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Linux Tutorial - System Monitoring - Finding Out About Your System - Network Files
  User Files ---- Important System Files  

Network Files

If you are running TCP/IP, you can look in a couple of places for information about your system. First, check out the file /etc/resolv.conf. If you don't find it and you know you are running TCP/IP, don't worry! The fact that it is missing tells you that you are not running a nameserver in your network. If it is not there, you can find a list of machines that your machine knows about and can contact by name in /etc/hosts. If you are running a nameserver, this information is kept on the nameserver itself.

The content of the /etc/hosts file is the IP address of a system followed by its fully qualified name, and then any aliases you might want to use. A common alias simply uses the node name and omits the domain name. Each line in the /etc/resolv.conf file contains one of a couple different types of entries. The two most common entries are the domain entry, which is set to the local domain name, and the nameserver, which is followed by the IP address of the name "resolver." See the section on TCP/IP for more information on both of these files.

It's possible that your machine is the nameserver itself. To find this out, look at the file /etc/named.conf. If this exists, it is probably a nameserver. The /etc/named.conf file will tell you the directory where the name server database information is kept. For information about the meaning of these entries, check out the named man-page, as well as the section on TCP/IP.

Another place to look is the start-up scripts in /etc/rc.d. Often, static routes are added there. One likely place is /etc/rc.d/init.d/network. If these static routes use tokens from either /etc/networks or /etc/gateways that are incorrect, then the routes will be incorrect. By using the -f option to the route command, you can flush all of the entries and start over.

Although not as often corrupted or otherwise goofed up, a couple other files require a quick peek. If you think back to our telephone switchboard analogy for TCP, you can think of the /etc/services file as the phone book that the operator uses to match names to phone numbers. Rather than names and phone numbers, /etc/services matches the service requested to the appropriate port. To determine the characteristics of the connection, inetd uses /etc/inetd.conf, which contains such information as whether to wait for the first process to finish before allowing new connections.

Other common places for confusion are incorrect entries and the inevitable calls to support deals with user equivalence. As I talked about in the section on TCP/IP, when user equivalence is set up between machines, many remote commands can be executed without the user having to produce a password. One more common misconception is the universality of the /etc/hosts.equiv file. Though this file determines what user equivalence should be established with what other machine, it does not apply to one user: root. To me, this is rightly so. Though it does annoy administrators who are not aware of this, it is nothing compared to the problems that might occur if it did apply to root, and this is not what you would expect.

To allow root access, you need to create a .rhosts file in roots home directory (usually /) that contains the same information as /etc/hosts.equiv but instead applies to the root account. The most common mistake made with this file is the permission. If the permission is such that any user other than root (as the owner of the file) can read it, the user-equivalent mechanism will fail. See /etc/hosts.equiv and $HOME/.rhosts to see which remote users have access to which user accounts.

A list of the essential system files can be found here.

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Copyright 2002-2009 by James Mohr. Licensed under modified GNU Free Documentation License (Portions of this material originally published by Prentice Hall, Pearson Education, Inc). See here for details. All rights reserved.

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