On 20/10/2022 20:33, Павел Балашов wrote:
So now the questions:
(1) If we receive some dtls data at the line above with '????' what
should we do in terms of OpenSSL API calls ? I assume this dtls data
could be a client's retransmission due to server's last flight was lost
or this could be client receiving server's last flight duplicated
(theoretically could happen as long as lower layer protocol is UDP) or
this could be DTLS-encrypted real-app data or this could be
DTLS-renegotiation, this also could be a DTLS shutdown alert and
anything else DTSL-related. What is the supposed way of inferring and
reacting to those different events with API ?
Call SSL_read(). If it is app data it will be returned. If it is
something else it will be handled.
(2) Is the whole usage of OpenSSL even right for this scenario - maybe
the structure and sequence of API calls should be rearranged somehow ?
Seems approximately right except that BIO_s_mem() is not a good choice
for DTLS. It does not respect packet semantics, so if you receive 2
packets and they both get pushed into the BIO_s_mem(), the reader will
just get both packets together.
Unfortunately there is no equivalent of BIO_s_mem() that respects
packets in 3.0/1.1.1. There is in the master branch (what will become
3.2), where we have BIO_s_dgram_mem().
Also note that the only BIO that provides the various ctrls for querying
the underlying MTU is BIO_s_dgram(). If you use something else that
doesn't have those ctrls it will fallback to some worse case MTU.
It might be possible to write a custom BIO equivalent of BIO_s_mem()
that does the right thing with respect to packets and has the right MTU
ctrls.
On the server side you might want to consider whether DTLSv1_listen()
should be used in your scenario, and whether you should set the cookie
callback.
You should check the return value from SSL_do_handshake() and use
SSL_get_error() to interpret any failure return codes.
(3) There is an option to pass custom info_callback
with SSL_CTX_set_info_callback(). Would there be a proper usage of
this kind of callback in this scenario ?
Not sure what you're asking here. There is a man page for this function
here:
https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_info_callback.html
Matt
Any other input, links to any kind of relevant supplemental material is
really appreciated.
Thanks a lot for reading, very special thanks to authors and maintainers
for all the hard work on this project.
---
Regards,
Pavel.
