The proxy_arp flag allows arp requests to proxy through an interface in a Linux system.

This wiki page has some more information on proxy_arp.

Check if proxy_arp is enabled for eth0 (for other interfaces, simply replace eth0 with your interface).

cat /proc/sys/net/ipv4/conf/eth0/proxy_arp

Enable arp proxy on eth0.

echo "1" > /proc/sys/net/ipv4/conf/eth0/proxy_arp

Disable arp proxy on eth0.

echo "0" > /proc/sys/net/ipv4/conf/eth0/proxy_arp