FreeBSD aliases, done well

While consulting on a FreeBSD server, the owner mentioned that some of his IP aliases had to be manually applied after the server rebooted. He is using the ipv4_addrs_[name] syntax, which is deprecated, according to the rc.conf man page.

While reading the man page, it also states that the syntax that I’ve been using for years ifconfig_[name]_alias[N] is also deprecated, and instead suggests the use of ifconfig_[name]_aliases, which appears to be a better solution.

The man page also mentions the /etc/start_if.[name] method, but provides no examples or explanation. So I searched and found UNIXgod’s guide to sane IP aliases. His example shows an example start_if file:

#/sbin/ifconfig $1 alias <public_ip> netmask 0xffffffff
/sbin/ifconfig $1 alias netmask 0xffffffff


It looked like a reasonable approach so I removed my IP alias definitions from /etc/rc.conf and put them into /etc/start_if.[name] files. After booting, I noticed that the first alias on each interface was not defined.

I believe I understand why. Since the start_if.[name] script runs before the interfaces are configured, the first alias entry is treated as the primary IP and not an alias. Later, when the network device is configured by the ifconfig_[name] entry in rc.conf, it overwrites the address defined by that first alias. The easy workaround is to put the primary IP (or a placeholder) as the first entry in each /etc/start_if.[name] file.

I further improved upon UNIXgod’s example by declaring a couple variables. Doing so reduces the duplicate tokens and increases the clarity of each entry. Here’s an example of one of my files:

IFC=”/sbin/ifconfig lo0 inet”
AMASK=”netmask 0xffffffff”

$IFC netmask 0xff000000
$IFC alias $AMASK
$IFC alias $AMASK


I have seen or experienced the problems described by the deprecated methods. Perhaps this new technique will see fewer issues in the coming years.

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