Mini How-to on Setting Up IP Aliasing On A Linux Machine

GNU Copyleft 1996/1997 Harish Pillay (h.pillay@ieee.org)

Primary site: http://home.pacific.net.sg/~harish/linuxipalias.html

Synopsis:

This is a cook book recipe on how to set up and run IP aliasing on a Linux box. In addition, there are instructions on how to also set up the machine to receive e-mail on the aliased IP #s.

My setup:

Latest kernel (2.0.27 - from ftp.funet.fi:/pub/Linux/kernel/src/v2.0) - has worked since 1.3.7x.
IP Alias compiled as a loadable module. You would have indicated in the "make config" command to make your kernel, that you want the IP Masq to be compiled as a (M)odule. Check the Modules HOW-TO (if that exists) or check the info in /usr/src/linux/Documentation/modules.txt.
I have to support 2 additional IPs over and above the IP already allocated to me.
A D-Link DE620 pocket adapter (not important, works with any Linux supported network adapter).

Commands:

First 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 
```
Second, setup the loopback, eth0 and all the IP #s beginning with the main IP # 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 #, while .10 and .100 are the aliases. The magic is the eth0:x where x=0,1,2,...n for the different IP #s. The main IP # does not need to be aliased.

Third, setup the routes. First route the loopback, then the net and, finally, the various IP #s 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 # above, I am using the Private IP #s (RFC 1918) for illustrative purposes. Substitute them with your own official or private IP #s.

The example shows only 3 IP #s. The max is defined to be 256 in /usr/include/linux/net_alias.h. 256 IP #s 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!

Question: How can I keep the settings through a reboot?

Answer: Whether you are using BSD-style or SysV-style (Redhat for example) init, you can always include it in /etc/rc.d/rc.local. Here's what I have on my SysV init system (Redhat 3.0.3 and 4.0):

My /etc/rc.d/rc.local: (edited to show the relevant portions)

#setting up IP alias interfaces
echo "Setting 172.16.3.1, 172.16.3.10, 172.16.3.100 IP Aliases ..."
/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
#setting up the routes
echo "Setting IP routes ..."
/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 eth0
/sbin/route add -host 172.16.3.10 eth0:0
/sbin/route add -host 172.16.3.100 eth0:1
/sbin/route add default gw 172.16.3.200 
#

Question: How do I set up the IP aliased machine to receive e-mail on the various aliased IP #s (on a machine using sendmail)?

Answer: Create (if not already existing) a file called for example, /etc/mynames.cw. It does not have to be this exact name nor in the /etc directory.

In that file, place the official domain names of the aliased IP #s. If these aliased IP #s do not have a domain name, then you can place the IP # itself.

/etc/mynames.cw:
----------------
# /etc/mynames.cw - include all aliases for your machine here; # is a comment.
domain.one.net
domain.two.com
domain.three.org
4.5.6.7

In your sendmail.cf file, where it defines a file class macro Fw, add the following:

.
.
.
##################
#   local info   #
##################
.
.
# file containing names of hosts for which we receive email
Fw/etc/mynames.cw
.
.
.

That should do it. Test out the new setting by invoking sendmail in test mode for example:

ganymede$ /usr/lib/sendmail -bt
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter < ruleset> < address>
> 0 me@4.5.6.7
rewrite: ruleset  0   input: me @ 4 . 5 . 6 . 7
rewrite: ruleset 98   input: me @ 4 . 5 . 6 . 7
rewrite: ruleset 98 returns: me @ 4 . 5 . 6 . 7
rewrite: ruleset 97   input: me @ 4 . 5 . 6 . 7
rewrite: ruleset  3   input: me @ 4 . 5 . 6 . 7
rewrite: ruleset 96   input: me < @ 4 . 5 . 6 . 7 >
rewrite: ruleset 96 returns: me < @ 4 . 5 . 6 . 7 . >
rewrite: ruleset  3 returns: me < @ 4 . 5 . 6 . 7 . >
rewrite: ruleset  0   input: me < @ 4 . 5 . 6 . 7 . >
rewrite: ruleset 98   input: me < @ 4 . 5 . 6 . 7 . >
rewrite: ruleset 98 returns: me < @ 4 . 5 . 6 . 7 . >
rewrite: ruleset  0 returns: $# local $: me
rewrite: ruleset 97 returns: $# local $: me
rewrite: ruleset  0 returns: $# local $: me
> 0 me@4.5.6.8
rewrite: ruleset  0   input: me @ 4 . 5 . 6 . 8
rewrite: ruleset 98   input: me @ 4 . 5 . 6 . 8
rewrite: ruleset 98 returns: me @ 4 . 5 . 6 . 8
rewrite: ruleset 97   input: me @ 4 . 5 . 6 . 8
rewrite: ruleset  3   input: me @ 4 . 5 . 6 . 8
rewrite: ruleset 96   input: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset 96 returns: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset  3 returns: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset  0   input: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset 98   input: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset 98 returns: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset 95   input: < > me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset 95 returns: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset  0 returns: $# smtp $@ 4 . 5 . 6 . 8 $: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset 97 returns: $# smtp $@ 4 . 5 . 6 . 8 $: me < @ 4 . 5 . 6 . 8 >
rewrite: ruleset  0 returns: $# smtp $@ 4 . 5 . 6 . 8 $: me < @ 4 . 5 . 6 . 8 >
>

Notice when I tested me@4.5.6.7, it delivered the mail to the local machine, while me@4.5.6.8 was to be handed off to the smtp mailer. That is the correct response.

You are all set now.

Hope the preceding is useful to someone.

Thanks to all those who have done this great work on Linux and IP Aliasing. And especially to Juan Jose Ciarlante for clarifying my questions.

Kudos to the ace programmers!

If you do find this document useful or have suggestions on improvements, do send me an e-mail at h.pillay@ieee.org.

Enjoy.

Questions?

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