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}
58 \item 2.0.19apple1 on OS X Server 10.8.5 (without ssl\_prefer\_server\_ciphers)
61 \subsubsection{Settings}
62 % Example: http://dovecot.org/list/dovecot/2013-October/092999.html
64 \begin{lstlisting}[breaklines]
65 ssl_cipher_list = '@@@CIPHERSTRINGB@@@'
66 ssl_prefer_server_ciphers = yes
69 \subsubsection{Additional info}
71 Dovecot 2.0, 2.1: Almost as good as dovecot 2.2. Dovecot does not ignore unknown configuration parameters. Does not support
72 ssl\_prefer\_server\_ciphers
74 \subsubsection{Limitations}
76 Dovecot currently does not support disabling TLS compression. Furthermore, DH
77 parameters greater than 1024bit are not supported. The most recent version
78 2.2.7 of Dovecot implements configurable DH parameter length
79 \footnote{\url{http://hg.dovecot.org/dovecot-2.2/rev/43ab5abeb8f0}}.
81 %\item[Justification for special settings (if needed):] \mbox{}
83 % 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
85 \subsubsection{References} \url{http://wiki2.dovecot.org/SSL}
87 % add any further references or best practice documents here
89 \subsubsection{How to test}
90 % 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.
91 \begin{lstlisting}[breaklines]
92 openssl s_client -crlf -connect SERVER.TLD:993
96 %% ----------------------------------------------------------------------
98 \subsection{cyrus-imapd}
100 \subsubsection{Tested with Version}
103 \subsubsection{Settings}
104 \paragraph*{imapd.conf}\mbox{}\\
106 To activate SSL/TLS configure your certificate with
107 \begin{lstlisting}[breaklines]
108 tls_cert_file: .../cert.pem
109 tls_key_file: .../cert.key
112 Do not forget to add necessary intermediate certificates to the .pem file.\\
114 Limiting the ciphers provided may force (especially older) clients to connect without encryption at all! Sticking to the defaults is recommended.\\
116 If you still want to force strong encryption use
117 \begin{lstlisting}[breaklines]
118 tls_cipher_list: @@@CIPHERSTRINGB@@@
121 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.\\
123 To prevent unencrypted connections on the STARTTLS ports you can set
124 \begin{lstlisting}[breaklines]
127 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.\\
129 \paragraph*{cyrus.conf}\mbox{}\\
131 To support POP3/IMAP on ports 110/143 with STARTTLS add
132 \begin{lstlisting}[breaklines]
133 imap cmd="imapd" listen="imap" prefork=3
134 pop3 cmd="pop3d" listen="pop3" prefork=1
136 to the SERVICES section.\\
138 To support POP3S/IMAPS on ports 995/993 add
139 \begin{lstlisting}[breaklines]
140 imaps cmd="imapd -s" listen="imaps" prefork=3
141 pop3s cmd="pop3d -s" listen="pop3s" prefork=1
145 \subsubsection{Limitations}
147 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.\\
149 Currently there is no way to prefer server ciphers or to disable compression.\\
151 There is a working patch for all three features:
152 \url{https://bugzilla.cyrusimap.org/show_bug.cgi?id=3823}\\
154 \subsubsection{How to test}
155 \begin{lstlisting}[breaklines]
156 openssl s_client -crlf -connect SERVER.TLD:993
161 % XXX config von Adi?
163 % ciphers = EDH+CAMELLIA256:EDH+aRSA:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:-AES128:!CAMELLIA128:!ECDSA:AES256-SHA:EDH+AES128;
164 % options = CIPHER_SERVER_PREFERENCE
169 \subsubsection{Tested with Version} Postfix 2.9.6 (Debian Wheezy)
170 \subsubsection{Settings}
172 %% I (cm) consider the generation of own DH parameters to be voodoo until
173 %% someone can explain the contrary. They are, after all, public, and
174 %% I found no research that would show that long-term use of a
175 %% parameter set would weaken the DH exchange. Also notice that IPSEC
176 %% uses fixed parameter sets only.
178 %% also notice the following comment from src/tls/tls_dh.c:
179 %% * Compiled-in EDH primes (the compiled-in generator is always 2). These are
180 %% * used when no parameters are explicitly loaded from a site-specific file.
182 %% * 512-bit parameters are used for export ciphers, and 1024-bit parameters are
183 %% * used for non-export ciphers. An ~80-bit strong EDH key exchange is really
184 %% * too weak to protect 128+ bit keys, but larger DH primes are
185 %% * computationally expensive. When greater security is required, use EECDH.
187 %% First, you need to generate Diffie Hellman parameters (please first take a look at the section \ref{section:RNGs}):
189 %% \todo{FIXME: this is a really weak setting! See also: http://postfix.1071664.n5.nabble.com/postfix-hardening-what-can-we-do-td61874.html}
190 %% \begin{lstlisting}[breaklines]
191 %% % openssl gendh -out /etc/postfix/dh_param_512.pem -2 512
192 %% % openssl gendh -out /etc/postfix/dh_param_1024.pem -2 1024
195 %% Next, we specify these DH parameters in \verb|main.cf|:
197 %% \begin{lstlisting}[breaklines]
198 %% smtpd_tls_dh512_param_file = /etc/postfix/dh_param_512.pem
199 %% smtpd_tls_dh1024_param_file = /etc/postfix/dh_param_1024.pem
202 \paragraph*{MX and SMTP client configuration}\mbox{}\\
204 As discussed in section \ref{subsection:smtp_general}, because of opportunistic encryption we do not
205 restrict the list of ciphers. There are still some steps needed to
206 enable TLS, all in \verb|main.cf|:
208 \begin{lstlisting}[breaklines]
209 smtpd_tls_cert_file = /etc/postfix/server.pem
210 smtpd_tls_key_file = /etc/postfix/server.key
211 # use 0 for Postfix >= 2.9, and 1 for earlier versions
212 smtpd_tls_loglevel = 0
213 # enable opportunistic TLS support in the SMTP server and client
214 smtpd_tls_security_level = may
215 smtp_tls_security_level = may
216 # if you have authentication enabled, only offer it after STARTTLS
217 smtpd_tls_auth_only = yes
218 tls_ssl_options = NO_COMPRESSION
221 \paragraph*{MSA}\mbox{}\\
223 For the MSA \verb|smtpd| process, we first define the ciphers that are
224 acceptable for the ``mandatory'' security level, again in
227 \begin{lstlisting}[breaklines]
228 smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
229 smtpd_tls_mandatory_ciphers=high
230 tls_high_cipherlist=@@@CIPHERSTRINGB@@@
233 Then, we configure the MSA smtpd in \verb|master.cf| with two
234 additional options that are only used for this instance of smtpd:
236 \begin{lstlisting}[breaklines]
237 587 inet n - - - - smtpd
238 -o smtpd_tls_security_level=encrypt -o tls_preempt_cipherlist = yes
241 For those users who want to use ECC key exchange, it is possible to specify this via:
242 \begin{lstlisting}[breaklines]
243 smtpd_tls_eecdh_grade = ultra
246 \subsubsection{Limitations}
248 tls\_ssl\_options is supported from Postfix 2.11 onwards. You can
249 leave the statement in the configuration for older versions, it will
252 tls\_preempt\_cipherlist is supported from Postfix 2.8 onwards. Again,
253 you can leave the statement in for older versions.
255 \subsubsection{References}
257 Refer to \url{http://www.postfix.org/TLS_README.html} for an in-depth
260 \subsubsection{Additional settings}
262 Postfix has two sets of built-in DH parameters that can be overridden
263 with the \verb|smtpd_tls_dh512_param_file|
264 and \verb|smtpd_tls_dh1024_param_file| options. The ``dh512''
265 parameters are used for export ciphers, while the ``dh1024'' ones are
266 used for all other ciphers.
268 The ``bit length'' in those parameter names is just a name, so one
269 could use stronger parameter sets; it should be possible to e.g. use the
270 IKE Group14 parameters (see section \ref{section:DH}) without much
271 interoperability risk, but we have not tested this yet.
273 % \item[Justification for special settings (if needed):]
274 % no special settings
277 \subsubsection{How to test}
279 You can check the effect of the settings with the following command:
280 \begin{lstlisting}[breaklines]
281 $ 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
282 1 SSLv3 with cipher DHE-RSA-AES256-SHA
283 23 TLSv1.2 with cipher DHE-RSA-AES256-GCM-SHA384
284 60 TLSv1 with cipher ECDHE-RSA-AES256-SHA
285 270 TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384
286 335 TLSv1 with cipher DHE-RSA-AES256-SHA
289 \begin{lstlisting}[breaklines]
290 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
293 %% ----------------------------------------------------------------------
295 \subsection{Exim (based on 4.82)}
297 It is highly recommended to read
298 \url{http://exim.org/exim-html-current/doc/html/spec_html/ch-encrypted_smtp_connections_using_tlsssl.html}
301 \paragraph*{MSA mode (submission)}\mbox{}\\
303 In the main config section of Exim add:
305 \begin{lstlisting}[breaklines]
306 tls_certificate = ..../cert.pem
307 tls_privatekey = ..../cert.key
309 don't forget to add intermediate certificates to the .pem file if needed.\\
311 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
312 \begin{lstlisting}[breaklines]
313 tls_advertise_hosts = *
316 If you want to support legacy SMTPS on port 465, and STARTTLS on smtp(25)/submission(587) ports set
317 \begin{lstlisting}[breaklines]
318 daemon_smtp_ports = smtp : smtps : submission
319 tls_on_connect_ports = 465
322 It is highly recommended to limit SMTP AUTH to SSL connections only. To do so add
323 \begin{lstlisting}[breaklines]
324 server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}}
326 to every authenticator defined.\\
328 Add the following rules on top of your acl\_smtp\_mail:
329 \begin{lstlisting}[breaklines]
331 control = submission/sender_retain
333 This switches Exim to submission mode and allows addition of missing ``Message-ID'' and ``Date'' headers.\\
335 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.\\
336 % Exim maintainers do not recommend to change default ciphers
337 % I think we shouldn't, too
339 %\begin{lstlisting}[breaklines]
340 % tls_require_ciphers = <...recommended ciphersuite...>
343 The cipher used is written to the logfiles by default. You may want to add
344 \begin{lstlisting}[breaklines]
345 log_selector = <....whatever your log_selector already contains...> \
346 +tls_certificate_verified +tls_peerdn +tls_sni
348 to get even more TLS information logged.
351 \paragraph*{server mode (incoming)}\mbox{}\\
353 In the main config section of Exim add:
355 \begin{lstlisting}[breaklines]
356 tls_certificate = ..../cert.pem
357 tls_privatekey = ..../cert.key
359 don't forget to add intermediate certificates to the .pem file if needed.\\
361 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
362 \begin{lstlisting}[breaklines]
363 tls_advertise_hosts = *
366 Listen on smtp(25) port only
367 \begin{lstlisting}[breaklines]
368 daemon_smtp_ports = smtp
371 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.\\
372 % Exim maintainers do not recommend to change default ciphers
375 %\begin{lstlisting}[breaklines]
376 % tls_require_ciphers = <...recommended ciphersuite...>
379 If you want to request and verify client certificates from sending hosts set
380 \begin{lstlisting}[breaklines]
381 tls_verify_certificates = /etc/pki/tls/certs/ca-bundle.crt
382 tls_try_verify_hosts = *
385 tls\_try\_verify\_hosts only reports the result to your logfile. If you want to disconnect such clients you have to use
386 \begin{lstlisting}[breaklines]
390 The cipher used is written to the logfiles by default. You may want to add
391 \begin{lstlisting}[breaklines]
392 log_selector = <....whatever your log_selector already contains...> \
393 +tls_certificate_verified +tls_peerdn +tls_sni
395 to get even more TLS information logged.
397 \paragraph*{client mode (outgoing)}\mbox{}\\
399 Exim uses opportunistic encryption in the SMTP transport by default.
401 Client mode settings have to be done in the configuration section of the smtp transport (driver = smtp).
403 If you want to use a client certificate (most server certificates can be used as client certificate, too) set
404 \begin{lstlisting}[breaklines]
405 tls_certificate = .../cert.pem
406 tls_privatekey = .../cert.key
408 This is recommended for MTA-MTA traffic.\\
410 %If you want to limit used ciphers set
411 %\begin{lstlisting}[breaklines]
412 % tls_require_ciphers = <...recommended ciphersuite...>
414 % Exim Maintainers do not recommend ciphers. We shouldn't do so, too.
415 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.
417 \paragraph*{OpenSSL}\mbox{}\\
418 Exim already disables SSLv2 by default. We recommend to add
419 \begin{lstlisting}[breaklines]
420 openssl_options = +all +no_sslv2 +no_compression +cipher_server_preference
422 to the main configuration.\\
423 Note: +all is misleading here since OpenSSL only activates the most common workarounds. But that's how SSL\_OP\_ALL is defined.\\
425 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).
426 If you want to set your own DH parameters please read the TLS documentation of exim.\\
430 \paragraph*{GnuTLS}\mbox{}\\
432 GnuTLS is different in only some respects to OpenSSL:
434 \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.
435 \item There is no option like openssl\_options
438 \paragraph*{Exim string expansion}\mbox{}\\
440 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.
442 \paragraph*{Limitations}\mbox{}\\
444 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.
445 There already is a working patch to provide support:\\
446 \url{http://bugs.exim.org/show_bug.cgi?id=1397}
448 \paragraph*{How to test}\mbox{}\\
449 \begin{lstlisting}[breaklines]
450 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
454 %% ----------------------------------------------------------------------
455 %\subsection{Exchange}
457 %\todo{FIXME: write this section}