10. Appendix A - Netmask 255.255.255.255, proxy arp and bridging

Here we view some Linux advantages in Wireless Internetworking.

Linux let you specify a netmask like 255.255.255.255 for an interface which can help you assign IP addresses in to any interface you want, for example one in eth0, another in eth1 and so on...

This has not particularly side-effects.

In addition you have proxy arp setting under /proc/sys/net/ipv4/conf/ethx/proxy_arp where ethx if your interface.

If you "echo 1 > proxy_arp" you enable proxy_arp for that interface while with "echo 0 > proxy_arp" you disable it.

What's proxy_arp? Quickly proxy arp help you when you want a router answer to an ARP request if the destination address is in another interface of the linux router.

Example:

192.168.1.1 ---- 192.168.1.2 Linux router 192.168.2.2 ----192.168.2.1 

To get this example working you should:

Without proxy-arp 

1. In 192.168.1.1 host to set 192.168.1.2 as gateway
2. In 192.168.2.1 host to set 192.168.2.2 as gateway
3. pinging with success from any edge.

With proxy-arp 

1. In 192.168.1.1 host to set 192.168.1.2 as gateway
2. do not set gateway for 192.168.2.1 host but enable proxy_arp for right interface of the router.
3. pinging with success from any edge

Proxy-arp in 2. case let the linux router answer when you ping from 192.168.2.1 host, saying that it has the 192.168.1.1 host so it can answer for it. After, when the source start sending ICMP packet, Linux router knows that it have to redirect it to the real host 192.168.1.1

In Wireless network proxy arp can help you if you have many Linux boxes that acts as IP Forwarders and you wouldn't set to all hosts a number of static route.

You also can experiment Linux bridging in Wireless network:

1. install a recent stable kernel
2. download good bridge utils at http link or ftp link

Bridging should be more simple to manage.