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Re: Stumper

  • From: Stephen J. Wilcox
  • Date: Tue Jan 21 17:34:03 2003


Are there sub-1500 byte MTUs anywhere and is one of the devices
(Linksys?) dropping the relevant icmp fragments?

Morpheus might be working by not having DF bit set..

just a possibility

test by removing any filtering of icmp

Steve


On Tue, 21 Jan 2003, Mark J. Scheller wrote:

> 
> 
> I have run into a problem that has me completely stumped, so I'm tossing it
> out to NANOG for some help.
> 
> Before I lay out the specifics, I'm not trying to point fingers at any
> particular ISP or vendor here, but this problem only exhibits itself in very
> specific configurations.  Unfortunately, the configuration is common enough as
> to get unwanted attention from the higher-ups.
> 
> Here's the particulars:
> 
> Users that have Verizon DSL and a Linksys cable/DSL router have difficulties
> accessing sites on my network -- whether they are trying with http, https,
> smtp, pop3, ssh, ftp, etc., etc.  Oh, but pings seem to be fine.  Low latency,
> no loss.  This is true even for access to a server brought up in the DMZ, to
> keep the firewalls out of the equation.
> 
> Doing some packet sniffing on the ethernet side of my router, I could see
> specific http requests never showed up (and the user saw the broken image
> icon).  This was for an mrtg graph page with +/- 30 images.  I saw the request
> for almost all the image files, save for one and the user reported the broken
> image icon for the one.  So this looks and smells like a packet loss
> issue..... but who/where/how?
> 
> Taking the Linksys out of the pictures (connecting their PC directly to the
> Verizon DSL modem) makes the problem go away.
> 
> These same users report no trouble whatsoever accessing many other common
> sites across the internet.
> 
> Here's another interesting data point:  when one user runs Morpheus (on
> any machine in his home network) he then has absolutely no problems accessing
> servers/services on my network.
> 
> Other users with Linksys routers and, say cable modem, do not have this
> problem!
> 
> So I'm looking for some pointers.  What could I have done to my edge router (a
> Cisco 3640 if that helps any) that would make it drop packets from Verizon DSL
> customers with Linksys routers so long as they aren't running Morpheus?
> 
> Mark J. Scheller (scheller@u1.net)
> 
> 
> 



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