shadow: Capture only 40 bytes of packets

I've had this commit sitting in my arti repo since last August, so figured I'd make an MR. @jnewsome If there's any reason to not include this change, feel free to close, I don't have a strong opinion either way.

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The pcaps are only used to check for leaks by reading the packet source/destination, so we don't need the entire packet. This should make CI artifacts a bit smaller. I see a current pcap artifact is about 46 MiB.

The fixed-size portion of an IP header is 20 bytes for IPv4 and 40 bytes for IPv6. Shadow doesn't support IPv6, but I used the IPv6 header size anyways.

requested review from @jnewsome

assigned to @opara

changed title from shadow: capture only 40 bytes of packets to shadow: Capture only 40 bytes of packets

approved this merge request

Makes sense; the rest of the data is of pretty limited usefulness anyway unless we were to somehow rig up a wireshark plugin that could decrypt the packets

enabled an automatic merge when all merge checks for 4cde5f56 pass

canceled the automatic merge

enabled an automatic merge when all merge checks for 4cde5f56 pass

This should make CI artifacts a bit smaller. I see a current pcap artifact is about 46 MiB.

shadow.data/hosts/articlient-bridge-obfs4/lo.pcap has shrunk from 46.2 MiB to 3.59 MiB, and eth0.pcap from 13.6 MiB to 658 KiB.

merged

mentioned in commit 5c25c851