Linux Networking Setup

Posted: September 27, 2010 in REDHAT 5 LinUX

Required Information

To enable networking, you must configure your network interface card or cards 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, IP=, broadcast=, netmask=, gateway-, nameserver=

  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)
    Class B
  3. Your network address which is your IP address bit wise anded with the network mask.
    Ex: IP: Mask: Net Addr::
  4. Your broadcast address. Used to broadcast packets to every machine on your subnet.
    Ex: IP: Mask: Net Addr:
  5. Your gateway address. The address of the machine that is your gateway to the outside world.
    In many cases: Ex: IP: Gateway:
  6. Your nameserver address. Translates host names into IP addresses.

Configuration tools

There are many network configuration tools today. They are:

netconf A GUI interactive interface available on Redhat 6.1
linuxconf A GUI interactive interface available on Redhat 6.1 which 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(1) – Send domain name query packets to name servers for debugging or testing.

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 netmask up”.
  3. Tell your machine that a hub is ready for information with the command “route add -net netmask eth0”
  4. To contact hosts outside your network if a machine with IP address is the gateway use the command “route add default gw eth0”
  5. If using a dialup connection use the command “route add default ppp0” The word default says if the packet is not for a machine on your local network, send it to the default device.

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:


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 netmask

This uses the dummy system interface capability supported in the kernel to setup another virtual interface which operates at IP address 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

    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.	localhost	localhost.localdomain	mymachine
  • /etc/services – Internet network services list. It associates port numbers with names of services. The file contains three fields which are name, port/protocol, and aliases with an optional comment.
  • /etc/protocols – Describes DARPA internet protocols available from the TCP/IP subsystem. Maps protocol ID numbers to protocol names. It includes protocol name, number, and aliases. The protocol file on my system:
    # /etc/protocols:
    # $Id: protocols,v 1.1 1995/02/24 01:09:41 imurdock Exp $
    # Internet (IP) protocols
    #	from: @(#)protocols	5.1 (Berkeley) 4/17/89
    # Updated for NetBSD based on RFC 1340, Assigned Numbers (July 1992).
    ip	0	IP		# internet protocol, pseudo protocol number
    icmp	1	ICMP		# internet control message protocol
    igmp	2	IGMP		# Internet Group Management
    ggp	3	GGP		# gateway-gateway protocol
    ipencap	4	IP-ENCAP	# IP encapsulated in IP (officially ``IP'')
    st	5	ST		# ST datagram mode
    tcp	6	TCP		# transmission control protocol
    egp	8	EGP		# exterior gateway protocol
    pup	12	PUP		# PARC universal packet protocol
    udp	17	UDP		# user datagram protocol
    hmp	20	HMP		# host monitoring protocol
    xns-idp	22	XNS-IDP		# Xerox NS IDP
    rdp	27	RDP		# "reliable datagram" protocol
    iso-tp4	29	ISO-TP4		# ISO Transport Protocol class 4
    xtp	36	XTP		# Xpress Tranfer Protocol
    ddp	37	DDP		# Datagram Delivery Protocol
    idpr-cmtp	39	IDPR-CMTP	# IDPR Control Message Transport
    rspf	73	RSPF		#Radio Shortest Path First.
    vmtp	81	VMTP		# Versatile Message Transport
    ospf	89	OSPFIGP		# Open Shortest Path First IGP
    ipip	94	IPIP		# Yet Another IP encapsulation
    encap	98	ENCAP		# Yet Another IP encapsulation
  • /etc/named.conf – Used for domain name service to configure named. Other files used are dependent on this file. This file is explained further in the DNS section
  • /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.

    The third line specifies that DNS should be tried on my machine first then use the normal nameserver on the fifth line. The fourth line specifies that my machine is running nameservices on another network which is using interface This assumes the nameserver is set up on my machine which is explained in another section.

  • /etc/host.conf – Specifies the order nameservice looks to resolve names. An example file:
    	order hosts, bind
    	multi on
    	nospoof on

    The order specifies that when resolving names to first look in the /etc/host file, then use BIND8 (DNS) to resolve the name. The line “multi on” specifies that all valid addresses for a host found in the hosts file should be returned.

The files in /etc/sysconfig/network-scripts control your network interfaces. The network interface file is described above in the section “Manual Configuration”. If you want or need more in depth knowledge about how these files are used, you will need to read the document “How Linux Works CTDP Guide” or “The CTDP Linux Startup Manual”. Otherwise you will need to analyze the system startup scripts which is no small task.

Older X windows configuration

In Xwindows a working configuration is set up as follows:

IP –
Name –
Interface – eth0
proto – none
atboot – yes
Default gateway:
Default gateway device: eth0
Interface –
Network Address –
Network gateway
Netmask –


Routing table information is used to route incoming and outgoing network diagrams to other machines. On most simple configurations, there are three routes. One for sending packets to your own machine, one for sending packets to other machines on your network and one for sending packets to other machines outside your network through the gateway. Two programs (ifconfig and route) are used to configure these parameters. They are described in more detail in the routing section.


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