Linux Network Configuration and Troubleshooting Commands

Linux Network Configuration and Troubleshooting Commands

NIC: Network Interface Card

Use “ipconfig” command to determine IP address, interface devices, and change NIC configuration

Determining NIC IP Address

[root@tmp]# ifconfig -a

eth0 Link encap:Ethernet HWaddr 00:08:C7:10:74:A8

BROADCAST MULTICAST MTU:1500 Metric:1

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

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

collisions:0 txqueuelen:100

RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)

Interrupt:11 Base address:0x1820

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX packets:787 errors:0 dropped:0 overruns:0 frame:0

TX packets:787 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

RX bytes:82644 (80.7 Kb) TX bytes:82644 (80.7 Kb)

Changing IP Address

We could give this eth0 interface an IP address using the ifconfig command.

[root@tmp]# ifconfig eth0 10.0.0.1 netmask 255.255.255.0 up

The "up" at the end of the command activates the interface.

To make this permanent each time boot up by add this command in /etc/rc.local file which is run at the end of every reboot.

Permanent IP configuration

—  Fedora Linux also makes life a little easier with interface configuration files located in the /etc/sysconfig/network-scripts directory.

—  Interface eth0 has a file called ifcfg-eth0, eth1 uses ifcfg-eth1, and so on.

—  Admin can place your IP address information in these files

File formats for network-scripts

root@network-scripts]# less ifcfg-eth0

DEVICE=eth0

IPADDR=192.168.1.100

NETMASK=255.255.255.0

BOOTPROTO=static

ONBOOT=yes

#

# The following settings are optional

#

BROADCAST=192.168.1.255

NETWORK=192.168.1.0

[root@network-scripts]#

Activate config change

—  After change the values in the configuration files for the NIC you have to deactivate and activate it for the modifications to take effect.

—   The ifdown and ifup commands can be used to do this:

[root@network-scripts]# ifdown eth0

[root@network-scripts]# ifup eth0

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 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.

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.

Enable or Disable Specific Interface

 To enable or disable specific Interface, we use example command as follows.

—  

Setting MTU Size

By default MTU size is 1500. We can set required MTU size with below command. Replace XXXX with size.

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 analyse later. For this you may require super user access.

how do I Configure Static IP Address Internet Protocol (IPv4)?

—  Assign Static IP Address to eth0 interface editing configuration file  /etc/network/interfaces to make permanent changes as shown below.

Next, restart network services after entering all the details using the following command.

  How to Remove an IP Address

—  The following command will remove an assigned IP address from the given interface (eth1).

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.

—  In Linux ping command keep executing until you interrupt.

—  Ping with -c option exit after N number of request (success or error respond).

3.       TRACEROUTE Command

—  traceroute is a network troubleshooting utility which shows number of hops taken to reach destination

—   also determine packets travelling path.

—  Below we are tracing route to global DNS server IP Address and able to reach destination also shows path of that packet is travelling.

4.       NETSTAT Command

—  Netstat (Network Statistic) command display connection info, routing table information etc.

—  To displays routing table information use option as -r.

Listing all ports (both TCP and UDP) using netstat -a option.

Listing only TCP (Transmission Control Protocol) port connections using netstat -at.

Listing all active listening ports connections with netstat -l.

5.       DIG Command

—  Dig (domain information groper ) is a tool for querying DNS nameservers for information about host addresses, mail exchanges, nameservers, and related information.

—   This command mainly use to troubleshoot DNS related query.

6.       NSLOOKUP Command

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

7.       ROUTE Command

—  route command also shows and manipulate ip routing table.

—   To see default routing table in Linux, type the following command.

Adding, deleting routes and default Gateway with following commands.

8.       HOST Command

—  host command to find name to IP or IP to name in IPv4 or IPv6 and also query DNS records.

9.       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_OPTSvariable.

By : mogtaba altyib 
Modification by : Mohammed Bakry PhD