These files contain the hardware configurations of network cards and other devices. They contain the needed parameters, such as the kernel module, start mode, and script associations. Refer to the manual page of hwup for details. Regardless of the existing hardware, the hwcfg-static-* configurations are applied when coldplug is started.

These files contain the configurations for network interface. They include information such as the start mode and the IP address. Possible parameters are described in the manual page of ifup. Additionally, all variables from the files dhcp, wireless, and config can be used in the ifcfg-* files if a general setting should be used for only one interface.

The file config contains general settings for the behavior of ifup, ifdown, and ifstatus. dhcp contains settings for DHCP and wireless for wireless LAN cards. The variables in all three configuration files are commented and can also be used in ifcfg-* files, where they are treated with higher priority.

The static routing of TCP/IP packets is determined here. All the static routes required by the various system tasks can be entered in the /etc/sysconfig/network/routes file: routes to a host, routes to a host via a gateway, and routes to a network. For each interface that needs individual routing, define an additional configuration file: /etc/sysconfig/network/ifroute-*. Replace * with the name of the interface. The entries in the routing configuration files look like this:

DESTINATION           GATEWAY NETMASK   INTERFACE [ TYPE ] [ OPTIONS ]
DESTINATION           GATEWAY PREFIXLEN INTERFACE [ TYPE ] [ OPTIONS ]
DESTINATION/PREFIXLEN GATEWAY -         INTERFACE [ TYPE ] [ OPTIONS ]

To omit GATEWAY, NETMASK, PREFIXLEN, or INTERFACE, write - instead. The entries TYPE and OPTIONS may just be omitted.

The route's destination is in the first column. This column may contain the IP address of a network or host or, in the case of reachable name servers, the fully qualified network or hostname.

The second column contains the default gateway or a gateway through which a host or network can be accessed. The third column contains the netmask for networks or hosts behind a gateway. For example, the mask is 255.255.255.255 for a host behind a gateway.

The last column is only relevant for networks connected to the local host such as loopback, ethernet, ISDN, PPP, and dummy device. The device name must be entered here.

The domain to which the host belongs is specified in this file (keyword search). Also listed is the status of the name server address to access (keyword nameserver). Multiple domain names can be specified. When resolving a name that is not fully qualified, an attempt is made to generate one by attaching the individual search entries. Use multiple name servers by entering several lines, each beginning with nameserver. Precede comments with # signs. YaST enters the specified name server in this file. Example 22.5, “/etc/resolv.conf” shows what /etc/resolv.conf could look like.

# Our domain
search example.com
#
# We use sun (192.168.0.20) as nameserver
nameserver 192.168.0.20

Some services, like pppd (wvdial), ipppd (isdn), dhcp (dhcpcd and dhclient), pcmcia, and hotplug, modify the file /etc/resolv.conf by means of the script modify_resolvconf. If the file /etc/resolv.conf has been temporarily modified by this script, it contains a predefined comment giving information about the service that modified it, the location where the original file has been backed up, and how to turn off the automatic modification mechanism. If /etc/resolv.conf is modified several times, the file includes modifications in a nested form. These can be reverted in a clean way even if this reversal takes place in an order different from the order in which modifications were introduced. Services that may need this flexibility include isdn, pcmcia, and hotplug.

If a service was not terminated in a normal, clean way, modify_resolvconf can be used to restore the original file. Also, on system boot, a check is performed to see whether there is an uncleaned, modified resolv.conf, for example, after a system crash, in which case the original (unmodified) resolv.conf is restored.

YaST uses the command modify_resolvconf check to find out whether resolv.conf has been modified and subsequently warns the user that changes will be lost after restoring the file. Apart from this, YaST does not rely on modify_resolvconf, which means that the impact of changing resolv.conf through YaST is the same as that of any manual change. In both cases, changes have a permanent effect. Modifications requested by the mentioned services are only temporary.

In this file, shown in Example 22.6, “/etc/hosts”, IP addresses are assigned to hostnames. If no name server is implemented, all hosts to which an IP connection will be set up must be listed here. For each host, enter a line consisting of the IP address, the fully qualified hostname, and the hostname into the file. The IP address must be at the beginning of the line and the entries separated by blanks and tabs. Comments are always preceded by the # sign.

127.0.0.1 localhost
192.168.0.20 sun.example.com sun
192.168.0.1 earth.example.com earth

Here, network names are converted to network addresses. The format is similar to that of the hosts file, except the network names precede the addresses. See Example 22.7, “/etc/networks”.

loopback     127.0.0.0
localnet     192.168.0.0

Name resolution—the translation of host and network names via the resolver library—is controlled by this file. This file is only used for programs linked to libc4 or libc5. For current glibc programs, refer to the settings in /etc/nsswitch.conf. A parameter must always stand alone in its own line. Comments are preceded by a # sign. Table 22.6, “Parameters for /etc/host.conf” shows the parameters available. A sample /etc/host.conf is shown in Example 22.8, “ /etc/host.conf ”.

# We have named running
order hosts bind
# Allow multiple addrs
multi on

The introduction of the GNU C Library 2.0 was accompanied by the introduction of the Name Service Switch (NSS). Refer to man 5 nsswitch.conf and The GNU C Library Reference Manual for details.

The order for queries is defined in the file /etc/nsswitch.conf. A sample nsswitch.conf is shown in Example 22.9, “/etc/nsswitch.conf”. Comments are introduced by # signs. In this example, the entry under the hosts database means that a request is sent to /etc/hosts (files) via DNS (see Chapter 24, The Domain Name System).

passwd:     compat
group:      compat

hosts:      files dns
networks:   files dns

services:   db files
protocols:  db files

netgroup:   files
automount:  files nis

The “databases” available over NSS are listed in Table 22.7, “Databases Available via /etc/nsswitch.conf”. In addition, automount, bootparams, netmasks, and publickey are expected in the near future. The configuration options for NSS databases are listed in Table 22.8, “Configuration Options for NSS “Databases””.

This file is used to configure nscd (name service cache daemon). See man 8 nscd and man 5 nscd.conf. By default, the system entries of passwd and groups are cached by nscd. This is important for the performance of directory services, like NIS and LDAP, because otherwise the network connection needs to be used for every access to names or groups. hosts is not cached by default, because the mechanism in nscd to cache hosts makes the local system unable to trust forward and reverse lookup checks. Instead of asking nscd to cache names, set up a caching DNS server.

If the caching for passwd is activated, it usually takes about fifteen seconds until a newly added local user is recognized. Reduce this waiting time by restarting nscd with the command rcnscd restart.

This contains the hostname without the domain name attached. This file is read by several scripts while the machine is booting. It may only contain one line in which the hostname is set.