The purpose of this chapter is not providing solutions to all possible network problems but more to point out that you have to do it methodically and with knowledge of the surrounding environment.

Knowledge of the surrounding environment is an absolute necessity because you can't solve a problem if you have no idea whatsoever where problems can occur. You have to know how you are connected to the network because every component that plays a role in facilitating your network access can fail.

The key files, terms and utilities will not be described in this chapter since most of them have already been described in Chapter 5, Networking Configuration (205) and other chapters.

Let's assume you're working on a PC which is connected to the Internet by means of a local area network and a firewall. Suddenly you can't view that web page anymore.

The first step in determining the problem is to make a list of the components that are involved, this could be something like:

+ your PC with eth0 network interface to the LAN
+ the firewall with eth0 interface to the LAN
               and  eth1 interface to the ISP
+ the site you are trying to reach
   

Then think about how everything works together. You enter the URL in the browser. Your machine uses DNS to find out what the IP address is of the web site you are trying to reach etc.

Packets travel through your eth0 interface over the LAN to the eth0 interface of the firewall and out the eth1 interface of the firewall to the ISP and from the ISP in some way to the web server.

Now that you know how the different components interact, you can take steps to determine the source of the malfunction.

The graphic below should not be treated as the way to solve all problems. The purpose of the graphic is to give an example of step-by-step troubleshooting given a certain situation, which in this case is fictitious :)

Besides the checklist given above, there are some other possibilities why the connection to another machine might fail. These possiblities are listed below.

Name resolution is the translation of a hostname into an IP address. If a user tries to connect to a machine based on the hostname of that machine and the hostname resolution doesn't function properly then there will be no connection made.

The file /etc/resolv.conf contains the IP addresses of the nameservers. The nameservers are the servers that do the name resolution for a external network. For small (local) networks a local lookup table can be made by using the /etc/hosts file. This file contains a list of aliases or FQDN (or both) per IP address.

Checking the name resolution can be done with the commands /usr/bin/dig (dig is an acronym for Domain Information Groper) or /usr/bin/host. Both of these commands return the IP address associated with the hostname.

# host crystal.snow.nl
crystal.snow.nl is an alias for imap.snow.nl.
imap.snow.nl has address 213.154.248.156
    

# dig crystal.snow.nl

; <<>> DiG 9.8.0-P1-RedHat-9.8.0-3.P1.fc15 <<>> crystal.snow.nl
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13354
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 3, ADDITIONAL: 6

;; QUESTION SECTION:
;crystal.snow.nl.  IN A

;; ANSWER SECTION:
crystal.snow.nl. 62664 IN CNAME imap.snow.nl.
imap.snow.nl.  67198 IN A 213.154.248.156

;; AUTHORITY SECTION:
snow.nl.  2344 IN NS ns2.transip.eu.
snow.nl.  2344 IN NS ns0.transip.net.
snow.nl.  2344 IN NS ns1.transip.nl.

;; ADDITIONAL SECTION:
ns0.transip.net. 67199 IN A 80.69.67.67
ns0.transip.net. 67199 IN AAAA 2a01:7c8:a::53
ns1.transip.nl.  75705 IN A 80.69.69.69
ns1.transip.nl.  314 IN AAAA 2a01:7c8:b::53
ns2.transip.eu.  67198 IN A 217.115.203.194
ns2.transip.eu.  67198 IN AAAA 2001:14a0:100:6::53

;; Query time: 2 msec
;; SERVER: 213.154.248.156#53(213.154.248.156)
;; WHEN: Mon May 16 15:43:13 2011
;; MSG SIZE  rcvd: 283
    

The information the dig command returns is more elaborate than the host command does. But both commands return the same IP address.

The hostname of a machine itself is stored in a file called /etc/hostname or /etc/HOSTNAME for Debian based systems. On Fedora systems the name is stored in the file /etc/sysconfig/network. For all systems the hostname can be found with the command /bin/hostname. When given no argument, this command gives replies with the hostname of the machine. In case an argument is given along with the command, the hostname of the machine will be changed.

Another possible cause of network problems can be the incorrect initialization of the system. To find any initialization errors check out the file /var/log/messages or read the kernel ring buffer by using the /bin/dmesg command.

Security settings can also be a source of connection problems. The server may have blocked access from or allow access from certain clients using the /etc/host.deny resp. /etc/host.allow

Also the network configuration may cause connectivity problems. For instance if the computer uses fixed network settings from another site. You can check these settings in the files in the directory /etc/sysconfig/network-scripts for Fedora based systems or in the file /etc/network for Debian based systems.