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RE: Linux PPTP VPN server - can't map Windows shares/printers
From: Peter Mueller <pmueller () sidestep com>
Date: Mon, 24 Nov 2003 12:46:19 -0800

Hello,

My MTU setting in /etc/ppp/options.pptpd is currently set to 
1450.  I wouldn't imagine that this is too low, but as this 
is my first VPN server, I'm open to suggestions.  However, I 
believe this to be a problem with routing the appropriate 
data through the server.

Theoretically MTU is supposed to resize automagically through the use of
certain ICMP packets.  I found this didn't occur for me with the latest
poptop-CVS, so..

<snip /etc/ppp/options.pptpd>
# MTU/MRU setting:
#            TCP header = 20 byte
#            IP header = 20 byte
#            GRE header = 24 byte
#            PPP header = 8 byte
#            MPPE header = 2 byte
#            GRE over Eth: 1500 - 24 = 1476 MTU (-20 -20 = 1436 MSS)
#            GRE over PPP over Eth: 1500 - 8 - 24 = 1468 MTU (-20 -20 = 1428
MSS) -> used with PPPoE adsl
#            GRE over PPP over Eth with MPPE: 1500 - 8 - 24 - 2 = 1466 MTU
(-20 -20 = 1426 MSS) -> used with PPPoE adsl
</snip>

I found it was necessary to change the MTU in /etc/ppp/ip-up in my
particular setup.  This was because I was trying to connect to an
IPSec-connected remoted network.  Note that this doesn't change the MRU, but
I found that it wasn't necessary in my setup.

<snip /etc/ppp/ip-up>
#!/bin/bash

# This file should not be modified -- make local changes to
# /etc/ppp/ip-up.local instead

LOGDEVICE=$6
REALDEVICE=$1

export PATH=/sbin:/usr/sbin:/bin:/usr/bin

[ -x /etc/ppp/ip-up.local ] && /etc/ppp/ip-up.local $*

# Used for clustering heartbeat monitoring stuff.
[ -x /etc/ppp/ip-up.heart ] && /etc/ppp/ip-up.heart $*

/etc/sysconfig/network-scripts/ifup-post ifcfg-${LOGDEVICE}

/sbin/ifconfig $1 mtu 1442    # <-- add this line

exit 0
</snip>

Hope it helps.

Peter M.

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