Ncat (netcat) network diagnostics cheat sheet

Confirm the installed executable is Nmap Ncat before relying on its option names. [1][2]

Local help reflects the installed build and its enabled features. [1][2]

Connect to a TCP service already listening on the local machine. [1][2]

Select IPv4 transport for this numeric loopback endpoint. [1][2]

Connect to IPv6 loopback when a matching listener exists. [1][2]

Report connection information as the TCP session starts. [1][2]

Repeated verbose flags provide additional diagnostic messages. [1][2]

Stop trying a connection after the specified wait interval. [1][2]

Zero-I/O mode reports whether a local TCP connection succeeds. [1][2]

A quick local listener check with connection-status details. [1][2]

The numeric address works without DNS resolution. [1][2]

Sends datagrams rather than opening a TCP stream; no handshake confirms a listener. [1][2]

Closes a connected session after the chosen period of inactivity. [1][2]

Handles Telnet option negotiation against a permitted local service. [1][2]

Binds only loopback; a separate local terminal can connect to this port. [1][2]

Accepts multiple clients; terminate it when the diagnostic task is complete. [1][2]

Displays connection notices while listening on localhost. [1][2]

Explicitly limits the listening socket to IPv4 loopback. [1][2]

Limits a listener to the IPv6 localhost address. [1][2]

Receives UDP datagrams addressed to the local test socket. [1][2]

Can continue handling local UDP test traffic until stopped. [1][2]

Prevents keyboard input from being transmitted back to the connected client. [1][2]

The listener writes outgoing data but ignores anything sent by a client. [1][2]

Restricts inbound connections to the numeric local IPv4 address. [1][2]

Caps concurrent loopback clients on a persistent listener. [1][2]

Stop an interactive local listener after testing; it otherwise holds the port. [1][2]

Sends a line to an already running local TCP listener and exits on input EOF. [1][3]

View data from the other endpoint without typing into the session. [1][3]

Interrogate a plain HTTP loopback server using correctly terminated request lines. [1][3]

Converts typed line endings into CRLF for text-based protocols. [1][3]

Records session data locally; logs can include private messages. [1][3]

Preserves hexadecimal data for binary-protocol diagnosis. [1][3]

Appends instead of replacing the selected session log. [1][3]

Avoid waiting for a reply when verifying basic payload receipt. [1][3]

Quit after the configured EOF delay while allowing short response time. [1][3]

Keep send-side socket behavior when stdin reaches EOF; protocol-specific. [1][3]

Pause one second between lines where supported; helps slow interactive diagnostics. [1][3]

Shell status reflects Ncat's result; zero indicates success of the attempted check. [1][3]

Authenticate the remote certificate and negotiate encryption; use a real trusted service hostname. [1][4][3]

This encrypts traffic but does NOT verify the peer; do not treat it as authenticated. [1][4][3]

Custom PEM trust chain must be appropriate for the expected service. [1][4][3]

Send explicit SNI for a TLS virtual host while requiring certificate verification. [1][4][3]

More logging helps diagnose handshake and certificate issues. [1][4][3]

Use only a permitted proxy and destination; transport is plain unless TLS is separately enabled. [1][4][3]

TCP connection traverses an explicitly configured HTTP proxy. [1][4][3]

Encrypt and authenticate destination TLS after proxy negotiation. [1][4][3]

A successful UDP send does not prove any application is receiving. [1][4][3]

Check local binding, protocol and service status rather than assuming a firewall. [1][4][3]

Narrow the diagnostic with local loopback checks and documented ports. [1][4][3]

OpenBSD netcat, traditional netcat and Nmap Ncat use different option flags. [1][4][3]