1 %%\subsection{Mail Servers}
3 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.
6 %% ----------------------------------------------------------------------
8 \subsection{SMTP in general}
9 \label{subsection:smtp_general}
11 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.\\
13 Furthermore a mailserver can operate in three modes:
15 \item As MSA (Mail Submission Agent) your mailserver receives mail from your clients MUAs (Mail User Agent).
16 \item As receiving MTA (Mail Transmission Agent, MX)
17 \item As sending MTA (SMTP client)
20 We recommend the following basic setup for all modes:
22 \item correctly setup MX, A and PTR RRs without using CNAMEs at all.
23 \item enable encryption (opportunistic TLS)
24 \item do not use self signed certificates
27 For SMTP client mode we additionally recommend:
29 \item the hostname used as HELO must match the PTR RR
30 \item setup a client certificate (most server certificates are client certificates as well)
31 \item either the common name or at least an alternate subject name of your certificate must match the PTR RR
32 \item do not modify the cipher suite for client mode
35 For MSA operation we recommend:
37 \item listen on submission port 587
38 \item enforce SMTP AUTH even for local networks
39 \item do not allow SMTP AUTH on unencrypted connections
40 \item optionally use the recommended cipher suites if (and only if) all your connecting MUAs support them
45 % 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.
47 We strongly recommend to allow all cipher suites for anything but MSA
48 mode, because the alternative is plain text transmission.
50 %% ----------------------------------------------------------------------
55 \subsubsection{Tested with Version}
57 \item Dovecot 2.1.17, Debian Wheezy (without ``ssl\_prefer\_server\_ciphers'' setting)
59 \item 2.0.19apple1 on OS X Server 10.8.5 (without ``ssl\_prefer\_server\_ciphers'' setting)
62 \subsubsection{Settings}
63 % Example: http://dovecot.org/list/dovecot/2013-October/092999.html
65 \begin{lstlisting}[breaklines]
66 ssl_cipher_list = '@@@CIPHERSTRINGB@@@'
67 ssl_prefer_server_ciphers = yes
70 \subsubsection{Additional info}
72 Dovecot 2.0, 2.1: Almost as good as dovecot 2.2. Dovecot does not ignore unknown configuration parameters. Does not support
73 ssl\_prefer\_server\_ciphers
75 \subsubsection{Limitations}
77 Dovecot currently does not support disabling TLS compression. Furthermore, DH
78 parameters greater than 1024bit are not supported. The most recent version
79 2.2.7 of Dovecot implements configurable DH parameter length
80 \footnote{\url{http://hg.dovecot.org/dovecot-2.2/rev/43ab5abeb8f0}}.
82 %\item[Justification for special settings (if needed):] \mbox{}
84 % 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
86 \subsubsection{References} \url{http://wiki2.dovecot.org/SSL}
88 % add any further references or best practice documents here
90 \subsubsection{How to test}
91 % 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.
92 \begin{lstlisting}[breaklines]
93 openssl s_client -crlf -connect SERVER.TLD:993
97 %% ----------------------------------------------------------------------
99 \subsection{cyrus-imapd}
101 \subsubsection{Tested with Version}
104 \subsubsection{Settings}
105 \paragraph*{imapd.conf}\mbox{}\\
107 To activate SSL/TLS configure your certificate with
108 \begin{lstlisting}[breaklines]
109 tls_cert_file: .../cert.pem
110 tls_key_file: .../cert.key
113 Do not forget to add necessary intermediate certificates to the .pem file.\\
115 Limiting the ciphers provided may force (especially older) clients to connect without encryption at all! Sticking to the defaults is recommended.\\
117 If you still want to force strong encryption use
118 \begin{lstlisting}[breaklines]
119 tls_cipher_list: @@@CIPHERSTRINGB@@@
122 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.\\
124 To prevent unencrypted connections on the STARTTLS ports you can set
125 \begin{lstlisting}[breaklines]
128 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.\\
130 \paragraph*{cyrus.conf}\mbox{}\\
132 To support POP3/IMAP on ports 110/143 with STARTTLS add
133 \begin{lstlisting}[breaklines]
134 imap cmd="imapd" listen="imap" prefork=3
135 pop3 cmd="pop3d" listen="pop3" prefork=1
137 to the SERVICES section.\\
139 To support POP3S/IMAPS on ports 995/993 add
140 \begin{lstlisting}[breaklines]
141 imaps cmd="imapd -s" listen="imaps" prefork=3
142 pop3s cmd="pop3d -s" listen="pop3s" prefork=1
146 \subsubsection{Limitations}
148 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.\\
150 Currently there is no way to prefer server ciphers or to disable compression.\\
152 There is a working patch for all three features:
153 \url{https://bugzilla.cyrusimap.org/show_bug.cgi?id=3823}\\
155 \subsubsection{How to test}
156 \begin{lstlisting}[breaklines]
157 openssl s_client -crlf -connect SERVER.TLD:993
162 % XXX config von Adi?
164 % ciphers = EDH+CAMELLIA256:EDH+aRSA:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:-AES128:!CAMELLIA128:!ECDSA:AES256-SHA:EDH+AES128;
165 % options = CIPHER_SERVER_PREFERENCE
170 \subsubsection{Tested with Version}
172 \item Postfix 2.9.6, Debian Wheezy
175 \subsubsection{Settings}
177 %% I (cm) consider the generation of own DH parameters to be voodoo until
178 %% someone can explain the contrary. They are, after all, public, and
179 %% I found no research that would show that long-term use of a
180 %% parameter set would weaken the DH exchange. Also notice that IPSEC
181 %% uses fixed parameter sets only.
183 %% also notice the following comment from src/tls/tls_dh.c:
184 %% * Compiled-in EDH primes (the compiled-in generator is always 2). These are
185 %% * used when no parameters are explicitly loaded from a site-specific file.
187 %% * 512-bit parameters are used for export ciphers, and 1024-bit parameters are
188 %% * used for non-export ciphers. An ~80-bit strong EDH key exchange is really
189 %% * too weak to protect 128+ bit keys, but larger DH primes are
190 %% * computationally expensive. When greater security is required, use EECDH.
192 %% First, you need to generate Diffie Hellman parameters (please first take a look at the section \ref{section:RNGs}):
194 %% \todo{FIXME: this is a really weak setting! See also: http://postfix.1071664.n5.nabble.com/postfix-hardening-what-can-we-do-td61874.html}
195 %% \begin{lstlisting}[breaklines]
196 %% % openssl gendh -out /etc/postfix/dh_param_512.pem -2 512
197 %% % openssl gendh -out /etc/postfix/dh_param_1024.pem -2 1024
200 %% Next, we specify these DH parameters in \verb|main.cf|:
202 %% \begin{lstlisting}[breaklines]
203 %% smtpd_tls_dh512_param_file = /etc/postfix/dh_param_512.pem
204 %% smtpd_tls_dh1024_param_file = /etc/postfix/dh_param_1024.pem
207 \paragraph*{MX and SMTP client configuration}\mbox{}\\
209 As discussed in section \ref{subsection:smtp_general}, because of opportunistic encryption we do not
210 restrict the list of ciphers. There are still some steps needed to
211 enable TLS, all in \verb|main.cf|:
213 \begin{lstlisting}[breaklines]
214 smtpd_tls_cert_file = /etc/postfix/server.pem
215 smtpd_tls_key_file = /etc/postfix/server.key
216 # use 0 for Postfix >= 2.9, and 1 for earlier versions
217 smtpd_tls_loglevel = 0
218 # enable opportunistic TLS support in the SMTP server and client
219 smtpd_tls_security_level = may
220 smtp_tls_security_level = may
221 # if you have authentication enabled, only offer it after STARTTLS
222 smtpd_tls_auth_only = yes
223 tls_ssl_options = NO_COMPRESSION
226 \paragraph*{MSA}\mbox{}\\
228 For the MSA \verb|smtpd| process, we first define the ciphers that are
229 acceptable for the ``mandatory'' security level, again in
232 \begin{lstlisting}[breaklines]
233 smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
234 smtpd_tls_mandatory_ciphers=high
235 tls_high_cipherlist=@@@CIPHERSTRINGB@@@
238 Then, we configure the MSA smtpd in \verb|master.cf| with two
239 additional options that are only used for this instance of smtpd:
241 \begin{lstlisting}[breaklines]
242 587 inet n - - - - smtpd
243 -o smtpd_tls_security_level=encrypt -o tls_preempt_cipherlist = yes
246 For those users who want to use ECC key exchange, it is possible to specify this via:
247 \begin{lstlisting}[breaklines]
248 smtpd_tls_eecdh_grade = ultra
251 \subsubsection{Limitations}
253 tls\_ssl\_options is supported from Postfix 2.11 onwards. You can
254 leave the statement in the configuration for older versions, it will
257 tls\_preempt\_cipherlist is supported from Postfix 2.8 onwards. Again,
258 you can leave the statement in for older versions.
260 \subsubsection{References}
262 Refer to \url{http://www.postfix.org/TLS_README.html} for an in-depth
265 \subsubsection{Additional settings}
267 Postfix has two sets of built-in DH parameters that can be overridden
268 with the \verb|smtpd_tls_dh512_param_file|
269 and \verb|smtpd_tls_dh1024_param_file| options. The ``dh512''
270 parameters are used for export ciphers, while the ``dh1024'' ones are
271 used for all other ciphers.
273 The ``bit length'' in those parameter names is just a name, so one
274 could use stronger parameter sets; it should be possible to e.g. use the
275 IKE Group14 parameters (see section \ref{section:DH}) without much
276 interoperability risk, but we have not tested this yet.
278 % \item[Justification for special settings (if needed):]
279 % no special settings
282 \subsubsection{How to test}
284 You can check the effect of the settings with the following command:
285 \begin{lstlisting}[breaklines]
286 $ 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
287 1 SSLv3 with cipher DHE-RSA-AES256-SHA
288 23 TLSv1.2 with cipher DHE-RSA-AES256-GCM-SHA384
289 60 TLSv1 with cipher ECDHE-RSA-AES256-SHA
290 270 TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384
291 335 TLSv1 with cipher DHE-RSA-AES256-SHA
294 \begin{lstlisting}[breaklines]
295 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
298 %% ----------------------------------------------------------------------
300 \subsection{Exim (based on 4.82)}
302 It is highly recommended to read
303 \url{http://exim.org/exim-html-current/doc/html/spec_html/ch-encrypted_smtp_connections_using_tlsssl.html}
306 \paragraph*{MSA mode (submission)}\mbox{}\\
308 In the main config section of Exim add:
310 \begin{lstlisting}[breaklines]
311 tls_certificate = ..../cert.pem
312 tls_privatekey = ..../cert.key
314 don't forget to add intermediate certificates to the .pem file if needed.\\
316 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
317 \begin{lstlisting}[breaklines]
318 tls_advertise_hosts = *
321 If you want to support legacy SMTPS on port 465, and STARTTLS on smtp(25)/submission(587) ports set
322 \begin{lstlisting}[breaklines]
323 daemon_smtp_ports = smtp : smtps : submission
324 tls_on_connect_ports = 465
327 It is highly recommended to limit SMTP AUTH to SSL connections only. To do so add
328 \begin{lstlisting}[breaklines]
329 server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}}
331 to every authenticator defined.\\
333 Add the following rules on top of your acl\_smtp\_mail:
334 \begin{lstlisting}[breaklines]
336 control = submission/sender_retain
338 This switches Exim to submission mode and allows addition of missing ``Message-ID'' and ``Date'' headers.\\
340 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.\\
341 % Exim maintainers do not recommend to change default ciphers
342 % I think we shouldn't, too
344 %\begin{lstlisting}[breaklines]
345 % tls_require_ciphers = <...recommended ciphersuite...>
348 The cipher used is written to the logfiles by default. You may want to add
349 \begin{lstlisting}[breaklines]
350 log_selector = <whatever your log_selector already contains> \
351 +tls_certificate_verified +tls_peerdn +tls_sni
353 to get even more TLS information logged.
356 \paragraph*{server mode (incoming)}\mbox{}\\
358 In the main config section of Exim add:
360 \begin{lstlisting}[breaklines]
361 tls_certificate = ..../cert.pem
362 tls_privatekey = ..../cert.key
364 don't forget to add intermediate certificates to the .pem file if needed.\\
366 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
367 \begin{lstlisting}[breaklines]
368 tls_advertise_hosts = *
371 Listen on smtp(25) port only
372 \begin{lstlisting}[breaklines]
373 daemon_smtp_ports = smtp
376 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.\\
377 % Exim maintainers do not recommend to change default ciphers
380 %\begin{lstlisting}[breaklines]
381 % tls_require_ciphers = <...recommended ciphersuite...>
384 If you want to request and verify client certificates from sending hosts set
385 \begin{lstlisting}[breaklines]
386 tls_verify_certificates = /etc/pki/tls/certs/ca-bundle.crt
387 tls_try_verify_hosts = *
390 tls\_try\_verify\_hosts only reports the result to your logfile. If you want to disconnect such clients you have to use
391 \begin{lstlisting}[breaklines]
395 The cipher used is written to the logfiles by default. You may want to add
396 \begin{lstlisting}[breaklines]
397 log_selector = <whatever your log_selector already contains> \
398 +tls_certificate_verified +tls_peerdn +tls_sni
400 to get even more TLS information logged.
402 \paragraph*{client mode (outgoing)}\mbox{}\\
404 Exim uses opportunistic encryption in the SMTP transport by default.
406 Client mode settings have to be done in the configuration section of the smtp transport (driver = smtp).
408 If you want to use a client certificate (most server certificates can be used as client certificate, too) set
409 \begin{lstlisting}[breaklines]
410 tls_certificate = .../cert.pem
411 tls_privatekey = .../cert.key
413 This is recommended for MTA-MTA traffic.\\
415 %If you want to limit used ciphers set
416 %\begin{lstlisting}[breaklines]
417 % tls_require_ciphers = <...recommended ciphersuite...>
419 % Exim Maintainers do not recommend ciphers. We shouldn't do so, too.
420 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.
422 \paragraph*{OpenSSL}\mbox{}\\
423 Exim already disables SSLv2 by default. We recommend to add
424 \begin{lstlisting}[breaklines]
425 openssl_options = +all +no_sslv2 +no_compression +cipher_server_preference
427 to the main configuration.\\
428 Note: +all is misleading here since OpenSSL only activates the most common workarounds. But that's how SSL\_OP\_ALL is defined.\\
430 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).
431 If you want to set your own DH parameters please read the TLS documentation of exim.\\
435 \paragraph*{GnuTLS}\mbox{}\\
437 GnuTLS is different in only some respects to OpenSSL:
439 \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.
440 \item There is no option like openssl\_options
443 \paragraph*{Exim string expansion}\mbox{}\\
445 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.
447 \paragraph*{Limitations}\mbox{}\\
449 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.
450 There already is a working patch to provide support:\\
451 \url{http://bugs.exim.org/show_bug.cgi?id=1397}
453 \paragraph*{How to test}\mbox{}\\
454 \begin{lstlisting}[breaklines]
455 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
459 %% ----------------------------------------------------------------------
460 %\subsection{Exchange}
462 %\todo{FIXME: write this section}