[Serusers] TLS comments

[Andrei Pelinescu-Onciul]

On Feb 01, 2006 at 14:43, Jan Janak <jan at iptel.org> wrote:
> Cesc wrote:
> > On 2/1/06, Klaus Darilion <klaus.mailinglists at pernau.at> wrote:
> >> Hi!
> >>
> >> I've tried the new TLS module:
> >>
> >> 1. It breaks compatibility with old TLS stack: Even when configured to
> >> use TLSv1, it sends an SSLv2 compatible HELLO:
> >>
> >> server2:~# ssldump
> >> New TCP connection #1: 10.10.0.41(33107) <-> 10.10.0.42(5063)
> >> 1 1  0.0088 (0.0088)  C>S SSLv2 compatible client hello
> >>    Version 3.1
> >>
> >>
> >> I do not know if this is a problem with the new or the old stack.
> >> Further I do not know what other TLS enabled SIP products use. Do they
> >> accept SSL compatible HELLOs?
> >>
> > Klaus, i don't think this is a bug ... i think that the hello is
> > always v2 and then (with the server hello message) the handshake is
> > upgraded to v3 or tlsv1. This way, you can have an sslv2-only client
> > try connecting to  any server, but the server will send back sslv3 or
> > tlsv1 server hello, thus disconnecting the client.
> 
>   Yes, I think this is correct. The protocol version should be set to
>   TLSv1  afterwards, you can test this with @tls.version:

No, he is right, this is a bug.

A normal TLSv1 server will accept only TLS hello messages (version =
3.1) and since sip is supposed to work only with TLS, this should be the
default (tls_method=TLS_USE_TLSv1).
The SSLv23 stuff will accept any type of hello. A client
 with the SSLv23 method will send a hello v2 message, indicating in it
 also tls and ssl 3.0 support (they will include tls specific ciphers
 a.s.o). The client sending v2 hellos is not exactly standard behaviour
 for TLS. It is a hack for backward compatibility, but a TLS only server
 will not accept it.
One of the clients that used a SSLv23 equivalent was Windows Messenger
 (it was doing this in 2003 when I wrote my ser tls code).

The tls method should be configurable to allow older/strange clients
support, but it should default to TLSv1 (btw: this is how you can reject
 v2 clients without SSL_OP_NO_SSLv2).
> 
>   if (@tls.version == "SSLv2") {
>        sl_send_reply("400", "Bad TLS protocol version");
>        break;
>   }

I think we should not handle TLS errors from the script. A TLS client
will expect the handshake phase to fail if it uses an unsupported SSL
version or the wrong certificate. Accepting the ssl connection and then
returning a SIP error or plainly dropping it, it's just wrong IMHO and
not very TLS frienldy/conformant.  That's what the handshake phase for.
Moreover if you go to the trouble to accept the connection just to
reject it immediately you will waste more resources.
If you don't want to accept V2, then just change the method.
For cetificates: you either verify them (you can have verify off, verify
 but don't check host name/ip, verify all) or not. If you need special
 verify on only for some clients, either make it per ip, or use another
  tls port on which you have verify turned on.


Andrei