The first step in troubleshooting a network service is to make sure the program is running.
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.
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.
The following example shows how to check the status of httpd on CentOS using the service command.
$ sudo service httpd status httpd is stopped
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]
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.
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.
Once you're sure the application is running you'll want to take a look at the server resources it is consuming.
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