Linux network 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.

Linux Network Configuration and Troubleshooting Commands

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 Gopher) 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 192.168..1

PING 192.168..1 (192.168..1) 56(84) bytes of data.

64 bytes from 192.168..1: icmp_seq=1 ttl=44 time=203 ms

64 bytes from 192.168..1: icmp_seq=2 ttl=44 time=201 ms

64 bytes from 192.168..1: icmp_seq=3 ttl=44 time=201 ms

OR

# ping www.raju.com

PING raju.com (192.168..1) 56(84) bytes of data.

64 bytes from 192.168..1: icmp_seq=1 ttl=47 time=284 ms

64 bytes from 192.168..16: icmp_seq=2 ttl=47 time=287 ms

64 bytes from 192.168..1: 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.raju.com

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

64 bytes from 192.168..1: icmp_seq=1 ttl=47 time=285 ms

64 bytes from 192.168..1: icmp_seq=2 ttl=47 time=285 ms

64 bytes from 192.168..1: icmp_seq=3 ttl=47 time=285 ms

--- raju.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.

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

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

;; global options: +cmd

;; Got answer:

;; ->>HEADER<

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.raju.com

Server:         4.2.2.2

Address:        4.2.2.2#53

Non-authoritative answer:

www.raju.com canonical name = raju.com.

Name:   raju.com

Address: 50.116.66.136

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 192.168.0.0/24 gw 192.168.0.1

Route Deleting

# route del -net 192.168.0.0/24 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

raju.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