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Lord Yupa

February 2010

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Danger Mouse

Network Problem solved!

I've figured out what the problem is. Finally.

First, a little background. A few days ago, a strange thing happened. One of my machines, a Linux box, stopped being able to make outbound TCP connections. What was even more strange about this, is that it could send out UDP/ICMP packets just fine, and all incoming connections were going fine.

Sounds odd, right?

Well, it gets more unusual. The machine in question (nexus.zyp.org) also acts as a firewall, and provides NAT[1] for a handful of other machines. The machines behind nexus, when going through nexus, were perfectly capable of making outbound TCP connections. That threw a serious wrench in my guesses of what might be going wrong.

A few moments ago, a random thought hit me, and upon further investigation, solved the problem. A couple months ago, I started including support for ECN[2] in the LInux kernels I built, because I'm planning on doing some testing with it, soon. Not a big deal, because my network initialization scripts disable it by default. Or, at least, they did. When I was updating my network scripts to set up NAT through this box, I typo'ed a line, and accidentally set the network scripts to enable ECN.

This was made more confusing because it wasn't enabled immediately, but upon my next reboot. Unfortunately, since this occurred in the network scripts, going back to a previous kernel didn't solve the problem, as it was having ECN enabled on it, too.

So, to sum up, my ISP is one of those that uses a broken router that harfs on ECN, and thus was having problems with my outgoing connections when it was enabled (Although, why things went through NAT fine still is a mystery to me). I've disabled ECN, and everything seems cool again. ;-)

[1] Network Address Translation. Basically, it allows the machines behind it to access the Internet, even though there is only a single Internet connection.

[2] Explicit Congestion Notification. A very good thing. . . in theory.

Comments

This is why I (try to) reboot machines immediately after making changes that affect boot-time stuff - then what you've done is still fresh in your mind. (It's also why I think uptime DSWs are silly.)

I do too, usually. This one got me because I was just making minor Network configuration changes, and I'd already manually enacted all of them on the machine. When I was making the changes, I accidentally deleted a quotation mark on a line, and that caused the script to eat the next line, which is the one disabling ECN. So I was just updating boot initialization stuff to match the current (new) setup. Next time I'll be more careful to check that kind of stuff immediately, if I'm not rebooting soon.

As for the ECN <-> NAT connection. . . yeah, I'm curious about that one. I would expect connections going through NAT to be treated the same way as locally originating connections, with regards to ECN. . . I can't think of any reason offhand why they wouldn't (of course, maybe that's why I'm curious about it, and not a kernel programmer;-).

I totally agree with you regarding uptime, too. Provided that the disruption of a reboot is acceptable, uptime should never be a reason to wait. Especially if it's in any way security related. That really gets me. I know a guy who waited 3 extra days to reboot a machine, despite the fact that a security upgrade required it, so tha the could break 200 days on his machine's uptime. Maybe I'm just paranoid, but how stupid can you get? Uptimes don't mean crap when your box just got rooted. ;-)
Oh my god, what the HELL are you people saying? Every once in a while there's a word from the english language thrown in, like 'thing' or 'silly,' these are words i understand. But what's that other crap? What language? To me, all it looks like is "geek geek geek, geek-geek geek geek. Geek geek? Geek geek geek!" ;-) Now that my brain has completely shut down, i think i'll get in the shower. See you at El Bees soon, Toph?

Techie-talk.

Sorry, Sweety. ;-)

These computer troubles have been causing me a lot of grief in the past few days, so I had to vent a little bit. I'll try to stick to other topics for a while, now. ;-)

Oh, by the way. . . did you throw in mention of a shower just to tease me? ;-)

Re: Techie-talk.

That's all right babe, i was just kidding.

And yes, yes i did. *giggles* You know me so well.