Andrew Hudson - Fedora™ Unleashed, 2008 edition

As previously stated, seven network configuration files can be modified to make changes to basic network interaction of your system. The files are as follows:

► /etc/hosts — A listing of addresses, hostnames, and aliases

► /etc/services — Network service and port connections

► /etc/nsswitch.conf — Linux network information service configuration

► /etc/resolv.conf — Domain name service domain (search) settings

► /etc/host.conf — Network information search order (by default, /etc/hostsand then DNS)

► /etc/sysconfig/network — The hostname, IP address, boot activation control, and gateway settings (along with optional IPv6 settings)

► /etc/sysconfig/network-scripts/ifcfg-eth0 — Network settings for the eth0 network device; see the file sysconfig.txtunder the /usr/share/doc/initscripts/directory for details about optional settings

After the first six of these files have been modified, the changes are active. As with most configuration files, comments can be added with a hash mark ( #) preceding the comment. The last file ( /etc/sysconfig/network) requires the networking daemons to be restarted before the file is used. All seven of these files have a man page written about them for more information.

The /etc/hosts file is a map of IP to hostnames. If you are not using DNS or another naming service, and you are connected to a large network, this file can get quite large and can be a real headache to manage. A small /etc/hostsfile can look something like this:

127.0.0.1 localhost.localdomain localhost

128.112.50.69 myhost.mydomain.com myhost

128.112.50.169 yourhost.mydomain.com yourhost

The first entry is for the loopback entry. The second is for the name of the machine. The third is another machine on the network. If no naming service is in use on the network, the only host that myhostrecognizes by name is yourhost. (IP addresses on the network can still be used.)

If your network is using a naming service, the last line is not needed and can be deleted. However, if myhostconnects to yourhostfrequently, it might be good to leave the entry so that myhostdoes not need to consult the naming service each time. This can save time and reduce the strain on the network or the name service server. Edit this file if you need to change your hostname or IP address or if you aren't using a naming service and a host has been added to your network.

The /etc/servicesfile maps port numbers to services. The first few lines look similar to this (the /etc/servicesfile can be quite long, more than 500 lines):

# Each line describes one service, and is of the form:

#

# service-name port/protocol [aliases ... ] [# comment]

tcpmux 1/tcp # TCP port service multiplexer

tcpmux 1/udp # TCP port service multiplexer

rje 5/tcp # Remote Job Entry

rje 5/udp # Remote Job Entry

echo 7/tcp

echo 7/udp

discard 9/tcp sink null

discard 9/udp sink null

systat 11/tcp users

Typically, there are two entries for each service because most services can use either TCP or UDP for their transmissions. Usually after /etc/servicesis initially configured, you will not need to change it.

This file was initially developed by Sun Microsystems to specify the order in which services are accessed on the system. A number of services are listed in the /etc/nsswitch.conffile, but the most commonly modified entry is the hostsentry. A portion of the file can look like this:

passwd: files

shadow: files

group: files

#hosts: db files nisplus nis dns

hosts: files dns

This tells services that they should consult standard UNIX/Linux files for passwd, shadow, and group( /etc/passwd, /etc/shadow, /etc/group, respectively) lookups. For hostlookups, the system checks /etc/hostsand if there is no entry, it checks DNS. The commented hostsentry lists the possible values for hosts.Edit this file only if your naming service has changed.

/etc/resolv.confis used by DNS, the domain name service. (DNS is covered in detail in Chapter 23.) The following is an example of resolv.conf:

nameserver 192.172.3.8

nameserver 192.172.3.9

search mydomain.com

This sets the nameservers and the order of domains for DNS to use. The contents of this file are set automatically if you use Dynamic Host Configuration Protocol, or DHCP (see the section on "DHCP" later in this chapter).

If you make use of NetworkManager to handle your network connections, don't make any changes to /etc/resolv.conf, because it will cause havoc for NetworkManager. Instead, let NetworkManager dynamically work with this file of its own accord.

The /etc/host.conffile lists the order in which your machine will search for hostname resolution. The following is the default /etc/host.conffile:

order hosts, bind

In this example, the host checks the /etc/hostsfile first and then performs a DNS lookup. A couple more options control how the name service is used. The only reason to modify this file is if you use NIS for your name service or you want one of the optional services. The nospoofoption can be a good option for system security. It compares a standard DNS lookup to a reverse lookup (host-to-IP then IP-to-host) and fails if the two don't match. The drawback is that often when proxy services are used, the lookup fails, so you want to use this with caution.

Changes to /etc/sysconfig/networkdo take effect until you restart the networking daemons or reboot the system. If you use Fedora's graphical configuration network tools (described in the next section), you should not edit this file. The file might look like this:

NETWORKING=yes

HOSTNAME=myhost

GATEWAY=192.112.50.99

A GATEWAYDEVsetting is also available to associate a specific network device (such as eth0 or eth1and so on). An additional optional entry to /etc/sysconfig/networkis for NIS domain machines and would look like this:

NISDOMAIN=rebel

The networkfile previously supported a FORWARD_IPV4value, which determined whether the host forwarded IP packets (usually "yes" for routers). This setting is now saved in /etc/sysctl.confas a net.ipv4.ip_forwardsetting, which can be modified if the forwarding changes are required. See the sysctl.confman page for more information.