Basic Network Commands for Linux

Required Information

To enable networking, you must configure your NIC (Network Interface Card/s) with an

IP address and netmask. The kernel must have support for your cards compiled in, either as modular support or direct support. If you don't have kernel support read the sections about the kernel and how to compile it. To set your cards up, do the following. In my example my network is 192.168.16.0, IP=192.168.16.100, broadcast=192.168.16.255, netmask=255.255.255.0, gateway-192.168.16.1, nameserver=192.168.16.10.

1. Determine your machines IP address from your network administrator
2. Your network mask. This determines which portion of the IP address specifies the subnetwork number and which portion specifies the host.

Class C (most networks) 255.255.255.0
Class B 255.255.0.0

3. Your network address which is your IP address bit wise anded with the network mask.
Ex: IP: 192.168.16.100 Mask: 255.255.255.0 Net Addr:: 192.168.16.0

4. Your broadcast address. Used to broadcast packets to every machine on your subnet.
Ex: IP: 192.168.16.100 Mask: 255.255.255.0 Net Addr: 192.168.16.255

5. Your gateway address. The address of the machine that is your gateway to the outside world.
In many cases: Ex: IP: 192.168.16.100 Gateway: 192.168.16.1

6. Your nameserver address. Translates host names into IP addresses. 192.168.16.10 or you can use 8.8.8.8 or 4.2.2.2

Configuration tools

There are many network configuration tools today. They are:

netconf A GUI interactive interface
linuxconf A GUI interactive interface includes netconf configuration.
netconfig A GUI step by step interface
ifconfig A text based program to configure the network interface. Type "man ifconfig" for info.

These programs will modify values in the following files:

* /etc/sysconfig/network - Defines your network and some of its characteristics.

* /etc/HOSTNAME - Shows the host name of this host. IF your name is "myhost" then that is exactly the text this file will contain.

* /etc/resolv.conf - Specifies the domain to be searched for host names to connect to, the nameserver address, and the search order for the nameservers.

* /etc/host.conf - Specifies the order nameservice looks to resolve names.

* /etc/hosts - Shows addresses and names of local hosts.

* /etc/networks - Provides a database of network names with network addresses similar to the /etc/hosts file. This file is not required for operation.

* /etc/sysconfig/network-scripts/ifcfg-eth* - There is a file for each network interface. This file contains the IP address of the interface and many other setup variables.

Analysis Tools

* netstat - Displays information about the systems network connections, including port connections, routing tables, and more. The command "netstar -r" will display the routing table.

* traceroute - This command can be used to determine the network route from your computer to some other computer on your network or the internet. To use it you can type
"route IPaddress" of the computer you want to see the route to.

* nslookup - Used to query DNS servers for information about hosts.

* arp - This program lets the user read or modify their arp cache.

* tcpdump - This program allows the user to see TCP traffic on their network.

* dig - Send domain name query packets to name servers for debugging or testing.

* ping

Manual Configuration

You can use one of the above tools or configure the network the old fashioned way as follows:

1. First to use networking on any permanent basis you should setup the file /etc/sysconfig/network similar to the example shown below.
2. Assign an ip address with "ifconfig eth0 192.168.16.100 netmask 255.255.255.0 up".
3. Tell your machine that a hub is ready for information with the command "route add -net 192.168.16.0 netmask 255.255.255.0 eth0"
4. To contact hosts outside your network if a machine with IP address 192.168.16.1 is the gateway use the command "route add default gw 192.168.1.1 eth0"

These settings are not permanent, but go away the next time you boot. They are normally set up in the directory /etc/sysconfig/network-scripts. Add the network interface to the file /etc/sysconfig/network-scripts/ifcfg-eth*. For example the file ifcfg-eth0 if for the first ethernet interface, ifcfg-eth1 for the second, ifcfg-lo is for the local interface. An example file from my system is:

DEVICE="eth0"
IPADDR="192.168.16.100"
NETMASK="255.255.255.0"
ONBOOT="yes"
BOOTPROTO="none"

Unless you know what you're doing it is best to use a network configuration tool. I cannot guarantee the accurateness of how to set these files up on your system.

Configuring an interface for multiple IP addresses

If you want to configure your network card to act as more than one IP address, issue the following command:

ifconfig dummy 192.168.16.102 netmask 255.255.255.0

This uses the dummy system interface capability supported in the kernel to setup another virtual interface which operates at IP address 192.168.1.102. Substitute the IP address that you want your virtual interface to be with an appropriate netmask for your network. To disable this, issue the following command.

ifconfig dummy down

Another way to use multiple IP addresses on one ethernet card is to set up a new file in your /etc/sysconfig/network-scripts directory. Copy your ifcfg-eth0 role to ifcfg-eth0:0. Edit that file and rename the device to "eth0:0" and the IP address to the desired IP address. You may also want to modify BROADCAST, NETWORK, or NETMASK. You can continue adding IP addresses by using :1, :2, etc such as ifcfg-eth0:2.

To make it effective, you must reboot your system or issue the command "/etc/rc.d/init.d/network restart" as root.

Dynamically allocated IP addresses

To get the IP address of a dynamically allocated network interface in a script file enter the following:

dynip=`/sbin/ifconfig | grep -A 4 ppp0 | awk '/inet/ { print $2 } ' | sed -e s/addr://`

Substitute the correct interface that you get your dynamic IP address in for ppp0 in the example above. This script line gets your dynamic IP address for use in a masquerade script. You can use the variable $dynip as in any other configuration. The next time you make a new connection you will need to extract the dynip value again and re-run the masquerade script.

Networking file formats, examples and considerations

Below are listed some more in depth information about the networking files.

• /etc/sysconfig/network
  The /etc/inittab file contains the entry "si::sysinit:/etc/rc.d/rc.sysinit" which causes the system at startup to run the rc.sysinit script. The rc.sysinit file expects to find the file /etc/sysconfig/network if networking is to be enabled.
  The network file looks like this:

  NETWORKING=yes
  FORWARD_IPV4=false
  HOSTNAME=mymachine.mycompany.com
  DOMAINNAME=mycompany.com
  GATEWAY=192.168.16.1
  GATEWAYDEV=eth0

  Where GATEWAYDEV is the network interface card that is attached to the network the gateway machine is on. The GATEWAY is the actual IP address of the gateway machine.
  

• /etc/hosts - Defines local hosts.

  127.0.0.1 localhost localhost.localdomain
  192.168.16.100 mymachine.mycompany.com mymachine
  

• /etc/resolv.conf - Specifies the domain to be searched for host names to connect to, the nameserver address, and the search order for the nameservers.

  domain mycompany.com
  search mycompany.com mynet.net
  8.8.8.8
  nameserver 192.168.16.10 

• /etc/host.conf - Specifies the order nameservice looks to resolve names. An example file:

   order hosts, bind
   multi on
   nospoof on
  

Older X windows configuration

In Xwindows a working configuration is set up as follows:

NAMES:

hostname: mymachine.mycompany.com

Domain: mycompany.com

Nameservers: 192.168.16.10

nameserver: 8.8.8.8

HOSTS:

IP - 192.168.16.100

Name - mymachine.mycompany.com

INTERFACES:

Interface - eth0

IP- 192.168.16.100

proto - none

Netmask: 255.255.255.0

Network: 192.168.16.0

Broadcast: 192.168.16.255

ROUTING:

Default gateway: 192.168.16.1

Default gateway device: eth0