Linux Networking Configuration

Adding a network interface card (NIC):
Manual method: This does not alter the permanent configuration and will only configure support until the next reboot.
cd /lib/modules/2.2.5-15/net/ - Use kernel version for your system. This example uses 2.2.5-15 Here you will find the modules supported by your system. It can be permanently added to /etc/modules.conf (or for older systems: /etc/conf.modules) Example: alias eth0 3c59x

/sbin/insmod -v 3c59x (For a 3Com ethernet card)
ifconfig ...
The easy way: Red Hat versions 6.2 and later, ship with Kudzu, a device detection program which runs during system initialization. (/etc/rc.d/init.d/kudzu) This can detect a newly installed NIC and load the appropriate driver. Then use /usr/sbin/netconfig to configure the IP address and network settings. The configuration will be stored so that it will be utilized upon system boot.
Systems with two NIC cards: Typically two cards are used when connecting to two networks. In this case the device must be defined using one of three methods:
Use the GUI tool /usr/bin/netcfg
OR
Define network parameters in configuration files:
Define new device in file /etc/sysconfig/network-scripts/ifcfg-eth1
DEVICE=eth1
BOOTPROTO=static
IPADDR=192.168.10.12
NETMASK=255.255.255.0
GATEWAY=XXX.XXX.XXX.XXX
HOSTNAME=node-name.name-of-domain.com
DOMAIN=name-of-domain.com
Special routing information may be specified, if necessary, in the file: /etc/sysconfig/static-routes Example:
eth1 net XXX.XXX.XXX.0 netmask 255.255.255.0 gw XXX.XXX.XXX.XXX
OR
Define network parameters using Unix command line interface:
Define IP address: ifconfig eth0 XXX.XXX.XXX.XXX netmask 255.255.255.0 broadcast XXX.XXX.XXX.255
ifconfig eth1 192.168.10.12 netmask 255.255.255.0 broadcast 192.168.10.255

If necessary, define route with with the route command: Examples: route add default gw XXX.XXX.XXX.XXX dev eth0
route add -net XXX.XXX.XXX.0 netmask 255.255.255.0 gw XXX.XXX.XXX.XXX dev eth0
Where XXX.XXX.XXX.XXX is the gateway to the internet as defined by your ISP or network operator.
If a mistake is made just repeat the route command substituting "del" in place of "add".
Configuration files:

File: /etc/resolv.conf - 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, 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.

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)

/etc/sysconfig/network
Red Hat network configuration file used by the system during the boot process.
/etc/nsswitch In the past this file has had the following names: /etc/nsswitch.conf, /etc/svc.conf, /etc/netsvc.conf, ... depending on the distribution.

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.
File: /etc/sysconfig/network-scripts/ifcfg-eth0 Configuration settings for your first ethernet port (0). Your second port is eth1.

File: /etc/modules.conf (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)

Network GUI Configuration Tools:
/usr/bin/redhat-config-network (/usr/bin/neat) (RH 7.2+)
/usr/bin/redhat-config-network-tui (text console RH 9.0)
/usr/sbin/netconfig (console. First interface only - eth0)
/usr/bin/netcfg (GUI) (no longer available with RH 7.2)
/usr/bin/redhat-control-network (Start/Stop network interfaces RH 9.0)
redhat-config-network-druid (Configure CIPE,Ethernet,modem,token Ring,Wireless or DSL)
/usr/bin/gnome-network-preferences Gnome Desktop Network Configuration (RH 9.0) - proxy configuration

The Red Hat configuration tools store the configuration information in the file /etc/sysconfig/network. They will also allow one to configure routing information.
File: /etc/sysconfig/network
Static IP address Configuration: (Configure gateway address)
NETWORKING=yes
HOSTNAME=my-hostname - Hostname is defined here and by command hostname
FORWARD_IPV4=true - True for NAT firewall gateways and linux routers. False for everyone else - desktops and servers.
GATEWAY="XXX.XXX.XXX.YYY" - Used if your network is connected to another network or the internet.
Static IP configuration. Gateway not defined here for DHCP.
OR for DHCP configuration:
NETWORKING=yes
HOSTNAME=my-hostname - Hostname is defined here and by command hostname
(Gateway is assigned by DHCP.)
File: /etc/sysconfig/network-scripts/ifcfg-eth0
Static IP address configuration:
DEVICE=eth0
BOOTPROTO=static
BROADCAST=XXX.XXX.XXX.255
IPADDR=XXX.XXX.XXX.XXX
NETMASK=255.255.255.0
NETWORK=XXX.XXX.XXX.0
ONBOOT=yes
OR for DHCP configuration:
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
(Used by script /etc/sysconfig/network-scripts/ifup to bring the various network interfaces on-line) To disable DHCP change BOOTPROTO=dhcp to BOOTPROTO=none
In order for updated information in any of these files to take effect, one must issue the command: service network restart

Changing the host name:
This is a three step process:
Issue the command: hostname new-host-name
Change network configuration file: /etc/sysconfig/network Edit entry: HOSTNAME=new-host-name
Restart systems which relied on the hostname (or reboot):
Restart network services: service network restart
Restart desktop:
Bring down system to console mode: init 3
Bring up X-Windows: init 5 One may also want to check the file /etc/hosts for an entry using the system name which allows the system to be self aware.

Usefull Linux networking commands:
/etc/rc.d/init.d/network start - command to start, restart or stop the network
netstat - Display connections, routing tables, stats etc
List externally connected processes: netstat -punta
List all connected processes: netstat -nap
Show network statistics: netstat -s
Kernel interface table info: netstat -a -i eth0
ping - send ICMP ECHO_REQUEST packets to network hosts. Use Cntl-C to stop ping.
traceroute - print the route packets take to network host
mtr - a network diagnostic tool - Like traceroute except it gives more network quality and network diagnostic info.
whois - Lookup a domain name in the internic whois database.
finger - Display information on a system user. i.e. finger user@host Uses $HOME/.plan and $HOME/.project user files. Often used by game developers. See http://finger.planetquake.com/
ipchains - IP firewall administration
tcpdump - dump traffic on a network
iptraf - Interactive Colorful IP LAN Monitor
socklist - Display list of open sockets, type, port, process id and the name of the process. Kill with fuser or kill.
nslookup - Give a host name and the command will return IP address. Also see Testing your DNS (YoLinux Tutorial) Note that nslookup does not use the /etc/hosts file.
host - Give a host name and the command will return IP address. Unlike nslookup, the host command will use both /etc/hosts as well as DNS.
nmap - Network exploration tool and security scanner
List pingable nodes on network: nmap -sP 192.168.0.0/24 Scans network for IP addresses 192.168.0.0 to 192.168.0.255 using ping.