Setting up IP Aliasing on A Linux Machine Mini-HOWTO
Harish Pillay
h.pillay@ieee.org
Joy Yokley - Converted document from HTML to DocBook v4.1 (SGML)
2001-01-23
Revision History
Revision 1.2 2001-01-26 Revised by: JEY
Revision 1.1 2001-01-24 Revised by: JEY
Revision 1.0 1997-01-13 Revised by: HP
This is a cookbook recipe on how to set up and run IP aliasing on a Linux box
and how to set up the machine to receive e-mail on the aliased IP addresses.
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Table of Contents
1. My Setup
2. Commands
3. Troubleshooting: Questions and Answers
3.1. Question: How can I keep the settings through a reboot?
3.2. Question: How do I set up the IP aliased machine to receive e-mail
on the various aliased IP addresses (on a machine using sendmail)?
4. Acknowledgements
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1. My Setup
* IP Alias is standard in kernels 2.0.x and 2.2.x, and available as a
compile-time option in 2.4.x (IP Alias has been deprecated in 2.4.x and
replaced by a more powerful firewalling mechanism.)
* 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).
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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!
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3. Troubleshooting: Questions and Answers
3.1. 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
#
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3.2. Question: How do I set up the IP aliased machine to receive e-mail on
the various aliased IP addresses (on a machine using sendmail)?
Answer: Create (if it doesn't already exist) a file called, /etc/
mynames.cw,for example. The file 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 addresses. If
these aliased IP addresses do not have a domain name, then you can place the
IP address itself.
The /etc/mynames.cw might look like this:
# /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. The following is an 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 handed off to the smtp mailer. That is the correct
response.
You are all set now.
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4. Acknowledgements
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 find this document useful or have suggestions on improvements, email
me at <[mailto:h.pillay@ieee.org] h.pillay@ieee.org>.
Enjoy.
For additional information on networking, you may want to consult the [http:/
/www.linuxdoc.org/HOWTO/Networking-Overview-HOWTO.html] The Linux Networking
Overview HOWTO.
