Bogus networking

[Marcelo A. Ferreira Gomes]

At 15:58 -0500 19/12/2002, Ross Williams wrote:
>Hello,
>I'm new to this list. I started working with MOL two days ago, and 
>it really is the best thing since sliced bread. I'm having a bit of 
>trouble, however, with networking.
>
>From Mac OS 10.2.2 I am able to ping my host and even 
>www.google.com, but I cannot access web pages, SSH, or anything that 
>requires a sustained tcp connection. DNS obviously works fine 
>because I can resolve any host I choose. Curiously, I am able to 
>retrieve the source of www.google.com if I telnet to it on port 80 
>and issue the "GET /" command. It still will not load in the web 
>browser. I believe that eliminates iptables MASQUERADE problems, 
>though.

Yes, that's correct. If you can telnet to port 80, there's no problem 
with your masquerade and firewalling rules. But if you look closely 
to the page source you get from the "GET /" command, you'll probably 
see that some parts of the HTML code are missing.

>I ran tcpdump on the linux side, and I came up with traffic to 
>www.google.com that listed the source ip address as "truncated ip". 
>I don't have those logs at my fingertips because I am writing this 
>mail from OS X. I'd be glad to send them if someone requests.

=46or what you describe, I guess I can diagnose it right away. TCP/IP 
has a parameter called MTU, for maximum transmit unit. This is the 
biggest chunk of information that can be sent over your particular 
physical medium in a single packet. On ethernet media, it is close to 
1500 bytes, but can vary a little bit above or below it, depending on 
whether you have some other protocol on top of it (such as PPPoE, for 
instance). This value is normally guessed right by the OS, in a 
process called PMTU discovery (for path MTU), so you don't need to 
bother wit it.

Your problem seems to be that Mac OS is getting a wrong value for 
your particular setup. I am currently in the middle of a big mess 
here at home, so I can't conduct networking experiences right now, 
and thus can't even try to reproduce your problem here. But try 
checking out Apple's support or developers sites to find a way to 
force Mac OS X to use a lower value for the PMTU. That way, you won't 
be getting truncated packets, and browsing will work again.

This is an interim solution, until the cause for the wrong PMTU 
discovery can be diagnosed and eliminated. Perhaps there's a problem 
with the sheep net driver, or Linux's tunnel driver. Exactly how are 
you connecting to the network?

>I know that this issue has been brought up on the list for months, 
>now, but I've yet to see a definitive solution. No one has seemed to 
>mail back and say, "Yes, it's working."

In a few days, I hope to be back with everything working here, so if 
you can't find the solution you need until then, I'll be glad to 
help, and we'll probably end up with a step-by-step solution to your 
problems.

>I'm running MOL 0.9.67 on Debian Woody with a stock 2.4.18-newpmac 
>kernel on a Dual USB iBook. I am aware that the 0.9.67 seems to be a 
>bogus version number or at least not released on the maconlinux.org 
>site. If someone suggests, I could try 0.9.65.

I don't believe this has to do with your particular version of 
hardware, Linux, or maybe even MoL. It seems to be some conflicting 
aspect between your network drivers and your network setup. Do you 
use cable/DSL/somethingelse with PPPoE?

>Thanks,
>Ross Williams
>overhacked {at} bigfoot {dot} com

You're welcome. ;-)

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