Bennett, Joe wrote:

While poking and proding around I've found that my particular linksys router is very sensitive to mtu size. I'm running Suse 9.0 which defaults the mtu size to 1500 (as I would assume all other do)upon reboot. Since I'm a slave to sbc dsl, and am living with the 8 byte overhead of PPPOE, my mtu size needs to be 1492. If I reboot the machine, the mtu reverts back to the default and becomes basically useless on the net until I open up a terminal and perform an /sbin/ifconfig eth0 mtu 1492. After that, all is well with the world.

Now, on with the question. What is the best way to automate the mtu change to 1492 or is there a way to just change the default???? I'm attempting to limit my maintenance on this machine in the event that someone reboots it, or the UPS runs to the end of its rope when the power fails....

The IP MTU size defaults to 1500 because that's the maximum payload size for standard Ethernet. This changes if one of your hops is over PPPoE, IPsec, PPTP, SLIP, etc. You might try:

- Forcing the MTU in your network settings. A quick Google search says that you need to put "MTU='1492'" in /etc/sysconfig/network/ifcfg-eth0. I don't have any Suse machines handy, so I can't verify this.

- Assuming path MTU discovery is enabled, figuring out what's keeping it from working properly. PMTU works by sending out IP packets with the DF (don't fragment) bit set. If the packet is too big to traverse one of the links in the path, an ICMP message (destination unreachable, fragmentation needed) is sent back.

If the ICMP message is lost or dropped (e.g. by a firewall), the connection locks up. This is a common problem for people with overly-zealous firewall rules.

- Again assuming PMTU discovery enabled, disable it. This can be done in modern kernels with

echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc

Large data transfers won't be as efficient due to fragmentation, however.

- Pestering SBC. A friend of mine asked nicely, and SBC support configured his connection for straight bridging connection, with no PPPoE. Tell them you're running VMS or Windows ME or something.