Date: Sat, 3 Feb 2007 03:20:20 GMT From: Bruce M Simpson <bms@incunabulum.net> To: freebsd-bugs@FreeBSD.org Subject: Re: bin/103762: some tun interfaces with a mtu of 1500 while i should never exceed 1472 with ppp Message-ID: <200702030320.l133KKC9073974@freefall.freebsd.org>
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The following reply was made to PR bin/103762; it has been noted by GNATS.
From: Bruce M Simpson <bms@incunabulum.net>
To: freebsd-gnats-submit@FreeBSD.org
Cc: Dominic Blais <dblais@interplex.ca>
Subject: Re: bin/103762: some tun interfaces with a mtu of 1500 while i should
never exceed 1472 with ppp
Date: Sat, 03 Feb 2007 03:19:50 +0000
Hi,
I'm a FreeBSD committer looking into this PR.
Not having access to your network infrastructure, I can't really
reproduce the problem.
This does not appear to be a tun(4) problem, however; it should simply
do what it's told via the TUNSIFINFO ioctl or SIOCSIFMTU interface ioctl.
It sounds like you're using pppoed(8) to serve PPPoE sessions. It relies
on Netgraph to do the low-level plumbing, then forks ppp(8) to handle
the PPP session itself.
I would draw your attention to the following in the ppp(8) man page:
set mru [max[imum]] [value]
The default MRU (Maximum Receive Unit) is 1500. If it is
increased, the other side *may* increase its MTU. In theory
there is no point in decreasing the MRU to below the default as
the PPP protocol says implementations *must* be able to accept
packets of at least 1500 octets.
It's entirely possible there's a bug in ppp(8) and its MRU/MTU
negotiation, however I lack the time and resources to track that down.
Perhaps try using mpd from ports instead?
Regards,
BMS
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