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I have some troubles with my server! After I asked for support, I get the following answer:

You can use the serial port to reset your network configuration.

I have a client machine with Ubuntu Oneiric Ocelot (11.10) installed on it and a cable for serial access with 9 pins in a head and a network connector in the other. Can anyone give me some hints on how can I access my server via serial port?

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What type of server is it ? Do you have physical access to the server? Do you have a laptop and if so what OS is on the Laptop. – Iain♦ Dec 27 '11 at 9:25
@Iain I do have phisical access to server. The server OS is Centos 5.6. On my client machine (a desktop PC) I have installed ubuntu 11.10 – artaxerxe Dec 27 '11 at 9:31
Why not solve your problems via a monitor and keyboard connected to your server? – poplitea Dec 27 '11 at 9:41
@poplitea Because I don't have a username and a password to login. Just a password for serial console. – artaxerxe Dec 27 '11 at 9:44
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@artaxerxe See my answer below. Other than that, maybe re-tag your question with CentOS rather than Ubuntu, since your question regards your server and not your client machine... – poplitea Dec 27 '11 at 9:56
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Even though it's not access via serial port, you could probably solve your problem by connecting a monitor and keyboard physically to your server, then reboot into single-user mode. In this mode, you get root access without a password, and you could reset your user/password.

For single user boot, see: http://centos.org/docs/5/html/5.1/Installation_Guide/s1-rescuemode-booting-single.html

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But will affect this my system? For example if I don't want to change the root password. – artaxerxe Dec 27 '11 at 10:00
Nothing else than what you manually change will be changed (I think). Just reboot into "normal" mode when you're finished "repairing" :) – poplitea Dec 27 '11 at 10:03
But, my server has some functionality. What if the server functionality is based on the root password? (I'm relatively newbie to linux, so please excuse me if I have some bothering questions). – artaxerxe Dec 27 '11 at 10:08
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@artaxerxe I can't think of anything that's "based" on the root password, but this is nonetheless an approach for repairing/doing maintenance, and when you've done that, you reboot into normal mode again (single-user mode is not a normal operating mode in which you should be running your server for long term). If you regularly need (shell) access to your server, you should create a user with the privileges you need, so that you f.ex. can login remotely from your client machine... This is pretty Linux/Unix basics; you should read up on this! – poplitea Dec 27 '11 at 10:19
Thanks for your answers! Finally I solved my problem well with your help. Have a nice day! – artaxerxe Dec 27 '11 at 11:53
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