liable to incompatible change.
-PRDR support
---------------------------------------------------------------
-
-Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall
-in a (now-expired) IETF draft from 2007. It's not hit mainstream
-use, but has apparently been implemented in the META1 MTA.
-
-There is mention at http://mail.aegee.org/intern/sendmail.html
-of a patch to sendmail "to make it PRDR capable".
-
- ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt
-
-If Exim is built with EXPERIMENTAL_PRDR there is a new config
-boolean "prdr_enable" which controls whether PRDR is advertised
-as part of an EHLO response, a new "acl_data_smtp_prdr" ACL
-(called for each recipient, after data arrives but before the
-data ACL), and a new smtp transport option "hosts_try_prdr".
-
-PRDR may be used to support per-user content filtering. Without it
-one must defer any recipient after the first that has a different
-content-filter configuration. With PRDR, the RCPT-time check
-for this can be disabled when the MAIL-time $smtp_command included
-"PRDR". Any required difference in behaviour of the main DATA-time
-ACL should however depend on the PRDR-time ACL having run, as Exim
-will avoid doing so in some situations (eg. single-recipient mails).
-
-
-
-OCSP Stapling support
---------------------------------------------------------------
-
-X.509 PKI certificates expire and can be revoked; to handle this, the
-clients need some way to determine if a particular certificate, from a
-particular Certificate Authority (CA), is still valid. There are three
-main ways to do so.
-
-The simplest way is to serve up a Certificate Revocation List (CRL) with
-an ordinary web-server, regenerating the CRL before it expires. The
-downside is that clients have to periodically re-download a potentially
-huge file from every certificate authority it knows of.
-
-The way with most moving parts at query time is Online Certificate
-Status Protocol (OCSP), where the client verifies the certificate
-against an OCSP server run by the CA. This lets the CA track all
-usage of the certs. This requires running software with access to the
-private key of the CA, to sign the responses to the OCSP queries. OCSP
-is based on HTTP and can be proxied accordingly.
-
-The only widespread OCSP server implementation (known to this writer)
-comes as part of OpenSSL and aborts on an invalid request, such as
-connecting to the port and then disconnecting. This requires
-re-entering the passphrase each time some random client does this.
-
-The third way is OCSP Stapling; in this, the server using a certificate
-issued by the CA periodically requests an OCSP proof of validity from
-the OCSP server, then serves it up inline as part of the TLS
-negotiation. This approach adds no extra round trips, does not let the
-CA track users, scales well with number of certs issued by the CA and is
-resilient to temporary OCSP server failures, as long as the server
-starts retrying to fetch an OCSP proof some time before its current
-proof expires. The downside is that it requires server support.
-
-If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
-then it gains a new global option: "tls_ocsp_file".
-
-The file specified therein is expected to be in DER format, and contain
-an OCSP proof. Exim will serve it as part of the TLS handshake. This
-option will be re-expanded for SNI, if the tls_certificate option
-contains $tls_sni, as per other TLS options.
-
-Exim does not at this time implement any support for fetching a new OCSP
-proof. The burden is on the administrator to handle this, outside of
-Exim. The file specified should be replaced atomically, so that the
-contents are always valid. Exim will expand the "tls_ocsp_file" option
-on each connection, so a new file will be handled transparently on the
-next connection.
-
-Exim will check for a valid next update timestamp in the OCSP proof;
-if not present, or if the proof has expired, it will be ignored.
-
-Also, given EXPERIMENTAL_OCSP and OpenSSL, the smtp transport gains
-a "hosts_require_ocsp" option; a host-list for which an OCSP Stapling
-is requested and required for the connection to proceed. The host(s)
-should also be in "hosts_require_tls", and "tls_verify_certificates"
-configured for the transport.
-
-For the client to be able to verify the stapled OCSP the server must
-also supply, in its stapled information, any intermediate
-certificates for the chain leading to the OCSP proof from the signer
-of the server certificate. There may be zero or one such. These
-intermediate certificates should be added to the server OCSP stapling
-file (named by tls_ocsp_file).
-
-At this point in time, we're gathering feedback on use, to determine if
-it's worth adding complexity to the Exim daemon to periodically re-fetch
-OCSP files and somehow handling multiple files.
-
- A helper script "ocsp_fetch.pl" for fetching a proof from a CA
- OCSP server is supplied. The server URL may be included in the
- server certificate, if the CA is helpful.
-
- One fail mode seen was the OCSP Signer cert expiring before the end
- of vailidity of the OCSP proof. The checking done by Exim/OpenSSL
- noted this as invalid overall, but the re-fetch script did not.
-
-
-
-
Brightmail AntiSpam (BMI) suppport
--------------------------------------------------------------
This means the queried domain has published
a SPF record, but wants to allow outside
servers to send mail under its domain as well.
- o err_perm This indicates a syntax error in the SPF
- record of the queried domain. This should be
- treated like "none".
- o err_temp This indicates a temporary error during all
+ This should be treated like "none".
+ o permerror This indicates a syntax error in the SPF
+ record of the queried domain. You may deny
+ messages when this occurs. (Changed in 4.83)
+ o temperror This indicates a temporary error during all
processing, including Exim's SPF processing.
You may defer messages when this occurs.
+ (Changed in 4.83)
+ o err_temp Same as permerror, deprecated in 4.83, will be
+ removed in a future release.
+ o err_perm Same as temperror, deprecated in 4.83, will be
+ removed in a future release.
You can prefix each string with an exclamation mark to invert
-is meaning, for example "!fail" will match all results but
+its meaning, for example "!fail" will match all results but
"fail". The string list is evaluated left-to-right, in a
short-circuit fashion. When a string matches the outcome of
the SPF check, the condition succeeds. If none of the listed
$spf_result
This contains the outcome of the SPF check in string form,
- one of pass, fail, softfail, none, neutral, err_perm or
- err_temp.
+ one of pass, fail, softfail, none, neutral, permerror or
+ temperror.
$spf_smtp_comment
This contains a string that can be used in a SMTP response
rejecting the email.
o quarantine The DMARC check failed and the library recommends
keeping it for further inspection.
+ o none The DMARC check passed and the library recommends
+ no specific action, neutral.
o norecord No policy section in the DMARC record for this
sender domain.
o nofrom Unable to determine the domain of the sender.
- o none There is no DMARC record for this sender domain.
- o error Library error or dns error.
+ o temperror Library error or dns error.
+ o off The DMARC check was disabled for this email.
You can prefix each string with an exclamation mark to invert its
meaning, for example "!accept" will match all results but
Of course, you can also use any other lookup method that Exim
supports, including LDAP, Postgres, MySQL, etc, as long as the
-result is a list of colon-separated strings;
+result is a list of colon-separated strings.
Several expansion variables are set before the DATA ACL is
processed, and you can use them in this ACL. The following
o $dmarc_status
This is a one word status indicating what the DMARC library
- thinks of the email.
+ thinks of the email. It is a combination of the results of
+ DMARC record lookup and the SPF/DKIM/DMARC processing results
+ (if a DMARC record was found). The actual policy declared
+ in the DMARC record is in a separate expansion variable.
o $dmarc_status_text
This is a slightly longer, human readable status.
This is the domain which DMARC used to look up the DMARC
policy record.
+ o $dmarc_domain_policy
+ This is the policy declared in the DMARC record. Valid values
+ are "none", "reject" and "quarantine". It is blank when there
+ is any error, including no DMARC record.
+
o $dmarc_ar_header
This is the entire Authentication-Results header which you can
add using an add_header modifier.
warn !domains = +screwed_up_dmarc_records
control = dmarc_enable_forensic
+ warn condition = (lookup if destined to mailing list)
+ set acl_m_mailing_list = 1
+
(DATA ACL)
warn dmarc_status = accept : none : off
!authenticated = *
set $acl_m_quarantine = 1
# Do something in a transport with this flag variable
+ deny condition = ${if eq{$dmarc_domain_policy}{reject}}
+ condition = ${if eq{$acl_m_mailing_list}{1}}
+ message = Messages from $dmarc_used_domain break mailing lists
+
deny dmarc_status = reject
!authenticated = *
message = Message from $domain_used_domain failed sender's DMARC policy, REJECT
-Transport post-delivery actions
+Event Actions
--------------------------------------------------------------
-An arbitrary per-transport string can be expanded on successful delivery,
-and (for SMTP transports) a second string on deferrals caused by a host error.
+(Renamed from TPDA, Transport post-delivery actions)
+
+An arbitrary per-transport string can be expanded upon various transport events.
+Additionally a main-section configuration option can be expanded on some
+per-message events.
This feature may be used, for example, to write exim internal log information
(not available otherwise) into a database.
-In order to use the feature, you must set
+In order to use the feature, you must compile with
-EXPERIMENTAL_TPDA=yes
+EXPERIMENTAL_EVENT=yes
in your Local/Makefile
-and define the expandable strings in the runtime config file, to
-be executed at end of delivery.
+and define one or both of
+- the event_action option in the transport
+- the event_action main option
+to be expanded when the event fires.
+
+A new variable, $event_name, is set to the event type when the
+expansion is done. The current list of events is:
-Additionally, there are 6 more variables, available at end of
-delivery:
+ msg:complete after main per message
+ msg:delivery after transport per recipient
+ msg:host:defer after transport per attempt
+ msg:fail:delivery after main per recipient
+ msg:fail:internal after main per recipient
+ tcp:connect before transport per connection
+ tcp:close after transport per connection
+ tls:cert before both per certificate in verification chain
+ smtp:connect after transport per connection
-tpda_delivery_ip IP of host, which has accepted delivery
-tpda_delivery_port Port of remote host which has accepted delivery
-tpda_delivery_fqdn FQDN of host, which has accepted delivery
-tpda_delivery_local_part local part of address being delivered
-tpda_delivery_domain domain part of address being delivered
-tpda_delivery_confirmation SMTP confirmation message
+The expansion is called for all event types, and should use the $event_name
+value to decide when to act. The variable data is a colon-separated
+list, describing an event tree.
-In case of a deferral caused by a host-error:
-tpda_defer_errno Error number
-tpda_defer_errstr Error string possibly containing more details
+There is an auxilary variable, $event_data, for which the
+content is event_dependent:
-The $router_name and $transport_name variables are also usable.
+ msg:delivery smtp confirmation mssage
+ msg:host:defer error string
+ tls:cert verification chain depth
+ smtp:connect smtp banner
+The msg:host:defer event populates one extra variable, $event_defer_errno.
-To take action after successful deliveries, set the following option
-on any transport of interest.
+The following variables are likely to be useful depending on the event type:
+
+ router_name, transport_name
+ local_part, domain
+ host, host_address, host_port
+ tls_out_peercert
+ lookup_dnssec_authenticated, tls_out_dane
+ sending_ip_address, sending_port
+ message_exim_id, verify_mode
-tpda_delivery_action
An example might look like:
-tpda_delivery_action = \
-${lookup pgsql {SELECT * FROM record_Delivery( \
+event_action = ${if eq {msg:delivery}{$event_name} \
+{${lookup pgsql {SELECT * FROM record_Delivery( \
'${quote_pgsql:$sender_address_domain}',\
'${quote_pgsql:${lc:$sender_address_local_part}}', \
- '${quote_pgsql:$tpda_delivery_domain}', \
- '${quote_pgsql:${lc:$tpda_delivery_local_part}}', \
- '${quote_pgsql:$tpda_delivery_ip}', \
- '${quote_pgsql:${lc:$tpda_delivery_fqdn}}', \
- '${quote_pgsql:$message_exim_id}')}}
-
-The string is expanded after the delivery completes and any
-side-effects will happen. The result is then discarded.
+ '${quote_pgsql:$domain}', \
+ '${quote_pgsql:${lc:$local_part}}', \
+ '${quote_pgsql:$host_address}', \
+ '${quote_pgsql:${lc:$host}}', \
+ '${quote_pgsql:$message_exim_id}')}} \
+} {}}
+
+The string is expanded when each of the supported events occur
+and any side-effects of the expansion will happen.
Note that for complex operations an ACL expansion can be used.
-In order to log host deferrals, add the following option to an SMTP
-transport:
-
-tpda_host_defer_action
-
-This is a private option of the SMTP transport. It is intended to
-log failures of remote hosts. It is executed only when exim has
-attempted to deliver a message to a remote host and failed due to
-an error which doesn't seem to be related to the individual
-message, sender, or recipient address.
-See section 45.2 of the exim documentation for more details on how
-this is determined.
-
-Example:
-
-tpda_host_defer_action = \
-${lookup mysql {insert into delivlog set \
- msgid = '${quote_mysql:$message_exim_id}', \
- senderlp = '${quote_mysql:${lc:$sender_address_local_part}}', \
- senderdom = '${quote_mysql:$sender_address_domain}', \
- delivlp = '${quote_mysql:${lc:$tpda_delivery_local_part}}', \
- delivdom = '${quote_mysql:$tpda_delivery_domain}', \
- delivip = '${quote_mysql:$tpda_delivery_ip}', \
- delivport = '${quote_mysql:$tpda_delivery_port}', \
- delivfqdn = '${quote_mysql:$tpda_delivery_fqdn}', \
- deliverrno = '${quote_mysql:$tpda_defer_errno}', \
- deliverrstr = '${quote_mysql:$tpda_defer_errstr}' \
- }}
+The expansion of the event_action option should normally
+return an empty string. Should it return anything else the
+following will be forced:
+
+ msg:delivery (ignored)
+ msg:host:defer (ignored)
+ msg:fail:delivery (ignored)
+ tcp:connect do not connect
+ tcp:close (ignored)
+ tls:cert refuse verification
+ smtp:connect close connection
+
+No other use is made of the result string.
+
+
+Known issues:
+- the tls:cert event is only called for the cert chain elements
+ received over the wire, with GnuTLS. OpenSSL gives the entire
+ chain including thse loaded locally.
+
+
+Redis Lookup
+--------------------------------------------------------------
+
+Redis is open source advanced key-value data store. This document
+does not explain the fundamentals, you should read and understand how
+it works by visiting the website at http://www.redis.io/.
+
+Redis lookup support is added via the hiredis library. Visit:
+
+ https://github.com/redis/hiredis
+
+to obtain a copy, or find it in your operating systems package repository.
+If building from source, this description assumes that headers will be in
+/usr/local/include, and that the libraries are in /usr/local/lib.
+
+1. In order to build exim with Redis lookup support add
+
+EXPERIMENTAL_REDIS=yes
+
+to your Local/Makefile. (Re-)build/install exim. exim -d should show
+Experimental_Redis in the line "Support for:".
+
+EXPERIMENTAL_REDIS=yes
+LDFLAGS += -lhiredis
+# CFLAGS += -I/usr/local/include
+# LDFLAGS += -L/usr/local/lib
+
+The first line sets the feature to include the correct code, and
+the second line says to link the hiredis libraries into the
+exim binary. The commented out lines should be uncommented if you
+built hiredis from source and installed in the default location.
+Adjust the paths if you installed them elsewhere, but you do not
+need to uncomment them if an rpm (or you) installed them in the
+package controlled locations (/usr/include and /usr/lib).
+
+
+2. Use the following global settings to configure Redis lookup support:
+
+Required:
+redis_servers This option provides a list of Redis servers
+ and associated connection data, to be used in
+ conjunction with redis lookups. The option is
+ only available if Exim is configured with Redis
+ support.
+
+For example:
+
+redis_servers = 127.0.0.1/10/ - using database 10 with no password
+redis_servers = 127.0.0.1//password - to make use of the default database of 0 with a password
+redis_servers = 127.0.0.1// - for default database of 0 with no password
+
+3. Once you have the Redis servers defined you can then make use of the
+experimental Redis lookup by specifying ${lookup redis{}} in a lookup query.
+
+4. Example usage:
+
+(Host List)
+hostlist relay_from_ips = <\n ${lookup redis{SMEMBERS relay_from_ips}}
+
+Where relay_from_ips is a Redis set which contains entries such as "192.168.0.0/24" "10.0.0.0/8" and so on.
+The result set is returned as
+192.168.0.0/24
+10.0.0.0/8
+..
+.
+
+(Domain list)
+domainlist virtual_domains = ${lookup redis {HGET $domain domain}}
+
+Where $domain is a hash which includes the key 'domain' and the value '$domain'.
+
+(Adding or updating an existing key)
+set acl_c_spammer = ${if eq{${lookup redis{SPAMMER_SET}}}{OK}}
+
+Where SPAMMER_SET is a macro and it is defined as
+
+"SET SPAMMER <some_value>"
+
+(Getting a value from Redis)
+
+set acl_c_spam_host = ${lookup redis{GET...}}
+
+
+Proxy Protocol Support
+--------------------------------------------------------------
+
+Exim now has Experimental "Proxy Protocol" support. It was built on
+specifications from:
+http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt
+Above URL revised May 2014 to change version 2 spec:
+http://git.1wt.eu/web?p=haproxy.git;a=commitdiff;h=afb768340c9d7e50d8e
+
+The purpose of this function is so that an application load balancer,
+such as HAProxy, can sit in front of several Exim servers and Exim
+will log the IP that is connecting to the proxy server instead of
+the IP of the proxy server when it connects to Exim. It resets the
+$sender_address_host and $sender_address_port to the IP:port of the
+connection to the proxy. It also re-queries the DNS information for
+this new IP address so that the original sender's hostname and IP
+get logged in the Exim logfile. There is no logging if a host passes or
+fails Proxy Protocol negotiation, but it can easily be determined and
+recorded in an ACL (example is below).
+
+1. To compile Exim with Proxy Protocol support, put this in
+Local/Makefile:
+
+EXPERIMENTAL_PROXY=yes
+
+2. Global configuration settings:
+
+proxy_required_hosts = HOSTLIST
+
+The proxy_required_hosts option will require any IP in that hostlist
+to use Proxy Protocol. The specification of Proxy Protocol is very
+strict, and if proxy negotiation fails, Exim will not allow any SMTP
+command other than QUIT. (See end of this section for an example.)
+The option is expanded when used, so it can be a hostlist as well as
+string of IP addresses. Since it is expanded, specifying an alternate
+separator is supported for ease of use with IPv6 addresses.
+
+To log the IP of the proxy in the incoming logline, add:
+ log_selector = +proxy
+
+A default incoming logline (wrapped for appearance) will look like this:
+
+ 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
+ H=mail.example.net [1.2.3.4] P=esmtp S=433
+
+With the log selector enabled, an email that was proxied through a
+Proxy Protocol server at 192.168.1.2 will look like this:
+
+ 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
+ H=mail.example.net [1.2.3.4] P=esmtp PRX=192.168.1.2 S=433
+
+3. In the ACL's the following expansion variables are available.
+
+proxy_host_address The (internal) src IP of the proxy server
+ making the connection to the Exim server.
+proxy_host_port The (internal) src port the proxy server is
+ using to connect to the Exim server.
+proxy_target_address The dest (public) IP of the remote host to
+ the proxy server.
+proxy_target_port The dest port the remote host is using to
+ connect to the proxy server.
+proxy_session Boolean, yes/no, the connected host is required
+ to use Proxy Protocol.
+
+There is no expansion for a failed proxy session, however you can detect
+it by checking if $proxy_session is true but $proxy_host is empty. As
+an example, in my connect ACL, I have:
+
+ warn condition = ${if and{ {bool{$proxy_session}} \
+ {eq{$proxy_host_address}{}} } }
+ log_message = Failed required proxy protocol negotiation \
+ from $sender_host_name [$sender_host_address]
+
+ warn condition = ${if and{ {bool{$proxy_session}} \
+ {!eq{$proxy_host_address}{}} } }
+ # But don't log health probes from the proxy itself
+ condition = ${if eq{$proxy_host_address}{$sender_host_address} \
+ {false}{true}}
+ log_message = Successfully proxied from $sender_host_name \
+ [$sender_host_address] through proxy protocol \
+ host $proxy_host_address
+
+ # Possibly more clear
+ warn logwrite = Remote Source Address: $sender_host_address:$sender_host_port
+ logwrite = Proxy Target Address: $proxy_target_address:$proxy_target_port
+ logwrite = Proxy Internal Address: $proxy_host_address:$proxy_host_port
+ logwrite = Internal Server Address: $received_ip_address:$received_port
+
+
+4. Recommended ACL additions:
+ - Since the real connections are all coming from your proxy, and the
+ per host connection tracking is done before Proxy Protocol is
+ evaluated, smtp_accept_max_per_host must be set high enough to
+ handle all of the parallel volume you expect per inbound proxy.
+ - With the smtp_accept_max_per_host set so high, you lose the ability
+ to protect your server from massive numbers of inbound connections
+ from one IP. In order to prevent your server from being DOS'd, you
+ need to add a per connection ratelimit to your connect ACL. I
+ suggest something like this:
+
+ # Set max number of connections per host
+ LIMIT = 5
+ # Or do some kind of IP lookup in a flat file or database
+ # LIMIT = ${lookup{$sender_host_address}iplsearch{/etc/exim/proxy_limits}}
+
+ defer message = Too many connections from this IP right now
+ ratelimit = LIMIT / 5s / per_conn / strict
+
+
+5. Runtime issues to be aware of:
+ - The proxy has 3 seconds (hard-coded in the source code) to send the
+ required Proxy Protocol header after it connects. If it does not,
+ the response to any commands will be:
+ "503 Command refused, required Proxy negotiation failed"
+ - If the incoming connection is configured in Exim to be a Proxy
+ Protocol host, but the proxy is not sending the header, the banner
+ does not get sent until the timeout occurs. If the sending host
+ sent any input (before the banner), this causes a standard Exim
+ synchronization error (i.e. trying to pipeline before PIPELINING
+ was advertised).
+ - This is not advised, but is mentioned for completeness if you have
+ a specific internal configuration that you want this: If the Exim
+ server only has an internal IP address and no other machines in your
+ organization will connect to it to try to send email, you may
+ simply set the hostlist to "*", however, this will prevent local
+ mail programs from working because that would require mail from
+ localhost to use Proxy Protocol. Again, not advised!
+
+6. Example of a refused connection because the Proxy Protocol header was
+not sent from a host configured to use Proxy Protocol. In the example,
+the 3 second timeout occurred (when a Proxy Protocol banner should have
+been sent), the banner was displayed to the user, but all commands are
+rejected except for QUIT:
+
+# nc mail.example.net 25
+220-mail.example.net, ESMTP Exim 4.82+proxy, Mon, 04 Nov 2013 10:45:59
+220 -0800 RFC's enforced
+EHLO localhost
+503 Command refused, required Proxy negotiation failed
+QUIT
+221 mail.example.net closing connection
+
+
+
+
+DANE
+------------------------------------------------------------
+DNS-based Authentication of Named Entities, as applied
+to SMTP over TLS, provides assurance to a client that
+it is actually talking to the server it wants to rather
+than some attacker operating a Man In The Middle (MITM)
+operation. The latter can terminate the TLS connection
+you make, and make another one to the server (so both
+you and the server still think you have an encrypted
+connection) and, if one of the "well known" set of
+Certificate Authorities has been suborned - something
+which *has* been seen already (2014), a verifiable
+certificate (if you're using normal root CAs, eg. the
+Mozilla set, as your trust anchors).
+
+What DANE does is replace the CAs with the DNS as the
+trust anchor. The assurance is limited to a) the possibility
+that the DNS has been suborned, b) mistakes made by the
+admins of the target server. The attack surface presented
+by (a) is thought to be smaller than that of the set
+of root CAs.
+
+It also allows the server to declare (implicitly) that
+connections to it should use TLS. An MITM could simply
+fail to pass on a server's STARTTLS.
+
+DANE scales better than having to maintain (and
+side-channel communicate) copies of server certificates
+for every possible target server. It also scales
+(slightly) better than having to maintain on an SMTP
+client a copy of the standard CAs bundle. It also
+means not having to pay a CA for certificates.
+
+DANE requires a server operator to do three things:
+1) run DNSSEC. This provides assurance to clients
+that DNS lookups they do for the server have not
+been tampered with. The domain MX record applying
+to this server, its A record, its TLSA record and
+any associated CNAME records must all be covered by
+DNSSEC.
+2) add TLSA DNS records. These say what the server
+certificate for a TLS connection should be.
+3) offer a server certificate, or certificate chain,
+in TLS connections which is traceable to the one
+defined by (one of?) the TSLA records
+
+There are no changes to Exim specific to server-side
+operation of DANE.
+
+The TLSA record for the server may have "certificate
+usage" of DANE-TA(2) or DANE-EE(3). The latter specifies
+the End Entity directly, i.e. the certificate involved
+is that of the server (and should be the sole one transmitted
+during the TLS handshake); this is appropriate for a
+single system, using a self-signed certificate.
+ DANE-TA usage is effectively declaring a specific CA
+to be used; this might be a private CA or a public,
+well-known one. A private CA at simplest is just
+a self-signed certificate which is used to sign
+cerver certificates, but running one securely does
+require careful arrangement. If a private CA is used
+then either all clients must be primed with it, or
+(probably simpler) the server TLS handshake must transmit
+the entire certificate chain from CA to server-certificate.
+If a public CA is used then all clients must be primed with it
+(losing one advantage of DANE) - but the attack surface is
+reduced from all public CAs to that single CA.
+DANE-TA is commonly used for several services and/or
+servers, each having a TLSA query-domain CNAME record,
+all of which point to a single TLSA record.
+
+The TLSA record should have a Selector field of SPKI(1)
+and a Matching Type field of SHA2-512(2).
+
+At the time of writing, https://www.huque.com/bin/gen_tlsa
+is useful for quickly generating TLSA records; and commands like
+
+ openssl x509 -in -pubkey -noout <certificate.pem \
+ | openssl rsa -outform der -pubin 2>/dev/null \
+ | openssl sha512 \
+ | awk '{print $2}'
+
+are workable for 4th-field hashes.
+
+For use with the DANE-TA model, server certificates
+must have a correct name (SubjectName or SubjectAltName).
+
+The use of OCSP-stapling should be considered, allowing
+for fast revocation of certificates (which would otherwise
+be limited by the DNS TTL on the TLSA records). However,
+this is likely to only be usable with DANE-TA. NOTE: the
+default of requesting OCSP for all hosts is modified iff
+DANE is in use, to:
+
+ hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \
+ {= {4}{$tls_out_tlsa_usage}} } \
+ {*}{}}
+
+The (new) variable $tls_out_tlsa_usage is a bitfield with
+numbered bits set for TLSA record usage codes.
+The zero above means DANE was not in use,
+the four means that only DANE-TA usage TLSA records were
+found. If the definition of hosts_request_ocsp includes the
+string "tls_out_tlsa_usage", they are re-expanded in time to
+control the OCSP request.
+
+This modification of hosts_request_ocsp is only done if
+it has the default value of "*". Admins who change it, and
+those who use hosts_require_ocsp, should consider the interaction
+with DANE in their OCSP settings.
+
+
+For client-side DANE there are two new smtp transport options,
+hosts_try_dane and hosts_require_dane. They do the obvious thing.
+[ should they be domain-based rather than host-based? ]
+
+DANE will only be usable if the target host has DNSSEC-secured
+MX, A and TLSA records.
+
+A TLSA lookup will be done if either of the above options match
+and the host-lookup succeded using dnssec.
+If a TLSA lookup is done and succeeds, a DANE-verified TLS connection
+will be required for the host.
+
+(TODO: specify when fallback happens vs. when the host is not used)
+
+If DANE is requested and useable (see above) the following transport
+options are ignored:
+ hosts_require_tls
+ tls_verify_hosts
+ tls_try_verify_hosts
+ tls_verify_certificates
+ tls_crl
+ tls_verify_cert_hostnames
+
+If DANE is not usable, whether requested or not, and CA-anchored
+verification evaluation is wanted, the above variables should be set
+appropriately.
+
+Currently dnssec_request_domains must be active (need to think about that)
+and dnssec_require_domains is ignored.
+
+If verification was successful using DANE then the "CV" item
+in the delivery log line will show as "CV=dane".
+
+There is a new variable $tls_out_dane which will have "yes" if
+verification succeeded using DANE and "no" otherwise (only useful
+in combination with EXPERIMENTAL_EVENT), and a new variable
+$tls_out_tlsa_usage (detailed above).
+
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