Is the program running?

The first step in troubleshooting a network service is to make sure the program is running.

service

To check that the program is running we will start by using the command 'service'. You can use service to start, stop, and check status for an application that has an init script installed.

Service names

The service command references a service using its init script, stored in the /etc/init.d directory. Check that directory if you aren't sure what name the system uses for a service.

Some names vary depending on your distribution - apache is 'httpd' on CentOS, for exampe, while it's 'apache2' on Ubuntu.

Service status

The following example shows how to check the status of httpd on CentOS using the service command.

$ sudo service httpd status
httpd is stopped

Service control

If a service isn’t running you can use service to start it.

$ sudo service httpd start
Starting httpd:                                            [  OK  ]

If the application cannot be started the service command will report the failure and usually show a message explaining the reason.

$ sudo service httpd start
Starting httpd: (98)Address already in use: make_sock: could not bind to address [::]:80 
(98)Address already in use: make_sock: could not bind to address 0.0.0.0:80 
no listening sockets available, shutting down 
Unable to open logs 
[FAILED]

netstat

In the example above httpd cannot be started because something is already listening on the port. To find out what it is you can run netstat.

We will cover netstat in more detail later in this series but for this example it is enough to know that it can be used to display a list of listening programs and the ports they are using.

# netstat -plnt 
Active Internet connections (only servers) 
Proto Recv-Q Send-Q Local Address               Foreign Address             State       PID/Program name   
tcp        0      0 10.176.77.113:3306          0.0.0.0:*                   LISTEN      28509/mysqld        
tcp        0      0 0.0.0.0:80                  0.0.0.0:*                   LISTEN      2113/nc             
tcp        0      0 127.0.0.1:25                0.0.0.0:*                   LISTEN      1115/master         
tcp        0      0 :::22                       :::*                        LISTEN      1051/sshd

The output from netstat shows that nc (listed in the 'Program name' column) is listening on port 80 (in the 'Local Address' column) and so stopping it should allow httpd to be started.

Remember that if the service isn’t running it may be that a super-server, such as xinetd, is being used to launch the program when a connection is received.

Checking again

If the service wasn’t running, starting it may have resolved the issue. Let's give it a test to find out.

If the program is running you should see something similar to the following when you check it with service:

$ sudo service xinetd status
xinetd (pid  8795) is running...

If you cannot start your application take a look at your logs to see if they contain further information regarding the issue. This guide should help with making use of your logs.

Continuing onward

Once you're sure the application is running you'll want to take a look at the server resources it is consuming.