Linux 2.2.17 (only tested version, I assume all other 2.2 series suffer from
the same problem and possibly 2.4 as well - but I havent even looked at that).
Assuming a configuration with linuxbox1 eth0 has adresses 192.168.129.1 and
192.168.130.1, and IP forward being enabled, and another box on the same
ethernet with IP 192.168.129.10 and a route to 192.168.130.1 via 192.168.129.1
(eg that machine doesnt handle multiple logical nets on the same ether very
well). Now this machine pings eg 192.168.130.10.
The linux box will issue a redirect redirecting to 192.168.130.10, assuming
that 192.168.129.10 can talk directly to 192.168.130.10. Under RFC 1812
Rule 184.108.40.206 I think this is illegal (different IP networks...).
There actually is the "shared_media" sysctl, which is not properly documented
in Documentation/network/ip-sysctl.txt. Attached ip-sysctl.txt.patch fixes
that and also documents the behaviour of the ../all/.. and the ../default/..
sysctl directories as best to my knowledge (ie, I may be wrong)
That sysctl defaults to 1. It could be argued that it should be better turned
be 0 (so that RFC1812 compliance is the default).
But anyway, that sysctl does not work in the situation outlined above,
since the inet_addr_onlink check in net/ipv4/route.c will return true, because
FIB_RES_GW(res) will be 0 in that case (192.168.130.0 is directly connected).
Since I am not sure if patching that inet_addr_onlink routine may break other
stuff, I propose attached route.c.patch, which checks for this condition and
puts in the destination address (which is the next hop in this case) on that
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