On Wed, Oct 16, 2013 at 01:59:51PM +0000, Viktor Dukhovni wrote:

> > > exchangerelay unix - - n - - smtp
> > >   -o smtp_sasl_mechanism_filter=!gssapi,login
> To support Exchange MSAs on Windows 2003 generically (less critical
> state in per-relay policy entries):
>     exchangerelay unix - - n - - smtp
>       -o smtp_sasl_mechanism_filter=login
>       -o smtp_tls_security_level=secure
>       -o smtp_tls_mandatory_ciphers=medium
>       -o smtp_tls_mandatory_exclude_ciphers=3DES

So after further research it turns out that:

    - The Windows 2003 TLS implementation only looks at the first
      64 cipher-suites in the client SSL/TLS HELLO.

    - If RC4-MD5 is found among these, it uses RC4-MD5.
      Otherwise, if RC4-SHA is found among these, it uses RC4-SHA.
      Otherwise, if DES-CBC3-SHA is found among these, it uses DES-CBC3-SHA.

    - However, the DES-CBD3-SHA cipher-suite is broken (incorrect CBC padding)

So to use TLS with such a site "RC4-SHA" or "RC4-MD5" must be in
the first 64 ciphers-suites.  Since no other ciphers work, and MD5
is deprecated the cleanest solution is a radically trimmed down
set of ciphers:

        exchangerelay unix - - n - - smtp
          -o smtp_sasl_mechanism_filter=login
          -o smtp_tls_security_level=may
          -o smtp_tls_ciphers=medium
          -o smtp_tls_mandatory_ciphers=medium
          -o tls_medium_cipherlist=$tls_compat_cipherlist

        indexed = ${default_database_type}:${config_directory}/
        transport_maps = ${indexed}transport
        smtp_tls_policy_maps = ${indexed}tls_policy
        tls_compat_cipherlist = aNULL+AES128:aRSA+AES128:RC4-SHA:@STRENGTH

where we throw in some AES128 algorithms just in case the server
is some-day patched to support a more reasonable set of cipher-suites.

The security level for the transport can be set to a preferred value
globally as above, and if desired per-site settings can be used to
specify non-default security levels and/or match criteria.

        example.com     exchangerelay:example.com

        example.com     secure match=nexthop


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