telnetz

Telnet-like command line client for TCP and TLS sockets.

cargo install --path .
telnetz example.com 443 --tls

At a terminal it reads lines interactively (/quit exits). With piped stdin or -c it sends the input and prints the reply until the server closes, -w expires or --expect matches, so it can be used from scripts and AI agents.

telnetz -q --expect 220 -w 5 mail.example.com 25            # exit 0 if banner seen, 3 if not
telnetz -q -w 1 -c PING localhost 6379
printf 'GET / HTTP/1.0\r\n\r\n' | telnetz -q --raw -w 5 example.com 80

Exit codes: 0 ok, 1 connection/IO error, 2 usage error, 3 --expect not seen. See telnetz --help.

For a self-signed server, obtain its certificate first:

telnetz localhost 443 --tls --dump-cert server.crt
openssl x509 -in server.crt -noout -fingerprint -sha256

Compare the fingerprint with a value obtained through a trusted channel. Then trust the certificate for the connection:

telnetz localhost 443 --tls --cacert server.crt

--cacert accepts PEM certificates, PEM bundles, or a DER certificate. Repeat it to trust multiple files. These certificates are added to the built-in public roots for this invocation. Hostname and expiry checks still apply; the host must match the certificate's subject alternative name. --cacert requires --tls and cannot be combined with --insecure.

Use --no-default-ca to trust only certificates supplied with --cacert:

telnetz localhost 443 --tls --no-default-ca --cacert server.crt

This option requires --tls. Certificate verification remains enabled; without any --cacert files, no server certificate is trusted.

--dump-cert saves the server's leaf certificate as PEM and exits without reading stdin or sending application data. It fetches certificates without checking trust, hostname, or expiry. It requires --tls and cannot be combined with --cacert, --command, or --expect. Existing files are preserved. Use --dump-cert - to write PEM to stdout.

Agent skill

skills/telnetz/SKILL.md teaches coding agents to use telnetz. For Claude Code:

cp -r skills/telnetz ~/.claude/skills/