What the f#&* is MTU and why have Telstra changed it? More importantly why does my CounterStrike not work.

Well I think the heading pretty well spells it out, here I am everything travelling ok and one day i get an error from within my Counterstrike Source stating error after 4 tries – hmm strange, now everthing else is working, browsing the web, mail, even finding CSS servers and Steam connection, it all worked fine. The only thing was trying to connect to a server..whack thanks for coming..no CSS for you :).css-erro-cropr.png

css-error-2-crop.pngA little research pulled up a similiar problem from the Gamearena forums with a clue regarding the recent Telstra change of the MTU value – note there was very little info and related cases…. Now wtf is a mtu and why is telstra messing with it.

Referring to the wonderful Wikipedia, MTU stands for Maximum Transmission Unit. Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links). A higher MTU brings higher bandwidth efficiency. However, large packets can block up a slow interface for some time, increasing the lag for further packets.

Read on…it gets interesting 🙂

Now telstra has, in their infinte wisdom set the MTU for their DHCP servers to 576 – yep thats a random number ok and more importantly it is quite low and even more importantly blows for gaming…..

So apparently most DHCP clients just ignore what MTU value is specified by the server, however some real routers and gateways, like my debian based firewall/gateway request the MTU. So we need to get our gateway to be alittle more stupid and not ask for this…just ignore it.

So find out the DHCP client, in the case of Debian the defalut is dhcp3-client – DHCP Client and fire up the favorite editor and what do we find

# nano /etc/dhcp3/dhclient.conf

request subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, host-name,
netbios-name-servers, netbios-scope, interface-mtu;

yep that would probably be the one! remove that from the request list and.. very important DONT just restart the network interface, that does not seem to do it…. restart the box and fire up the CSS and get back to fragging…. now if only getting work stuff out of the way was that easy 🙂

For future reference:



and my dhclient.conf file

# Configuration file for /sbin/dhclient, which is included in Debian’s
# dhcp3-client package.
# This is a sample configuration file for dhclient. See dhclient.conf’s
# man page for more information about the syntax of this file
# and a more comprehensive list of the parameters understood by
# dhclient.
# Normally, if the DHCP server provides reasonable information and does
# not leave anything out (like the domain name, for example), then
# few changes must be made to this file, if any.

#send host-name “andare.fugue.com”;
#send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
#send dhcp-lease-time 3600;
#supersede domain-name “fugue.com home.vix.com”;
#prepend domain-name-servers;
request subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, host-name,
netbios-name-servers, netbios-scope, interface-mtu;

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