Network Configuration in Linux

This document 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 with two NICs will have two devices labeled eth0 and eth1.

Local DNS resolution

File: /etc/hosts contains a list of hosts that are to be resolved locally(not by DNS)

Sample contents of the file:

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

The file format for the hosts file is specified by RFC 952

Configure DNS servers for domain name resolution

File: /etc/resolv.conf contains a list of DNS servers for domain name resolution

Sample contents of the file:

nameserver 8.8.8.8  # IP address of the primary name server
nameserver 8.8.4.4  # IP address of the secondary name server

In case internal DNS server you can validate if this server resolve DNS names properly using dig command:

$ dig google.com @ your.dns.server.com +short

See and manipulate routes

Manipulate the IP routing table using route

Display routing table

$ route # Displays list or routes and also resolves host names
$ route -n # Displays list of routes without resolving host names for faster results

Add/Delete route

Option Description
add or del Add or delete a route
-host x.x.x.x Add route to a single host identified by the IP address
-net x.x.x.x Add route to a network identified by the network address
gw x.x.x.x Specify the network gateway
netmask x.x.x.xSpecify the network netmask
default Add a default route

Examples:

add route to a host $ route add -host x.x.x.x eth1
add route to a network $ route add -net 2.2.2.0 netmask 255.255.255.0 eth0
Alternatively, you could also use cidr format to add a route to network route add -net 2.2.2.0 / 24 eth0
add default gateway $ route add default gw 2.2.2.1 eth0
delete a route $ route del -net 2.2.2.0 / 24

Manipulate the IP routing table using ip

Display routing table

$ ip route show # List routing table

Add/Delete route

• add or del: Add or delete a route
• -host x.x.x.x: Add route to a single host identified by the IP address
• -net x.x.x.x: Add route to a network identified by the network address
• gw x.x.x.x: Specify the network gateway
• netmask x.x.x.x: Specify the network netmask
• default: Add a default route

Examples:

• add route to a host route add -host x.x.x.x eth1
• add route to a network route add -net 2.2.2.0 netmask 255.255.255.0 eth0
• Alternatively, you could also use cidr format to add a route to network route add -net 2.2.2.0/24 eth0
• add default gateway route add default gw 2.2.2.1 eth0
• delete a route route del -net 2.2.2.0/24

Configure a hostname for some other system on your network

You can configure your Linux (or macOS) system in order to tie in an identifier **** to some other system's IP address in your network. You can configure it:

• Systemwide. You should modify the /etc/hosts file. You just have to add to that file a new line containing:
  1. the remote system’s IP address **** ,
  2. one or more blank spaces, and
  3. the identifier ****.
• For a single user. You should modify the ~/.hosts file — you’d have to create it. It is not as simple as for systemwide. Here you can see an explanation.

For instance, you could add this line using the cat Unix tool. Suppose that you want to make a ping to a PC in your local network whose IP address is 192.168.1.44 and you want to refer to that IP address just by remote_pc. Then you must write on your shell:

$ sudo cat 192.168.1.44 remote_pc

Then you can make that ping just by:

$ ping remote_pc

Interface details

ifconfig

List all the interfaces available on the machine

$ ifconfig -a

List the details of a specific interface

Syntax: $ ifconfig <interface>

Example:

$ ifconfig eth0
eth0        Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
            inet addr:x.x.x.x Bcast:x.x.x.x Mask:x.x.x.x
            inet6 addr: xxxx::xxx:xxxx:xxxx:xxxx / 64 Scope:Link
            UP BROADCAST RUNNING MULTICAST MTU: 1500 Metric: 1
            RX packets: 4426618 errors: 0 dropped: 1124 overruns: 0 frame: 0
            TX packets: 189171 errors: 0 dropped: 0 overruns: 0 carrier: 0
            collisions: 0 txqueuelen: 1000
            RX bytes: 382611580 ( 382.6 MB ) TX bytes: 36923665 ( 36.9 MB )
            Interrupt: 16 Memory:fb5e0000-fb600000

Ethtool - query the network driver and hardware settings

Syntax: $ ethtool<interface>

Example:

$ ethtool eth0

Settings for eth0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised pause frame use: No
Advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on (auto)
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000007 (7)
drv probe link
Link detected: yes

ip - show / manipulate routing, devices, policy routing and tunnels

Examples

List network interfaces

$ ip link show

Rename interface eth0 to wan

$ ip link set dev eth0 name wan

Bring interface eth0 up (or down)

$ ip link set dev eth0 up

List addresses for interfaces

$ ip addr show

Add (or del) ip and mask (255.255.255.0)

$ ip addr add 1.2.3.4 / 24 brd + dev eth0

Adding IP to an interface

An IP address to an interface could be obtained via DHCP or Static assignment

DHCP If you are connected to a network with a DHCP server running, dhclient command can get an IP address for

your interface

$ dhclient <interface>

or alternatively, you could make a change to the /etc/network/interfaces file for the interface to be brought up on boot and obtain DHCP IP

auto eth0
iface eth0 inet dhcp

Static configuration(Permanent Change) using /etc/network/interfaces file

If you want to statically configure the interface settings(permanent change), you could do so in the /etc/network/interfaces file.

Example:

auto eth0 # Bring up the interface on boot
iface eth0 inet static
    address 10.10.70.10
    netmask 255.255.0.0
    gateway 10.10.1.1
    dns-nameservers 10.10.1.20
    dns-nameservers 10.10.1.30

These changes persist even after system reboot.

Static configuration(Temporary change) using ifconfig utility

A static IP address could be added to an interface using the ifconfig utility as follows

$ ifconfig <interface> <ip-address>/<mask> up

Example:

$ ifconfig eth0 10.10.50.100/16 up