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List:       poptop-server
Subject:    [Poptop-server] MTU is always 8 bytes too low
From:       "Florian Effenberger" <floeff () arcor ! de>
Date:       2003-11-30 1:42:53
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Hi,

I use the MPPE kernel module 0.7.1 with Kernel 2.4.22, Poptop (PPTPD) 1.1.3
and PPPD 2.4.2b3, but I guess I've discovered a bug.

I have the typcial MTU problem, but I cannot fix it. Neither by setting
"mru" and "mtu" in options.pptd, nor by manually changing the MTU and MRU on
my XP Pro client.

I always receive this error message:

kernel: mppe_compress[1]: osize too small! (have: 1400 need: 1404)

No matter which value I set the MTU and MRU to, the "need" value is always
+4 than I have set. I tried fixing it manually via firewall rules as well,
but it does not help. As soon as I disable MPPE in the options.pptd file on
the server, everything works great.

Do you have any idea what could be wrong?

I poked around a little bit, and if I issue an "ifconfig ppp1 mtu 1408" to
set my VPN ppp interface to [current MTU]+8, it works just fine. I have no
clue why, but the MTU seems to be ALWAYS 8 byte too small. Maybe it's
because both sides of the VPN are behind a masquerading router? Right now, I
have added this command to the ip-up script, but this is not fine solution,
I think.

Thanks!
Florian



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