Proxy ARP in Linux

In my previous post, you have seen how proxy arp was configured on cisco router.  Now, we do a demo on a linux machines FC19.

Briefly,

• Without proxyarp, Multihost will respond for all configured IPs
• With proxyarp, Multihost will respond for all connected network IPs(except the interface on which it receives arp request)

Here, I will discuss only about how multicast responds to ARP and not about Ping.  To simplify our explanation we use a tool 'arping'.  It is usually inbuilt in all machines.

Without proxyarp, Multihost will respond for all configured IPs

No proxyarp is enabled in Multihost.  'eth0' on Multihost will arp respond to its configured IP addresses '172.16.0.10' and '192.168.1.8'.  The way I use arping below to direct arp request is self-explanatory.

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[root@abc12end1 ~]$arping -I enp0s20u4 -f 172.16.0.10 ARPING 172.16.0.10 from 172.16.0.1 enp0s20u4 Unicast reply from 172.16.0.10 [00:FF:18:B4:CC:6F] 0.811ms Sent 1 probes (1 broadcast(s)) Received 1 response(s) [root@abc12end1 ~]$arping -I enp0s20u4 -f 192.168.1.8 ARPING 192.168.1.8 from 172.16.0.1 enp0s20u4 Unicast reply from 192.168.1.8 [00:FF:18:B4:CC:6F] 0.799ms Sent 1 probes (1 broadcast(s)) Received 1 response(s) [root@abc12end1 ~]$arping -I enp0s20u4 -f 192.168.1.1 ARPING 192.168.1.1 from 172.16.0.1 enp0s20u4 ^CSent 2 probes (2 broadcast(s)) Received 0 response(s)

Observe that I requested for 192.168.1.1 for which Multihost didn't respond.

With proxyarp, Multihost will respond for all connected network IPs


Now, I enable proxyarp using the following commands.

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# cat /proc/sys/net/ipv4/conf/all/proxy_arp 0 # cat /proc/sys/net/ipv4/ip_forward 0 # echo 1 > /proc/sys/net/ipv4/conf/all/proxy_arp # echo 1 > /proc/sys/net/ipv4/ip_forward # cat /proc/sys/net/ipv4/conf/all/proxy_arp 1 # cat /proc/sys/net/ipv4/ip_forward 1

Now issue arp request for any of the IP address in 192.168.1.x network.  Irrespective of whether the host exists or not, eth0 will respond with its MAC address.

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[root@abc12end1 ~]$arping -I enp0s20u4 -f 192.168.1.8 ARPING 192.168.1.8 from 172.16.0.1 enp0s20u4 Unicast reply from 192.168.1.8 [00:FF:18:B4:CC:6F] 0.802ms Sent 1 probes (1 broadcast(s)) Received 1 response(s) [root@abc12end1 ~]$arping -I enp0s20u4 -f 192.168.1.1 ARPING 192.168.1.1 from 172.16.0.1 enp0s20u4 Unicast reply from 192.168.1.1 [00:FF:18:B4:CC:6F] 444.585ms Sent 1 probes (1 broadcast(s)) Received 1 response(s) [root@abc12end1 ~]$arping -I enp0s20u4 -f 192.168.1.2 ARPING 192.168.1.2 from 172.16.0.1 enp0s20u4 Unicast reply from 192.168.1.2 [00:FF:18:B4:CC:6F] 126.458ms Sent 1 probes (1 broadcast(s)) Received 1 response(s)

Here 192.168.1.1 and 192.168.1.2 hosts do not exist.  But Multihost's eth0 responded with its MAC Address.