Networking in Linux

Linux TCP/IP Network configuration files

File	Description
/etc/resolve.conf	List DNS servers for internet domain name resolution. Manual page for: /etc/resolv.conf
/etc/hosts	Lists hosts to be resolved locally (not by DNS). Manual page for: /etc/hosts
/etc/nsswitch.conf	List order of host name search. Typically look at local files, then NIS server, then DNSserver. Manual page for: /etc/nsswitch.conf
Red Hat/Fedora/CentOS: /etc/sysconfig/network	Specify network configuration. eg. Static IP, DHCP, NIS, etc.
Red Hat/Fedora/CentOS: /etc/sysconfig/network-scripts/ifcfg-device	Specify TCP network information.
Ubuntu/Debian: /etc/network/interfaces	Specify network configuration and devices. eg. Static IP and info, DHCP, etc.

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• DOMAIN RESOLUTION CONFIGURATION FILES

• File: /etc/resolv.conf - host name resolver configuration file
  

  search name-of-domain.com - Name of your domain or ISP's domain if using their name server nameserver XXX.XXX.XXX.XXX - IP address of primary name server nameserver XXX.XXX.XXX.XXX - IP address of secondary name server

  This configures Linux so that it knows which DNS server will be resolving domain names into IP addresses. If using DHCP client, this will automatically be sent to you by the ISP and loaded into this file as part of the DHCP protocol. If using a static IP address, ask the ISP or check another machine on your network.
  Red Hat/Fedora GUI: /usr/sbin/system-config-network (select tab "DNS").
• File: /etc/hosts - locally resolve node names to IP addresses

  127.0.0.1 your-node-name.your-domain.com localhost.localdomain localhost XXX.XXX.XXX.XXX node-name

  Note when adding hosts to this file, place the fully qualified name first. (It helps sendmail identify your server correctly) i.e.:

      XXX.XXX.XXX.XXX  superserver.yolinux.com  superserver
      

  This informs Linux of local systems on the network which are not handled by the DNS server. (or for all systems in your LAN if you are not using DNS or NIS)The file format for the hosts file is specified by RFC 952.
  Red Hat/Fedora configuration GUI: /usr/sbin/system-config-network (select tab "Hosts").
  
• File: /etc/nsswitch.conf - System Databases and Name Service Switch configuration file

  hosts: files dns nisplus nis

  This example tells Linux to first resolve a host name by looking at the local hosts file(/etc/hosts), then if the name is not found look to your DNS server as defined by/etc/resolv.conf and if not found there look to your NIS server.In the past this file has had the following names: /etc/nsswitch.conf, /etc/svc.conf, /etc/netsvc.conf, ... depending on the distribution.

Assigning IP addresses

Computers may be assigned a static IP address or assigned one dynamically. Typically a server will require a static IP while a workstation will use DHCP (dynamic IP assignment). The Linux server requires a static IP so that those who wish to use its resources can find the system. It is more easily found if the IP address does not change and is static. This is not important for the Linux client workstation and thus it is easier to use an automated Dynamic Host Configuration Protocol (DHCP) for IP address assignment.

Static IP address assignment:

Choose one of the following methods:

• Command Line:

      /sbin/ifconfig eth0 192.168.10.12 netmask 255.255.255.0 broadcast 192.168.10.255
      

  Network address by convention would be the lowest: 192.168.10.0
  Broadcast address by convention would be the highest: 192.168.10.255
  The gateway can be anything, but following convention: 192.168.10.1Note: the highest and lowest addresses are based on the netmask. The previous example is based on a netmask of 255.255.255.0
  
• Red Hat / Fedora GUI tools:
  • /usr/bin/neat Gnome GUI network administration tool. Handles all interfaces. Configure for Static IP or DHCP client.
    (First available with Red Hat 7.2.)
  • /usr/bin/netcfg (Handles all interfaces) (last available in Red Hat 7.1)
• Red Hat / Fedora Console tools:
  • /usr/sbin/system-config-network-tui (Text User Interface)
  • /usr/sbin/netconfig (Only seems to work for the first network interface eth0 but not eth1,...)
• Directly edit configuration files/scripts. See format below.

The ifconfig command does NOT store this information permanently. Upon reboot this information is lost. Manually add the network configuration to /etc/sysconfig/network-scripts/ifcfg-eth0 (Red Hat/Fedora/CentOS) for the first NIC, ifcfg-eth1 for the second, etc, or /etc/network/interfaces (Ubuntu) as shown below. Any other commands you may want to add to the system boot sequence can be added to the end of the file /etc/rc.d/rc.local. The commands netcfg and netconfig make permanent changes to system network configuration files located in /etc/sysconfig/network-scripts/, so that this information is retained and used upon system boot.

Static and Dynamic IP Configuration

Static IP address configuration:

DEVICE=eth0 BOOTPROTO=static BROADCAST=XXX.XXX.XXX.255 IPADDR=XXX.XXX.XXX.XXX NETMASK=255.255.255.0 NETWORK=XXX.XXX.XXX.0 ONBOOT=yes - Will activate upon system boot

RHEL4/FC3 additions:

• TYPE=Ethernet
• HWADDR=XX:XX:XX:XX:XX:XX
• GATEWAY=XXX.XXX.XXX.XXX

OR for DHCP client configuration:

DEVICE=eth0 ONBOOT=yes BOOTPROTO=dhcp

Changing the hostname

One may also want to check the file /etc/hosts for an entry using the system name which allows the system to be self aware.The hostname may be changed at runtime using the command: sysctl -w kernel.hostname="superserver"