File:  [NetBSD Developer Wiki] / wikisrc / tutorials / services / sendmail_auth_tls.mdwn
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Sat Nov 12 16:26:45 2011 UTC (2 years, 5 months ago) by imil
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## Setting up a secure SMTP server with AUTH and TLS enabled in Sendmail

While *postfix* is the basesystem's [SMTP](http://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol) server, it is still possible to use the venerable [Sendmail](http://www.sendmail.com/sm/open_source/) as your mail server of choice.
Securing a *sendmail* SMTP gateway in order to use it from anywhere using your system's credentials is an easy task, here is how to achieve it.

### Enabling Sendmail as the system's SMTP server

First thing is to disable *postfix* as the system's SMTP server. This action is controlled by the *postfix* parameter in */etc/rc.conf*:

	postfix=NO

We will then Install *sendmail* from *pkgsrc* with [SASL](http://en.wikipedia.org/wiki/Simple_Authentication_and_Security_Layer) for the authentication mechanism and [TLS](http://en.wikipedia.org/wiki/Transport_Layer_Security) as the secure transport layer:

	$ grep sendmail /etc/mk.conf
	PKG_OPTIONS.sendmail=   tls sasl
	ACCEPTABLE_LICENSES+=   sendmail-license

### AUTH with SASL

Enabling *SASL* will build *security/cyrus-sasl*, but this package build failed with the following on my NetBSD 5.0.2 box:

	db_ndbm.c:95: warning: passing argument 3 of 'utils->getcallback' from incompatible pointer type

So we will specify that *cyrus-sasl* should use *berkeley* as its database type:

	$ grep SASL /home/bulk/etc/mk.conf
	SASL_DBTYPE=            berkeley

We can now install *sendmail* with *TLS* and *SASL* support the classic way:

	$ cd /usr/pkgsrc/mail/sendmail && sudo make install clean

*cyrus-sasl* package does now include any authentication plugin, it's up to us to pick one that will suit our needs. As we want to authenticate over system's login/password, we will use *cy2-login*:

	$ cd /usr/pkgsrc/security/cy2-login && sudo make install

In order to use this method, we will have to install the *saslauthd* package. *Saslauthd* is in charge of plaintext authentications on behalf of the SASL library.

	$ cd /usr/pkgsrc/security/cyrus-saslauthd && sudo make install clean

Of course, we want this daemon to start at every boot of this mail server:

	# cp /usr/pkg/share/examples/rc.d/saslauthd /etc/rc.d
	# echo "saslauthd=YES" >> /etc/rc.conf
	# /etc/rc.d/saslauthd start

Now we have to inform the *SASL* library that it should use *saslauthd* whenever *sendmail* asks for an authentication:

	# echo "pwcheck_method:saslauthd" > /usr/pkg/lib/sasl2/Sendmail.conf

### Setting up the secure transport layer

As everything is in place for authentication, we will now prepare the *TLS* prerequisites.
Instead of generating a self-signed certificate, I use to rely on [CACert](http://www.cacert.org/), "a community driven, Certificate Authority that issues certificates to the public at large for free." (from CACert.org).

In order to generate the certificate signing request (CSR), you can use the [CSRGenerator](http://wiki.cacert.org/CSRGenerator) script from CACert, which is really handy.

Once you have generated your server's private key with *CSRGenerator* and received your server certificate from CACert, simply copy them to */etc/mail/certs*, along with [CACert root certificate](http://www.cacert.org/certs/root.crt). Make sure your private key has strict permissions, *sendmail* will refuse to start if it is readable by everyone.

### Configuring sendmail

It is now time to write our *sendmail* configuration. Create a *mc* file corresponding to your needs in */usr/pkg/share/sendmail/cf*, for example:

	# cat > /usr/pkg/share/sendmail/cf/korriban.mc << EOF
	divert(0)dnl
	VERSIONID(`Mustafar')
	OSTYPE(bsd4.4)dnl
	DOMAIN(generic)dnl

	FEATURE(access_db, `hash -T<TMPF> /etc/mail/access')
	FEATURE(blacklist_recipients)
	FEATURE(mailertable, `hash -o /etc/mail/mailertable')
	FEATURE(virtusertable, `hash -o /etc/mail/virtusertable')
	FEATURE(genericstable, `hash -o /etc/mail/genericstable')
	FEATURE(local_procmail)

	dnl ### I use procmail as my MDA
	define(`PROCMAIL_MAILER_PATH',`/usr/pkg/bin/procmail')
	dnl ### and dspam as my antispam
	define(`LOCAL_MAILER_PATH', `/usr/pkg/bin/dspam')
	define(`LOCAL_MAILER_ARGS', `dspam -t -Y -a $h "--deliver=innocent" --user $u -d %u')

	define(`confMAX_MESSAGE_SIZE', 5000000)

	dnl ### here begins the secure SMTP gateway parameters
	dnl ###
	dnl ### enable SMTP AUTH with LOGIN mechanism
	define(`confAUTH_MECHANISMS', `LOGIN')dnl
	TRUST_AUTH_MECH(`LOGIN')dnl
	dnl ### enable STARTTLS
	define(`confCACERT_PATH',`/etc/mail/certs/')dnl
	define(`confCACERT', `/etc/mail/certs/cacert.crt')
	define(`confSERVER_CERT',`/etc/mail/certs/korriban_server.pem')dnl
	define(`confSERVER_KEY',`/etc/mail/certs/korriban_privatekey.pem')dnl
	dnl ### end of secure SMTP gateway parameters

	MAILER(local)dnl
	MAILER(smtp)dnl
	MAILER(procmail)
	EOF

Once your configuration is ready, build and install it using the following:

	# make install-cf CF=korriban
	rm -f korriban.cf
	m4 ../m4/cf.m4 korriban.mc > korriban.cf || ( rm -f korriban.cf && exit 1 )
	echo "### korriban.mc ###" >>korriban.cf
	sed -e 's/^/# /' korriban.mc >>korriban.cf
	chmod 444 korriban.cf
	/usr/bin/install -c -o root -g wheel -m 0444 korriban.cf /etc/mail/sendmail.cf
	/usr/bin/install -c -o root -g wheel -m 0444 korriban.cf /etc/mail/submit.cf

Now that *sendmail* is configured, fire it up by invoking:

	# /etc/rc.d/sendmail start

And test that the features we've added are working:

	# sendmail -d0.1 -bv root | grep SASL
		SASLv2 SCANF SOCKETMAP STARTTLS TCPWRAPPERS USERDB XDEBUG
	$ telnet localhost 25
	Trying 127.0.0.1...
	Connected to localhost.
	Escape character is '^]'.
	220 korriban.imil.net ESMTP Sendmail 8.14.5/8.14.5; Sat, 12 Nov 2011 16:43:40 +0100 (CET)
	ehlo localhost
	250-korriban.imil.net Hello localhost [127.0.0.1], pleased to meet you
	250-ENHANCEDSTATUSCODES
	250-PIPELINING
	250-EXPN
	250-VERB
	250-8BITMIME
	250-SIZE 5000000
	250-DSN
	250-ETRN
	250-AUTH LOGIN
	250-STARTTLS
	250-DELIVERBY
	250 HELP

There you go! now configure your [MUA](http://en.wikipedia.org/wiki/Mail_user_agent) so it always tries *TLS* for sending mail, using the *LOGIN* authentication method.

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