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Re: STARTTLS & EHLO: Errata text?

2009-01-29 16:35:14


Hi Tony,

Some of the text in this message is from from RFC 3207.

At 09:04 29-01-2009, Tony Hansen wrote:

>If we were to write an Errata against RFC 3207, I'd suggest text such as
>the following (in Errata format):
>
>Section:
>    4.2 Result of the STARTTLS Command
>
>Old text:
>    The server MUST discard any knowledge obtained from the client, such
>    as the argument to the EHLO command, which was not obtained from the
>    TLS negotiation itself.
>
>New text:
>    The server MUST discard any knowledge obtained from the client that
>    was not obtained from the TLS negotiation itself. The server state
>    is otherwise as if the connection had just been opened.
>
>Reason:
>    The example is misleading and has lead some people to think that
>    knowledge of an EHLO having been sent previously should be
>    remembered.

Quoting the entire paragraph from the RFC:

   "Upon completion of the TLS handshake, the SMTP protocol is reset to
    the initial state (the state in SMTP after a server issues a 220
    service ready greeting).  The server MUST discard any knowledge
    obtained from the client, such as the argument to the EHLO command,
    which was not obtained from the TLS negotiation itself.  The client
    MUST discard any knowledge obtained from the server, such as the list
    of SMTP service extensions, which was not obtained from the TLS
    negotiation itself.  The client SHOULD send an EHLO command as the
    first command after a successful TLS negotiation."

Updated text:

    Upon completion of the TLS handshake, the SMTP protocol is reset to
    the initial state (the state in SMTP after a server issues a 220
    service ready greeting).  The server MUST discard any knowledge obtained
    from the client, such as command verbs and their arguments, that was
    not obtained from the TLS negotiation itself.  The client MUST discard any
    knowledge obtained from the server, such as the list of SMTP service 
extensions,
    which was not obtained from the TLS  negotiation itself.  As the server 
state is
    as when a SMTP session is initiated, the client SHOULD send an EHLO command 
as the
    first command after a successful TLS negotiation.

While I have no objection to making this change, I note in passing that quite a
few servers, ours included, violate the "the server MUST discard any knowledge
obtained from the client" part of this and will continue to do so no matter
what is written in any standard.

The reason for this is simple: Limits used in controlling spam and DOS attacks.
Servers impose limits on all sorts of things, including but not limited to the
number of transactions in a session, the total number of recipients, the total
time a session has taken, and so on. If a server follows this MUST it turns
STARTTLS into a one time "reset all my rate limits" pass. That's simply not
acceptable in today's email climate.

Please note that I am not arguing that such limits are effective in controlling
spam or deflecting DOS attacks. (Nor am I arguing that they aren't.) My point
is rather that mail system administrators insist on having these sort of limits
available as part of their security toolbox. If they aren't available or are
there in a form that can easily be bypassed they will run a different mail
server product instead.

The simplest fix to bring this text in line with reality is to change the MUST
into a SHOULD. Beyond that lies a slippery slope where we attempt to categorize
what sorts of information a server can or cannot retain. I really don't think
we want to go there.

>New text:
>    The client MUST send either an EHLO command or a HELO command as the
>    first command after a successful TLS negotiation.

As the two ends are support EHLO, there is no need to have HELO
there.  I suggest leaving the MUST to the next revision.

Works for me.

>Reason:
>    Since the state is reset to that of a connection having just been
>    opened, the requirement from RFC 5321 applies:
>
>         In any event, a client MUST issue HELO or EHLO before starting a
>         mail transaction.

I didn't use MUST for the EHLO because there isn't any restriction on
whether the client can only perform mail transactions in RFC 3207 or
RFC 5321.  In the updated text, it is implied that the client must
not forget that the server supports the STARTTLS extension.  Do we
expressly need to say that? :-)

It seems pretty obvious so I see no need to say it.

>Section:
>    4. The STARTTLS Command
>
>Old text:
>    The format for the STARTTLS command is:
>
>    STARTTLS
>
>    with no parameters.
>
>New text:
>    The format for the STARTTLS command is:
>
>    STARTTLS
>
>    with no parameters.
>
>    Because the server state machine is reset to an initial connection
>    state after negotiating TLS, and any modifications to the server
>    state will be lost, the client SHOULD NOT issue any MAIL
>    FROM or RCPT TO commands prior to using the STARTTLS command.

Agreed.

While this clarification exercise is all well and good, if we're actually going
to issue a revision to RFC 3207 we should consider fixing its most serious flaw
(IMO) - the lack of a domain parameter on the STARTTLS command, in order to
allow a single SMTP server to provide "virtual hosting" support for multiple
domains.

This would have to be done by advertising a separate extension, say
STARTTLSDOMAIN, which if present says the STARTTLS command accept a single,
optional parameter specifying the domain associated with the certificate the
client would like to see the server assert. Text for this can be adapted from
section 6 of RFC 3887.

As things stand now these sorts of virtual hosting setups require the use of
multiple separate ports or IP addresses (usually the latter given how many
clients don't support alternate ports). That's fine and dandy when you  have
two or three domains, but unacceptable when you're hosting thousands of them or
more.

>Now for the $64k questions:
>
>1) Is there consensus behind this viewpoint?

See updated text.

>3) If so, who wants to file the Errata?

I guess I have no problem with an errata clarifing this one point, but a
revision is going to be needed to get at the real issues in RFC 3207.

You have already volunteered. :-)

I'd be happy to help with revision.

                                Ned