2 \gdef\currentsectionname{MailServers}
3 This section documents the most common mail servers. Mail servers may usually be
4 grouped into three categories:
6 \item the mail submission agent (MSA)
7 \item the mail transfer agent (MTA)/mail exchanger (MX)
8 \item the mail delivery agent (MDA)
11 An e-mail client (mail user agent, MUA) submits mail to the MSA. This is usually
12 been done using the Simple Mail Transfer Protocol (SMTP). Afterwards, the mail
13 is transmitted by the MTA over the Internet to the MTA of the receiver. This
14 happens again via SMTP. Finally, the mail client of the receiver will fetch mail
15 from an MDA usually via the Internet Message Access Protocol (IMAP) or the Post
16 Office Protocol (POP).
18 As MSAs and MTAs both use SMTP as transfer protocols, both functionalities may
19 often be implemented with the same software. On the other hand, MDA software
20 might or might not implement both IMAP and POP.
22 %% ----------------------------------------------------------------------
23 \subsection{TLS usage in mail server protocols}
25 E-mail protocols support TLS in two different ways. It may be added as a
26 protocol wrapper on a different port. This method is referred to as Implicit TLS
27 or as protocol variants SMTPS, IMAPS and POP3S. The other method is to establish
28 a cleartext session first and switch to TLS afterwards by issuing the STARTTLS
31 SMTP between MTAs usually makes use of opportunistic TLS. This means that an
32 MTA will accept TLS connections when asked for it but will not require it.
33 MTAs should always try opportunistic TLS handshakes outgoing and always accept
34 incoming opportunistic TLS.
36 %% ----------------------------------------------------------------------
37 \subsection{Recommended configuration}
38 % TODO: Check configurations for compliance
40 We recommend to use the following settings for Mail Transfer Agents:
42 \item correctly setup MX, A and PTR RRs without using CNAMEs at all.
43 \item the hostname used as HELO/EHLO in outgoing mail shall match the PTR RR
44 \item enable opportunistic TLS, using the STARTTLS mechanism on port 25
45 \item Implicit TLS on port 465 may be offered additionally
46 \item use server and client certificates (most server certificates are client
48 \item either the common name or at least an alternate subject name of the
49 certificate shall match the PTR RR (client mode) or the MX RR (server mode)
50 \item do not use self signed certificates
51 \item accept all cipher suites, as the alternative would be to fall back to
52 cleartext transmission
53 \item an exception to the last sentence is that MTAs \textit{MUST NOT}
54 enable SSLv2 protocol support, due to the DROWN attack\footnote{\url{https://drownattack.com/drown-attack-paper.pdf}}.
57 For MSA operation we recommend:
59 \item listen on submission port 587 with mandatory STARTTLS
60 \item optionally listen on port 465 with Implicit TLS
61 \item enforce SMTP AUTH even for local networks
62 \item ensure that SMTP AUTH is not allowed on unencrypted connections
63 \item only use the recommended cipher suites if all connecting MUAs support them
66 For MDA operation we recommend:
68 \item listen on the protocol port (143 for IMAP, 110 for POP3) with mandatory
70 \item optionally listen on Implicit TLS ports (993 for IMAPS, 995 for POP3S)
71 \item enforce authentication even for local networks
72 \item make sure that authentication is not allowed on unencrypted connections
73 \item use the recommended cipher suites if all connecting MUAs support them
74 \item turn off SSLv2 (DROWN attack\footnote{\url{https://drownattack.com/drown-attack-paper.pdf}})
77 %% ----------------------------------------------------------------------
81 \subsubsection{Tested with Version}
83 \item Dovecot 2.1.7, Debian Wheezy (without: ``\texttt{ssl\_prefer\_server\_ciphers}``)
84 \item Dovecot 2.2.9, Debian Jessie
85 \item Dovecot 2.2.13, Debian 8.2 Jessie
86 \item Dovecot 2.0.19apple1 on OS X Server 10.8.5 (without: ``\texttt{ssl\_prefer\_server\_ciphers}``)
87 \item Dovecot 2.2.9 on Ubuntu 14.04 trusty
88 \item Dovecot 2.2.31 on Ubuntu 16.04.3 LTS
91 \subsubsection{Settings}
92 % Example: https://dovecot.org/list/dovecot/2013-October/092999.html
94 \configfile{10-ssl.conf}{48-64}{Dovecot SSL configuration}
96 \subsubsection{Additional info}
97 Dovecot 2.0, 2.1: Almost as good as dovecot 2.2. Dovecot does not ignore unknown configuration parameters. Does not support
98 ssl\_prefer\_server\_ciphers
100 \subsubsection{Limitations}
102 \item Dovecot <2.2.14 does not support disabling TLS compression.\\
103 {\(\geq\)2.2.14}\footnote{\url{https://www.dovecot.org/doc/NEWS-2.2}}
104 use: \texttt{ssl\_options = no\_compression}
105 \item Dovecot <2.2.7 uses fixed DH parameters.\\
106 {\(\geq\)2.2.7}\footnote{\url{https://hg.dovecot.org/dovecot-2.2/rev/43ab5abeb8f0}}
107 greater DH-Parameters are supported: \texttt{ssl\_dh\_parameters\_length = 2048}.
110 %\subsubsection{Justification for special settings (if needed)}
112 % 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
114 \subsubsection{References}
116 \item \url{https://wiki2.dovecot.org/SSL}
119 % add any further references or best practice documents here
121 \subsubsection{How to test}
122 % 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.
124 openssl s_client -crlf -connect SERVER.TLD:993
125 openssl s_client -crlf -connect SERVER.TLD:995
126 openssl s_client -crlf -starttls imap -connect SERVER.TLD:143
127 openssl s_client -crlf -starttls pop3 -connect SERVER.TLD:110
130 SSLyze\footnote{\url{https://github.com/nabla-c0d3/sslyze/releases}} offers scanning for common vulnerabilities and displays Protocols and Cipher-Suites.
132 sslyze.exe --regular SERVER.TLD:993
133 sslyze.exe --regular SERVER.TLD:995
134 sslyze.exe --regular --starttls=imap SERVER.TLD:143
135 sslyze.exe --regular --starttls=pop3 SERVER.TLD:110
140 %% ----------------------------------------------------------------------
141 \subsection{cyrus-imapd}
142 \subsubsection{Tested with Versions}
147 \subsubsection{Settings}
149 To activate SSL/TLS configure your certificate with
150 \configfile{imapd.conf}{206-206,209-209}{Activating TLS in cyrus}
152 Do not forget to add necessary intermediate certificates to the .pem file.
154 Limiting the ciphers provided may force (especially older) clients to connect without encryption at all! Sticking to the defaults is recommended.
156 If you still want to force strong encryption use
157 \configfile{imapd.conf}{263-263}{TLS cipher selection in cyrus}
159 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.
161 To prevent unencrypted connections on the STARTTLS ports you can set
162 \configfile{imapd.conf}{131-131}{Force encrypted connections in cyrus}
163 This way MUAs can only authenticate with plain text authentication schemes after issuing the STARTTLS command. Providing CRAM-MD5 or DIGEST-MD5 methods is not recommended.
165 To support POP3/IMAP on ports 110/143 with STARTTLS and POP3S/IMAPS on ports
166 995/993 check the SERVICES section in \texttt{cyrus.conf}
167 \configfile{cyrus.conf}{28-28,31-34,71-71}{STARTTLS for POP3/IMAP and POP3S/IMAPS in cyrus}
170 \subsubsection{Limitations}
171 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.
173 Currently there is no way to prefer server ciphers or to disable compression.
175 There is a working patch for all three features:
176 \url{https://bugzilla.cyrusimap.org/show_bug.cgi?id=3823}
178 \subsubsection{How to test}
180 openssl s_client -crlf -connect SERVER.TLD:993
183 % XXX config von Adi?
185 % ciphers = EDH+CAMELLIA256:EDH+aRSA:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:-AES128:!CAMELLIA128:!ECDSA:AES256-SHA:EDH+AES128;
186 % options = CIPHER_SERVER_PREFERENCE
190 %% ----------------------------------------------------------------------
193 \subsubsection{Tested with Versions}
195 \item Postfix 2.9.6, Debian Wheezy with OpenSSL 1.0.1e
196 \item Postfix 2.11.0 on Ubuntu 14.04.02 with OpenSSL 1.0.1f
197 \item Postfix 3.1.0 on Ubuntu 16.04.3 LTS
201 \subsubsection{Settings}
203 Postfix has five internal lists of ciphers, and the possibility to switch
204 between those with \emph{smtpd\_tls\_ciphers}. However, we leave this at its
205 default value for server to server connections, as many mail servers only
206 support outdated protocols and ciphers. We consider bad encryption still better
207 than plain text transmission. For connections to MUAs, TLS is mandatory and the
208 ciphersuite is modified.
210 %% I (cm) consider the generation of own DH parameters to be voodoo until
211 %% someone can explain the contrary. They are, after all, public, and
212 %% I found no research that would show that long-term use of a
213 %% parameter set would weaken the DH exchange. Also notice that IPSEC
214 %% uses fixed parameter sets only.
216 %% also notice the following comment from src/tls/tls_dh.c:
217 %% * Compiled-in EDH primes (the compiled-in generator is always 2). These are
218 %% * used when no parameters are explicitly loaded from a site-specific file.
220 %% * 512-bit parameters are used for export ciphers, and 1024-bit parameters are
221 %% * used for non-export ciphers. An ~80-bit strong EDH key exchange is really
222 %% * too weak to protect 128+ bit keys, but larger DH primes are
223 %% * computationally expensive. When greater security is required, use EECDH.
225 %% First, you need to generate Diffie Hellman parameters (please first take a look at the section \ref{section:RNGs}):
227 %% \todo{FIXME: this is a really weak setting! See also: https://postfix.1071664.n5.nabble.com/postfix-hardening-what-can-we-do-td61874.html}
228 %% \begin{lstlisting}
229 %% % openssl gendh -out /etc/postfix/dh_param_512.pem -2 512
230 %% % openssl gendh -out /etc/postfix/dh_param_1024.pem -2 1024
233 %% Next, we specify these DH parameters in \verb|main.cf|:
235 %% \begin{lstlisting}
236 %% smtpd_tls_dh512_param_file = /etc/postfix/dh_param_512.pem
237 %% smtpd_tls_dh1024_param_file = /etc/postfix/dh_param_1024.pem
240 \paragraph{MX and SMTP client configuration:}
241 As discussed in section \ref{subsection:smtp_general}, because of opportunistic
242 encryption we do not restrict the list of ciphers or protocols for communication
243 with other mail servers to avoid transmission in plain text. There are still
244 some steps needed to enable TLS, all in \verb|main.cf|:
246 \configfile{main.cf}{20-31}{Opportunistic TLS in Postfix}
249 For the MSA \verb|smtpd| process which communicates with mail clients, we first
250 define the ciphers that are acceptable for the ``mandatory'' security level,
251 again in \verb|main.cf|:
253 \configfile{main.cf}{34-44}{MSA TLS configuration in Postfix}
255 Then, we configure the MSA smtpd in \verb|master.cf| with two
256 additional options that are only used for this instance of smtpd:
258 \configfile{master.cf}{12-14}{MSA smtpd service configuration in Postfix}
260 For those users who want to use EECDH key exchange, it is possible to customize this via:
261 \configfile{main.cf}{45-45}{EECDH customization in Postfix}
262 The default value since Postfix 2.8 is ``strong''.
264 \subsubsection{Limitations}
265 tls\_ssl\_options is supported from Postfix 2.11 onwards. You can
266 leave the statement in the configuration for older versions, it will
269 tls\_preempt\_cipherlist is supported from Postfix 2.8 onwards. Again,
270 you can leave the statement in for older versions.
272 \subsubsection{References}
273 Refer to \url{http://www.postfix.org/TLS_README.html} for an in-depth
276 \subsubsection{Additional settings}
277 Postfix has two sets of built-in DH parameters that can be overridden
278 with the \verb|smtpd_tls_dh512_param_file|
279 and \verb|smtpd_tls_dh1024_param_file| options. The ``dh512''
280 parameters are used for export ciphers, while the ``dh1024'' ones are
281 used for all other ciphers.
283 The ``bit length'' in those parameter names is just a name, so one
284 could use stronger parameter sets; it should be possible to e.g. use the
285 IKE Group14 parameters (see section \ref{section:DH}) without much
286 interoperability risk, but we have not tested this yet.
288 % \subsubsection{Justification for special settings (if needed)}
289 % no special settings
292 \subsubsection{How to test}
293 You can check the effect of the settings with the following command:
295 $ 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
296 1 SSLv3 with cipher DHE-RSA-AES256-SHA
297 23 TLSv1.2 with cipher DHE-RSA-AES256-GCM-SHA384
298 60 TLSv1 with cipher ECDHE-RSA-AES256-SHA
299 270 TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384
300 335 TLSv1 with cipher DHE-RSA-AES256-SHA
304 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
307 %% ----------------------------------------------------------------------
311 \subsubsection{Tested with Versions}
313 \item Exim 4.82, Debian Jessie
314 \item Exim 4.82, Ubuntu 14.04.2 with OpenSSL 1.0.1e
318 It is highly recommended to read
319 \url{https://exim.org/exim-html-current/doc/html/spec_html/ch-encrypted_smtp_connections_using_tlsssl.html}
322 \paragraph{MSA mode (submission):}
323 In the main config section of Exim add:
324 \configfile{configure.msa}{153-154}{Certificate selection in Exim (MSA)}
325 Don't forget to add intermediate certificates to the .pem file if needed.
327 Tell Exim to advertise STARTTLS in the EHLO answer to everyone:
328 \configfile{configure.msa}{145-145}{TLS advertise in Exim (MSA)}
330 If you want to support legacy SMTPS on port 465, and STARTTLS on smtp(25)/submission(587) ports set
331 \configfile{configure.msa}{165-166}{STARTTLS and SMTPS in Exim (MSA)}
333 It is highly recommended to limit SMTP AUTH to SSL connections only. To do so add
334 \configfile{configure.msa}{813-813}{SSL-only authentication in Exim (MSA)}
335 to every authenticator defined.
337 Add the following rules on top of your acl\_smtp\_mail:
338 \configfile{configure.msa}{111-111,501-505}{Submission mode in Exim (MSA)}
339 This switches Exim to submission mode and allows addition of missing ``Message-ID'' and ``Date'' headers.
341 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.
342 % Exim maintainers do not recommend to change default ciphers
343 % I think we shouldn't, too
346 % tls_require_ciphers = <...recommended ciphersuite...>
349 The cipher used is written to the logfiles by default. You may want to add
351 log_selector = <whatever your log_selector already contains> +tls_certificate_verified +tls_peerdn +tls_sni
353 to get even more TLS information logged.
356 \paragraph{Server mode (incoming):}
357 In the main config section of Exim add:
358 \configfile{configure.server}{152-153}{Certificate selection in Exim (Server)}
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 \configfile{configure.server}{144-144}{TLS advertise in Exim (Server)}
364 Listen on smtp(25) port only
365 \configfile{configure.server}{166-166}{STARTTLS on SMTP in Exim (Server)}
367 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.
368 % Exim maintainers do not recommend to change default ciphers
372 % tls_require_ciphers = <...recommended ciphersuite...>
375 If you want to request and verify client certificates from sending hosts set
376 \configfile{configure.server}{154-155}{TLS certificate verifiaction in Exim (Server)}
378 tls\_try\_verify\_hosts only reports the result to your logfile. If you want to disconnect such clients you have to use
383 The cipher used is written to the logfiles by default. You may want to add
385 log_selector = <whatever your log_selector already contains> +tls_certificate_verified +tls_peerdn +tls_sni
387 to get even more TLS information logged.
389 \paragraph{Client mode (outgoing):}
390 Exim uses opportunistic encryption in the SMTP transport by default.
392 Client mode settings have to be done in the configuration section of the smtp transport (driver = smtp).
394 If you want to use a client certificate (most server certificates can be used as client certificate, too) set
395 \configfile{configure.client}{152-153}{Certificate selection in Exim (Client)}
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.
406 Exim already disables SSLv2 by default. We recommend to add
408 openssl_options = +all +no_sslv2 +no_sslv3 +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.
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 \paragraph{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 \paragraph{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{https://bugs.exim.org/show_bug.cgi?id=1397}
433 \subsubsection{How to test}
435 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
439 %% ----------------------------------------------------------------------
440 %\subsection{Exchange}
442 %\todo{FIXME: write this section}
444 %% ----------------------------------------------------------------------
445 \subsection{Cisco ESA/IronPort}
446 \subsubsection{Tested with Version}
450 \item AsyncOS 9.0.0, 9.5.0, 9.6.0, 9.7.0
453 \subsubsection{Settings}
454 Import your certificate(s) using the WEBUI (Network -> Certificates).
456 From AsyncOS 9.0 and up, SSL parameters for inbound SMTP, outbound SMTP and GUI access can be configured in one step via the WEBUI (System Administration -> SSL Configuration, see figure \ref{fig:ach_ironport_ssl_settings} on page \pageref{fig:ach_ironport_ssl_settings}). \\
457 For all versions prior to 9.0, you have to connect to the CLI and configure the SSL parameters separately, as shown below using inbound SMTP as example.
458 \begin{lstlisting}{foo}
459 ironport.example.com> sslconfig
461 GUI HTTPS method: sslv3tlsv1
462 GUI HTTPS ciphers: RC4-SHA:RC4-MD5:ALL
463 Inbound SMTP method: sslv3tlsv1
464 Inbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
465 Outbound SMTP method: sslv3tlsv1
466 Outbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
468 Choose the operation you want to perform:
469 - GUI - Edit GUI HTTPS ssl settings.
470 - INBOUND - Edit Inbound SMTP ssl settings.
471 - OUTBOUND - Edit Outbound SMTP ssl settings.
472 - VERIFY - Verify and show ssl cipher list.
475 Enter the inbound SMTP ssl method you want to use.
481 6. SSL v2, v3 and TLS v1
484 Enter the inbound SMTP ssl cipher you want to use.
485 [RC4-SHA:RC4-MD5:ALL]> EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA
488 GUI HTTPS method: sslv3tlsv1
489 GUI HTTPS ciphers: RC4-SHA:RC4-MD5:ALL
490 Inbound SMTP method: tlsv1
491 Inbound SMTP ciphers: EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA
492 Outbound SMTP method: sslv3tlsv1
493 Outbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
495 Note that starting with AsyncOS 9.0 SSLv3 is disabled by default, whereas the default cipher set is still \texttt{RC4-SHA:RC4-MD5:ALL} (see figure \ref{fig:ach_ironport_ssl_settings} on page \pageref{fig:ach_ironport_ssl_settings}).
499 \includegraphics[width=0.8\textwidth]{img/ach_ironport_ssl_settings.png}
500 \caption{Default SSL Settings}
501 \label{fig:ach_ironport_ssl_settings}
504 After committing these changes in the CLI, you have to activate the use of TLS in several locations.
506 For inbound connections, first select the appropriate certificate in the settings of each listener you want to have TLS enabled on (Network -> Listeners, see figure \ref{fig:ach_ironport_listener_cert} on page \pageref{fig:ach_ironport_listener_cert}). Afterwards, for each listener, configure all Mail Flow Policies which have their Connection Behavior set to ``Accept'' or ``Relay'' to at least prefer TLS (Mail Policies -> Mail Flow Policies, see figure \ref{fig:ach_ironport_mail_flow_tls} on page \pageref{fig:ach_ironport_mail_flow_tls}). \\
507 It is recommended to also enable TLS in the default Mail Flow Policy, because these settings will be inherited by newly created policies, unless specifically overwritten. \\
508 TLS can be enforced by creating a new Mail Flow Policy with TLS set to ``required'', creating a new Sender Group defining the addresses of the sending mail servers for which you want to enforce encryption (Mail Policies -> HAT Overview) and using this new Sender Group in conjunction with the newly created Mail Flow Policy.
512 \includegraphics[width=0.8\textwidth]{img/ach_ironport_listener_cert.png}
513 \caption{Listener Settings}
514 \label{fig:ach_ironport_listener_cert}
519 \includegraphics[width=0.8\textwidth]{img/ach_ironport_mail_flow_tls.png}
520 \caption{Mail Flow Policy Security Features}
521 \label{fig:ach_ironport_mail_flow_tls}
524 TLS settings for outbound connections have to be configured within the Destination Controls (Mail Policies -> Destination Controls). Choose the appropriate SSL certificate within the global settings and configure TLS to be preferred in the default profile to enable it for all outbound connections. After these two steps the Destination Control overview page should look like figure \ref{fig:ach_ironport_dest_control} on page \pageref{fig:ach_ironport_dest_control}.
525 To enforce TLS for a specific destination domain, add an entry to the Destination Control Table and set ``TLS Support'' to ``required''.
529 \includegraphics[width=0.8\textwidth]{img/ach_ironport_dest_control.png}
530 \caption{Destination Control overview}
531 \label{fig:ach_ironport_dest_control}
534 \subsubsection{Limitations}
535 All AsyncOS releases prior to version 9.5 use OpenSSL 0.9.8. Therefore TLS 1.2 is not supported in these versions and some of the suggested ciphers won't work. Starting with AsyncOS 9.5 TLS 1.2 is fully supported.\footnote{\url{https://www.cisco.com/c/dam/en/us/td/docs/security/esa/esa9-5/ESA_9-5_Release_Notes.pdf}, Changed Behaviour, page 4} You can check the supported ciphers on the CLI by using the option \texttt{verify} from within the \texttt{sslconfig} command:
536 \begin{lstlisting}{foo}
539 Enter the ssl cipher you want to verify.
540 []> EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA
542 DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(256) Mac=SHA1
543 DHE-RSA-CAMELLIA128-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(128) Mac=SHA1
544 DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1
545 DHE-RSA-AES128-SHA SSLv3 Kx=DH Au=RSA Enc=AES(128) Mac=SHA1
546 CAMELLIA128-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(128) Mac=SHA1
547 AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1
550 \subsubsection{How to test}
552 openssl s_client -starttls smtp -crlf -connect SERVER.TLD:25
555 \FloatBarrier % the preceding section has several figures. Floating them too far away might get confusing for readers.