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icmp: fix reply4() #183

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Merged
ignoramous merged 1 commit into from
Aug 27, 2025
Merged

icmp: fix reply4() #183

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36 changes: 12 additions & 24 deletions intra/netstack/icmp.go
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Original file line number Diff line number Diff line change
Expand Up @@ -117,41 +117,29 @@ func (f *icmpForwarder) reply4(id stack.TransportEndpointID, pkt *stack.PacketBu
if !f.h.Ping(data, src, dst) { // unreachable
err = f.icmpErr4(pkt, header.ICMPv4DstUnreachable, header.ICMPv4HostUnreachable)
} else { // reachable
newOptions := f.ipOpts(pkt, ipHdr)

// Correct IP header length (IHL in 32-bit words).
replyHeaderLength := uint8(header.IPv4MinimumSize + len(newOptions))
replyIPHdrView := buffer.NewView(int(replyHeaderLength))
replyIPHdrView.Write(ipHdr[:header.IPv4MinimumSize])
replyIPHdrView.Write(newOptions)

replyIPHdr := header.IPv4(replyIPHdrView.AsSlice())
replyIPHdr.SetHeaderLength(replyHeaderLength >> 2) // IHL in 32-bit words.
replyIPHdr.SetSourceAddress(route.LocalAddress())
replyIPHdr.SetDestinationAddress(route.RemoteAddress())
replyIPHdr.SetTTL(route.DefaultTTL())
replyIPHdr.SetTotalLength(uint16(len(replyIPHdr) + len(replyData.AsSlice())))
replyIPHdr.SetChecksum(0)
replyIPHdr.SetChecksum(^replyIPHdr.CalculateChecksum())

replyICMPHdr := header.ICMPv4(replyData.AsSlice())
originRef := replyData.AsSlice()
replyBuf := buffer.NewViewSize(len(originRef))
replyRef := replyBuf.AsSlice()

copy(replyRef[4:], originRef[4:])
replyICMPHdr := header.ICMPv4(replyRef)
replyICMPHdr.SetType(header.ICMPv4EchoReply)
replyICMPHdr.SetCode(0) // EchoReply must have Code=0.
replyICMPHdr.SetChecksum(0)
replyICMPHdr.SetChecksum(^checksum.Checksum(replyData.AsSlice(), 0))
replyICMPHdr.SetChecksum(^checksum.Checksum(replyRef, 0))

replyBuf := buffer.MakeWithView(replyIPHdrView)
replyBuf.Append(replyData.Clone())
replyPkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
ReserveHeaderBytes: int(route.MaxHeaderLength()),
Payload: replyBuf,
Payload: buffer.MakeWithView(replyBuf),
})

log.D("icmp: v4: %s: ok type %v/%v sz[%d] from %v <= %v",
f.o, replyICMPHdr.Type(), replyICMPHdr.Code(), len(replyICMPHdr), src, dst)

// github.com/google/gvisor/blob/738e1d995f/pkg/tcpip/network/ipv4/icmp.go#L794
err = route.WriteHeaderIncludedPacket(replyPkt)
err = route.WritePacket(stack.NetworkHeaderParams{
Protocol: header.ICMPv4ProtocolNumber,
TTL: route.DefaultTTL(),
}, replyPkt)
}
loge(err)("icmp: v4: %s: wrote reply to tun; err? %v", f.o, err)
})
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