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292840Re: GSSAPI SMTPD Authentication and MS Active Directory

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  • Matthew Larsen
    Apr 25, 2013
      On 4/25/2013 1:02 PM, Viktor Dukhovni wrote:
      > What evidence do you have that the server is "doing" GSSAPI? It
      > seems likely you're mistaken. Simply listing GSSAPI as a supported
      > SASL AUTH mechanism is not "doing" GSSAPI, the client would actually
      > have to use GSSAPI. It is quite possible your client's IP address
      > was whitelisted on the Exchange servers, or access was unrestricted, ...

      My apologies. I am mistaken about how this is happening. Sometimes it's
      a challenge to get accurate information from a different division that
      takes care of this client system.

      The computers running the SMTP client software are members of a child
      domain in our AD forest, there's a VPN between those computers and a
      different segment of our network housing the child domain AD
      infrastructure, but for some reason (probably bandwidth and latency) the
      SMTP client is connecting over the public Internet connection at the
      client sites rather than the VPN. I think that mostly explains how the
      infrastructure is there to use Kerberos for authentication.

      Here's what I see it doing with wireshark on the server.

      A screen shot of some of what I see:

      The gist of it is

      S: 220 mail.exch01.com ...
      S: 250-mail.exch01.com Hello [ip.addr.of.client] | 250- ... several
      items including AUTH GSSAPI NTLM LOGIN among others ....
      C: AUTH gssapi ...long string...
      S: 334 ...long string...
      C: ...long string...
      S: 235 2.7.0 Authentication successful.
      C: MAIL FROM:<sending@...>
      S: 250 2.1.0 sending@... ... Sender OK
      C: RCPT TO:<somebody@...>
      S: 250 2.1.5 somebody@...
      C: DATA
      S: 354 Start mail input; end with <CRLF>.<CRLF>
      ... blah blah blah ...

      > The clients may be doing NTLM or PLAIN or nothing at all. You need
      > to figure out what's actually used. If TLS is not in use a simple
      > packet capture plus wireshark or similar will show you exactly what
      > the client and server are doing.

      In addition to what I see in Wireshark, the event log shows it's using
      GSSAPI when I turn on the MSTransport authentication logging level to debug.

      Event Type: Information
      Event Source: MSExchangeTransport
      Event Category: Authentication
      Event ID: 1708
      Date: 4/25/2013
      Time: 11:17:49 AM
      User: N/A
      Computer: EXCH01
      SMTP Authentication was performed successfully with client "A510E". The
      authentication method was "GSSAPI" and the username was "MYDOMAIN\AAA".

      >> I'm not sure that my understanding of the security of the GSSAPI
      >> method is accurate, or that the infrastructure is there in this case
      >> to support doing this with Postfix?
      > The Postfix SMTP client if compiled with Cyrus SASL support, and
      > provided the Cyrus SASL gssapi plugin is installed will do GSSAPI.
      > There is no GSSAPI-specific code in Postfix, all the logic is in
      > Cyrus SASL. However, you need to specify a KRB5CCNAME in the
      > client's environment that is readable by the "postfix" user and
      > contains valid tickets at all times. To do this, run a cron-job
      > periodically that uses a keytab file to populate the credential
      > cache with freshly valid tickets.
      > If the above is just a bunch of greek to you, you want to look for
      > alternatives to GSSAPI.

      It's not entirely greek, but I'm trying to learn more greek. However, I
      don't believe that I need the Postifix client to do any authentication
      other than anonymous. It would be relaying messages from authenticated
      clients to Internet recipients via MX records. I'm only trying to
      configure the stmpd portion of Postfix for secure authentication.

      > If you replace the Exchange servers with Postfix, you can support
      > any of the following authorization methods:
      > - Allow any client to send anywhere (internal open relay).
      > - Whitelist the particular sending IPs.
      > - Allow the clients to send via authorized TLS client certs.
      > - Allow the clients to send via any mutually supported SASL
      > mechanism, including PLAIN and/or GSSAPI.
      > For server-side GSSAPI support the server will need a keytab file
      > containing shared keys with the appropriate realm's KDCs.

      The fourth option listed is what I'm trying to accomplish with GSSAPI,
      but have been finding challenging to get working. I'll go back over my
      configuration a time or two and try and find something specific that
      will point to where it's not working.
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