TCP port monitoring
DoubleCheck connects to any TCP port you choose, from the outside, and alerts you when it stops accepting connections.
The problem
Not everything that matters is a website. Mail servers, SFTP servers, remote desktop gateways and VPN endpoints listen on their own ports, and when one of them stops answering, users notice first.
Why it matters
A closed port is often the first visible sign of a crashed service, a changed firewall rule or a provider problem.
The connect time is a simple, honest measure of the network path to the server.
How to check it manually
On Windows, test a port with PowerShell:
Test-NetConnection mail.example.com -Port 25On Linux or macOS, use netcat:
nc -vz mail.example.com 25How DoubleCheck checks it
DoubleCheck resolves the host name and opens a TCP connection to the port at the interval you choose. If the connection succeeds within two seconds the check is OK, and the connect time is recorded.
It checks that the port answers, not what the service behind it says. For websites and APIs, use the HTTP check, which looks at the response too.
- Measures
- A TCP connection to the host and port you set, with the DNS lookup and connect time measured separately.
- Runs
- Every minute by default. You set the interval per check.
- Status
-
- OK when the port accepts the connection within 2 seconds.
- Error when the connection is refused, times out, or the host name does not resolve.
- A problem is only reported after a set number of failed runs in a row (2 by default).
Frequently asked questions
Which ports can I check?
Any TCP port on a host that is reachable from the internet.
Does DoubleCheck test the service behind the port?
No, only that the port accepts a connection. For web services the HTTP check also looks at the status code and content.
My firewall only allows known addresses. What do I allow?
The addresses returned by monitor.doublecheck.live, on the port of the check. See Firewall and network requirements.
Can I check UDP ports?
Not yet. This check covers TCP.
Start monitoring with DoubleCheck
Alerts by email, SMS, Slack, Microsoft Teams or Discord. You pay per check run: cost management in the app shows what every check costs, and a longer interval lowers it.