layers.inet.fragment: use fragsize=8 if argument is lower than that

[lpenz]

fragment was raising a ZeroDivisionError when given a frag size < 8. This is a regression, as versions older than 2.4.4 didn't have this problem. It was a side-effect of fixing fragment to support frag sizes that are not multiples of 8.

This commit simply tops up frag size to 8 if it's lower, as IP can't really handle fragments smaller than that anyway, due to the way the Fragment Offset field works.

[codecov]

Codecov Report

Merging #3970 (3040c89) into master (4aaed1d) will decrease coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3970      +/-   ##
==========================================
- Coverage   81.65%   81.19%   -0.47%     
==========================================
  Files         326      326              
  Lines       75466    75468       +2     
==========================================
- Hits        61625    61274     -351     
- Misses      13841    14194     +353     

Impacted Files	Coverage Δ
scapy/layers/inet.py	71.85% <100.00%> (+0.04%)	⬆️

... and 16 files with indirect coverage changes

[gpotter2]

Thanks for the PR.

Just a note on why this is taking so long: you are absolutely right and your fix is mostly right, but I'm not looking forward merging it. This PR pointed out that the fragment/frag implementation is duplicated and kinda terrible, so I think that the best option would be to actually rewrite the fragmentation functions (and session integration entirely). I'm working on that but it is taking longer than anticipated.

You can keep the PR open because it shows there's an issue, and use your patch in the meantime. Sorry for the wait !

[lpenz]

I understand your point 100%, it took me quite a bit of digging to figure the problem. I had a bigger patch for this but it not also not quite easy to understand. I'm using this one at the moment, it fits my immediate needs - so no worries about that.

Thank you for working on this!