Re: Is there a way to tell s_client to just "send a command" and quit?

On Thu, Sep 25, 2025 at 10:42:16PM +0000, 'Michael Wojcik' via openssl-users wrote:

> Even the newline from echo is unnecessary, so you can save a few characters:
> 
> $ openssl s_client -connect ... </dev/null
> 
> (The </dev/null can go anywhere after the command name, actually, so "openssl </dev/null s_client ..." works too.)

And, for matter, also before the command name,

    </dev/null openssl s_client ...

but this isn't a POSIX shell beginner forum, so the reason I'm replying
is in fact to add a bit more substance on two potentially relevant
points.

    1.  With some combinations of options s_client will ignore
        end-of-file on standard input, and remain connected to
        the remote end.  An explicit "-no_ign_eof" as a final
        option can be helpful.  From the docs:

           -ign_eof
               Inhibit shutting down the connection when end of file is
               reached in the input.

           -quiet
               Inhibit printing of session and certificate information.
               This implicitly turns on -ign_eof as well.

           -no_ign_eof
               Shut down the connection when end of file is reached in
               the input.  Can be used to override the implicit -ign_eof
               after -quiet.

    2.  With TLS 1.3, resumption PSKs (session tickets) are sent *after*
        the handshake completes, sometimes when it is the server's
        first turn to send application data (to avoid potential deadlock
        if both sides are writing large messages without concurrently
        reading).  So if processing of resumption PSKs is part of the
        diagnostic goals, the client may need to solicit an application
        layer response from the server.  Therefore, e.g. for an SMTP
        connection, one might:

            (printf 'QUIT\r\n'; sleep 2) |
                openssl s_client -starttls smtp \
                -connect foo.example:25 ... \
                -no_ign_eof

-- 
    Viktor.  🇺🇦 Слава Україні!

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