Managing port security

Ports of Call

© Photo by Jamie O'Sullivan on Unsplash

© Photo by Jamie O'Sullivan on Unsplash

Article from Issue 258/2022
Author(s):

A few basic commands for working with ports can help you make your small network or standalone system more secure.

Ports are a core feature of modern computing. I'm not talking about transferring versions of applications to another architecture or operating system. Instead, I mean the kind of port that is an address for a virtual connection point to or from a computer and another device or server, including the Internet. Ports direct external traffic to the correct application, and this function makes them important for troubleshooting and security. How do you find which ports are open or listening (i.e., currently in use) when there is no need for them to be? What ports are associated with which application or server? How do you know whether any ports are hidden and being used by an intruder? Even if you are working on a standalone computer, knowing how to answer these questions is a basic administrative skill.

Ports can be either hardware or software. Either way, they are treated similarly. Most ports are managed by two protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Both TCP and UDP have a range of port numbers, divided into three categories:

  • System ports (numbers 0-1023): These are the most common ports and essential to external communication. Sometimes called well-known ports, system ports can only be used by root or privileged users. For example, ssh is port 22, and the Network Time Protocol (NTP) is port 123. While these ports can be changed, the changed ports can only communicate with other machines that have also reassigned their ports accordingly.
  • Registered ports (numbers 1024-49151): These ports are available for processes and applications run by ordinary users. Registered ports are sometimes called user ports.
  • Dynamic ports (numbers 49152-65535): These ports can be used by any processes or applications as needed, and they are assigned on the fly. Dynamic ports are also called private or ephemeral ports.

[...]

Use Express-Checkout link below to read the full article (PDF).

Buy this article as PDF

Express-Checkout as PDF
Price $2.95
(incl. VAT)

Buy Linux Magazine

SINGLE ISSUES
 
SUBSCRIPTIONS
 
TABLET & SMARTPHONE APPS
Get it on Google Play

US / Canada

Get it on Google Play

UK / Australia

Related content

  • Nmap Methods

    How does the popular Nmap scanner identify holes in network security? In this article, we examine some Nmap analysis techniques.

  • Single-Packet Port Knocking

    If you are looking for an extra layer of remote access security, try single-packet port knocking.

  • Nmap Scripting

    Nmap is rolling out a new scripting engine to automatically investigate vulnerabilities that turn up in a security scan. We’ll show you how to protect your network with Nmap and NSE.

  • Customizing PortSentry

    PortSentry monitors your ports and lets you know when they’ve been scanned.

  • Backdoors

    Backdoors give attackers unrestricted access to a zombie system. If you plan to stop the bad guys from settling in, you’ll be interested in this analysis of the tools they might use for building a private entrance.

comments powered by Disqus

Direct Download

Read full article as PDF:

Price $2.95

Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Find SysAdmin Jobs

News