Re: [Openvpn-users] Windows client, mysterious routes and MTU issues

[Erich Titl <erich.titl@xxxxxxxx>]

John A. Sullivan III wrote:
> Please pardon the top post but I'll use the previous body as additional
> information to this post.  We now know what the problem is but it's
> uglier than we thought.  We are getting severe fragmentation on our
> OpenVPN network.  Large, do-no-fragment pings work perfectly fine until
> we launch a TCP session.  Then the tunnel MTU plummets to 576.  The
> problem is PMTU discovery.  Either we have something very badly
> misconfigured (configuration files are below) or we have hit a bug (far
> less likely!).
> 
> The bottom line is that, on a tunnel with a perfectly fine MTU as we can
> see from the 1472 byte pings, OpenVPN is returning a fragmentation
> needed ICMP packet for any sized TCP packet and, worse yet, it says that
> the acceptable MTU is zero bytes! (Unless I am misreading my ethereal
> traces).
> 
> Here's what I saw and what I tried:
> 
> We established that we could ping the full 1500 byte MTU with:
> ping -f -t -l 1472 10.7.254.1 - all replies successful until we launched
> an ssh session with the same host.  The TCP sequence began as follows:
> 
> No.Time  Packet Size Source   Destination   Protocol Info
> 1 0.000000  62     172.20.98.2  10.7.254.1  TCP      1349 > 22 [SYN]
> Seq=0 Ack=0 Win=16384 Len=0 MSS=1460
> 
> 2 0.049058  90     172.20.98.1  172.20.98.2 ICMP     Destination
> unreachable (Fragmentation needed)
> 
> Notice the very small packet size. This was only a 62 byte packet.  MTU
> is 1500.  Worse yet, here is the expansion of the ICMP packet returned
> by the OpenVPN server:
> 
> Internet Control Message Protocol
>     Type: 3 (Destination unreachable)
>     Code: 4 (Fragmentation needed)
>     Checksum: 0x133e [correct]
>     MTU of next hop: 0
>     Internet Protocol, Src: 172.20.98.2 (172.20.98.2), Dst: 10.7.254.1
> (10.7.254.1)
>     Transmission Control Protocol, Src Port: 1349 (1349), Dst Port: 22
> (22), Seq: 2813985907, Ack: 0
> 
> Notice the MTU next hop! Is that normal? I would assume the Windows
> default behavior when receiving such a request is to go to the old
> default MTU of 576.  That's what we are seeing.
> 
> I wondered if the bogus MTU might be coming from some other source as
> this is a fairly complex setup.  The path is from the tun device to the
> local eth device to the eth device on the OpenVPN gateway to the tun
> device of the gateway to the ipsec0 device on the gateway to the eth on
> the gateway to the eth on the remote network gateway to ipsec0 to eth,
> etc.  The problem could be anywhere so we traced at multiple points.
> 
> The do not fragment appears to be coming from the OpenVPN server.  We
> see the ICMP frag-needed packets on the gateway tun device.  We do not
> see them on ipsec0 (we even check lo just in case), so they do not
> appear to be reflected back from a downstream problem (I'm not even sure
> if these messages are reflected backwards).
> 
> Thus, it seems that the OpenVPN gateway advertising an MTU of 0 is the
> problem.  If we disable PMTU discovery on the Windows client, all works
> well and there is no fragmentation.

What is the requested Size in this case? I would expect 576.
It is possible that the smaller size now fits the other end of the
tunnel and no ICMP frag needed is sent?

Have you tried to restrict the MTU size of the windoze client to let's
say 1300? It is an ugly registry hack, but nonetheless.

When you observe the severe fragmented traffic, what is the negotiated
MTU size?

> 
> The configuration files are included below from the original post.  Have
> we misconfigured our systems or stumbled across a bug? We are using
> OpenVPN 2.1 beta 7 on the client and beta 8 on the server.  The server
> side is running in a Xen domU (virtual machine) running Fedora Core 3.

Did you try with real machines? I heard rumours that virtual machines
handled network interfaces differently.

cheers

Erich


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