Re: proxy_arp

[Vigneswaran R <vignesh@atc.tcs.com>]

On 04/25/2014 11:05 PM, richard lucassen wrote:
> Hello list,
>
> I have two identical Linux servers, two identical vanilla kernels
> (3.2.57). I start an arping on srv1 to the *external* ip of srv2 using
> the *internal* rfc1918 addressed NIC (eth2)
>
>             +------+                        +------+
> eth0       |      |  eth2            eth2  |      |  eth0
> ip1a-------+ srv1 +--ip1b--<------>--ip2b--+ srv2 +--ip2a
> outside    |      |  inside        inside  |      |  outside
>             +------+                        +------+
>
>      arping using eth1 ^^  --via-> eth2 ^^ to this ip --^^^
>
> root@srv1# arping -I eth1 ip2a
> ARPING 213.34.90.190 from 172.31.255.249 eth2
> Unicast reply from 213.34.90.190 [00:15:17:F4:41:46]  0.891ms
> Unicast reply from 213.34.90.190 [00:15:17:F4:41:46]  0.799ms
> ^CSent 2 probes (1 broadcast(s))
> Received 2 response(s)
>
> No problem, it works as expected. The other way round however:
>
>             +------+                        +------+
> eth0       |      |  eth2            eth2  |      |  eth0
> ip1a-------+ srv1 +--ip1b--<------>--ip2b--+ srv2 +--ip2a
> outside    |      |  inside        inside  |      |  outside
>             +------+                        +------+
>
>   ^^--<- arping to this ip <--via--- ^^ using eth2
>
> root@srv2# arping -I eth2 213.34.90.130
> ARPING 213.34.90.130 from 172.31.255.250 eth2
> ^CSent 15 probes (15 broadcast(s))
> Received 0 response(s)
>
> srv1 does NOT reply to arp requests, even if I add an:
>
> "arp -sD eth1 ip1a" (which is not necessary)
>
> I compared all sysctl settings, they are equal. ip_forward is set to 1
> on both machines. The srv1 has a large iptables rulebase, the srv2 just
> some simple rules. A tcpdump shows that srv1 receives the arp requests
> but is not willing to honour the arp requests of srv2.

It seems, iptable rules will not affect the ARP. By any chance you have 
arptables or ebtables installed on srv1 which is causing the problem?

http://www.linuxcommand.org/man_pages/arptables8.html
http://ebtables.sourceforge.net/examples/basic.html#ex_config


Regards,
Vignesh
>
> The goal is proxy_arping (which unexpectedly did not work), and I
> found out that the machine srv1 even does not reply to arp requests of
> its own ip addresses.
>
> Any thoughts or hints on this matter?
>
> R.
>