topic Re: Unsuccesfull TCP session does not time out in Operating System - Linux

Unsuccesfull TCP session does not time out

Paul Ambro — Thu, 04 Nov 2004 10:38:35 GMT

I am seeing a problem with my TCP stack in which I try to connect to a machine which is listed on DNS but is not "pinagble" and
the port is not active. The problem is that the connection doesn't time-out. If I do it from windows, the connection times out.

Here is an example:

[root@xxxxxxxx root]# telnet nyxxxxxadm3
Trying 10.221.7.135...

***On Linux it just sits there forever.

H:\>telnet nyxxxxxadm3
Connecting To nyxxxxxadm3...Could not open a connection to host on port 23 : Conn
ect failed

******On windows it disconnects after a while.

Any clues why, and how it can be fixed on Linux so it times out??

thanks,
paul

Re: Unsuccesfull TCP session does not time out

Steven E. Protter — Thu, 04 Nov 2004 11:05:40 GMT

You have no network connectivity.

On the console, look at the /var/log/messages file.

Check your network for a duplicate IP address. If found, change your linux configuration.

SEP

Re: Unsuccesfull TCP session does not time out

Paul Ambro — Thu, 04 Nov 2004 11:22:08 GMT

I am not concerned about the network connectivity. The issue is why Linux does not times-out a TCP connection (e.g telnet) if it is not able establish a session. Windows sends me a message and returns to the prompt, Linux hangs until I have to manually intervene with crtl-C.

Re: Unsuccesfull TCP session does not time out

Jan Sladky — Thu, 04 Nov 2004 14:16:23 GMT

Hi Paul,

I just tried telnet, ssh and ftp on nonexisting address and after cca 5 min got message Connection timed out
I know it isn't exactly same case, but you can try it also.

without developer's knowledge of TCP/IP stack and telnet aplication is hard to say, maybe you have longer timeout or some kind of philosophy on your Linux distro ;-)

Jan

Re: Unsuccesfull TCP session does not time out

Ermin Borovac — Thu, 04 Nov 2004 22:28:02 GMT

You can try setting tcp_syn_retries parameter to a lower value with

# echo 3 > /proc/sys/net/ipv4/tcp_syn_retries

Now telnet to unreachable host should take about 45 seconds.

If you want to make it permanent put an entry in /etc/sysctl.conf.

Re: Unsuccesfull TCP session does not time out

Paul Ambro — Fri, 05 Nov 2004 09:42:17 GMT

Ermin,

You suggestion seems to solve the issue. I also found some documentation on Microsoft explaining the TCP stack differences and it mentions the retry limit.

Thanks for your help

pa