Introduction#

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.

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

Syntax: $ ip { link | ... | route | macsec } (please see man ip for full list of objects)

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

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

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

Examples:

• Set default gateway to 1.2.3.254 $ ip route add default via 1.2.3.254
• Adds a default route (for all addresses) via the local gateway 192.168.1.1 that can be reached on device eth0 $ ip route add default via 192.168.1.1 dev eth0

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 <hostname> 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 <ip_rem>,
  2. one or more blank spaces, and
  3. the identifier <hostname>.

• 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 yout 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