Many protocols today allow you to upgrade to TLS from within a cleartext version of the protocol. This often falls under the rubric of "STARTTLS", though different protocols have different ways of doing it.

I often forget the exact steps, and when i'm debugging a TLS connection (e.g. with tools like gnutls-cli) i need to poke a remote peer into being ready for a TLS handshake. So i'm noting the different mechanisms here. lines starting with C: are from the client, lines starting with S: are from the server.

many of these are (roughly) built into openssl s_client, using the -starttls option. Sometimes this doesn't work because the handshake needs tuning for a given server; other times you want to do this with a different TLS library. To use the techniques below with gnutls-cli from the gnutls-bin package, just provide the --starttls argument (and the appropriate --port XXX argument), and then hit Ctrl+D when you think it's ok to start the TLS negotiation.


The polite SMTP handshake (on port 25 or port 587) that negotiates a TLS upgrade looks like:

C: EHLO myhostname.exampleS: [...]S: 250-STARTTLSS: [...]S: 250 [somefeature]C: STARTTLSS: 220 2.0.0 Ready to start TLS<Client can begin TLS handshake>


The polite IMAP handshake (on port 143) that negotiates a TLS upgrade looks like:

S: OK [CAPABILITY IMAP4rev1 [...] STARTTLS [...]] [...]C: A STARTTLSS: A OK Begin TLS negotiation now<Client can begin TLS handshake>


The polite POP handshake (on port 110) that negotiates a TLS upgrade looks like:

S: +OK POP3 readyC: STLSS: +OK Begin TLS <Client can begin TLS handshake>


The polite XMPP handshake (on port 5222 for client-to-server, or port 5269 for server-to-server) that negiotiates a TLS upgrade looks something like (note that the domain requested needs to be the right one):

C: <?xml version="1.0"?><stream:stream to=""C:  xmlns="jabber:client" xmlns:stream="" version="1.0">S: <?xml version='1.0'?>S: <stream:streamS:  xmlns:db='jabber:server:dialback'S:  xmlns:stream=''S:  version='1.0'S:  from=''S:  id='d34edc7c-22bd-44b3-9dba-8162da5b5e72'S:  xml:lang='en'S:  xmlns='jabber:server'>S: <stream:features>S: <dialback xmlns='urn:xmpp:features:dialback'/>S: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>S: </stream:features>C: <starttls xmlns="urn:ietf:params:xml:ns:xmpp-tls" id="1"/>S: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/><Client can begin TLS handshake>


RogerBW (in the comments below) points out that NNTP has TLS support:

C: CAPABILITIESS: [...]S: STARTTLSS: [...]S: .C: STARTTLSS: 382 Continue with TLS negotiation<Client can begin TLS handshake>


I got mail from James Cloos suggesting how to negotiate an upgrade to TLS over the PostgreSQL RDBMS. He points to the protocol docs, and in particular, to multiple protocol flow documents, and SSLRequest and StartupMessage chunks of the protocol spec (and clarification that data is sent in network byte order). It won't work in a text-mode communication, but it's worth noting here anyway:

The client starts by sending these eight octets:

0x00 0x00 0x00 0x08 0x04 0xD2 0x16 0x2F

and the server replies with 'S' for secure or 'N' for not. If the reply is S, TLS negotiation follows.

The message represents int32(8) specifying that there are 8 octets and int16(1234),int16(5678). All sent in network order.

(The non-TLS case starts with a similar message with int16(3),int16(0) for protocol version 3.0. Starttls is essentially pg protocol version 1234.5678.)

what else?

I don't know (but would like to) how to do:

If you know other mechanisms, or see bugs with the simple handshakes i've posted above, please let me know either by e-mail or on the comments here.

Other interesting notes: RFC 2817, a not-widely-supported mechanism for upgrading to TLS in the middle of a normal HTTP session.

Tags: gnutls, imap, pop, postgresql, smtp, starttls, tls, xmpp