gnutls-cli-debug(1)

💖 gnutls-cli-debug — the GnuTLS debug client, bestie edition 💖

okay so welcome to gnutls-cli-debug(1), a User Commands girlie who exists purely to poke at TLS servers and tell you what they can and can't do. she's the bestie who asks the nosy questions so you don't have to 🫶

---

✨ SYNOPSIS ✨

gnutls-cli-debug [-flags] [-flag [value]] [--option-name[[=| ]value]]
[hostname]

Operands and options may be intermixed. They will be reordered. (yes bestie, she'll shuffle them around for you, no cap 💅)

---

🌸 DESCRIPTION 🌸

okay so this is a TLS debug client. she sets up multiple TLS connections to a server and queries its capabilities, one by one, like a vibe check but for TLS stacks.

she was created to assist in debugging GnuTLS, but honestly she might be useful to extract a TLS server's capabilities too. like, the receipts are right there.

she connects to a TLS server, performs tests, and prints the server's capabilities. and listen — if you call her with the -V parameter, more checks will be performed. more checks!! she's thorough like that.

also, she can be used to check for servers with special needs or bugs. yes, servers with special needs. iconic. 👑

---

💅 OPTIONS 💅

• -d num, --debug=num — Enable debugging. This option takes an integer number as its argument. The value of num is constrained to being:
  • in the range 0 through 9999

Specifies the debug level.

• -V, --verbose — More verbose output. she will simply say more things. slay.
• -p num, --port=num — The port to connect to. This option takes an integer number as its argument. The value of num is constrained to being:
  • in the range 0 through 65536
• --app-proto — This is an alias for the --starttls-proto option. same girl, different name 🎀
• --starttls-proto=str — The application protocol to be used to obtain the server's certificate (https, ftp, smtp, imap, ldap, xmpp, lmtp, pop3, nntp, sieve, postgres).

Specify the application layer protocol for STARTTLS. If the protocol is supported, gnutls-cli will proceed to the TLS negotiation. periodt.

• --attime=timestamp — Perform validation at the timestamp instead of the system time.

timestamp is an instance in time encoded as Unix time or in a human readable timestring such as "29 Feb 2004", "2004-02-29". Full documentation available at <https://www.gnu.org/software/coreutils/manual/html_node/Date-input-formats.html> or locally via info '(coreutils) date invocation'.

• -v arg, --version=arg — Output version of program and exit. The default mode is v, a simple version. The c mode will print copyright information and n will print the full copyright notice. pick your fighter 💜
• -h, --help — Display usage information and exit.
• -!, --more-help — Pass the extended usage information through a pager.

---

🦋 EXAMPLES 🦋

here's her in her natural habitat, doing the whole little dance:

$ gnutls-cli-debug localhost
GnuTLS debug client 3.5.0
Checking localhost:443
                                     for SSL 3.0 (RFC6101) support... yes
                                whether we need to disable TLS 1.2... no
                                whether we need to disable TLS 1.1... no
                                whether we need to disable TLS 1.0... no
                                whether %NO_EXTENSIONS is required... no
                                       whether %COMPAT is required... no
                                     for TLS 1.0 (RFC2246) support... yes
                                     for TLS 1.1 (RFC4346) support... yes
                                     for TLS 1.2 (RFC5246) support... yes
                                          fallback from TLS 1.6 to... TLS1.2
                                for RFC7507 inappropriate fallback... yes
                                             for HTTPS server name... Local
                                       for certificate chain order... sorted
                          for safe renegotiation (RFC5746) support... yes
                             for Safe renegotiation support (SCSV)... no
                            for encrypt-then-MAC (RFC7366) support... no
                           for ext master secret (RFC7627) support... no
                                   for heartbeat (RFC6520) support... no
                               for version rollback bug in RSA PMS... dunno
                          for version rollback bug in Client Hello... no
                    whether the server ignores the RSA PMS version... yes
        whether small records (512 bytes) are tolerated on handshake... yes
            whether cipher suites not in SSL 3.0 spec are accepted... yes
        whether a bogus TLS record version in the client hello is accepted... yes
                 whether the server understands TLS closure alerts... partially
                    whether the server supports session resumption... yes
                              for anonymous authentication support... no
                              for ephemeral Diffie-Hellman support... no
                           for ephemeral EC Diffie-Hellman support... yes
                            ephemeral EC Diffie-Hellman group info... SECP256R1
                          for AES-128-GCM cipher (RFC5288) support... yes
                          for AES-128-CCM cipher (RFC6655) support... no
                        for AES-128-CCM-8 cipher (RFC6655) support... no
                          for AES-128-CBC cipher (RFC3268) support... yes
                     for CAMELLIA-128-GCM cipher (RFC6367) support... no
                     for CAMELLIA-128-CBC cipher (RFC5932) support... no
                             for 3DES-CBC cipher (RFC2246) support... yes
                          for ARCFOUR 128 cipher (RFC2246) support... yes
                                               for MD5 MAC support... yes
                                              for SHA1 MAC support... yes
                                            for SHA256 MAC support... yes
                                      for ZLIB compression support... no
                             for max record size (RFC6066) support... no
                        for OCSP status response (RFC6066) support... no
                      for OpenPGP authentication (RFC6091) support... no

You could also use the client to debug services with starttls capability. like, she's flexible bestie 😌

$ gnutls-cli-debug --starttls-proto smtp --port 25 localhost

---

🚪 EXIT STATUS 🚪

One of the following exit values will be returned:

• 0 (EXIT_SUCCESS) — Successful program execution. we love a win 💖
• 1 (EXIT_FAILURE) — The operation failed or the command syntax was not valid. oop 😭

---

👀 SEE ALSO 👀

gnutls-cli(1), gnutls-serv(1) — go read those too, they're the friend group 🌈

---

🎀 AUTHORS 🎀

(nothing listed here in the original, bestie — the section is just vibes and empty space) 🙈

---

📜 COPYRIGHT 📜

Copyright (C) 2020-2023 Free Software Foundation, and others all rights reserved. This program is released under the terms of the GNU General Public License, version 3 or later.

---

💀 BUGS 💀

Please send bug reports to: bugs@gnutls.org — tell them everything, they want the tea fr fr 🍓

---

and that's the whole page, bestie. this was gnutls-cli-debug from GnuTLS 3.8.13, dated 29 Apr 2026. she ate and left no crumbs ✨👑💅