Computers are connected in a network to exchange information or resources each other. Two or more computer connected through network media called computer network. There are number of network devices or media are involved to form computer network. Computer loaded with Linux Operating System can also be a part of network whether it is small or large network by its multitasking and multiuser natures. Maintaining of system and network up and running is a task of System / Network Administrator’s job. In this article we are going to review frequently used network configuration and troubleshoot commands in Linux.
ifconfig (interface configurator) command is use to initialize an interface, assign IP Address to interface and enable or disable interface on demand. With this command you can view IP Address and Hardware / MAC address assign to interface and also MTU (Maximum transmission unit) size.
ifconfig with interface (eth0) command only shows specific interface details like IP Address, MAC Address etc. with -a options will display all available interface details if it is disable also.
# ifconfig eth0
Assigning IP Address and Gateway
Assigning an IP Address and Gateway to interface on the fly. The setting will be removed in case of system reboot.
# ifconfig eth0 192.168.50.5 netmask 255.255.255.0
Enable or Disable Specific Interface
To enable or disable specific Interface, we use example command as follows.
# ifup eth0
# ifdown eth0
Setting MTU Size
By default MTU size is 1500. We can set required MTU size with below command. Replace XXXX with size.
# ifconfig eth0 mtu XXXX
Set Interface in Promiscuous mode
Network interface only received packets belongs to that particular NIC. If you put interface in promiscuousmode it will received all the packets. This is very useful to capture packets and analyze later. For this you may require superuser access.
# ifconfig eth0 -promise
PING (Packet INternet Groper) command is the best way to test connectivity between two nodes. Whether it is Local Area Network (LAN) or Wide Area Network (WAN). Ping use ICMP (Internet Control Message Protocol) to communicate to other devices. You can ping host name of ip address using below command.
# ping 220.127.116.11 # ping www.hentati.com
In Linux ping command keep executing until you interrupt. Ping with -c option exit after N number of request (success or error respond).
# ping -c 5 www.hentati.com
traceroute is a network troubleshooting utility which shows number of hops taken to reach destination also determine packets traveling path. Below we are tracing route to global DNS server IP Address and able to reach destination also shows path of that packet is traveling.
# traceroute 18.104.22.168
Netstat (Network Statistic) command display connection info, routing table information etc. To displays routing table information use option as -r.
# netstat -r Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 192.168.50.0 * 255.255.255.0 U 0 0 0 eth0 link-local * 255.255.0.0 U 0 0 0 eth0 default 192.168.50.1 0.0.0.0 UG 0 0 0 eth0
Dig (domain information groper) query DNS related information like A Record, CNAME, MX Record etc. This command mainly use to troubleshoot DNS related query.
# dig www.hentati.com; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.10.rc1.el6 <<>> www.hentati.com
nslookup command also use to find out DNS related query. The following examples shows A Record (IP Address) of tecmint.com.
# nslookup www.hentati.com Server: 22.214.171.124 Address: 126.96.36.199#53 Non-authoritative answer: www.hentati.com canonical name = tecmint.com. Name: hentati.com Address: 188.8.131.52
route command also shows and manipulate ip routing table. To see default routing table in Linux, type the following command.
# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.50.0 * 255.255.255.0 U 0 0 0 eth0 link-local * 255.255.0.0 U 1002 0 0 eth0 default 192.168.50.1 0.0.0.0 UG 0 0 0 eth0 Adding, deleting routes and default Gateway with following commands. Route Adding # route add -net 10.10.10.0/24 gw 192.168.0.1 Route Deleting # route del -net 10.10.10.0/24 gw 192.168.0.1 Adding default Gateway # route add default gw 192.168.0.1
host command to find name to IP or IP to name in IPv4 or IPv6 and also query DNS records.
# host www.google.com www.google.com has address 184.108.40.206 www.google.com has address 220.127.116.11 www.google.com has address 18.104.22.168 www.google.com has address 22.214.171.124 www.google.com has address 126.96.36.199 www.google.com has IPv6 address 2404:6800:4003:802::1014
Using -t option we can find out DNS Resource Records like CNAME, NS, MX, SOA etc.
# host -t CNAME www.redhat.com
www.redhat.com is an alias for wildcard.redhat.com.edgekey.net.
ARP (Address Resolution Protocol) is useful to view / add the contents of the kernel’s ARP tables. To see default table use the command as.
# arp -e Address HWtype HWaddress Flags Mask Iface 192.168.50.1 ether 00:50:56:c0:00:08 C eth0
ethtool is a replacement of mii-tool. It is to view, setting speed and duplex of your Network Interface Card (NIC). You can set duplex permanently in /etc/sysconfig/network-scripts/ifcfg-eth0 with ETHTOOL_OPTS variable.
# ethtool eth0 Settings for eth0: Current message level: 0x00000007 (7) Link detected: yes
iwconfig command in Linux is use to configure a wireless network interface. You can see and set the basic Wi-Fi details like SSID channel and encryption. You can refer man page of iwconfig to know more.
# iwconfig [interface]
hostname is to identify in a network. Execute hostname command to see the hostname of your box. You can set hostname permanently in /etc/sysconfig/network. Need to reboot box once set a proper hostname.
# hostname hentati.com
13-GUI tool system-config-network
Type system-config-network in command prompt to configure network setting and you will get nice Graphical User Interface (GUI) which may also use to configure IP Address, Gateway, DNS etc. as shown below image.