Linux Network Configuration

Networking, set-up and administration

This Linux tutorial covers TCP/IP networking, network administration and system configuration basics. Linux can support multiple network devices. The device names are numbered and begin at zero and count upwards. For example, a computer running two ethernet cards will have two devices labeled /dev/eth0 and /dev/eth1. Linux network configuration, management, monitoring and system tools are covered in this tutorial.

Tutorial Contents:

Other YoLinux Networking Tutorials:

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 DNS server. 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.

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.

Fedora / Red Hat Network Configuration Files:

/etc/sysconfig/network
Red Hat network configuration file used by the system during the boot process.
File: /etc/sysconfig/network-scripts/ifcfg-eth0
Configuration settings for your first ethernet port (0). Your second port is eth1.
File:
- /etc/modprobe.conf (kernel 2.6)
- /etc/modules.conf (kernel 2.4)
- (or for older systems: /etc/conf.modules)
Example statement for Intel ethernet card:
alias eth0 eepro100
Modules for other devices on the system will also be listed. This tells the kernel which device driver to use if configured as a loadable module. (default for Red Hat)

Fedora / Red Hat Network GUI Configuration Tools:

The following GUI tools edit the system configuration files. There is no difference in the configuration developed with the GUI tools and that developed by editing system configuration files directly.

TCP/IP ethernet configuration:

Network configuration:
/usr/sbin/system-config-network (FC-2/3) GUI shown here --->
/usr/bin/redhat-config-network (/usr/bin/neat) (RH 7.2+ FC-1)
Text console configuration tool:
/usr/sbin/system-config-network-tui (Text User Interface (TUI) for Fedora Core 2/3)
/usr/bin/redhat-config-network-tui (RH 9.0 - FC-1)
Text console network configuration tool.
First interface only - eth0: /usr/sbin/netconfig
/usr/bin/netcfg (GUI) (last available with RH 7.1)

Gnome Desktop:

Gnome Desktop Network Configuration
/usr/bin/gnome-network-preferences (RH 9.0 - FC-3)
Proxy configuration. Choose one of three options:
1. Direct internet connection
2. Manual proxy configuration (specify proxy and port)
3. Automatic proxy configuration (give URL)

Assigning an IP address:

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: 192.168.10.1
Note: 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.

The IANA has allocated IP addresses in the range of 192.168.0.0 to 192.168.255.255 for private networks.

Helpful tools:

Network Calculators: Subnet mask calculator, node calculator, mask inverter, ...
IP subnet calculator

Command line IP Configuration: ifconfig

ifconfig interface [aftype] options | address ...

where:

interface: eth0, eth1, eth2 represent the computer ethernet interfaces
aftype: inet (TCP/IP, default), inet6 (IPv6), ax25 (AMPR Packet Radio), ddp (Appletalk Phase 2), ipx (Novell IPX) or netrom (AMPR Packet radio)

Options:

Option	Description
up	Activate the interface. Implied if IP addresses are specified.
down	Shut down interface
arp	Enable ARP protocol on this interface. Allow ARP to detect the addresses of computer hosts attached to the network.
-arp	Disable ARP protocol on this interface
promisc	Enable promiscuous mode. Receive all packets on the network not just those destined for this interface.
-promisc	Disable promiscuous mode.
mtu ##	Specify the Maximum Transfer Unit (MTU) of the interface. The MTU is the maximum number of octets the interface is able to handle in a single transaction. Defaults: Ethernet: 1500 SLIP: 296
broadcast XXX.XXX.XXX.XXX	Set the network broadcast address for this interface.
netmask XXX.XXX.XXX.XXX	Set the IP network mask for this interface.

Man page: ifconfig

Ubuntu / Debian IP Configuration Files:

File: /etc/network/interfaces

Static IP example:

auto lo iface lo inet loopback  auto eth0 iface eth0 inet static         address 208.88.34.106         netmask 255.255.255.248         broadcast 208.88.34.111         network 208.88.34.104         gateway 208.88.34.110

Dynamic IP (DHCP) example:

auto lo iface lo inet loopback  auto eth0 iface eth0 inet dhcp  auto eth1 iface eth1 inet dhcp  auto eth2 iface eth2 inet dhcp  auto ath0 iface ath0 inet dhcp  auto wlan0 iface wlan0 inet dhcp

Interfaces:

lo: Loopback interface (network within your system without slowing down for the real ethernet based network)
eth0: First ethernet interface card
wlan0: First wireless network interface

Also see "man interfaces"

Ubuntu GUI Network Tools: