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XiotexStudios

Unreachable port on second network card

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Hi, After putting up with a dodgy on-board network card (NForce2 - which gives seemingly random response times on packets) I finally installed a second network card. After getting the IP address I begin to send packets to it using UDP - and for every packet sent I get an ICMP port unreachable back. Have I missed something? I am definately sending to the right ip address and the port number is 50000. I can ping the card in windows and do all the other windowsy type stuff (like browse the web - talk to our fileserver etc...).

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I hate to be obvious, but do you have a UDP service running on that port? Where are you pinging it from?

I like using nmap to test my service / firewall/nat configurations.

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My setup is two PC's running XP Home.

As I mentioned in the above post - I have been happily communicating with the second PC over UDP on port 50000 but the card was a little unreliable on a LAN. It was okay for the small packets I was sending but it was losing data on large transfers and doing a ping from machine 1 to machine 2 would result in seeming random timescales being returned - never the same twice.

So, I thought I would install this second card. Now all I get in response to UDP is ICMP: port unreachable.

Now, as a test I have attemped to fire UDP packets to all of the other machines in the office and they all get through except for the server - which I would accept.

I have never come across an ICMP Port unreachable before and I am sure the UDP service is running.

Never seen nmap before... any good?

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Check the windows firewall settings - SP2 enables it by default on a new connection (ie your new card).

nmap is great for security checking, generally being nosey and anything else you might want to throw a stealthy port scanner at. It even makes guesses at the OS of the remote host by looking for port profiles. Nice.

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This nmap sounds cool - will look into it.

I thought about firewalls. I have disabled the SP2 one and also uninstalled the nvidia network manager thingy that I found too.

Still the same problem.

its an odd one.

Going to try another network card tomorrow just in case.

I will kick myself in the nads if this turns out to be a PEBKAC....

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I found the problem!

I wasn't listening to the correct port on the second card. And the card/driver reported that there was nothing listening on that port.

Bizzare thing is that if I read on a port that isn't being listened to on other machines I don't get that message - is this to stop port scanners from being effective?

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Quote:
Bizzare thing is that if I read on a port that isn't being listened to on other machines I don't get that message


I'm assuming you're using UDP. You don't know whether another machine will send a datagram on that port. Datagrams are stateless (connectionless). Someone, somewhere in the world might send you a datagram on that port at some time. Thus, it's perfectly valid (and common) to receive on a port that maybe nobody is currently sending any datagrams to.

TCP is connected, so there, it doesn't make sense to send or receive without a connection (but you can still listen() for connections while there aren't any).

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Guest Anonymous Poster
Quote:
Original post by XiotexStudios
Hi,
After putting up with a dodgy on-board network card (NForce2 - which gives seemingly random response times on packets) I finally installed a second network card.
After getting the IP address I begin to send packets to it using UDP - and for every packet sent I get an ICMP port unreachable back.
Have I missed something? I am definately sending to the right ip address and the port number is 50000. I can ping the card in windows and do all the other windowsy type stuff (like browse the web - talk to our fileserver etc...).


Your card was not dodgy, but the nforce series have an on board hardware firewall on the network card. To use the machine as a realtime server, you have to disable it. (the older series also had a bug which often resulted in random packet drops)

If you are running winxp-sp2, then an udp port is only opened implicitly after a program sends data out on it. Before running a server, you have to open ports in your system. The firewall has no effect for localhost, so you can test the server with the loopback address.

Viktor

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