Running a server means you’re essentially operating a house with multiple doors and windows. Some you need open for legitimate visitors, but others might be unlocked without you even realizing it. That’s where port scanning comes in – it’s like doing a security check of all your entry points before someone with bad intentions finds them first.
Understanding Port Scanning Basics
Port scanning is the process of probing a server or network to identify which ports are open and what services are running on them. Think of ports as numbered doorways on your server – there are 65,535 of them in total. Some common ones include port 80 for HTTP traffic, port 443 for HTTPS, and port 22 for SSH access.
When you run a port scan, you’re essentially knocking on each door to see which ones answer. The scan tells you not just which ports are open, but often what’s running behind them – web servers, databases, email services, or other applications.
Why Open Ports Matter for Security
Every open port represents a potential attack surface. I learned this the hard way a few years back when I discovered an old test database was still accessible through port 3306. I’d forgotten about it after a development project, but automated scanners hadn’t. By the time I noticed unusual traffic patterns, attackers had already found it.
The reality is that hackers use automated tools that continuously scan the internet for vulnerable servers. They’re looking for:
Unnecessary open ports – Services you’re not actively using but forgot to close
Outdated software versions – Applications with known vulnerabilities
Misconfigured services – Databases or admin panels accidentally exposed to the public
Default credentials – Services running with factory settings
Common Vulnerable Ports to Watch
While any port can potentially be exploited, certain ones are more frequently targeted. Port 22 (SSH) is a prime target because it provides direct server access. If you’re running it with default settings or weak passwords, you’re basically leaving your front door unlocked with a welcome mat outside.
Port 3389 (Remote Desktop Protocol) is another favorite among attackers. I’ve seen countless servers compromised because RDP was left exposed to the internet without proper protection. Database ports like 3306 (MySQL) and 5432 (PostgreSQL) should almost never be open to the public internet, yet they often are.
Port 23 (Telnet) is particularly dangerous because it transmits everything in plain text, including passwords. If you find this port open on your server, close it immediately – there’s no legitimate reason to use Telnet anymore.
How Regular Port Scanning Protects You
Performing regular port scans gives you visibility into your actual security posture. It’s not about what you think is open – it’s about what’s actually accessible from the outside world.
Here’s what regular scanning helps you catch:
Services you thought were firewalled but aren’t. Configuration changes don’t always work as intended, and a scan confirms what’s really happening.
Software that opened unexpected ports. Some applications create their own ports during installation, and you might not even know about them.
Failed security updates. If your firewall rules didn’t apply correctly after a server restart, you’ll discover it through scanning rather than through a breach.
External vs Internal Scanning
There’s an important distinction between scanning from inside your network and from outside. Internal scans show you everything running on your server, but they don’t show you what the rest of the internet can actually see.
External scanning – the kind that views your server the same way an attacker would – is crucial. Your firewall might be blocking certain ports internally, but if they’re accessible from the public internet, that’s what matters for security. Always scan from an external perspective to get the real picture.
Making Port Scanning Part of Your Routine
The biggest mistake I see is treating port scanning as a one-time task. Your server configuration changes – you install new software, update services, modify firewall rules. Each change potentially affects your security posture.
Set up continuous monitoring that automatically scans your server and alerts you when something changes. This way, if a software update accidentally opens a new port or a misconfiguration exposes a service, you’ll know within hours instead of months.
What to Do When You Find Open Ports
Finding open ports isn’t automatically bad – the question is whether they should be open. For each port you discover, ask yourself:
Do I need this service accessible from the internet? If it’s a database or admin panel, probably not.
Is this software up to date? Check the version against known vulnerabilities.
Is it properly configured? Default configurations are often insecure.
Can I restrict access by IP address? Many services only need to be accessible from specific locations.
The goal isn’t to close everything – it’s to ensure that what’s open is intentional, necessary, and properly secured.
Beyond Just Scanning
Port scanning is your first line of defense, but it’s part of a broader security strategy. Once you know what’s open, you need to ensure those services are hardened – strong authentication, latest security patches, proper firewall rules, and ideally, additional layers like fail2ban to block repeated attack attempts.
Regular port scanning transforms security from guesswork into something measurable. You’ll sleep better knowing exactly what’s exposed and having confidence that unexpected changes will be caught immediately rather than discovered during a security incident.