Subject: Re: tun(4) minimum mtu is 576, can I safely make this smaller?
To: Tad Hunt <>
From: Thor Lancelot Simon <>
List: tech-net
Date: 10/09/2002 02:27:22
On Tue, Oct 08, 2002 at 03:02:53PM -0700, Tad Hunt wrote:
> I would like to set the MTU on the tun(4) interface to a number
> smaller than the currently allowed minimum which is
> hardcoded to 576.
> Can I safely reduce this number, or will it cause problems?

An MTU of less than 576 violates the IPv4 standard, and will cause
all kinds of obscure (and some not-so-obscure!) problems, even in
the presence of Path MTU discovery.

Since you're going to need to split the packets smaller than the
tun interface MTU anyway (fragment and reassemble in your "middle
layer" above LAPD, I suggest), it would be strongly advisable to
use an MTU of 1500 bytes, which will avoid some _other_ problems.

 Thor Lancelot Simon	                            
   But as he knew no bad language, he had called him all the names of common
 objects that he could think of, and had screamed: "You lamp!  You towel!  You
 plate!" and so on.              --Sigmund Freud