STARTTLS(8) BSD System Manager's Manual STARTTLS(8)NAMEstarttls - ESMTP over TLS/SSL
DESCRIPTION
STARTTLS is an ESMTP option, defined in RFC 2487, which is used to con-
duct ESMTP transactions over TLS circuits. This is used to increase the
security of mail server transactions. As of version 8.11, sendmail(8) has
supported the use of TLS to protect ESMTP communications.
STARTTLS allows for the combination of several security solutions for MTA
(mail transport agent) level services through the TLS suite. These secu-
rity features include:
Confidentiality
Encryption is used to protect data from passive monitoring. An
attacker would have to recover the encryption key used to decode
the transmitted data.
Integrity
Hash algorithms are used to ensure the integrity of the transmit-
ted data, and alternatively the timestamp, protecting against a
replay attack. This protects data from modification in transit.
Authentication
The use of public key encryption allows for the strong authenti-
cation of either, or both, communicating parties. This can be
used to allow for select features, such as relaying, to be con-
trolled more securely.
A new ESMTP option, STARTTLS, has been added. This is presented by the
server when an ESMTP session is initiated. The client then begins the TLS
portion of the ESMTP session by issuing the command "STARTTLS". The
remaining portion of the ESMTP session occurs over a TLS channel.
Creating a private key and certificate for sendmail
This example assumes you are creating your own self-signed certificates
for use with sendmail and STARTTLS. If you have an existing private key
and you simply wish to generate a new certificate (for example, if your
old certificate has expired), see the section entitled Creating a
certificate with an existing private key.
For the purposes of this example the certificates will be stored in
/etc/mail/certs, though it is possible to use a different directory if
needed. If this directory does not already exist, you must create it:
# mkdir /etc/mail/certs
Next, you must generate a DSA parameter set with a command like the fol-
lowing:
# openssl dsaparam 1024 -out dsa1024.pem
This would generate DSA parameters for 1024-bit DSA keys, and save them
to the file dsa1024.pem.
Once you have the DSA parameters generated, you can generate a certifi-
cate and unencrypted private key using the command:
# openssl req -x509 -nodes -days 365 -newkey dsa:dsa1024.pem \
-out /etc/mail/certs/mycert.pem -keyout /etc/mail/certs/mykey.pem
You may adjust the lifetime of the certificate via the -days parameter
(one year in this example).
Make sure to leave the private key files unencrypted, using the -nodes
option. Otherwise, sendmail(8) will be unable to initiate TLS server
functions.
You can verify that the newly-generated certificate has correct informa-
tion with the following command:
# openssl x509 -in /etc/mail/certs/mycert.pem -text
If you don't intend to use TLS for authentication (and if you are using
self-signed certificates you probably don't) you can simply link your new
certificate to CAcert.pem.
# ln -s /etc/mail/certs/mycert.pem /etc/mail/certs/CAcert.pem
If, on the other hand, you intend to use TLS for authentication you
should install your certificate authority bundle as
/etc/mail/certs/CAcert.pem.
At this point, you no longer need the dsa1024.pem file and it can be re-
moved.
# rm dsa1024.pem
Because the private key files are unencrypted, sendmail(8) is picky about
using tight permissions on those files. The certificate directory and the
files therein should be readable and writable only by the owner (root). A
simple way to ensure this is to run the following:
# chmod -R go-rwx /etc/mail/certs
Creating a certificate with an existing private key
This example assumes you already have an existing private key,
/etc/mail/certs/mykey.pem. You can generate a new certificate based on
this key using the command:
# openssl req -x509 -new -days 365 -key /etc/mail/certs/mykey.pem \
-out /etc/mail/certs/mycert.pem
# chmod 600 /etc/mail/certs/mycert.pem
You may adjust the lifetime of the certificate via the -days parameter
(one year in this example).
Configuring sendmail to utilize TLS
The global sendmail configuration files, /etc/mail/sendmail.cf and
/etc/mail/localhost.cf must be modified to support TLS functionality. An
example .mc file which has entries for TLS options, such as certificates,
is available as /usr/share/sendmail/cf/knecht.mc.
The pertinent options are:
+ CERT_DIR
+ confCACERT_PATH
+ confCACERT
+ confSERVER_CERT
+ confSERVER_KEY
+ confCLIENT_CERT
+ confCLIENT_KEY
By default, the directory /etc/mail/certs, defined by CERT_DIR, is used
to store certificates, and the server will use the same certificates both
as a client (outgoing mail) and as a server (for incoming mail). This can
be changed by having different entries for the respective roles.
The next step is to edit the .mc files your sendmail.cf and localhost.cf
files are generated from. First, change to the directory where your .mc
files are stored. You will need to make TLS-enabled versions of the fol-
lowing files: openbsd-proto.mc and openbsd-localhost.mc.
# cd /usr/share/sendmail/cf
# cp openbsd-proto.mc openbsd-proto-tls.mc
# cp openbsd-localhost.mc openbsd-localhost-tls.mc
You must then edit openbsd-proto-tls.mc, and openbsd-localhost-tls.mc to
add the following lines after the "VERSIONID" definition (the actual
placement within the file is not critical as long as it is after the
"divert(0)dnl" line).
define(`CERT_DIR', `MAIL_SETTINGS_DIR`'certs')
define(`confCACERT_PATH', `CERT_DIR')
define(`confCACERT', `CERT_DIR/CAcert.pem')
define(`confSERVER_CERT', `CERT_DIR/mycert.pem')
define(`confSERVER_KEY', `CERT_DIR/mykey.pem')
define(`confCLIENT_CERT', `CERT_DIR/mycert.pem')
define(`confCLIENT_KEY', `CERT_DIR/mykey.pem')
Now that you have the TLS-enabled versions of the .mc files you must gen-
erate .cf files from them and install the .cf files in /etc/mail.
# make openbsd-proto-tls.cf openbsd-localhost-tls.cf
# cp openbsd-proto-tls.cf /etc/mail/sendmail.cf
# cp openbsd-localhost-tls.cf /etc/mail/localhost.cf
Finally, restart sendmail with the new configuration by sending it a
SIGHUP.
# kill -HUP `head -1 /var/run/sendmail.pid`
Note that those are backticks and not single quotes in the example above.
After having installed the certificates and configuration files and hav-
ing restarted the sendmail daemon, a new option should be presented for
ESMTP transactions, STARTTLS. You can test this by connecting to the lo-
cal host and issuing the "EHLO" command.
# telnet localhost 25
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 localhost ESMTP Sendmail 8.12.1/8.12.1 ready
EHLO localhost
After typing EHLO localhost you should receive something like the follow-
ing back.
250-localhost Hello localhost [IPv6:::1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-STARTTLS
250-DELIVERBY
250 HELP
You should see "STARTTLS" listed along with the other options. If so,
congratulations, sendmail will now use TLS to encrypt your mail traffic
when the remote server supports it. If not, check /var/log/maillog to see
whether sendmail has reported any security problems or other errors.
Uses for TLS-Equipped sendmail
The most obvious use of a cryptographically enabled sendmail installation
is for confidentiality of the electronic mail transaction and the in-
tegrity checking provided by the cipher suite. All traffic between the
two mail servers is encrypted, including the sender and recipient ad-
dresses. TLS also allows for authentication of either or both systems in
the transaction.
One use of public key cryptography is for strong authentication. We can
use this authentication to selectively relay clients, including other
mail servers and mobile clients like laptops. However, there have been
some problems getting some mail clients to work using certificate-based
authentication. Note that your clients will have to generate certificates
and have them signed (for trust validation) by a CA (certificate authori-
ty) you also trust, if you configure your server to do client certificate
checking.
The use of the access map (usually /etc/mail/access), which is normally
used to determine connections and relaying, can also be extended to give
server level control for the use of TLS. Two new entries are available
for TLS options:
VERIFY contains the status of the level of verification (held in
the macro {verify})
ENCR the strength of the encryption (in the macro {cipher_bits})
VERIFY can also accept the argument for {cipher_bits}. Here are a few ex-
ample entries that illustrate these features, and the role based granu-
larity as well:
Force strong (112-bit) encryption for communications for this server:
server1.example.net ENCR:112
For a TLS client, force string verification depths to at least 80 bits:
TLS_Clt:desktop.example.net VERIFY:80
Much more complicated access maps are possible, and error conditions
(such as permanent or temporary, PERM+ or TEMP+) can be set on the basis
of various criteria. This allows you fine-grained control over the types
of connections you can allow.
Note that it is unwise to force all SMTP clients to use TLS, as it is not
yet widespread. The RFC document notes that publicly referenced SMTP
servers, such as the MX servers for a domain, must not refuse non-TLS
connections. However, restricted access SMTP servers, such as those for a
corporate intranet, can use TLS as an access control mechanism.
LIMITATIONS
One often forgotten limitation of using TLS on a mail server is the pay-
load of the mail message and the resulting security there. Many virus and
worm files are now distributed via electronic mail. While the mail may be
encrypted and the servers authenticated, the payload can still be mali-
cious. The use of a good content protection program on the desktop is
therefore still of value even with TLS at the MTA level.
Because sendmail with TLS can only authenticate at the server level, true
end-to-end authentication of the mail message cannot be performed with
only the use of STARTLS on the server. The use of S/MIME or PGP e-mail
and trustworthy key hierarchies can guarantee full confidentiality and
integrity of the entire message path.
Furthermore, if a mail message traverses more than just the starting and
ending servers, there is no way to control interactions between the in-
tervening mail servers, which may use non-secure connections. This intro-
duces a point of vulnerability in the chain.
Additionally, SMTP over TLS is not yet widely implemented. The standard,
in fact, doesn't require it, leaving it only as an option, though specif-
ic sites can configure their servers to force it for specific clients. As
such, it is difficult to foresee the widespread use of SMTP using TLS,
despite the fact that the standard is, at the date of this writing, over
two years old.
Lastly, interoperability problems can appear between different implemen-
tations.
SEE ALSOmail(1), openssl(1), afterboot(8), sendmail(8), ssl(8)
DARPA Internet Request for Comments RFC 2487
http://www.sendmail.org/~ca/email/starttls.html
HISTORY
TLS features first appeared in sendmail 8.11.
MirOS BSD #10-current January 11, 2002 4
