1 This section documents the most common mail (SMTP) and IMAPs/POPs servers. Another option to secure IMAPs/POPs servers is to place them behind an stunnel server.
4 %% ----------------------------------------------------------------------
5 \subsection{SMTP in general}
6 \label{subsection:smtp_general}
7 SMTP usually makes use of opportunistic TLS. This means that an MTA will accept TLS connections when asked for it during handshake but will not require it. One should always support incoming opportunistic TLS and always try TLS handshake outgoing.
9 Furthermore a mailserver can operate in three modes:
11 \item As MSA (Mail Submission Agent) your mailserver receives mail from your clients MUAs (Mail User Agent).
12 \item As receiving MTA (Mail Transmission Agent, MX)
13 \item As sending MTA (SMTP client)
15 We recommend the following basic setup for all modes:
17 \item correctly setup MX, A and PTR RRs without using CNAMEs at all.
18 \item enable encryption (opportunistic TLS)
19 \item do not use self signed certificates
22 For SMTP client mode we additionally recommend:
24 \item the hostname used as HELO must match the PTR RR
25 \item setup a client certificate (most server certificates are client certificates as well)
26 \item either the common name or at least an alternate subject name of your certificate must match the PTR RR
27 \item do not modify the cipher suite for client mode
30 For MSA operation we recommend:
32 \item listen on submission port 587
33 \item enforce SMTP AUTH even for local networks
34 \item do not allow SMTP AUTH on unencrypted connections
35 \item optionally use the recommended cipher suites if (and only if) all your connecting MUAs support them
39 % Note that (with the exception of MSA mode), it might be better to allow any cipher suite -- since any encryption is better than no encryption when it comes to opportunistic TLS.
41 We strongly recommend to allow all cipher suites for anything but MSA
42 mode, because the alternative is plain text transmission.
44 %% ----------------------------------------------------------------------
48 \subsubsection{Tested with Version}
50 \item Dovecot 2.1.17, Debian Wheezy (without ``ssl\_prefer\_server\_ciphers'' setting)
52 \item 2.0.19apple1 on OS X Server 10.8.5 (without ``ssl\_prefer\_server\_ciphers'' setting)
55 \subsubsection{Settings}
56 % Example: http://dovecot.org/list/dovecot/2013-October/092999.html
59 ssl_cipher_list = '%*\cipherStringB*)'
60 ssl_prefer_server_ciphers = yes
63 \subsubsection{Additional info}
64 Dovecot 2.0, 2.1: Almost as good as dovecot 2.2. Dovecot does not ignore unknown configuration parameters. Does not support
65 ssl\_prefer\_server\_ciphers
67 \subsubsection{Limitations}
68 Dovecot currently does not support disabling TLS compression. Furthermore, DH
69 parameters greater than 1024bit are not supported. The most recent version
70 2.2.7 of Dovecot implements configurable DH parameter length
71 \footnote{\url{http://hg.dovecot.org/dovecot-2.2/rev/43ab5abeb8f0}}.
73 %\subsubsection{Justification for special settings (if needed)}
75 % in case you have the need for further justifications why you chose this and that setting or if the settings do not fit into the standard Variant A or Variant B schema, please document this here
77 \subsubsection{References}
79 \item \url{http://wiki2.dovecot.org/SSL}
82 % add any further references or best practice documents here
84 \subsubsection{How to test}
85 % describe here or point the admin to tools (can be a simple footnote or \ref{} to the tools section) which help the admin to test his settings.
87 openssl s_client -crlf -connect SERVER.TLD:993
91 %% ----------------------------------------------------------------------
92 \subsection{cyrus-imapd}
93 \subsubsection{Tested with Versions}
98 \subsubsection{Settings}
99 \paragraphDiamond{imapd.conf}
100 To activate SSL/TLS configure your certificate with
102 tls_cert_file: .../cert.pem
103 tls_key_file: .../cert.key
106 Do not forget to add necessary intermediate certificates to the .pem file.
108 Limiting the ciphers provided may force (especially older) clients to connect without encryption at all! Sticking to the defaults is recommended.
110 If you still want to force strong encryption use
112 tls_cipher_list: %*\cipherStringB*)
115 cyrus-imapd loads hardcoded 1024 bit DH parameters using get\_rfc2409\_prime\_1024() by default. If you want to load your own DH parameters add them PEM encoded to the certificate file given in tls\_cert\_file. Do not forget to re-add them after updating your certificate.
117 To prevent unencrypted connections on the STARTTLS ports you can set
121 This way MUAs can only authenticate after STARTTLS if you only provide plaintext and SASL PLAIN login methods. Therefore providing CRAM-MD5 or DIGEST-MD5 methods is not recommended.
123 \paragraphDiamond{cyrus.conf}
125 To support POP3/IMAP on ports 110/143 with STARTTLS add
127 imap cmd="imapd" listen="imap" prefork=3
128 pop3 cmd="pop3d" listen="pop3" prefork=1
130 to the SERVICES section.
132 To support POP3S/IMAPS on ports 995/993 add
134 imaps cmd="imapd -s" listen="imaps" prefork=3
135 pop3s cmd="pop3d -s" listen="pop3s" prefork=1
139 \subsubsection{Limitations}
141 cyrus-imapd currently (2.4.17, trunk) does not support elliptic curve cryptography. Hence, ECDHE will not work even if defined in your cipher list.
143 Currently there is no way to prefer server ciphers or to disable compression.
145 There is a working patch for all three features:
146 \url{https://bugzilla.cyrusimap.org/show_bug.cgi?id=3823}
148 \subsubsection{How to test}
150 openssl s_client -crlf -connect SERVER.TLD:993
153 % XXX config von Adi?
155 % ciphers = EDH+CAMELLIA256:EDH+aRSA:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:-AES128:!CAMELLIA128:!ECDSA:AES256-SHA:EDH+AES128;
156 % options = CIPHER_SERVER_PREFERENCE
160 %% ----------------------------------------------------------------------
163 \subsubsection{Tested with Versions}
165 \item Postfix 2.9.6, Debian Wheezy
169 \subsubsection{Settings}
171 %% I (cm) consider the generation of own DH parameters to be voodoo until
172 %% someone can explain the contrary. They are, after all, public, and
173 %% I found no research that would show that long-term use of a
174 %% parameter set would weaken the DH exchange. Also notice that IPSEC
175 %% uses fixed parameter sets only.
177 %% also notice the following comment from src/tls/tls_dh.c:
178 %% * Compiled-in EDH primes (the compiled-in generator is always 2). These are
179 %% * used when no parameters are explicitly loaded from a site-specific file.
181 %% * 512-bit parameters are used for export ciphers, and 1024-bit parameters are
182 %% * used for non-export ciphers. An ~80-bit strong EDH key exchange is really
183 %% * too weak to protect 128+ bit keys, but larger DH primes are
184 %% * computationally expensive. When greater security is required, use EECDH.
186 %% First, you need to generate Diffie Hellman parameters (please first take a look at the section \ref{section:RNGs}):
188 %% \todo{FIXME: this is a really weak setting! See also: http://postfix.1071664.n5.nabble.com/postfix-hardening-what-can-we-do-td61874.html}
189 %% \begin{lstlisting}
190 %% % openssl gendh -out /etc/postfix/dh_param_512.pem -2 512
191 %% % openssl gendh -out /etc/postfix/dh_param_1024.pem -2 1024
194 %% Next, we specify these DH parameters in \verb|main.cf|:
196 %% \begin{lstlisting}
197 %% smtpd_tls_dh512_param_file = /etc/postfix/dh_param_512.pem
198 %% smtpd_tls_dh1024_param_file = /etc/postfix/dh_param_1024.pem
201 \paragraphDiamond{MX and SMTP client configuration}
202 As discussed in section \ref{subsection:smtp_general}, because of opportunistic encryption we do not
203 restrict the list of ciphers. There are still some steps needed to
204 enable TLS, all in \verb|main.cf|:
207 smtpd_tls_cert_file = /etc/postfix/server.pem
208 smtpd_tls_key_file = /etc/postfix/server.key
209 # use 0 for Postfix >= 2.9, and 1 for earlier versions
210 smtpd_tls_loglevel = 0
211 # enable opportunistic TLS support in the SMTP server and client
212 smtpd_tls_security_level = may
213 smtp_tls_security_level = may
214 # if you have authentication enabled, only offer it after STARTTLS
215 smtpd_tls_auth_only = yes
216 tls_ssl_options = NO_COMPRESSION
219 \paragraphDiamond{MSA}
220 For the MSA \verb|smtpd| process, we first define the ciphers that are
221 acceptable for the ``mandatory'' security level, again in
225 smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
226 smtpd_tls_mandatory_ciphers=high
227 tls_high_cipherlist=%*\cipherStringB*)
230 Then, we configure the MSA smtpd in \verb|master.cf| with two
231 additional options that are only used for this instance of smtpd:
234 587 inet n - - - - smtpd
235 -o smtpd_tls_security_level=encrypt -o tls_preempt_cipherlist = yes
238 For those users who want to use ECC key exchange, it is possible to specify this via:
240 smtpd_tls_eecdh_grade = ultra
243 \subsubsection{Limitations}
244 tls\_ssl\_options is supported from Postfix 2.11 onwards. You can
245 leave the statement in the configuration for older versions, it will
248 tls\_preempt\_cipherlist is supported from Postfix 2.8 onwards. Again,
249 you can leave the statement in for older versions.
251 \subsubsection{References}
252 Refer to \url{http://www.postfix.org/TLS_README.html} for an in-depth
255 \subsubsection{Additional settings}
256 Postfix has two sets of built-in DH parameters that can be overridden
257 with the \verb|smtpd_tls_dh512_param_file|
258 and \verb|smtpd_tls_dh1024_param_file| options. The ``dh512''
259 parameters are used for export ciphers, while the ``dh1024'' ones are
260 used for all other ciphers.
262 The ``bit length'' in those parameter names is just a name, so one
263 could use stronger parameter sets; it should be possible to e.g. use the
264 IKE Group14 parameters (see section \ref{section:DH}) without much
265 interoperability risk, but we have not tested this yet.
267 % \subsubsection{Justification for special settings (if needed)}
268 % no special settings
271 \subsubsection{How to test}
272 You can check the effect of the settings with the following command:
274 $ zegrep "TLS connection established from.*with cipher" /var/log/mail.log | awk '{printf("%s %s %s %s\n", $12, $13, $14, $15)}' | sort | uniq -c | sort -n
275 1 SSLv3 with cipher DHE-RSA-AES256-SHA
276 23 TLSv1.2 with cipher DHE-RSA-AES256-GCM-SHA384
277 60 TLSv1 with cipher ECDHE-RSA-AES256-SHA
278 270 TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384
279 335 TLSv1 with cipher DHE-RSA-AES256-SHA
283 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
286 %% ----------------------------------------------------------------------
288 \subsection{Exim (based on 4.82)}
289 It is highly recommended to read
290 \url{http://exim.org/exim-html-current/doc/html/spec_html/ch-encrypted_smtp_connections_using_tlsssl.html}
293 \paragraphDiamond{MSA mode (submission)}
294 In the main config section of Exim add:
297 tls_certificate = ..../cert.pem
298 tls_privatekey = ..../cert.key
300 don't forget to add intermediate certificates to the .pem file if needed.
302 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
304 tls_advertise_hosts = *
307 If you want to support legacy SMTPS on port 465, and STARTTLS on smtp(25)/submission(587) ports set
309 daemon_smtp_ports = smtp : smtps : submission
310 tls_on_connect_ports = 465
313 It is highly recommended to limit SMTP AUTH to SSL connections only. To do so add
315 server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}}
317 to every authenticator defined.
319 Add the following rules on top of your acl\_smtp\_mail:
322 control = submission/sender_retain
324 This switches Exim to submission mode and allows addition of missing ``Message-ID'' and ``Date'' headers.
326 It is not advisable to restrict the default cipher list for MSA mode if you don't know all connecting MUAs. If you still want to define one please consult the Exim documentation or ask on the exim-users mailinglist.
327 % Exim maintainers do not recommend to change default ciphers
328 % I think we shouldn't, too
331 % tls_require_ciphers = <...recommended ciphersuite...>
334 The cipher used is written to the logfiles by default. You may want to add
336 log_selector = <whatever your log_selector already contains> +tls_certificate_verified +tls_peerdn +tls_sni
338 to get even more TLS information logged.
341 \paragraphDiamond{server mode (incoming)}
343 In the main config section of Exim add:
346 tls_certificate = ..../cert.pem
347 tls_privatekey = ..../cert.key
349 don't forget to add intermediate certificates to the .pem file if needed.
351 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
353 tls_advertise_hosts = *
356 Listen on smtp(25) port only
358 daemon_smtp_ports = smtp
361 It is not advisable to restrict the default cipher list for opportunistic encryption as used by SMTP. Do not use cipher lists recommended for HTTPS! If you still want to define one please consult the Exim documentation or ask on the exim-users mailinglist.
362 % Exim maintainers do not recommend to change default ciphers
366 % tls_require_ciphers = <...recommended ciphersuite...>
369 If you want to request and verify client certificates from sending hosts set
371 tls_verify_certificates = /etc/pki/tls/certs/ca-bundle.crt
372 tls_try_verify_hosts = *
375 tls\_try\_verify\_hosts only reports the result to your logfile. If you want to disconnect such clients you have to use
380 The cipher used is written to the logfiles by default. You may want to add
382 log_selector = <whatever your log_selector already contains> +tls_certificate_verified +tls_peerdn +tls_sni
384 to get even more TLS information logged.
386 \paragraphDiamond{client mode (outgoing)}
387 Exim uses opportunistic encryption in the SMTP transport by default.
389 Client mode settings have to be done in the configuration section of the smtp transport (driver = smtp).
391 If you want to use a client certificate (most server certificates can be used as client certificate, too) set
393 tls_certificate = .../cert.pem
394 tls_privatekey = .../cert.key
396 This is recommended for MTA-MTA traffic.
398 %If you want to limit used ciphers set
400 % tls_require_ciphers = <...recommended ciphersuite...>
402 % Exim Maintainers do not recommend ciphers. We shouldn't do so, too.
403 Do not limit ciphers without a very good reason. In the worst case you end up without encryption at all instead of some weak encryption. Please consult the Exim documentation if you really need to define ciphers.
405 \paragraphDiamond{OpenSSL}
406 Exim already disables SSLv2 by default. We recommend to add
408 openssl_options = +all +no_sslv2 +no_compression +cipher_server_preference
410 to the main configuration.
412 Note: +all is misleading here since OpenSSL only activates the most common workarounds. But that's how SSL\_OP\_ALL is defined.
414 You do not need to set dh\_parameters. Exim with OpenSSL by default uses parameter initialization with the "2048-bit MODP Group with 224-bit Prime Order Subgroup" defined in section 2.2 of RFC 5114~\cite{rfc5114} (ike23).
415 If you want to set your own DH parameters please read the TLS documentation of exim.
418 \paragraphDiamond{GnuTLS}
419 GnuTLS is different in only some respects to OpenSSL:
421 \item tls\_require\_ciphers needs a GnuTLS priority string instead of a cipher list. It is recommended to use the defaults by not defining this option. It highly depends on the version of GnuTLS used. Therefore it is not advisable to change the defaults.
422 \item There is no option like openssl\_options
425 \paragraphDiamond{Exim string expansion}
426 Note that most of the options accept expansion strings. This way you can e.g. set cipher lists or STARTTLS advertisement conditionally. Please follow the link to the official Exim documentation to get more information.
428 \paragraphDiamond{Limitations}
429 Exim currently (4.82) does not support elliptic curves with OpenSSL. This means that ECDHE is not used even if defined in your cipher list.
430 There already is a working patch to provide support:
431 \url{http://bugs.exim.org/show_bug.cgi?id=1397}
433 \paragraphDiamond{How to test}
435 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
439 %% ----------------------------------------------------------------------
440 %\subsection{Exchange}
442 %\todo{FIXME: write this section}