MOCA's audit shape #1482 / [MOCA SBX] Caller identity from the connection, carried into every bridged request and every plugin #1476-77.
• extproc and the transparent listener are out of scope: MOCA shape A is the proxy-sidecar forward proxy, and the issue names CONNECT + absolute-form only.
• Full off_allowlist_named_403 conformance runs when moca grows a Target adapter for cortex (moca main has only conformance/ today). The in-tree tests pin the equivalent: the wire shape through the real Handler(), upstream-accepts-0, and the pre-dial placement — Go resolves inside the dial, which these refusal paths never reach.

Verification

• New tests: core/listener/egressallow matcher table; TestMocaRefusal (pipeline); seven TestEgressAllow_* gate tests through the real forward-proxy handler (off-list CONNECT, off-list absolute-form GET, portless-URL port default, on-list flow-through, nil-matcher behavior unchanged, allowlist-wins-over-skip_hosts, redirect-to-off-list); TestResolveEgressAllow (config-over-env precedence).
• Full suites: core go test ./... 73 ok / 0 fail; cmd/cortex ok; gofmt, go vet, go mod tidy -diff clean on both touched modules.
• Manual smoke: binary booted with MOCA_EGRESS_ALLOW=127.0.0.1:<origin> — on-list GET flows through (200); off-list GET and CONNECT both answered 403 with X-Moca-Egress-Error: destination_denied and the exact 29-byte body destination_denied: off.test; portless http://off.test/ judged at :80 and refused.
• Reload: any change to egress_allow_hosts is restart-required automatically — it lives under listener.*, where the reloader refuses every difference.

Docs: CLAUDE.md event-schema section (the destination_denied refusal shape) and the listener gotcha entry with config and env examples.

Assisted-By: Claude (Anthropic AI) noreply@anthropic.com

Summary by CodeRabbit

• New Features
  • Added an optional outbound destination allow list. When configured, requests to destinations not on the list are refused with a 403 before reaching upstream services.
  • Allow-list settings can be provided in configuration or through an environment variable. With neither set, outbound behavior remains unchanged.
• Documentation
  • Added guidance on allow-list entries, configuration precedence, request denials, and restart requirements.

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Commits

Reviewing files that changed from the base of the PR and between c4c96ae and 8091931.

Files selected for processing: 10

• CLAUDE.md
• cmd/cortex/egressallow_test.go
• cmd/cortex/main.go
• core/config/config.go
• core/listener/egressallow/egressallow.go
• core/listener/egressallow/egressallow_test.go
• core/listener/forwardproxy/egressallow_gate_test.go
• core/listener/forwardproxy/server.go
• core/pipeline/action.go
• core/pipeline/action_test.go

Walkthrough

The forward proxy now supports an optional exact host-and-port egress allow list from listener configuration or MOCA_EGRESS_ALLOW. It checks destinations before pipeline execution and checks the final destination after pipeline processing. Off-list requests receive a named 403 refusal and a recorded denial.

Changes: Egress destination allow list

Layer / File(s)	Summary
Exact host-and-port matching core/listener/egressallow/*	The new matcher validates and normalizes entries, including bracketed IPv6 addresses. It uses port 80 for portless HTTP destinations and 443 for other schemes. Tests cover exact matching, invalid entries, and malformed destinations.
Configuration and startup core/config/config.go, cmd/cortex/main.go, cmd/cortex/egressallow_test.go, CLAUDE.md	ListenerConfig.EgressAllowHosts takes precedence over the comma-separated environment fallback. Startup constructs the matcher when entries are present. The documentation describes configuration, precedence, and restart requirements.
Proxy checks and refusals core/listener/forwardproxy/*, core/pipeline/action.go, core/pipeline/action_test.go, core/pipeline/client.go, core/pipeline/context.go, CLAUDE.md	The proxy checks CONNECT and absolute-form HTTP destinations before the pipeline, then checks the final destination after pipeline processing. Denials return a named 403 and record a destination_denied invocation. Tests cover allowed and denied requests, redirects, skip_hosts precedence, and unset-list behavior.

Priority: Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant ForwardProxyServer
  participant EgressAllowMatcher
  participant PluginPipeline
  participant Upstream
  Client->>ForwardProxyServer: Send absolute-form HTTP request
  ForwardProxyServer->>EgressAllowMatcher: Check request destination
  EgressAllowMatcher-->>ForwardProxyServer: Return allow or deny
  ForwardProxyServer->>PluginPipeline: Process allowed request
  PluginPipeline-->>ForwardProxyServer: Return processed destination
  ForwardProxyServer->>EgressAllowMatcher: Check final destination
  EgressAllowMatcher-->>ForwardProxyServer: Return allow or deny
  ForwardProxyServer->>Client: Return 403 for denied destination
  ForwardProxyServer->>Upstream: Send request for allowed destination

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Possibly related issues

• #1489 — The conformance gate includes an off-allow-list named 403 case, which exercises the refusal behavior added here.
• #1490 — The epic identifies the allow-list requirement as part of the deployment-shape decision; this change implements the allow list in the listener.

Suggested labels: ready-for-ai-review

Suggested reviewers: huang195, araujof, alan-cha

Merge Risk: Low · previous review c4c96

Operators may overestimate which traffic the allow list protects. Clarify its listener scope; this is a bounded documentation issue rather than a reason to block merging.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title "Feat: Deny-by-default egress allow list for the forward proxy" matches the main change. The PR adds an exact host:port egress allowlist to the forward proxy. Off-list CONNECT targets, absol…
Linked Issues check		Issue #1483 requires named 403 refusals for off-list CONNECT and absolute-form destinations, an X-Moca-Egress-Error header, a destination_denied: <host> text body, and one audit event per refusal.…
Out of Scope Changes check		The changes stay within issue #1483. The matcher, configuration and environment fallback, forward-proxy enforcement, refusal constructor, audit recording, and tests directly support the allowlist beha…
Docstring Coverage		Docstring coverage is 96.97% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentati…

Full details: Title check

Explanation

The title "Feat: Deny-by-default egress allow list for the forward proxy" matches the main change. The PR adds an exact host:port egress allowlist to the forward proxy. Off-list CONNECT targets, absolute-form HTTP hosts, and plugin-redirect destinations are refused with a 403 before DNS or dialing. The title names the feature, the default-deny behavior, and the affected component. The title follows a conventional commit prefix and does not list files or use vague terms.

Full details: Linked Issues check

Explanation

Issue #1483 requires named 403 refusals for off-list CONNECT and absolute-form destinations, an X-Moca-Egress-Error header, a destination_denied: <host> text body, and one audit event per refusal. The pull request adds listener gates before pipeline, DNS, and dialing, records denied session rows, and adds real-handler tests for these paths. The reported equivalent tests cover the requested behavior because the MOCA Cortex Target adapter required for off_allowlist_named_403 is not available in the current conformance harness.

Full details: Out of Scope Changes check

Explanation

The changes stay within issue #1483. The matcher, configuration and environment fallback, forward-proxy enforcement, refusal constructor, audit recording, and tests directly support the allowlist behavior. Sanitizing refusal names supports safe logging and response rendering. Documentation records the new listener feature. No unrelated feature, extproc change, transparent-listener change, or MOCA JSON audit implementation is included.

Full details: Docstring Coverage

Explanation

Docstring coverage is 96.97% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 33 functions across 11 files. (1 skipped: 1 unsupported.)

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[mrsabath]

The high-severity XSS alert this check was failing on is fixed in 6bd3bc6 (html.EscapeString on the name in MocaRefusal — byte-identical for every legitimate host, so the MOCA conformance bytes are unchanged). This medium log-injection alert stays open by design, matching #1539's precedent for the identical finding: the value already passes through pipeline.SanitizeControlRunes, which neutralises control runes at runtime; CodeQL just cannot prove a custom rune loop is a barrier, and a new medium does not fail the check.

[mrsabath]

Same resolution as the sibling alert on this call: the value is wrapped in pipeline.SanitizeControlRunes (real runtime neutralisation, not a provable CodeQL barrier — see the reply on the host line). The high-severity alert that was failing the check is fixed in 6bd3bc6.

[mrsabath]

Same resolution as the request-path gate: the value is wrapped in pipeline.SanitizeControlRunes, which neutralises control runes at runtime but is not a barrier CodeQL can prove — the same accepted medium alert #1539 carries on its sanitized refusal line. The high-severity reflected-XSS alert that was failing this check is fixed in 6bd3bc6.

[huang195]

Dobra robota, Mariusz. It's checked before skip_hosts, checked again at the dial after a redirect, it refuses before DNS runs, and when the feature is off the matcher is nil and behavior doesn't change. The tests go through the real handler. The one blocker is a panic that any client can trigger in splitPort. The fix is one line, so it's quick to land.

Author: mrsabath (MEMBER — maintainer)
Areas reviewed: Go (listener, pipeline, config, cmd), Docs (CLAUDE.md)
Agent/IDE config (.claude/.vscode): none
Commits: 3 commits, all signed off: yes
CI status: passing

Verified locally on the PR head: go test for egressallow, forwardproxy, pipeline and TestResolveEgressAllow passes; gofmt -l is clean.

[huang195]

must-fix: Mariusz, the ] branch assumes there's a [ at s[0], and the client gets to pick the input 😄. Sending CONNECT ]:443 (or GET http://]:80/, since url.ParseRequestURI accepts it) means LastIndex("]") returns 0, then s[1:0] panics with slice bounds out of range [1:0]. I ran both through the real Handler() (newAllowHarness) and got http: panic serving … for each one. net/http recovers the panic, so the proxy stays up and nothing is dialed. Still, any client can write a stack trace into the log on every request, and the refusal row never lands.

Fix: only take this branch when strings.HasPrefix(s, "["), and treat a stray ] as not host:port. A "]:443" row in the Allows deny table would hold it. Nie ma tak dobrze, a parser at a trust boundary doesn't get to panic 😉

[mrsabath]

Masz całkowitą rację — fixed in c4c96ae, and thank you for running it through the real Handler(); that is exactly where it bites.

Reproduced both of yours before touching anything: ]:443 and ] panic with slice bounds out of range [1:0], because LastIndex(s, "]") returns 0 and the branch then indexes s[1:0]. Your reading of the consequences was right on all three counts — net/http recovers, the proxy stays up, nothing is dialed — and the third is the one that actually decided the fix for me: the refusal row never lands. A denial whose audit event is the thing the panic skips is the worst shape for this gate to fail in, since the denied row is today's audit surface for the feature.

The fix is the one-liner you proposed: the bracketed branch is taken only on a leading [, and a stray ] is simply not host:port, falling through to be denied like any other unparseable destination — with its audit row. One extra case while I was in there: [::1 (opened, never closed) used to reach s[1:i] with a different i, so it now errors explicitly rather than relying on the bound.

I went a bit past the "]:443" deny-table row you suggested, because the panic was a handler-level symptom and I wanted the regression pinned where you found it:

• TestEgressAllow_RefusesStrayBracketDestination — ]:443, ], ]]:443, a]b:443 through the real Handler() via newAllowHarness, pinning the 403, upstream-accepts-0, and the denied row's egress-allowlist invocation.
• TestEgressAllow_StrayBracketGETRefused — the absolute-form half, since url.ParseRequestURI accepts http://]:80/ as you noted.
• TestAllows_StrayBracketDeniedNotPanic / TestNew_RejectsUnbracketedIPv6Entry — the matcher directly, including that [::1]:443 still parses.

Worth recording that the test earns its keep: reverting only the source fix and keeping the tests reproduces http: panic serving … slice bounds out of range [1:0] and fails, so this cannot regress silently.

One case that is not in the 403 list, in case it looks like an omission: [::1 never reaches the gate over CONNECT — net/http rejects that request line with a 400 first. Refused either way, just not by us, so the test says so rather than asserting a 403 we do not produce.

core go test ./..., gofmt, go vet and go mod tidy -diff clean on both touched modules. Nie ma tak dobrze — zgoda, a parser at a trust boundary doesn't get to panic. 😉

防微杜渐 — "block the small leak before it becomes a flood": a one-character typo in an authority is about as small as it gets, and it was costing us the audit row on every denial.

[huang195]

suggestion: An IPv6 entry written without brackets, like 2001:db8::1 or ::1, gets through New without an error. LastIndex(":") splits it into host 2001:db8: and port 1, so that entry can never match. It fails closed, so it's safe. But the PR says entries that "silently empty the boundary are startup errors", and here the operator thinks they allowed a host and actually allowed nothing. Rejecting any host that still contains : after unbracketing would make this a startup error.

[mrsabath]

Good catch, and I took the suggestion — fixed in the same commit, c4c96ae.

Confirmed your analysis exactly: normalizeEntry("2001:db8::1") returned no error, with host 2001:db8: and port 1. The detail that makes it nastier than it first looks is that the resulting match key reads back as 2001:db8::1 — it looks completely correct in anything that prints the key — while the host half is 2001:db8: and no destination can ever match it, because a request carries a v6 literal bracketed and that normalizes to 2001:db8::1. ::1 behaves the same way (host :, port 1).

You are right that it fails closed and is therefore safe, and right that it contradicts what the PR body claims. That line — entries that "silently empty the boundary are startup errors" — was the standard I wrote for this list, so an entry that quietly allows nothing while the operator believes they listed a host has to be an error too. Implemented as you suggested: an entry whose host still contains : after unbracketing is rejected, and the message names the spelling that works ([<address>]:<port>).

One subtlety the tests caught and that is worth flagging, since the obvious version of this check is wrong: the guard must key on whether the entry was bracketed, not on the normalized host. My first attempt tested only strings.Contains(host, ":"), which rejected [::1]:443 as well — its host is legitimately ::1 after the brackets come off and netip canonicalization. So the condition is !strings.HasPrefix(trimmed, "[") && strings.Contains(host, ":"), and TestNew_RejectsUnbracketedIPv6Entry asserts both directions: the unbracketed spellings are startup errors, and [::1]:443 / [2001:db8::1]:8443 still compile.

Two v6 shapes land on a different error than the bracket message, which I left alone rather than reordering the guards: fe80::1%eth0 and ::ffff:203.0.113.7 trip the port check first (1%eth0 and 203.0.113.7 are not numbers 1–65535). Still startup errors, just reported as a bad port. The test pins them separately so the distinction is recorded rather than looking like a gap.

Allows is deliberately unchanged — an unbracketed v6 destination still fails closed rather than erroring, which is what TestAllows_IPv6Literals already pinned. The asymmetry is the point: an entry is operator input and gets told it is wrong at startup; a destination is attacker input and just gets denied.

Also updated the New doc comment and the CLAUDE.md egress entry so the documented contract matches the code.

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Actionable comments posted: 1

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Inline comments:
Review comments at @core/config/config.go:
- Around line 942-951: Clarify the scope of EgressAllowHosts in its field
comment and CLAUDE.md: state that the allow list applies only to the forward
proxy, not the transparent (enforce-redirect) or ext_proc listeners, and that
iptables-redirected egress to transparent_proxy_addr is not filtered by it. Keep
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Commits

Reviewing files that changed from the base of the PR and between 43cf553 and c4c96ae.

Files selected for processing: 12

• CLAUDE.md
• cmd/cortex/egressallow_test.go
• cmd/cortex/main.go
• core/config/config.go
• core/listener/egressallow/egressallow.go
• core/listener/egressallow/egressallow_test.go
• core/listener/forwardproxy/egressallow_gate_test.go
• core/listener/forwardproxy/server.go
• core/pipeline/action.go
• core/pipeline/action_test.go
• core/pipeline/client.go
• core/pipeline/context.go

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[huang195]

Both findings from my last round are fixed, each with a test, so I'm approving.

• splitPort panic (c4c96ae): the bracket branch now runs only when the string starts with [, so a stray ] gets denied instead of crashing. ]:443, ], ]]:443 and [::1 are covered in the matcher table, and the gate test sends one through the real Handler().
• Unbracketed IPv6 entry (c4c96ae): 2001:db8::1, ::1 and fd00::2 are now startup errors, and the message shows the [addr]:port form.

What else I checked on this head:

• The redirect re-check runs on every request, skip_hosts and self-traffic included, before client.Do. A redirected or skipped request can't dial a host that isn't on the list.
• The re-check sets pctx.StatusCode = 403, so the deferred RunFinish counts the refusal as an allow, not an error. That's the same as the 426 and destination_private cases.
• The reload claim holds: reloader.go:488 refuses any Listener change (reflect.DeepEqual).
• The new scope note (forward proxy only, not enforce-redirect or extproc) matches the code. HandleTransparentConn never looks at EgressAllow.

Author: mrsabath (MEMBER — maintainer)
Areas reviewed: Go (listener, pipeline, config, cmd), Docs (CLAUDE.md)
Agent/IDE config (.claude/.vscode): none
Commits: 8 commits, all signed off: yes
CI status: passing; a few long jobs (core, CodeQL Go, cpex) still pending when I posted

Ran locally on the PR head with GOWORK=off: go test passes for core/listener/egressallow, core/listener/forwardproxy, core/pipeline and cmd/cortex's TestResolveEgressAllow. gofmt -l is clean on every touched package.