tech-net: SIOCIFMTU on lo (faith, ...) -- how to update route MTU?

Subject: SIOCIFMTU on lo (faith, ...) -- how to update route MTU?
To: None <tech-net@netbsd.org>
From: Jonathan Stone <jonathan@DSG.Stanford.EDU>
List: tech-net
Date: 04/19/2000 19:24:11

Last-year-ish, I added SIOCSIFMTU support to the loopback driver.
Works like a champ, and makes it easy to test MTU-specific hacks over
a loopback interface. (e.g., looking for spikes in TCP throughput as a
function of MTU).

The one gotcha is that if I change the MTU after bringing the interface
up and ifconfig'ing 127.0.0.1 on it, the route MTU does *not* get updated.
I tried copying the code from if_faith, and lo and behold, that seems
broken, too.

Not getting this right is the only reason I havent committed the change.
Anyone know how to fix it?