Diagnosing Local Area Network

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Diagnosing Local Area Network

Functional network may go out of order at some point of time. This is when we require some kind of troubleshooting tools so that we may come to know where exactly the problem is? To troubleshoot a network problem first of all we should make sure that there is no physical problem like poor connections, problematic cables and like these.

After this activity we should use any of the following tools:

dmesg, ifconfig, netstat, ping or traceroute etc.

We also can check the log messages or use debugging tools like Ethereal.

Using dmesg

We can type the following command to check whether Linux system has found the LAN card and installed its driver properly:

# dmesg | grep eth

This command will list all the messages those were output by Linux at booting time. But the output will not be displayed on the monitor as this command is piped to grep command. The grep eth command will allow only those lines to print which contain eth word.

Network configuration files

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Network configuration files:

/etc/resolv.conf

Alternative to /etc/hosts file is Domain Name Service (DNS). In Linux the DNS is implemented using bind i.e. (Berkley Internet Name Domain). It uses named daemon. If we have IP addresses of DNS servers, we can make entry of all of them in /etc/resolv.conf file. This also is a simple file. In this file the DNS server’s entry is mentioned as nameserver and can be ISP defined DNS server’s address. This is shown as follows:

nameserver 202.56.230.6

/etc/sysconfig/network

This file stores the basic network configuration data. If we have any problem in our network, we must have a look in this file. The first entry in this file is NETWORKING=yes. Few of the other variables are show hereunder:

Variables Details

NETWORKING This can be YES for starting networking or NO for stopping it.

HOSTNAME This defines the host name of the computer

GATEWAY Defines the computer’s gateway IP address

GATEWAYDEV This defines the network device, as eth0, which is connected to the network with gateway. It is required if we have two or more cards on the system.

NISDOMAIN It happens to be the domain name of NIS system

network configuration Files

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Files for network configuration

There are many files for configuring network. These are: /etc/hosts, /etc/host.conf, and /etc/resolv.conf. Many other new configuration files are stored in /etc/sysconfig directory.

/etc/hosts file

If no DNS server is present on the network, this file is still useful in resolving the hostnames. It is very simple file. Each line comprises of an IP address, a fully qualified domain name (FQDN) and optional hostname. Example of one of the entry is shown as below:

192.168.1.72 linuxserver72.mydomain.com linuxserver72

/etc/host.conf

We have two databases of IP addresses and hostnames: 1. DNS servers and 2. /etc/hosts. The searching order is determined by /etc/host.conf file. This file contains only one line, which is as follows:

order hosts,bind

This line tells the Linux system to search for the correct IP address in /etc/hosts file first and then search bind i.e. the DNS server. Here we also can include NIS, in above list.

Address resolution protocol

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Using address resolution protocol (arp):

The arp associates hardware address of network card with IP address. Once any computer makes the contact with another computer on the network, both the systems exchange hardware addresses. Thereafter these are stored in ARP database. We can find this arp database on our computer by using arp command:

Based on how the contact was made, address column lists either host name or the IP address of the computer. The computer name is taken from /etc/hosts file. HWtype column shows the type of network adapter. HWaddress column shows the hardware address of the adapter in hexadecimal. If we see a duplicate IP address on two different MAC addresses then we must remove the associated system’s entry from the arp table as it will stop communication on the network. To remove the entry we should use the syntax arp -d computername.The computername should either be the name of computer or the IP address of the computer.