David Boreham wrote: ...
There seem to be a number of preconditions that must be met before the arp...pub form of proxy arp will work. If these conditions are not met the kernel silently fails to answer the arp request (as oposed to for example the user seeing an error message when they run the user-space program).
First, the /proc/sys/net/ipv4/conf/<dev>/proxy_arp must be enabled on the interface where you desire arp responses to be sent.
...
I don't have this enabled and the syntax "arp -i eth0 -Ds 1.2.3.4 eth0 pub" is working as expected.
With /proc/.../proxy_arp enabled it seems that linux answers arp request for all ip addresses, which are routed to other interfaces, e.g. with this routing table:
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1 10.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 0.0.0.0 10.0.0.1 0.0.0.0 UG 0 0 0 eth0
it answers arp request on eth1 for _all_ foreign ip addresses except 192.168.1.0/24.
I saw it once, when a machine with ms windows was trying to find out its ip address from dhcp server, which I didn't have. It didn't get any and was trying to find unused one from the subnet 169.254.0.0/16 (link local addresses, RFC 3330) - it didn't succeed, because my linux router, which had /proc/.../proxy_arp enabled, answered all arp requests for this subnet...
-- Martin
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