[Proxy ARP] What is Proxy ARP?

Why I mention Proxy ARP is because in OpenStack environment, this function exists in FIP namespace with DVR enabled. The way to check whether the FIP namespace's Proxy ARP enabled is here:
# ip netns exec fip-545b57e2-0fa5-46da-89a2-591f7a5474ce cat /proc/sys/net/ipv4/conf/fg-2f3f4992-23/proxy_arp
1

We also can see the arp and ip mapping:
# ip netns exec fip-545b57e2-0fa5-46da-89a2-591f7a5474ce ip neighbor
10.12.20.32 dev fpr-6ddabb95-1 lladdr 42:72:88:ca:07:72 STALE

The following is the explaination about Proxy ARP
http://linux-ip.net/html/ether-arp-proxy.html

Occasionally, an IP network must be split into separate segments. Proxy ARP can be used for increased control over packets exchanged between two hosts or to limit exposure between two hosts in a single IP network. The technique of proxy ARP is commonly used to interpose a device with higher layer functionality between two other hosts. From a practical standpoint, there is little difference between the functions of a packet-filtering bridge and a firewall performing proxy ARP. The manner by which the interposed device receives the packets, however, is tremendously different.

Example 2.10. Proxy ARP Network Diagram



The device performing proxy ARP (masq-gw) responds for all ARP queries on behalf of IPs reachable on interfaces other than the interface on which the query arrives.

FIXME; manual proxy ARP (see also Section 9.3, “Breaking a network in two with proxy ARP”), kernel proxy ARP, and the newly supported sysctl net/ipv4/conf/$DEV/medium_id.

For a brief description of the use of medium_id, see Julian's remarks.

FIXME; Kernel proxy ARP with the sysctl net/ipv4/conf/$DEV/proxy_arp.