Netcat (nc)
Check TCP port reachability; optionally send a payload and read the reply.
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NetTests can run this check on a schedule, preserve historical results, compare changes over time, and alert you the moment something breaks.
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What does the TCP/UDP Connection Tester do?
It attempts a TCP connection to a host and port, optionally sends a text payload, and reads the server's reply. This is the network equivalent of nc host port (netcat). Use it to verify a port is open, test raw protocol banners (SMTP, FTP, Redis, Memcached), or check if a service accepts custom payloads.
How is this different from a port scanner?
A port scanner (like Port Scanner) checks many ports quickly to map what's open. This tool tests a single port in depth — it can send a payload and capture the response, letting you interact with the service at the raw protocol level. It's better for banner grabbing, protocol debugging, and verifying a specific service.
What can I learn from a service banner?
Many services announce their software and version in the initial connection banner. An SMTP server might return 220 mail.example.com Postfix ESMTP, an SSH server returns its version string, Redis returns a pong to a PING command. Banners help identify software versions for security assessments, but production servers should suppress or fake banner information.
Can I test UDP ports?
UDP testing is limited — unlike TCP, UDP has no handshake to confirm a port is open. A sent packet either receives a response (port is open and service replied) or no response (filtered, or the service doesn't respond to your payload). DNS (port 53) and NTP (port 123) are good candidates to test with appropriate payloads.