Detect TCPError/UDPerror/SCTPerror after IPv6 Extension Headers

[guedou]

Fixes #4904

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (f303033) to head (ca622f8).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4914      +/-   ##
==========================================
- Coverage   80.84%   80.64%   -0.21%     
==========================================
  Files         369      370       +1     
  Lines       90968    91737     +769     
==========================================
+ Hits        73544    73981     +437     
- Misses      17424    17756     +332     

Files with missing lines	Coverage Δ
scapy/layers/inet6.py	88.64% <100.00%> (+0.11%)	⬆️
scapy/layers/sctp.py	95.83% <100.00%> (+0.02%)	⬆️

... and 19 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR fixes Scapy’s IPv6-in-ICMPv6 error dissection so that when an IPerror6 payload contains IPv6 extension headers, the final L4 layer is parsed as TCPerror/UDPerror/SCTPerror (as appropriate) rather than the non-error variants (e.g., TCP).

Changes:

• Update IPv6 extension header payload guessing to return TCPerror/UDPerror/SCTPerror when an IPerror6 exists in the underlayer chain.
• Extend IPv6 layer UTS coverage with a regression test for IPerror6 + IPv6 extension header + TCPerror.

Reviewed changes

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

File	Description
scapy/layers/inet6.py	Adds IPerror6-aware logic in _IPv6ExtHdr.guess_payload_class() to select *error layers after IPv6 extension headers.
test/scapy/layers/inet6.uts	Adds a regression test ensuring TCPerror is detected behind an IPv6 Fragment extension header within an ICMPv6 error citation.

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