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Adding IP aliases in FreeBSD

·214 words·2 mins·
Command Line

One question I hear quite often is “how do I add IP aliases in FreeBSD?” It’s not terribly intuitive, but you can follow these steps:

Example:

Server’s primary IP: 192.168.1.5

Additional IP’s to add: 192.168.1.10, 192.168.1.15, and 192.168.1.20

Boot-time configuration:

Add it to /etc/rc.conf first (so you don’t forget). In this example, we have a Realtek card called rl0:

ifconfig_rl0="inet 192.168.1.5 netmask 255.255.255.0"
ifconfig_rl0_alias0="inet 192.168.1.10 netmask 255.255.255.0"
ifconfig_rl0_alias1="inet 192.168.1.15 netmask 255.255.255.0"
ifconfig_rl0_alias2="inet 192.168.1.20 netmask 255.255.255.0"

UBER-IMPORTANT NOTE: Start with the number 0 (zero) any time that you make IP alias configurations in /etc/rc.conf.

This is BAD form:

ifconfig_rl0="inet 192.168.1.5 netmask 255.255.255.0"
ifconfig_rl0_alias1="inet 192.168.1.10 netmask 255.255.255.0"
ifconfig_rl0_alias2="inet 192.168.1.15 netmask 255.255.255.0"
ifconfig_rl0_alias3="inet 192.168.1.20 netmask 255.255.255.0"

If you do it the wrong way (which means starting alias with anything but alias0), only the primary comes up. Keep that in mind.

Bringing up the new IP’s:

You can do things the extraordinarily dangerous way:

# /etc/rc.network restart

Or, you can follow the recommended steps:

# ifconfig rl0 alias 192.168.1.10 netmask 255.255.255.0
# ifconfig rl0 alias 192.168.1.15 netmask 255.255.255.0
# ifconfig rl0 alias 192.168.1.20 netmask 255.255.255.0

Test your work:

Any good system administrator knows to test things once their configured. Make sure to ping your new IP’s from a source on your network and outside your network (if possible/applicable).