Shutdown examples/explanations

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Shutdown examples/explanations

Paul Smith
Does anyone have a straightforward example of the canonical way to
handle SSL_shutdown() in OpenSSL 1.1.1?  I mean both when my code is
the initiator of the shutdown and also when I'm the peer, and also for
both blocking and non-blocking BIOs?

I've read and re-read the SSL_shutdown() man page and it seems to me
that there's a lot of conflicting content there.  It says you can call
SSL_shutdown() twice, and it also says "However, it is recommended to
wait for it using SSL_read() instead".  It says "It is not possible to
call SSL_write() after calling SSL_shutdown()", but it also says that
for non-blocking BIOs, we might get an SSL_ERROR_WANT_WRITE and the
calling process "must take appropriate action to satisfy the needs of
SSL_shutdown()"... what "appropriate action" is is not clearly
specified but I assume it means invoke SSL_write().

Under RETURN VALUES <0 it says the shutdown was not successful, but
then says we can get this if an action is needed to continue the
operation for non-blocking BIOs... by "continue the operation" does
that mean call SSL_read() again?  If so what's the difference in
behavior of my code between SSL_shutdown() returning 0 or <0?

I just wish the docs would say what the recommended best practices are
in a clear way, rather than offering lots of not-very-clear
alternatives, or that there was example code somewhere of how the
OpenSSL devs designed this to be handled.


Also as a separate question, if I'm always going to be closing my
socket once the shutdown is complete is there any advantage in worrying
about bi-directional shutdown, versus simply calling SSL_shutdown(),
ignoring the return value, and closing the socket?  I would like my
code to be a good neighbor and play nicely with others but if it
doesn't matter I guess I could side-step all the above confusion and
simply get out.

Let me also assume that the peer may not be reading the socket
constantly.  For example suppose the peer is an interactive shell of
some kind and it's sitting at a prompt waiting for user input and not
invoking SSL_read() etc.  Is it possible to ever complete a bi-
directional handshake shutdown with that peer anyway?  If we need to
wait for it to reply I don't see how.