Update inet.py

[Shu-xueyuan]

Fix UDP packet unable to calculate validity when carrying AH extension header

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant
• I executed the regression tests (using cd test && ./run_tests or tox)
• If the PR is still not finished, please create a Draft Pull Request

fixes #xxx

[codecov]

Codecov Report

Merging #4227 (45aee45) into master (d71014a) will decrease coverage by 32.48%.
Report is 94 commits behind head on master.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4227       +/-   ##
===========================================
- Coverage   81.77%   49.29%   -32.48%     
===========================================
  Files         331      343       +12     
  Lines       76721    77435      +714     
===========================================
- Hits        62736    38175    -24561     
- Misses      13985    39260    +25275     

Files	Coverage Δ
scapy/layers/inet.py	23.74% <0.00%> (-46.88%)	⬇️

... and 273 files with indirect coverage changes

[guedou]

@Shu-xueyuan thanks for your PR. Could you share an example that triggers the issue that you are fixing?

[Shu-xueyuan]

@Shu-xueyuan thanks for your PR. Could you share an example that triggers the issue that you are fixing?

@guedou I'm very glad that you can reply to me. When I use the official scapy library to construct a UDP message carrying Authentication Header, the Checksum of the message is illegal. Using my modified scapy library to construct the same message will not have this problem. The following is my process of calling scapy: originalPacket = Ether(**ETH2_dicts) / IPv6(**IPV6_dicts) / AH(**Ah_dicts) / UDP(**UDP_dicts).

[Shu-xueyuan]

@Shu-xueyuan thanks for your PR. Could you share an example that triggers the issue that you are fixing?

@guedou This is the encapsulation format of AH in transport mode：

Looking forward to your reply.

[gpotter2]

Hi. This looks good, but you need to add tests in https://github.com/secdev/scapy/blob/master/test/scapy/layers/inet.uts to make sure that this does not regress. Thanks

[Shu-xueyuan]

@gpotter2 Hello, the test case has been submitted

[Shu-xueyuan]

@gpotter2 Hello, when will my changes be reviewed?

[polybassa]

LGTM

[polybassa]

Please fix the flake8 issues.

[gpotter2]

I noted I still need to check when this PR really applies, and compare to what's already implemented in ipsec.py. I will review this more in depth eventually, sorry for the delay. There were some higher priority issues blocking for 2.6.0

[Shu-xueyuan]

Thanks for your reply, I totally understand. I'll be happy to wait until you have time to review my edits. If there is anything you need further explanation or assistance from me, please feel free to let me know. I fully understand the high-priority issues encountered in version 2.6.0 and hope that the issues can be resolved smoothly. Thanks!

[Shu-xueyuan]

Please fix the flake8 issues.

@polybassa Thank you for your review, the issue has been resolved.