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I was testing something and stumbled on this weird oddity:

Ping using IPv4 and setting the source address to '127.0.0.1' (works):

# ping -I 127.0.0.1 192.168.1.1 PING 192.168.1.1 (192.168.1.1) from 127.0.0.1 : 56(84) bytes of data. 64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.130 ms

Ping using IPv6 and setting the source address to '::1' (doesn't work):

# ping -I ::1 fd00::1 PING fd00::1(fd00::1) from ::1 : 56 data bytes

The strange thing is that this used to work, at least on kernels ≤4.13.16. Is there any valid reason for this change in the Linux kernel, or am I missing something?

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  • Is 192.168.1.1 the system on which you are running ping, or another system on the network? Because on my system (kernel 6.9.x), ping -I 127.0.0.1 is unable to ping anything except local addresses, which is what I would expect. Commented Aug 1, 2024 at 17:25
  • Yes. 192.168.1.1 and fd00::1 are on the same interface and system. You can easily test this with a dummy interface. The problem is only with IPv6, but not on kernels <4.13. Commented Aug 1, 2024 at 18:38
  • Use ping6 -I ::1 fd00::1 The ICMP and ICMP6 protocols differ. Commented Aug 3, 2024 at 4:27
  • I tried your suggestion, but the outcome is the same. It's most definitely a bug in the Linux kernel starting with 4.14. Somehow fd00::1 is not being recognized as local. So in in essence, it would be the same as ping -6 -I ::1 www.google.com. Commented Aug 3, 2024 at 14:24

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