David Ramsden wrote:
> On Thu, Jan 25, 2007 at 11:54:08PM +0000, Chris Dennis wrote:
>> Hello folks.
>>
>> A client of mine is mysteriously unable to access a particular website,
>> viz www.newforestliving.co.uk. The browser hangs for quite a while and
>> then says it can't display the page.
>>
>
> Recently I've had several issues like this for customer's and at our
> offices. The issue was an incorrect MTU size set on the firewall and
> ADSL equipment.
>
> You can determine your optimal MTU size on Windows by doing the
> following (I couldn't see an option to set the DF flag in the IP header
> for ping under Linux):
>
> ping -f -l 1500 www.newforestliving.co.uk
>
> If you see "Packet needs to be fragmented but DF set" this means 1500
> bytes is too large to transmit and needs to be fragmented. Reduce the
> 1500 value in the ping command by 100 until you get a reply. Then
> increase it until you find the largest value before packet needs to be
> fragmented.
>
> Take that value and add 28 bytes to it (28 bytes = 20 bytes of IP
> header without options plus 8 bytes of ICMP header). This is
> your optimal MTU size. Our optimal MTU works out at 1492. Our Cisco ADSL
> equipment set this by default to 1500 so we were seeing issues where certain
> websites wouldn't load (such as www.3com.com, passport.net).
>
> If you can't change the MTU size on the network equipment, you can set
> it at the workstation level. For example: ifconfig eth0 mtu 1492 for
> Linux or download DrTCP if you're using Windows.
>
> Try this if all else fails and you're stuck for suggestions.
>
> Regards,
> David.
>
Thanks David -- that sounds like a possibility, but I can't test it
until next week.
I've just discovered that
www.3com.com won't load from my Debian machine
- I'll try playing with MTUs and see what happens.
cheers
Chris
--
Chris Dennis cgdennis@???
Fordingbridge, Hampshire, UK
