Stumper
Stephen J. Wilcox
steve at telecomplete.co.uk
Tue Jan 21 22:30:21 UTC 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 at u1.net)
>
>
>
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