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.

1. ifconfig

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

eth0 Link encap:Ethernet HWaddr 00:0C:29:28:FD:4C

inet addr:192.168.50.2 Bcast:192.168.50.255 Mask:255.255.255.0

inet6 addr: fe80::20c:29ff:fe28:fd4c/64 Scope:Link

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:6093 errors:0 dropped:0 overruns:0 frame:0

TX packets:4824 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:1000

RX bytes:6125302 (5.8 MiB) TX bytes:536966 (524.3 KiB)

Interrupt:18 Base address:0x2000

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

inet6 addr: ::1/128 Scope:Host

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX packets:8 errors:0 dropped:0 overruns:0 frame:0

TX packets:8 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

RX bytes:480 (480.0 b) TX bytes:480 (480.0 b)

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

eth0 Link encap:Ethernet HWaddr 00:0C:29:28:FD:4C

inet addr:192.168.50.2 Bcast:192.168.50.255 Mask:255.255.255.0

inet6 addr: fe80::20c:29ff:fe28:fd4c/64 Scope:Link

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:6119 errors:0 dropped:0 overruns:0 frame:0

TX packets:4841 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:1000

RX bytes:6127464 (5.8 MiB) TX bytes:539648 (527.0 KiB)

Interrupt:18 Base address:0x2000

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.

Enable eth0

# ifup eth0

Disable 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 promiscuous mode 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 - promisc

2. PING Command

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 4.2.2.2

PING 4.2.2.2 (4.2.2.2) 56(84) bytes of data.

64 bytes from 4.2.2.2: icmp_seq=1 ttl=44 time=203 ms

64 bytes from 4.2.2.2: icmp_seq=2 ttl=44 time=201 ms

64 bytes from 4.2.2.2: icmp_seq=3 ttl=44 time=201 ms

OR

# ping www.tecmint.com

PING tecmint.com (50.116.66.136) 56(84) bytes of data.

64 bytes from 50.116.66.136: icmp_seq=1 ttl=47 time=284 ms

64 bytes from 50.116.66.136: icmp_seq=2 ttl=47 time=287 ms

64 bytes from 50.116.66.136: icmp_seq=3 ttl=47 time=285 ms

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.tecmint.com

PING tecmint.com (50.116.66.136) 56(84) bytes of data.

64 bytes from 50.116.66.136: icmp_seq=1 ttl=47 time=285 ms

64 bytes from 50.116.66.136: icmp_seq=2 ttl=47 time=285 ms

64 bytes from 50.116.66.136: icmp_seq=3 ttl=47 time=285 ms

64 bytes from 50.116.66.136: icmp_seq=4 ttl=47 time=285 ms

64 bytes from 50.116.66.136: icmp_seq=5 ttl=47 time=285 ms

--- tecmint.com ping statistics ---

5 packets transmitted, 5 received, 0% packet loss, time 4295ms

rtt min/avg/max/mdev = 285.062/285.324/285.406/0.599 ms

3. TRACEROUTE Command

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 4.2.2.2

traceroute to 4.2.2.2 (4.2.2.2), 30 hops max, 60 byte packets

1 192.168.50.1 (192.168.50.1) 0.217 ms 0.624 ms 0.133 ms

2 227.18.106.27.mysipl.com (27.106.18.227) 2.343 ms 1.910 ms 1.799 ms

3 221-231-119-111.mysipl.com (111.119.231.221) 4.334 ms 4.001 ms 5.619 ms

4 10.0.0.5 (10.0.0.5) 5.386 ms 6.490 ms 6.224 ms

5 gi0-0-0.dgw1.bom2.pacific.net.in (203.123.129.25) 7.798 ms 7.614 ms 7.378 ms

6 115.113.165.49.static-mumbai.vsnl.net.in (115.113.165.49) 10.852 ms 5.389 ms 4.322 ms

7 ix-0-100.tcore1.MLV-Mumbai.as6453.net (180.87.38.5) 5.836 ms 5.590 ms 5.503 ms

8 if-9-5.tcore1.WYN-Marseille.as6453.net (80.231.217.17) 216.909 ms 198.864 ms 201.737 ms

9 if-2-2.tcore2.WYN-Marseille.as6453.net (80.231.217.2) 203.305 ms 203.141 ms 202.888 ms

10 if-5-2.tcore1.WV6-Madrid.as6453.net (80.231.200.6) 200.552 ms 202.463 ms 202.222 ms

11 if-8-2.tcore2.SV8-Highbridge.as6453.net (80.231.91.26) 205.446 ms 215.885 ms 202.867 ms

12 if-2-2.tcore1.SV8-Highbridge.as6453.net (80.231.139.2) 202.675 ms 201.540 ms 203.972 ms

13 if-6-2.tcore1.NJY-Newark.as6453.net (80.231.138.18) 203.732 ms 203.496 ms 202.951 ms

14 if-2-2.tcore2.NJY-Newark.as6453.net (66.198.70.2) 203.858 ms 203.373 ms 203.208 ms

15 66.198.111.26 (66.198.111.26) 201.093 ms 63.243.128.25 (63.243.128.25) 206.597 ms 66.198.111.26 (66.198.111.26) 204.178 ms

16 ae9.edge1.NewYork.Level3.net (4.68.62.185) 205.960 ms 205.740 ms 205.487 ms

17 vlan51.ebr1.NewYork2.Level3.net (4.69.138.222) 203.867 ms vlan52.ebr2.NewYork2.Level3.net (4.69.138.254) 202.850 ms vlan51.ebr1.NewYork2.Level3.net (4.69.138.222) 202.351 ms

18 ae-6-6.ebr2.NewYork1.Level3.net (4.69.141.21) 201.771 ms 201.185 ms 201.120 ms

19 ae-81-81.csw3.NewYork1.Level3.net (4.69.134.74) 202.407 ms 201.479 ms ae-92-92.csw4.NewYork1.Level3.net (4.69.148.46) 208.145 ms

20 ae-2-70.edge2.NewYork1.Level3.net (4.69.155.80) 200.572 ms ae-4-90.edge2.NewYork1.Level3.net (4.69.155.208) 200.402 ms ae-1-60.edge2.NewYork1.Level3.net (4.69.155.16) 203.573 ms

21 b.resolvers.Level3.net (4.2.2.2) 199.725 ms 199.190 ms 202.488 ms

4. NETSTAT Command

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

For more examples of Netstat Command, please read our earlier article on 20 Netstat Command Examples in Linux.

5. DIG Command

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.tecmint.com; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.10.rc1.el6 <<>> www.tecmint.com

;; global options: +cmd

;; Got answer:

;; ->>HEADER<

For more examples of Dig Command, please read the article on 10 Linux Dig Commands to Query DNS.

6. NSLOOKUP Command

nslookup command also use to find out DNS related query. The following examples shows A Record (IP Address) of tecmint.com.

# nslookup www.tecmint.com

Server: 4.2.2.2

Address: 4.2.2.2#53

Non-authoritative answer:

www.tecmint.com canonical name = tecmint.com.

Name: tecmint.com

Address: 50.116.66.136

For more NSLOOKUP Command, read the article on 8 Linux Nslookup Command Examples.

7. ROUTE Command

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

8. HOST Command

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 173.194.38.180

www.google.com has address 173.194.38.176

www.google.com has address 173.194.38.177

www.google.com has address 173.194.38.178

www.google.com has address 173.194.38.179

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.

9. ARP Command

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

10. ETHTOOL Command

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

11. IWCONFIG Command

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]

12. HOSTNAME Command

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

tecmint.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.

# system-config-network

Linux GUI Network Configuration

Linux GUI Network Configuration Tool

This article can be useful for day to day use of Linux Network administrator in Linux / Unix-like operating system. Kindly share through our comment box if we missed out.