I have a typical "a can ping b but b cannot ping a" problem. I use IPv6 for the ping. What is special in my case is that "b" can ping "a" immediately after "b" pings "a". After some trials, I realised that everything works fine if I specify the network interface that should be used with "-I eth0" (yes, two interfaces on the Linux machine are in the same subnet). I would still like to understand what changes once machine "b" pings back and would appreciate some expert knowledge. Ping with IPv4 works fine in both directions. If I stop the ping from "b", "a" can still ping "b", until I stop it with Ctrl-C. Trying again fails even if I try immediately, so I believe it is not an upcache issue. Some info:
Machine "a"
OS: Windows (added "ICMPv6" in exceptions for firewall)
IPv6 address: fe80::21c:1cff:fe00:3de3
Machine "b":
OS: Linux Busybox
IPv6 address: fe80::21c:1cff:fe00:3de4
Tried both "ping" and "ping6" binary
address%zone
, e.g.fe80::21c:1cff:fe00:3de4%eth0
on Linux; the correct number can be found in a table fromroute print
on Windows