Linux Network Configuration and Troubleshooting Commands

Linux Network Configuration and Troubleshooting Commands

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. ReplaceXXXX 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 useICMP (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 Nnumber 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