Re: [lvs-users] ipvsadm and packets leaving a gre tunnel

To: " users mailing list." <lvs-users@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [lvs-users] ipvsadm and packets leaving a gre tunnel
From: Joseph Mack NA3T <jmack@xxxxxxxx>
Date: Wed, 23 Jul 2008 07:22:45 -0700 (PDT)
On Wed, 23 Jul 2008, Marco Lorig wrote:

If I understand correctly, nopmtudisc within GRE-tunnels masks fragmentation to 
client and server.

any fragmentation inside the gre tunnel is transparent to the networks feeding it? ie the gre tunnel just looks like a regular 1500 byte packet carrier?

I couldn't find anything useful with google on nopmtudisc. Everyone uses it blindly as a magic parameter. The only posting that tried to figure anything out was

but he didn't get any useful answers


shows the command for turning off pmtu discovery for Linux
about half way down the page. It's

echo  1  >/proc/sys/net/ipv4/ip_no_pmtu_disc

so I assume the gre nopmtudisc is a part of this problem.
Can you give the opposite of nopmtudisc? Can you allow fragmentation at the src.

(I know this is a lot of pain to fix a system that was working for 2.4.)

This article also gives the pmtu timeout as 10mins, which is the same as your timeout.

He also says to change the mtu size for the route involved (in his example, the default route).

new connection or the current connection as well?

The connection is terminated after the transfer, so only new connections don´t 
work , I reckon.

wierd. I would have thought that the gre tunnel would handle each packet separately, independantly of the connection. (I thought mtu was for each packet, not per connection.)

Maybe the gre tunnel is handling the connections OK and the src end of the tunnel is getting bad info about the mtu size after the time out when it sends a SYN packet. Can you filter out the "need fragmentation" icmp packets that the src gets from the gre tunnel? (just block all icmp packets for the moment)


Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at
Homepage It's GNU/Linux!

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