Fix an ad-hoc network hosted by a Windows XP machine (problems with MTU)

Question

I have the following problem. I use an old Windows XP laptop as some sort of a wireless router by creating an ad-hoc wireless network. It works OK with my Win7 laptop, but when I brought my MacBook Pro from work I realized that although it finds and connects to the network, all it could actually show were a few Google services, and nothing else.

Someone told me that I should experiment with reducing the MTU on the Mac. I did so and it worked - I reduced it from 1500 to 1272 and it worked.

the problem is still that another WinXP machine has basically the same accessibility problems as the MacBook had, and there I cannot really mess with the MTU settings. Same applies for my iPod touch, which I have to jailbreak in order to mess with the MTU. Plus, the MacBook occasionally goes back to 1500 by itself, and it is very annoying to reset it every single time.

The strangest thing is that I created this ad-hoc network in the most standard way possible (using the XP wizards), without making ay funky configuration or something. Yet, as it seems, it doesn't even allow a Win XP machine to connect to another Win XP machine.

Please, help. I really need to use this laptop as a router, and currently buying a standard WiFi one is not an option.

Answer 1

Using ICS will create a 'router' of sorts out of your XP machine, i.e., it will assign addresses (192.168.137.x, I believe) to the clients behind it (i.e., coming over your WiFi interface), so I don't know if changing the MTU settings on your hardline router will help because it's not handing out addresses to the wireless clients connecting to your XP PC.

You may want to try Connectify (http://www.connectify.me/), if you only need wireless clients to get access to the internet - it uses your WiFi card to create a hotspot. Essentially what ICS does, but works well from my (admittedly short-term) usage.

Answer 2

It sounds like you are using Windows Internet Connection Sharing (ICS) on the XP laptop. This adds an overhead to packets that can cause them to exceed the MTU the router is expecting.

I presume that also on your network you have a router that is running DHCP to hand out IP addresses to each of the devices. Have a look in the routers configuration and see you can configure the MTU value that the DHCP server is handing out. If so, set it low enough to allow for the overhead added to clients connecting through the XP machine.

If it's not possible to set the MTU in the routers DHCP configuration, another option would be to turn off the DHCP function of the router and install a DHCP server on the Windows XP machine. I haven't had to do this but I think that there are freeware DHCP servers for Windows out there. Get one that allows for configuration of the MTU value, of course. The MTU size is DHCP option 26.

Answer 3

I think, that this problem can be in XP firewall. MTU path discovery uses ICMP packets (like ping), and this type packet is on the Windows prohibited. Can you try to switch off firewall completely and see, if that works?