[kbouck]

If you disable ARP, you can have a group of servers on the same network configured with the same IP! and if a server acting as a routing frontend can forward packets to a backend server's network interface by mac address (need a kernel extension for this trickery), that backend server will recognize itself as the destination, swap the source/dest IP and respond directly back to the client (without going back through the routing frontend)

Alternatively, you can accomplish the same without disabling ARP and by just adding the common IP address as an alias to the loopback interface, which allows the backend to recognize itself as the destination, but avoids ARP conflicts.

This was a trick used by IBM's WebSphere software load balancer back in the 90's-00's

[citrin_ru]

> This was a trick used by IBM's WebSphere software load balancer back in the 90's-00's

Cicso IOS SLB can work in a similar way - a virtual IP added as an alias to loopback on each server in a farm. An advantage over more widely used L3 balancing that there is need to rewrite headers in IP packets.

[lmz]

Also known as DSR (Direct Server Return) https://www.haproxy.com/blog/layer-4-load-balancing-direct-s...

[Bluecobra]

>If you disable ARP, you can have a group of servers on the same network configured with the same IP!

The downside to this is that a switch/bridge will not learn the MAC address and continue to flood/broadcast these packets to every port in that segment. So if you do decide to do this make sure you make a dedicated VLAN. :)

[10000truths]

ARP is for the LAN devices. L2 switches don't rely on ARP to build up their forwarding tables, they can just inspect the source MAC of every Ethernet frame they receive, and correlate it with the port they receive it on. Frames with unknown destination MACs are broadcast, but that stops as soon as every device in the LAN has sent at least one frame.

[mannyv]

F5s have an arp proxy setting so you don't have to do this. The downside is it tends to break dhcp.

[KeplerBoy]

For such low level shenanigans one can also fiddle around with dpdk. ARP disabled by default.