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Old 07-23-2008, 11:07 PM
Alex Samad
Default Weird Internet routing / mtu setting / browser / packet fragmentation problem

On Wed, Jul 23, 2008 at 06:14:25PM +0100, j t wrote:
> Hi all.
> Does anyone here know how I can diagnose exactly where / why the
> following problem occurs...?
> I've got a typical adsl setup (in the UK) with my internal
> (192.168.0.*) network behind a nat router. Everything works perfectly
> (!) except for 1 problem, which is:
> If I try to view http://www.osnews.com from any of my linux (current
> debian lenny) boxen using iceweasel or epiphany, the page starts to
> load but never finishes (the spinner spins for ever).
> I'm baffled because the problem goes away if I do any one of the
> following things:
> a) I use "wget -p http://www.osnews.com" instead of iceweasel or epiphany,
> or
> b) I change my workstation's mtu from 1500 to 1400
> or
> c) I get my adsl router to request a public ip address in the address
> space 96.x.x.x or 97.x.x.x rather than 78.x.x.x (I can do this, since
> my ISP provides various disparate dhcp ranges).
> or
> d) I use firefox under WinXP (yeuchh).
> While I can get rid of the problem by doing any of these things, I
> would prefer to be able to go to my ISP a say "Hey, your network's
> broken, this one-line command (run from my linux machine) proves it's
> broken". Does anyone know what that one-line command is? (I'm thinking
> of some kind of "ping -s 1500" style thing, but I don't understand
> enough about tcp/ip to know the exact answer)
> Thanks for any help you can offer.

your modem/router (which ever terminates the pppoe link) should be
resetting the mtu on outgoing syn packets.

under linux, try tcpdumpt -pni eth0 host www.osnews.com
you will see what the mtu is being used

man ping

-s packetsize
Specifies the number of data bytes to be sent. The
default is 56, which translates into 64 ICMP data bytes when combined
with the 8 bytes
of ICMP header data.

-M hint
Select Path MTU Discovery strategy. hint may be
either do (prohibit fragmentation, even local one), want (do PMTU
discovery, fragment
locally when packet size is large), or dont (do not set DF

these are the two flags, set do not fragment and start at 1400 for the
packet size upto ?

have a google for adsl black hole detection

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- George W. Bush
Washington, DC

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