Understanding Network Services

Services on computers generally run on assigned network ports. A computer running services ensures that the services are available on these standard assigned ports, and "listens" for incoming requests from other computers.

So, if a computer is running the Apache Web server (httpd) on the default port of 80, a client machine can access Web pages through port 80, as long as there are no firewall rules preventing access to a client machine. Similarly, port 25 is the standard port used for SMTP mail access, port 143 is standard for IMAP mail, and so on.

Tip

For a complete list of services that OS X is familiar with, refer to the file /private/etc/services.

Since a service's communication protocol is well-defined, the only information one computer needs to start communicating with another computer is that computer's IP address (or hostname, which is resolved into an IP address by DNS), along with the port through which it would like to communicate. The combination of an IP address and a port is called a socket.

In the early days of networking, when trust was implied because very few had access to what was the beginning of the Internet, it was fine for network services to communicate over the network through unprotected sockets, since the only people who had access to (or understood) the network usually were trustworthy system administrators.

The commands that were developed to serve users include the following:

• telnet, for remote logins to other computers, and to test access to a socket when an optional port is specified

• rcp, for remote copy, to copy files to a local drive from a remote host

• login, also used to log in to remote computers (but does not provide socket access test capabilities)

• ftp (File Transfer Protocol), a common method of transferring files

Each of these protocols does its job, but without protecting the information as it's transferred. Worse, each of these protocols sends authentication passwords to the remote system in clear text, making it very easy for an attacker to grab passwords and use them. Fortunately, there are improved versions of these tools, which you'll learn about in this lesson, that are designed to ensure that information is secure as it is sent over a network. There are now more secure ways to transfer your data from one computer to another device over a network.