Re: Strange http client/MTU problem under linux

[Stephen Hemminger]

On Wed, 30 Jul 2008 01:52:06 +0100
"j t" <mark473@xxxxxxxxx> wrote:

> Hi all.
> 
> I have a strange networking problem. If I run (on linux):
> wget http://ocp.com.com/ad.js
> then wget just times out, retries, and eventually gives up, thus:
> 
> 
> $ wget http://ocp.com.com/ad.js
> --2008-07-29 20:15:16--  http://ocp.com.com/ad.js
> Resolving ocp.com.com... 216.239.122.193
> Connecting to ocp.com.com|216.239.122.193|:80... connected.
> HTTP request sent, awaiting response... Read error (Connection timed
> out) in headers.
> Retrying.
> 
> --2008-07-29 20:30:17--  (try: 2)  http://ocp.com.com/ad.js
> Connecting to ocp.com.com|216.239.122.193|:80... connected.
> HTTP request sent, awaiting response... Read error (Connection timed
> out) in headers.
> Retrying.
> 
> --2008-07-29 20:45:19--  (try: 3)  http://ocp.com.com/ad.js
> Connecting to ocp.com.com|216.239.122.193|:80... connected.
> HTTP request sent, awaiting response... Read error (Connection timed
> out) in headers.
> Retrying.
> ...
> ...
> --2008-07-30 01:02:43--  (try:20)  http://ocp.com.com/ad.js
> Connecting to ocp.com.com|216.239.122.193|:80... connected.
> HTTP request sent, awaiting response... Read error (Connection timed
> out) in headers.
> Giving up.
> 
> 
> 
> If I change my mtu from 1500 to 1499, then the same wget command
> succeeds almost instantly, thus:
> 
> 
> $ wget http://ocp.com.com/ad.js
> --2008-07-29 20:28:13--  http://ocp.com.com/ad.js
> Resolving ocp.com.com... 216.239.122.193
> Connecting to ocp.com.com|216.239.122.193|:80... connected.
> HTTP request sent, awaiting response... 200 OK
> Length: 1069 (1.0K) [text/html]
> Saving to: `ad.js'
> 
> 100%[======================================>] 1,069       --.-K/s   in 0s
> 
> 2008-07-29 20:28:14 (89.0 MB/s) - `ad.js' saved [1069/1069]
> 
> 
> I've tried Debian Etch (with 2.6.18) and Debian Lenny (with 2.6.24)
> and the problem remains with both kernel versions. I get the same
> problem (under Linux) using various http clients: wget, epiphany and
> iceweasel (firefox). If, however, I reboot into Windows XP and use a
> native wget (from http://users.ugent.be/~bpuype/wget/) or firefox, the
> problem does not occur.
> 
> The final twist in the tale is that I can force my router (using a
> dhcp option) to get a public-facing IP on either the 78.105.x.x
> network or the 93.96.x.x network, and the problem only ever occurs
> when I'm connected to the 78.105.x.x network.
> 
> So, to recap, if my workstation is booted into linux (instead of
> windows XP) and my ISP has given my router an external ip address of
> 78.105.x.x (instead of 93.96.x.x), then I can't retrieve the contents
> at the url http://ocp.com.com/ad.js (regardless of which http client I
> use).
> 
> I think my next step is to run tcpdump on my router and dump the tcp
> flows in both cases and then compare, but I'd love to know if anyone
> reading this has any better ideas (I've already spent about 3 weeks
> trying to get this far - if there's a shortcut, I'd be very grateful
> if someone would tell me).
> 
> Thanks in advance for any help you can offer, Jaime.
> 
> PS: In case you're wondering why I'm interested in that url, it's
> included in http://www.osnews.com homepage (which doesn't load for me,
> and I've managed to trace it back to that ad.js url).

You may be suffering with a site that has a window scaling problem.
This is due to some firewall or other middlebox that can't keep track
of window scaling properly. More information is in:
     http://lwn.net/Articles/92727/
    
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