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INTRO:

There's an IPIP tunnel (call it megatun0) to a machine connected to a private network. The megatun0 tunnel is bound with the address 192.168.15.1. One of the network interface on the machine remote machine has address 10.10.10.10.

PROBLEM:

ping -I megatun0 10.10.10.10 reports no ICMP echo replies received. Running strace ping -I megatun0 10.10.10.10 didn't show up anything interesting except that

recvmsg(3, {msg_namelen=128}, 0)        = -1 EAGAIN (Resource temporarily unavailable)

But when running tcpdump icmp -nn -v I got the following output:

22:29:32.220191 IP (tos 0x0, ttl 64, id 34344, offset 0, flags [DF], proto ICMP (1), length 84)
    192.168.15.1 > 10.10.10.10: ICMP echo request, id 25118, seq 6, length 64
22:29:32.388927 IP (tos 0x0, ttl 64, id 7361, offset 0, flags [none], proto ICMP (1), length 84)
    10.10.10.10 > 192.168.15.1: ICMP echo reply, id 25118, seq 6, length 64

So in fact I got the echo replies from the host, but they were not reported by ping though.

Why might that happen?

Some Name
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    Probably a missing route. With your network setup (`ip -br link; ip -4 -br addr; ip -4 route; ip -4 rule`), this should be be easier to figure out. – A.B Jun 30 '23 at 20:30

0 Answers0