2. Commands

1. Load the IP Alias module (you can skip this step if you compiled the module into the kernel):

   /sbin/insmod /lib/modules/`uname -r`/ipv4/ip_alias.o

2. Setup the loopback, eth0, and all the IP addresses beginning with the main IP address for the eth0 interface:

   /sbin/ifconfig lo 127.0.0.1 /sbin/ifconfig eth0 up /sbin/ifconfig eth0 172.16.3.1 /sbin/ifconfig eth0:0 172.16.3.10 /sbin/ifconfig eth0:1 172.16.3.100

   172.16.3.1 is the main IP address, while .10 and .100 are the aliases. The magic is the eth0:x where x=0,1,2,...n for the different IP addresses. The main IP address does not need to be aliased.

3. Setup the routes. First route the loopback, then the net, and finally, the various IP addresses starting with the default (originally allocated) one:

   /sbin/route add -net 127.0.0.0 /sbin/route add -net 172.16.3.0 dev eth0 /sbin/route add -host 172.16.3.1 dev eth0 /sbin/route add -host 172.16.3.10 dev eth0:0 /sbin/route add -host 172.16.3.100 dev eth0:1 /sbin/route add default gw 172.16.3.200

   That's it.

In the example IP address above, I am using the Private IP addresses (RFC 1918) for illustrative purposes. Substitute them with your own official or private IP addresses.

The example shows only 3 IP addresses. The max is defined to be 256 in /usr/include/linux/net_alias.h. 256 IP addresses on ONE card is a lot :-)!

Here's what my /sbin/ifconfig looks like:

lo        Link encap:Local Loopback
               inet addr:127.0.0.1  Bcast:127.255.255.255  Mask:255.0.0.0
                         UP BROADCAST LOOPBACK RUNNING  MTU:3584  Metric:1
                    RX packets:5088 errors:0 dropped:0 overruns:0
                    TX packets:5088 errors:0 dropped:0 overruns:0
                
eth0      Link encap:10Mbps Ethernet  HWaddr 00:8E:B8:83:19:20
                    inet addr:172.16.3.1  Bcast:172.16.3.255  Mask:255.255.255.0
                    UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
                    RX packets:334036 errors:0 dropped:0 overruns:0
                    TX packets:11605 errors:0 dropped:0 overruns:0
                    Interrupt:7 Base address:0x378
                
eth0:0    Link encap:10Mbps Ethernet  HWaddr 00:8E:B8:83:19:20
                    inet addr:172.16.3.10  Bcast:172.16.3.255  Mask:255.255.255.0
                    UP BROADCAST RUNNING  MTU:1500  Metric:1
                    RX packets:0 errors:0 dropped:0 overruns:0
                    TX packets:0 errors:0 dropped:0 overruns:0
                
eth0:1    Link encap:10Mbps Ethernet  HWaddr 00:8E:B8:83:19:20
                    inet addr:172.16.3.100  Bcast:172.16.3.255  Mask:255.255.255.0
                    UP BROADCAST RUNNING  MTU:1500  Metric:1
                    RX packets:1 errors:0 dropped:0 overruns:0
                    TX packets:0 errors:0 dropped:0 overruns:0

And /proc/net/aliases:

device         family    address
eth0:0           2      172.16.3.10
eth0:1           2      172.16.3.100

And /proc/net/alias_types:

type    name            n_attach
2       ip              2

Of course, the stuff in /proc/net was created by the ifconfig command and not by hand!