What is a Service?

Process States


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:


Example which port used the port 25?

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

This is a SMTP port

smtp            25/tcp          mai

How to manage services

on Windows

on Linux

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:

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