Forward RFC 3326 Reason header on CANCEL/BYE between B2B legs

[hecko]

Summary

This change implements selective forwarding of RFC 3326 "Reason:" headers from inbound CANCEL requests to outbound CANCEL/BYE messages on peer B2B legs in the SBC. This allows call termination reasons to propagate across the SBC while maintaining security boundaries by filtering out trust-sensitive headers.

Key Changes

• Added filterReasonHdr() helper function: Extracts only RFC 3326 "Reason:" headers from raw SIP header blocks, intentionally excluding other CANCEL headers (Via, CSeq, Route, Authorization, Privacy, P-A-I) that are either meaningless on different legs or would leak trust-boundary metadata.

• Introduced CallLegTerminateEvent struct: A custom B2B event that extends B2BEvent to carry filtered CANCEL headers between CallLegs. Reuses the existing B2BTerminateLeg event_id so non-CallLeg receivers still terminate correctly without the extra header.

• Added cancel_hdrs member variable: Stores filtered extension headers (only RFC 3326 "Reason:") captured from inbound CANCEL requests. Non-empty only for the duration of a single termination call chain.

• Modified terminateOtherLeg(): Now relays CallLegTerminateEvent with filtered headers to peer legs instead of plain termination events when cancel headers are present.

• Modified terminateNotConnectedLegs(): Propagates filtered CANCEL headers to all non-connected legs via CallLegTerminateEvent.

• Enhanced onB2BEvent() handler: Added case for B2BTerminateLeg to detect CallLegTerminateEvent and apply the propagated headers when terminating the local leg.

• Updated onCancel() handler: Captures and filters the "Reason:" header from inbound CANCEL requests into cancel_hdrs before processing call termination.

• Modified terminateLeg(): When cancel_hdrs is populated, sends BYE with the filtered headers using SIP_FLAGS_VERBATIM flag instead of the default termination path.

Implementation Details

• The filtering is conservative: only RFC 3326 "Reason:" headers are forwarded; all other headers are dropped to prevent leaking sensitive information across trust boundaries.
• The cancel_hdrs variable is temporary and scoped to the termination call chain, being saved and restored to prevent unintended side effects.
• The implementation maintains backward compatibility by falling back to standard termination when no filtered headers are present.

https://claude.ai/code/session_01PWHzum8cu3oJhct3A4YEdd

[Copilot]

Pull request overview

Implements selective propagation of RFC 3326 Reason: from inbound CANCEL to outbound CANCEL/BYE across SBC B2B legs, while avoiding forwarding trust-boundary-sensitive headers.

Changes:

• Added a cancel_hdrs per-leg temporary buffer to carry filtered Reason: headers through a termination chain.
• Introduced filterReasonHdr() and a CallLegTerminateEvent to transport the filtered headers between legs using the existing B2BTerminateLeg event id.
• Updated termination and CANCEL handling to attach the filtered Reason: header(s) to generated CANCEL/BYE.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
apps/sbc/CallLeg.h	Adds cancel_hdrs member to store filtered Reason: headers during termination propagation.
apps/sbc/CallLeg.cpp	Implements Reason: extraction, propagates it via a custom terminate event, and uses it when generating CANCEL/BYE.

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[Copilot]

The static_cast<B2BEvent*> around new CallLegTerminateEvent(...) is redundant (implicit upcast already applies) and makes the ternary harder to read. Consider dropping the cast and keeping both branches symmetric for clarity.

Suggested change

	: static_cast<B2BEvent*>(new CallLegTerminateEvent(cancel_hdrs));
	: new CallLegTerminateEvent(cancel_hdrs);

[Copilot]

filterReasonHdr() only matches Reason when the ':' immediately follows the 6-character name (i.e., it rejects Reason : / Reason\t:). Elsewhere in the codebase header matching explicitly tolerates optional whitespace before ':' (see findHeader() in core/AmSipMsg.cpp), so this stricter parsing may cause valid-in-the-wild Reason headers to be dropped and not propagated. Consider loosening the match to allow OWS between the header name and ':' (or reuse the existing header-finding logic).

Suggested change

	size_t colon = hdrs.find(':', pos);
	if (colon != string::npos && colon < content_end
	&& (colon - pos) == 6
	&& strncasecmp(hdrs.c_str() + pos, "Reason", 6) == 0) {
	out.append(hdrs, pos, content_end - pos);
	out.append("\r\n");
	if ((content_end - pos) >= 6
	&& strncasecmp(hdrs.c_str() + pos, "Reason", 6) == 0) {
	size_t colon = pos + 6;
	while (colon < content_end &&
	(hdrs[colon] == ' ' || hdrs[colon] == '\t')) {
	++colon;
	}
	if (colon < content_end && hdrs[colon] == ':') {
	out.append(hdrs, pos, content_end - pos);
	out.append("\r\n");
	}