Port checker
Is that port really open from the outside? Test any TCP port on any public host — with connection timing and service banner — from one of our monitoring nodes.
Test from the outside, not from localhost
Every sysadmin has been there: the service is up, curl localhost works, but the application can't reach it from the internet. Testing from inside your network can never prove that firewalls, security groups and NAT are letting traffic through.
This tool opens a real TCP connection from an external node, measures the handshake time and — when the service announces itself — captures the banner. A refused connection means nothing is listening; a timeout usually means a firewall is silently dropping packets.
With SiteFlux you can monitor critical ports continuously: SSH gates, database listeners behind a VPN, mail ports — and get an alert the moment one stops accepting connections.
Frequently asked questions
Why does my port appear closed even though the service is running?
The most common causes are a firewall rule (cloud security groups, ufw, iptables) dropping the traffic, the service listening only on 127.0.0.1 instead of 0.0.0.0, or NAT not forwarding the port. A check from outside your network, like this one, tells you which side the problem is on.
Which ports should be open on a public server?
Only what you actually serve: typically 80 and 443 for web, 22 for SSH (ideally restricted by IP), and nothing else. Databases (3306, 5432, 6379) and admin panels should never be reachable from the whole internet.
What is a banner grab?
After opening a TCP connection, some services announce themselves — SSH shows its version, SMTP shows a greeting. The tool reads the first bytes and shows them, which helps confirm that the expected service (not something else) is actually listening.
Can I check UDP ports with this tool?
No — this test opens a TCP connection, which is the reliable way to prove reachability. UDP is fire-and-forget and needs application-specific probes to be verified.
Tested your port once? SiteFlux monitors TCP ports 24/7 and alerts you before your users do.
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