Stephen Gildea <[EMAIL PROTECTED]> wrote:
>I just got the sendmail 8.11.2 sources and compiled and configured
>them for STARTTLS. It is great that you provide this functionality,
>and great that you make it freely available.
>I had some problems configuring, which I would like to share with you
>in the spirit of making it easier for those doing it in the future.
>Some of my comments are directed at the OpenSSL folks.
>Let me start by telling you my background, to give some context for my
>experience. A am very familiar and comfortable with sendmail, SMTP,
>cf and mc files. I understand Diffie-Hellman and public-key
>cryptography. I use PGP. I have compiled and configured ssh. When I
>started, I didn't know what "X.509" is or what a "request" is. (These
>terms come up a lot in the OpenSSL manual pages.) All in all, I think
>my profile is on the well-informed side of average for people who will
>be setting up sendmail.
>What I wanted is the same functionality I get from ssh:
> - I want to encrypt the SMTP conversation on the wire, so my site's
>email is safe from prying eyes in the network.
> - I want basic protection from man-in-the-middle attacks, obtained by
>remembering previous host behavior and host keys.
The second is difficult to do. The email world is large and complex.
The number of different hosts you talk to is very large. There isn't
a practical way of keeping track of all the host keys.
Moreover, whenever any host changed key, how should sendmail handle
that? With SSH, the user can be prompted for permission to
continue. But email is sent in the background.
Our general view is that message content encryption is best handled
by PGP and similar methods. STARTTLS does encrypt the contents,
but it would be difficult to completely avoid man-in-the-middle
attacks. Think of it as making life difficult for hackers, even
if it does not completely solve the problem.
The more urgent problem is the use of SMTP-AUTH. With STARTTLS we
protect the password. Client programs sending to our server can
guard against man-in-the-middle attacks. This does require that your
encryption certificate be signed by a CA. But you can be your own
CA, as long as you provide a way for your client users to install
your CA certificate in their browsers.
>I want these things because I can get them. I don't feel the need to
>authenticate--we never used to with SMTP and that's still fine. So I
>see no need for keys signed by a trusted CA. All in all, I think my
>requirements here are similar to those of many.
>What I felt to be missing from the documentation I read was how to
>create this basic setup. I had to get and build OpenSSL (version
>0.9.6) for the first time for this task. I did not find in the
>OpenSSL documents a basic introduction to X.509, CA's, requests, etc.
It does provide a perl script and a sh script for generating CA
certificates and for using it to sign server certificates.
>There is a section in the Sendmail Operations Guide on "Certificates
>for STARTTLS," which was very helpful. Without this section, I would
>have flailed around a lot more than I did. I'm still wondering, what
>is the DHParameters variable in sendmail.cf?
>By comparison, ssh (version ???) was simple to set up: I created a
>local private host key and I pointed ssh at a file to collect remote
>host public keys.
SSH is interactive, and is used to connect to a limited number of
hosts, all of which are known and relatively trusted.
Sendmail is non-interactive, and can connect to an unlimited number
of hosts, most of which are unknown and for which there is no basis
for trust.
>I'm not even sure I'm getting as much from sendmail as I am from ssh.
You really cannot compare.
>Is sendmail recording and remembering previous host keys presented?
No.
>This message describes some places where I felt I could have gotten a
>little more guidance in setting up STARTTLS. Where I asked specific
>questions, consider that a request to enhance the documentation,
>rather than a request for a personal reply to me. I fear that setting
>this up is currently so complicated that it will prevent wide-spread
>adoption of STARTTLS support, and that would be a shame.
Thanks. We will take a look at our documentation.
-NWR
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