HOSTtest Domain Report Q3/2026
7 Aug 2026
Port Monitoring: how to monitor services on your server
A server can be reachable in principle yet an important service may not be functioning correctly. For example, if the web server, mail server, SSH access or a database does not respond on the intended port, this can directly affect your website, application or internal processes. With port monitoring you regularly check whether a specific network port is reachable and whether the service behind it is responding at all. This lets you detect problems with individual services early, even if the server itself remains online. In this article you will learn exactly what port monitoring is, which faults it can uncover and how to set up monitoring step by step with Hosttest Plus.
Author:
Christopher | 24 Jul 2026
Christopher | 24 Jul 2026
via Gemini
In a Nutshell
Port monitoring actively checks whether specific network ports accept connections and thus allows the distinction between host reachability and the failure of individual services (e.g. HTTP(S), SMTP, SSH, RDP, databases). It complements ping checks with service-specific tests and enables prompt alerting in the event of timeouts or repeated failed connections.
- Funktion: Establishing a TCP/UDP connection to an IP/domain and port; a successful connection indicates port or service availability, while a timeout or refusal points to a service outage, a firewall rule, or a misconfiguration.
- Betrieb & Konfiguration: Set the target (domain/IP) and port, adjust the monitoring interval to the criticality (e.g. 1–5 minutes for production services), plan retries before alerting and configure notifications (escalation/channels).
- Grenzen & Best Practices: A reachable port is not a content-level functional check — combine with HTTP, SSL/TLS, DNS and keyword monitoring; secure administrative ports and monitor firewall/security-group rules and load balancer/proxy configurations as well.
Port Monitoring
Port Monitoring: Targeted monitoring of server services
A server can be online while individual services are no longer working correctly. Port Monitoring specifically checks whether a particular network port accepts connections and whether important services such as HTTP, HTTPS, SMTP, IMAP, SSH or RDP are reachable.
🔌
When only a particular service fails
Die Website lädt nicht, E-Mails kommen nicht an oder eine Anwendung kann keine Verbindung zum gewünschten System herstellen. Auf den ersten Blick wirkt es dann so, als sei der gesamte Server ausgefallen, obwohl möglicherweise nur ein einzelner Dienst betroffen ist.
This is exactly where Port Monitoring comes in. It does not only check whether a server is reachable in general, but whether a specific network port is accepting connections. That way it is possible to determine whether a particular service such as HTTP, HTTPS, SMTP, IMAP, SSH or an application interface is reachable.
Basics
What is Port Monitoring?
Port monitoring regularly checks whether a specific port on a server is reachable. A port can be simply understood as an access point for a particular network service.
Targeted Service Checks
While an IP address or domain describes the target system, the port determines which service on that system is being addressed.
Establishing a Connection
Port monitoring attempts to establish a connection to a specific port. If successful, the port is generally considered reachable.
Detecting Timeouts
If the connection attempt fails or times out, this may indicate an outage, a firewall rule, or a misconfiguration.
More Precise Than Ping
Ping monitoring only checks whether a server is reachable in general. Port monitoring, on the other hand, verifies whether a specific service is accepting connections.
Important for Server Services
Web servers, mail servers, databases, SSH access, APIs and gateways can be monitored via their respective ports.
Helpful for Troubleshooting
If a server is still reachable but an individual service fails, port monitoring helps to narrow down the problem more quickly.
Well-known ports
Which services use common network ports?
Many well-known services use standardised ports. This allows you to check whether a particular service is reachable.
| Service | Typical ports | What the service is used for |
|---|---|---|
| HTTP | Port 80 | Unencrypted website access via HTTP. |
| HTTPS | Port 443 | Encrypted website access via HTTPS. |
| SMTP | Port 25, 465 or 587 | Email delivery and sending. |
| IMAP | Port 143 or 993 | Retrieval and synchronisation of emails. |
| POP3 | Port 110 or 995 | Retrieval of emails from the mail server. |
| SSH | Port 22 | Administration of Linux servers. |
| RDP | Port 3389 | Remote access to Windows servers. |
| MySQL | Port 3306 | Database connections to MySQL servers. |
| PostgreSQL | Port 5432 | Database connections to PostgreSQL servers. |
Why is it important?
Why Port Monitoring is important
A server often consists of multiple services that can operate or fail independently. The server can generally be running while individual services are no longer reachable.
Without Port Monitoring
- A server may appear reachable even though important services have failed.
- Ping monitoring may still report a successful check.
- Website, mail server, SSH access or APIs might still not function.
- Troubleshooting often starts too generally at the whole server.
- Service problems remain undetected for longer.
With Port Monitoring
- You can specifically determine whether a particular service is reachable.
- Problems with HTTP, HTTPS, mail, SSH, RDP or APIs become visible more quickly.
- Firewall and configuration errors can be isolated more effectively.
- Failures of individual services are not mistaken for a complete server outage.
- The technical root-cause analysis becomes significantly more accurate.
Typical situations
When is Port Monitoring particularly helpful?
Port monitoring is particularly useful when the server itself is up but a specific service is not functioning as expected.
Website does not load
The server is online, but port 80 or port 443 is no longer accepting connections.
HTTPS is down
HTTP still works, but the encrypted connection via port 443 is not reachable.
Email issues
SMTP, IMAP or POP3 are not reachable, preventing emails from being sent or retrieved.
SSH or RDP inaccessible
Administrative access to the server is blocked or the corresponding service is not running.
Application cannot connect
An application or API cannot establish a connection to a required service.
Service stopped after update
After an update, a service was not restarted correctly or is no longer listening on the expected port.
Failure scenarios
Which problems does Port Monitoring detect?
Port Monitoring primarily detects whether a service is reachable on a specific port or whether the connection attempt fails.
01
A service has crashed
A common use case is a service that is no longer running on the server. The server itself remains active, but the affected service no longer accepts connections.
- the web server service has been stopped
- the mail server process is no longer running
- a database service has crashed
- the SSH service has been disabled
- an application is no longer listening on the expected port
- a proxy or gateway has failed
- a service was not restarted after an update
02
A firewall is blocking the port
Not every unreachable port means the service itself has failed. Access is often blocked by a firewall or security rule.
- new firewall rule
- changed network access
- hosting provider block
- security plugin
- incorrect security group on cloud servers
- IP-based access restriction
- changed rules on routers or load balancers
03
A port is no longer reachable after a change
Many port issues arise after technical changes. During a migration, services are moved, ports are changed, firewalls adjusted, or new systems integrated.
- server migration
- operating system update
- change to the web server configuration
- change of mail server
- switch from HTTP to HTTPS
- activation of a load balancer
- configuration of a new cloud firewall
- migration of an application
04
A service responds slowly or intermittently
Depending on the monitoring used, not only the basic reachability of a port can be checked, but also the duration of the connection establishment. If this takes unusually long or there are regular timeouts, it can indicate an unstable system.
- server overload
- insufficient resources
- network issues
- faulty service configuration
- too many concurrent connections
- issues with an upstream proxy
- load spikes in an application
05
The wrong port is open or closed
Port Monitoring can also help notice unexpected changes. It's not only about whether a required port is open. An unintentionally reachable port can also be a security concern.
- A required port has been closed.
- A service was accidentally started on a different port.
- A port opening was forgotten after a server change.
- An otherwise non-public port is reachable.
- An old application is still listening on a port.
- An administration service is publicly reachable although it should be restricted.
Process
How does Port Monitoring work?
Port Monitoring runs automatically in the background. The website or server operator specifies which domain or IP address and which port should be monitored.
1
Establish connection to the port
The monitoring system attempts to establish a connection to an IP address or domain on a specified port.
2
Check response from the target server
The target server accepts the connection, rejects it, or does not respond at all.
3
Evaluate reachability
The monitoring determines whether the port is fundamentally reachable.
4
Measure connection time
Optionally, it can measure how long the connection takes to establish.
5
Confirm errors
If the port is inaccessible or a timeout occurs, a repeated check is performed so that short-term network fluctuations do not immediately cause false alarms.
6
Trigger a notification
If the error persists, a notification is sent so that the affected service can be checked promptly.
ℹ️
A reachable port is not a full functional check
Port Monitoring initially only shows that a connection is possible. Whether the application behind it is functioning correctly must be verified by additional checks depending on the service.
In practice
Which ports should be monitored?
Which ports should be monitored depends on the project. A simple website requires different checks than a mail server, a customer portal or a complex application.
80
443
443
Ports for websites
For traditional websites, Port 80 for HTTP and Port 443 for HTTPS are most relevant. Since professional websites are usually delivered via HTTPS today, Port 443 is particularly important.
25
587
587
Ports for mail servers
For mail servers, several ports are relevant because different protocols are used for sending and retrieval. These include SMTP, IMAP and POP3.
- Port 25 for SMTP server-to-server delivery
- Port 465 or 587 for SMTP submission
- Port 143 or 993 for IMAP
- Port 110 or 995 for POP3
22
3389
3389
Ports for server administration
For server administration, Port 22 for SSH and Port 3389 for RDP are often used. These ports are important for managing the system but, for security reasons, should not always be exposed to the entire Internet.
API
Ports for applications and APIs
Many applications use their own ports or run behind a proxy, gateway or load balancer. You can monitor individual application ports, API endpoints, load balancer ports, proxy or gateway ports and communication ports between systems.
DB
Ports for databases
Databases such as MySQL or PostgreSQL also use specific ports, for example Port 3306 for MySQL or Port 5432 for PostgreSQL. However, database ports should generally not be publicly accessible on the Internet.
Interval
How often should port monitoring be performed?
The appropriate check interval depends on how important the monitored service is and how quickly a failure must be responded to.
Less critical services
For less important services, an interval of around 5 to 15 minutes may be sufficient.
Business websites and email services
For production websites, HTTPS availability and mail servers, an interval of 1 to 5 minutes is often appropriate.
Critical applications
For important APIs, customer portals or business-critical services, a check every minute can be appropriate.
Configure alerting correctly
Brief connection problems should not immediately trigger unnecessary alerts. It is often sensible to send a notification only after several failed checks.
Checklist
What should be checked in a port alert?
In a port alert, it should first be determined whether the service itself has failed or whether the connection is being blocked from outside.
Initial checks
- Is the affected service running on the server?
- Is the correct port configured?
- Has the firewall been changed?
- Has the service been restarted after an update?
- Is there an outage with the hosting provider?
- Is the monitored IP address or domain correct?
Check other possible causes
- Is the port only reachable from certain networks?
- Are there new security rules?
- Has a load balancer or proxy been changed?
- Does the service work locally on the server?
- Has the service been moved to a different port?
- Has there been maintenance or a migration?
💡
Service runs locally, but is not reachable externally?
If the service works locally on the server but is not reachable from outside, the cause is often firewall rules, network access controls or upstream systems. If the service does not respond locally either, check whether the process is running and whether error messages are present in the logs.
Context
Limitations of Port Monitoring
Port monitoring checks the reachability of a port. It therefore indicates whether a connection to a specific service is basically possible. However, this does not provide a complete functional test.
Port monitoring primarily detects
- whether a particular port is reachable
- whether a service is accepting connections
- whether a timeout occurs when establishing a connection
- whether firewall or network issues are likely
- whether individual services fail even though the server is online
Not fully tested
- whether port 443 serves a website without 500 errors
- whether port 25 correctly delivers email
- whether port 3306 allows successful database logins
- whether port 80 serves the correct website
- whether port 22 allows a login
- whether an API port provides functionally correct data
ℹ️
Port monitoring is an availability check
A reachable port does not automatically mean that the underlying application is working without errors. For websites, it is therefore sensible to combine it with HTTP monitoring. For HTTPS connections, SSL monitoring should also be used.
Combination
Combine Port monitoring with other checks
Port monitoring is particularly useful when combined with other types of monitoring via website monitoring tools. Each check examines a different technical layer.
Ping Monitoring
Checks whether a server is reachable over the network in general. It does not indicate whether individual services are functioning.
HTTP Monitoring
Checks whether a website is reachable at a specific URL, which status code the web server returns and how quickly it responds.
SSL Monitoring
Checks whether the certificate of an HTTPS connection is valid and is renewed in good time.
DNS Monitoring
Checks whether a domain resolves correctly and points to the expected target.
Keyword Monitoring
Checks whether the expected content is present on a website. This helps when a page responds technically but serves incorrect content.
Improved root cause analysis
If Ping is successful but port 443 is not reachable, the problem is likely with the web server service, the firewall or the HTTPS configuration. If port 443 is reachable but HTTP Monitoring reports an error, the connection generally works, but the website application has a problem.
HOSTtest Plus
Port monitoring with HOSTtest Plus
With HOSTtest Plus individual ports can be monitored regularly. Website and server operators can specify which IP address or domain and which port should be checked.
This lets you check whether important services are generally reachable. If a port fails or a timeout occurs, a notification can be triggered. That way, issues can be detected more quickly before they remain unnoticed for an extended period.
Port Monitoring
Checks whether specific services and network ports are reachable.
Ping Monitoring
Checks basic network reachability.
HTTP Monitoring
Checks availability, status code and response time.
SSL Monitoring
Monitors certificates, validity periods and HTTPS connections.
DNS Monitoring
Checks domain resolution, DNS records and expected values.
Keyword Monitoring
Checks whether the expected content is being delivered.
Step by step
How to use HOSTtest Plus as a port monitoring tool
A port monitor can be set up quickly in HOSTtest Plus. After logging in, create a new monitor, select the appropriate monitoring type and then specify which domain or IP address should be checked on which port.
1
Create an account or log in
First create a free HOSTtest Plus account or log in with your existing credentials.
2
Create a new monitor
Open the menu section Pulse → Monitors. There you can create an individual monitor for a domain, an IP address or a specific service.
Screenshot: Creating a new port monitor in HOSTtest Plus
3
Select Port as the monitor type
Select Port as the monitor type. HOSTtest Plus will then check whether a specific network port on the chosen target system is reachable.
4
Enter the domain or IP address
Enter the domain or IP address to be monitored. This could be, for example, a web server, mail server, gateway, load balancer or application system.
5
Specify the port
Specify which port should be checked. For websites, ports 80 and 443 are commonly relevant; for mail servers, for example, SMTP, IMAP or POP3 ports.
6
Select the check interval
Choose how frequently the port should be checked. The more important the monitored service, the shorter the interval should be.
7
Configure notifications
Decide how you want to be notified in the event of a timeout, a failed connection or repeated outages.
8
Activate port monitoring
Activate port monitoring by clicking "Create monitor". From that point, HOSTtest Plus will automatically check the selected port at the set interval.
Screenshot: Status and overview of your port monitor
✅
Conclusion: Port monitoring checks the reachability of individual services
Port monitoring is an important complement to traditional server and website monitoring. It shows whether a particular service is reachable on a specific network port.
This makes port monitoring more accurate than a simple ping. A server can generally be online while individual services such as web servers, mail servers, SSH, RDP or applications are no longer functioning. Port monitoring can detect exactly such cases more effectively.
At the same time, port monitoring does not replace a complete functional check. A reachable port does not automatically mean that the application behind it is working correctly. For reliable monitoring, port monitoring should therefore be combined with HTTP, SSL, DNS, ping and keyword monitoring.
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