Arne Kepp | 10 Aug 11:40 2004
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Re: GRE: read(fd=6,buffer=8055580,len=8260) ... error = Message too long

Thanks James, your reply kept me on the right track (I don't know much about
these layers). I'll look at HP again the next time I need a new server  ;) 

Removing MRU and MTU altogether from options.pptpd actually made the problem go
away in my case. According to ifconfig ppp0 has MTU set to 1450, which I believe
is the default.

Not sure how the MRU really playes into it, but I guess it makes some sense
that pptpd doesn't reject jumbo packets when MRU is not set. Anyway, try James'
approach first and try removing the MRU (and perhaps MTU) settings if
evertything else fails. Thanks again,

-Arne

Quoting James Cameron <james.cameron <at> hp.com>:

> The cause is that a packet that was sent over the external network link
> triggered an ICMP destination unreachable fragmentation needed message.
> 
> This causes the next read() on the GRE socket to fail with "EMSGSIZE",
> translated as "Message too long".
> 
> (a) find out why you have an external network interface with an
> MTU higher than one of the hops in the path to the PPTP server, and
> either fix the hop, or fix your external interface MTU.
> 
> or
> 
> (b) try brute force approach of lowering the MTU of your tunnel network
> interface (e.g. ppp0) by about 50%, in the hope that that will stop the
> problem from occurring.  Find the largest value that works.
> 
> Either of the solutions may work.  Just remember; it is caused by some
> other host between you and the PPTP server.  You can prove this by doing
> "tcpdump -i eth0 proto icmp" before the problem happens.  This may also
> tell you what network hop is causing the problem.
> 
> I doubt that encryption settings would have any effect.
> 
> -- 
> James Cameron                         http://quozl.netrek.org/
> HP Open Source, Volunteer             http://opensource.hp.com/
> PPTP Client Project, Release Engineer http://pptpclient.sourceforge.net/
> 
> 
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