What is a Service?

About

A service is a process that:

• runs continuously in the background
• are started by the operating system when the system starts.

It's declared and managed by the OS init system. With the init system, you may:

• start / stop a service
• get a list of all currently running services
• …

It is used remotely through some remote interface, either synchronous or asynchronous (eg web service, messaging system, RPC, or socket.)

How to list the well-known services

To have a list of the well-known services, you can inspect the services file (The services translation file) on linux, located at:

/etc/services

Example which port used the port 25?

cat /etc/services | grep ' 25/tcp'

This is a SMTP port

smtp            25/tcp          mai

How do you identify a service?

A service is identified:

• on the network by a socket. The most well-known is the netsocket (ie the hostname, the port and the protocol)
• on the application level by its scheme (application protocol)

For instance, if you known that there is a service on the port 80, you would expect it to be:

• a HTTP/Web server (because this is the default HTTP port
• over TCP (because HTTP works generally over it)