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Networking: MTU leads to madness
Sun, 08 Jan 2006

Once in awhile my ISP tells me to alter my DSL router's MTU; sometimes it helps, sometimes it kills the connection. MTU stands for "maximum transmission unit" and is the threshold at which the routing hardware decides to fragment a data 'packet' into smaller packets for transmission. ie: For DSL the normal MTU is 1492 (bytes), meaning if an application sends a larger packet it'll be subdivided and then automagicly re-assembled at the 'other end' somewhere. To my mind, this suggests altering MTU can impact performance depending on the characteristics of the network... however, stranger things occur.

In the past I've had some oddities, but I don't recall specifics; todays oddities are just plain weird -- if you're a network administrator type who can make sense of this, please let me know :)

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Networking: Its all in teh name
Thu, 28 Jul 2005

Instead of wringing the neck of my various PCs around the place I've been trying to catch up on some reading (If Chins Could Kill). However, the whole time, my wifes connectivity has been shot and it drives me nuts to no end to have a problem lieing around unfixed (not to mention her being angry as hell at this new network ;) Now, shes had some spyware recently so I was smugly chiding her and blaming that nasty stuff the whole time -- she's a developer herself so getting caught with spyware is like working as a lawyer and being found with an inflatable date in your briefcase..)

It did seem odd that she couldn't ping out and I couldn't ping in - spyware often crashes machines or slows them down (you'd think it would spy without being so crippling but the authors are obviously not caring folks..) - but this didn't sound right. I'll cut to the chase for a lesson in humility and not jumping to conclusions -- the problem was in fact that the machine was actually connecting to another wireless access point (presumably a neighbours) - despite our own WAP having a much higher signal strength. It seems the old operating system installed there selected only by channel number and took publicly announcing networks over sneaky private ones like ours. This is the sort of bad design in network drivers that leads to man in the middle attacks -- imagine if you can just drop a WAP outside the window of an office building and suddenly scoop all the traffic from naive users, then redirect it back onto their own WAP.

A little frightening and pretty much what was going on here - though I assume our neighbour wasn't being malicious. Still, the lesson is to do your research carefully - even when the published network name sounds the same as your own. I think I'll seek out new drivers, since this must have been fixed as its a terrible security and usability flaw - thankfully my (now maybe deceased?) laptop and Windows XP / FreeBSD default to highest signal strength instead.

(This does ring of that issue that came up in the news a few weeks back where a man was accused of illegally using someone elses network, though he claimed that foreign network invited his laptop in to use it (by offering out DHCP details) and his laptop was designed to just accept. The trick is - this is quite normal and as witnessed above can happen easily. Thoughts that fall out are - if something illegal is found on your computers, just play the 'someone else was using my LAN' card. Further - tighter coupling is needed in these devices so they don't whore connectivity around the district...)

Addition: It would seem that this machine will swap connections every few seconds if you sit in the wrong place or lean over just-so. Pretty badly designed if it just gives up a valid connection for a slightly-better one just-like-that, while the network is busy. (ie: Mid-download, thus breaking the connection and losing the download!) So I'll have to go talk to the neighbours for now and get them to clamp down on security a bit, to keep us out -- amazing. I guess the software is just too trusting. Cracking must be easy..

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