icmp: fix reply4()

[Lanius-collaris]

haha debug replyBuf: [65 0 0 84 237 199 64 0 128 1 190 171 9 9 9 9 10 0 2 100 0 0 239 131 12 60 0 0 74 174 185 145 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
E async.go:21>icmp.go:158: icmp: v4: 9: wrote reply to tun; err? header is malformed

Firestack misused IPv4.SetHeaderLength(), replyHeaderLength >> 2 is unnecessary.

firestack/intra/netstack/icmp.go

Line 129 in 271e3d3

replyIPHdr.SetHeaderLength(replyHeaderLength >> 2) // IHL in 32-bit words.

https://github.com/google/gvisor/blob/738e1d995f64d20fd11ab59a88d12b524acb672e/pkg/tcpip/header/ipv4.go#L298

The upstream has merged Amaindex's pull request google/gvisor#11609 , so my small changes should work.

[ignoramous]

Thanks!

[Lanius-collaris]

Oh, I forgot to call replyPkt.DecRef(), merging #142 will fix it.

[ignoramous]

Is #142 required when google/gvisor#11609 is present in upstream?

[Lanius-collaris]

Is #142 required when google/gvisor#11609 is present in upstream?

google/gvisor#11609 didn't touch ICMPv6, as current gvisor never delivers ICMPv6 Echo Request Messages to TransportProtocolHandler, I suggest using iptables hack for now.

😇 Maybe libslirp in AVD is too old, I can't test ping6 in emulator, but icmp-hack branch does work on physical devices.