NIC naming

Jake milios at
Mon Oct 26 21:05:33 UTC 2015

> On Oct 26, 2015, at 4:44 PM, Chad J. Milios <milios at> wrote:
>> On 10/26/2015 2:16 PM, Terje Elde wrote:
>>> On 26 Oct 2015, at 17:30, Matthew Seaman <matthew at> wrote:
>>> This probably doesn't solve your problem, which appears to be knowing
>>> which of the ethernet ports on your machine is em0 -- the usual method
>>> to do that is by ifconfig'ing everything else down and then testing with
>>> a network cable until you find the working port.  However it might help
>>> clarify the system configuration.
>> ifconfig will give you the MAC of the card. I'm not sure if there's a standard tool for it, but it should be easy to do something like "rename the card with MAC address X to Y"
> you can put the following one-liner (fix if your email client breaks) directly into /etc/rc.conf or /etc/rc.conf.local (replace 01:23:45:67:89:ab with your "main" card's MAC address and that interface will always be named, in this example net0:
> eval setvar `ifconfig | while read a b; do if [ "$b" != "${b#flags=}" ]; then c=${a%:}; fi; if [ "$a" = ether -a "$b" = 01:23:45:67:89:ab ]; then echo ifconfig_${c}_name net0; fi; done`
>> Personally, I've started renaming along the lines of renaming em0 to em_lan0, em1 to em_wan1 and so on. Makes it a bit easier to see what's going on, both for when another admin might inherit the box, and also for myself, if I haven't touched the box in a few years.
> you may replace net0 with legal names such as up0, up1, down0, down1, down2, down4 (or in#, out#, wan#, lan#) etc. whatever makes most sense to describe your use case (just not wlan#. use wifi# or wphy# for your wlan#'s phy interface):
> name_mac () {
> eval setvar `ifconfig | while read a b; do if [ "$b" != "${b#flags=}" ]; then c=${a%:}; fi; if [ "$a" = ether -a "$b" = $2 ]; then echo ifconfig_${c}_name $1; fi; done`
> }
> name_mac up0 aa:aa:aa:aa:aa:aa
> name_mac down0 bb:bb:bb:bb:bb:bb
> name_mac down1 cc:cc:cc:cc:cc:cc
>> Sure, you could argue all of this should be documented somewhere as well - and you wouldn't be wrong - but renaming makes things obvious where the next admin will look, not just in some doc that might be somewhere he didn't.
>> Terje
> -Chad J. Milios
> P.S. any snippets of code contained in this email are hereby placed in the public domain.

Both of those should say "elif" near the middle there instead of "fi; if" to be a tiny bit faster, but either way works. Doh! I just did this little hack now. I've always just went with the straight ifconfig_phy0_name=net0; ifconfig_phoo0_name=net1; ifconfig_phum0_name=net2; myself and don't move cards around much.

More information about the freebsd-questions mailing list