On 06/30/2009 05:39 PM, Jay Daniels wrote:
> Seems to only happen on certain sites repeatedly at times and at random.
>  However, while browsing using hardy and firefox I've been getting a
> lot of these.
>
> Connection reset by server...
>
> Shoddy satellite connection; however, I was wondering if there is an MTU
> or some other setting on the Linksys I could set to get rid of this
> problem? I believe it set to standard 1500.
>
>
> jay
>

You can test for when fragmentation starts:

ping -M do -s 1460 google.com

and you should get normal ping. That setting is MTU (Maximum
Transmission Unit - the maximum packet size for the link) 1488
(1460+28). Now adjust 1460 higher until you hit something like:

$ ping -M do -s 1465 google.com
PING google.com (74.125.45.100) 1465(1493) bytes of data.
From <machine> (192.168.4.100) icmp_seq=1 Frag needed and DF set (mtu =
1492)

So if I set my MTU higher than 1492 (1464+28) to 1493 (1465+28) I run
into fragmentation issues.

The default MTU set in Ubuntu is 1500 (it is set that way because
Ethernet has a maximum payload size of 1500 octets). However because my
DSL is using PPoE, my router and my dsl modem are set for 1492 per RFC
2516 (RFC2516 - A Method for Transmitting PPP Over Ethernet (PPPoE)). So
it makes no sense to have my client MTU(s) set at 1500[1]:

$ tracepath google.com
1: <machine> (192.168.4.100)  0.177ms pmtu 1500 <=== this is when I'm
set to the default 1500
1: 192.168.4.1 (192.168.4.1)  0.534ms pmtu 1492 <=== this is telling
me that my router set to 1492 per RFC 2516

So I can temporarily change the MTU on the client to 1492:

$ sudo ifconfig eth0 mtu 1492

And I now match my router:
$ tracepath google.com
1: <machine> (192.168.4.100)       0.175ms pmtu 1492
1: 192.168.4.1 (192.168.4.1)       0.564ms asymm 106

If I wish to change eth0 to 1492 permanently:

$ gksu gedit /etc/network/interfaces

and at the bottom of my iface secton I add 'mtu 1492':

iface eth0 inet static
address 192.168.4.100
netmask 255.255.255.0
gateway 192.168.4.1
mtu 1492

If you are using DHCP, I _think_ it's something along the lines of:

iface eth0 inet dhcp
pre-up /sbin/ifconfig $IFACE mtu 1492


[1] I only have a small local network, so I'm not concerned about the
difference between a 1500 MTU vs 1492 MTU on the ethernet between
machines.



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