Especially for resumed handshake, it's entirely possible for an epoch=0
ClientHello to be retransmitted or arrive so late that the server is already
at epoch=1. There is no good way to detect whether it's that or a reconnect.
However:
- a late ClientHello seems more likely that client going down and then up
again in the middle of a handshake
- even if that's the case, we'll time out on that handshake soon enough
- we don't want to break handshake flows that used to work
So the safest option is to not treat that as a reconnect.
Don't depend on srv.c in config.h, but add explicit checks. This is more
in line with other options that only make sense server-side, and also it
allows to test full config minus srv.c more easily.
Use a custom function that minimally parses the message an creates a reply
without the overhead of a full SSL context.
Also fix dependencies: needs DTLS_HELLO_VERIFY for the cookie types, and let's
also depend on SRV_C as is doesn't make sense on client.
This is not very useful for TLS as mbedtls_ssl_write() will automatically
fragment and return the length used, and the application should check for that
anyway, but this is useful for DTLS where mbedtls_ssl_write() returns an
error, and the application needs to be able to query the maximum length
instead of just guessing.
This is not required nor recommended by the protocol, and it's a layering
violation, but it's a know flaw in the protocol that you can't detect a PSK
auth error in any other way, so it is probably the right thing to do.
closes#227
We document that either of recv or recv_timeout may be NULL, but for TLS we
always used recv... Thanks Coverity for catching that.
(Not remotely trigerrable: local configuration.)
Also made me notice net_recv_timeout didn't do its job properly.
While at it, fix the following:
- on server with RSA_PSK, we don't want to set flags (client auth happens via
the PSK, no cert is expected).
- use safer tests (eg == OPTIONAL vs != REQUIRED)
- DTLS_HELLO_VERIFY no longer depends on SRV_C
- SSL_COOKIE_C no longer depends on DTLS_HELLO_VERIFY
Not that much work for us, and easier on users (esp. since it allows just
disabling SRV_C alone).
