Hitchhiker's Guide to Openbsd

6.2.8 - Setting up aliases on an interface
OpenBSD has a simple mechanism for setting up IP aliases on an interface. To do this simply edit the 
file 
/etc/hostname.
. This file is read upon boot by the 
/etc/netstart(8)
 script, which is part of the 
rc 
startup hierarchy
. For the example, we assume that the user has an interface dc0 and is on the network 
192.168.0.0. Other important information: 
●
IP for dc0 is 192.168.0.2 
●
NETMASK is 255.255.255.0 
A few side notes about aliases. In OpenBSD you use the interface name only. There is no difference 
between the first alias and the second alias. Unlike some other operating systems, OpenBSD doesn't 
refer to them as dc0:0, dc0:1. If you are referring to a specific aliased IP address with ifconfig, or adding 
an alias, be sure to say "
ifconfig int alias
" instead of just "
ifconfig int
" at the command 
line. You can delete aliases with "
ifconfig int delete
". 
Assuming you are using multiple IP addresses which are in the same IP subnet with aliases, your 
netmask setting for each alias becomes 255.255.255.255. They do not need to follow the netmask of the 
first IP bound to the interface. In this example, /etc/hostname.dc0, two aliases are added to the device 
dc0, which, by the way, was configured as 192.168.0.2 netmask 255.255.255.0. 
# cat /etc/hostname.dc0
inet 192.168.0.2 255.255.255.0 NONE media 100baseTX
inet alias 192.168.0.3 255.255.255.255
http://www.openbsd.org/faq/faq6.html (8 of 33)9/4/2011 10:02:06 AM

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6 - Networking
inet alias 192.168.0.4 255.255.255.255
Once you've made this file, it just takes a reboot for it to take effect. You can, however, bring up the 
aliases by hand using the 
ifconfig(8)
utility. To bring up the first alias you would use the command: 
# ifconfig dc0 inet alias 192.168.0.3 netmask 
255.255.255.255
(but again, a reboot is recommended to make sure you entered everything as you expected it to be!) 
To view these aliases you must use the command: 
$ ifconfig -A
dc0: flags=8863MULTICAST>
media: Ethernet manual
inet 192.168.0.2 netmask 0xffffff00 broadcast 
192.168.0.255
inet 192.168.0.3 netmask 0xffffffff broadcast 
192.168.0.3