. Update the Copyright year (only) when changing content.
. /////////////////////////////////////////////////////////////////////////////
-.set previousversion "4.90"
+.set previousversion "4.91"
.include ./local_params
.set ACL "access control lists (ACLs)"
.set I " "
.macro copyyear
-2017
+2018
.endmacro
. /////////////////////////////////////////////////////////////////////////////
.section "FTP and web sites" "SECID2"
.cindex "web site"
.cindex "FTP site"
-The primary site for Exim source distributions is currently the University of
-Cambridge's FTP site, whose contents are described in &'Where to find the Exim
-distribution'& below. In addition, there is a web site and an FTP site at
-&%exim.org%&. These are now also hosted at the University of Cambridge. The
-&%exim.org%& site was previously hosted for a number of years by Energis
-Squared, formerly Planet Online Ltd, whose support I gratefully acknowledge.
+The primary site for Exim source distributions is the &%exim.org%& FTP site,
+available over HTTPS, HTTP and FTP. These services, and the &%exim.org%&
+website, are hosted at the University of Cambridge.
.cindex "wiki"
.cindex "FAQ"
online information is the Exim wiki (&url(http://wiki.exim.org)),
which contains what used to be a separate FAQ, as well as various other
examples, tips, and know-how that have been contributed by Exim users.
+The wiki site should always redirect to the correct place, which is currently
+provided by GitHub, and is open to editing by anyone with a GitHub account.
.cindex Bugzilla
An Exim Bugzilla exists at &url(https://bugs.exim.org). You can use
this to report bugs, and also to add items to the wish list. Please search
first to check that you are not duplicating a previous entry.
-
+Please do not ask for configuration help in the bug-tracker.
.section "Mailing lists" "SECID3"
.section "Where to find the Exim distribution" "SECTavail"
.cindex "FTP site"
+.cindex "HTTPS download site"
.cindex "distribution" "ftp site"
-The master ftp site for the Exim distribution is
+.cindex "distribution" "https site"
+The master distribution site for the Exim distribution is
.display
-&*ftp://ftp.exim.org/pub/exim*&
+&*https://downloads.exim.org/*&
.endd
-The file references that follow are relative to the &_exim_& directories at
-these sites. There are now quite a number of independent mirror sites around
+The service is available over HTTPS, HTTP and FTP.
+We encourage people to migrate to HTTPS.
+
+The content served at &'https://downloads.exim.org/'& is identical to the
+content served at &'https://ftp.exim.org/pub/exim'& and
+&'ftp://ftp.exim.org/pub/exim'&.
+
+If accessing via a hostname containing &'ftp'&, then the file references that
+follow are relative to the &_exim_& directories at these sites.
+If accessing via the hostname &'downloads'& then the subdirectories described
+here are top-level directories.
+
+There are now quite a number of independent mirror sites around
the world. Those that I know about are listed in the file called &_Mirrors_&.
-Within the &_exim_& directory there are subdirectories called &_exim3_& (for
+Within the top exim directory there are subdirectories called &_exim3_& (for
previous Exim 3 distributions), &_exim4_& (for the latest Exim 4
distributions), and &_Testing_& for testing versions. In the &_exim4_&
subdirectory, the current release can always be found in files called
.display
+&_exim-n.nn.tar.xz_&
&_exim-n.nn.tar.gz_&
&_exim-n.nn.tar.bz2_&
.endd
-where &'n.nn'& is the highest such version number in the directory. The two
+where &'n.nn'& is the highest such version number in the directory. The three
files contain identical data; the only difference is the type of compression.
-The &_.bz2_& file is usually a lot smaller than the &_.gz_& file.
+The &_.xz_& file is usually the smallest, while the &_.gz_& file is the
+most portable to old systems.
.cindex "distribution" "signing details"
.cindex "distribution" "public key"
&_nigel-pubkey.asc_&. All keys used will be available in public keyserver pools,
such as &'pool.sks-keyservers.net'&.
-At time of last update, releases were being made by Phil Pennock and signed with
-key &'0x403043153903637F'&, although that key is expected to be replaced in 2013.
-A trust path from Nigel's key to Phil's can be observed at
-&url(https://www.security.spodhuis.org/exim-trustpath).
-
-Releases have also been authorized to be performed by Todd Lyons who signs with
-key &'0xC4F4F94804D29EBA'&. A direct trust path exists between previous RE Phil
-Pennock and Todd Lyons through a common associate.
+At time of last update, releases were being made by Jeremy Harris and signed
+with key &'0xBCE58C8CE41F32DF'&. Other recent keys used for signing are those
+of Heiko Schlittermann, &'0x26101B62F69376CE'&,
+and of Phil Pennock, &'0x4D1E900E14C1CC04'&.
The signatures for the tar bundles are in:
.display
+&_exim-n.nn.tar.xz.asc_&
&_exim-n.nn.tar.gz.asc_&
&_exim-n.nn.tar.bz2.asc_&
.endd
&_exim-texinfo-n.nn.tar.gz_&
.endd
These tar files contain only the &_doc_& directory, not the complete
-distribution, and are also available in &_.bz2_& as well as &_.gz_& forms.
+distribution, and are also available in &_.bz2_& and &_.xz_& forms.
.section "Limitations" "SECID6"
are available, similarly to the drivers. Because macros are sometimes used
for storing passwords, this option is restricted.
The output format is one item per line.
+For the "-bP macro <name>" form, if no such macro is found
+the exit status will be nonzero.
.vitem &%-bp%&
.oindex "&%-bp%&"
.vitem &%-MCK%&
.oindex "&%-MCK%&"
This option is not intended for use by external callers. It is used internally
-by Exim in conjunction with the &%-MC%& option. It signifies that an
+by Exim in conjunction with the &%-MC%& option. It signifies that a
remote host supports the ESMTP &_CHUNKING_& extension.
.vitem &%-MCP%&
.cindex "port" "465 and 587"
.cindex "port" "for message submission"
.cindex "message" "submission, ports for"
-.cindex "ssmtp protocol"
+.cindex "submissions protocol"
.cindex "smtps protocol"
+.cindex "ssmtp protocol"
+.cindex "SMTP" "submissions protocol"
.cindex "SMTP" "ssmtp protocol"
.cindex "SMTP" "smtps protocol"
These options provide better support for roaming users who wish to use this
server for message submission. They are not much use unless you have turned on
TLS (as described in the previous paragraph) and authentication (about which
-more in section &<<SECTdefconfauth>>&). The usual SMTP port 25 is often blocked
-on end-user networks, so RFC 4409 specifies that message submission should use
-port 587 instead. However some software (notably Microsoft Outlook) cannot be
-configured to use port 587 correctly, so these settings also enable the
-non-standard &"smtps"& (aka &"ssmtp"&) port 465 (see section
-&<<SECTsupobssmt>>&).
+more in section &<<SECTdefconfauth>>&).
+Mail submission from mail clients (MUAs) should be separate from inbound mail
+to your domain (MX delivery) for various good reasons (eg, ability to impose
+much saner TLS protocol and ciphersuite requirements without unintended
+consequences).
+RFC 6409 (previously 4409) specifies use of port 587 for SMTP Submission,
+which uses STARTTLS, so this is the &"submission"& port.
+RFC 8314 specifies use of port 465 as the &"submissions"& protocol,
+which should be used in preference to 587.
+You should also consider deploying SRV records to help clients find
+these ports.
+Older names for &"submissions"& are &"smtps"& and &"ssmtp"&.
Two more commented-out options settings follow:
.code
${lookup redis{get keyname}}
.endd
+As of release 4.91, "lightweight" support for Redis Cluster is available.
+Requires &%redis_servers%& list to contain all the servers in the cluster, all
+of which must be reachable from the running exim instance. If the cluster has
+master/slave replication, the list must contain all the master and slave
+servers.
+
+When the Redis Cluster returns a "MOVED" response to a query, Exim does not
+immediately follow the redirection but treats the response as a DEFER, moving on
+to the next server in the &%redis_servers%& list until the correct server is
+reached.
+
.ecindex IIDfidalo1
.ecindex IIDfidalo2
If the ACL returns defer the result is a forced-fail. Otherwise the expansion fails.
+.vitem "&*${authresults{*&<&'authserv-id'&>&*}}*&"
+.cindex authentication "results header"
+.cindex headers "authentication-results:"
+.cindex authentication "expansion item"
+This item returns a string suitable for insertion as an
+&'Authentication-Results"'&
+header line.
+The given <&'authserv-id'&> is included in the result; typically this
+will be a domain name identifying the system performing the authentications.
+Methods that might be present in the result include:
+.code
+none
+iprev
+auth
+spf
+dkim
+.endd
+
+Example use (as an ACL modifier):
+.code
+ add_header = :at_start:${authresults {$primary_hostname}}
+.endd
+This is safe even if no authentication results are available.
+
+
.vitem "&*${certextract{*&<&'field'&>&*}{*&<&'certificate'&>&*}&&&
{*&<&'string2'&>&*}{*&<&'string3'&>&*}}*&"
.cindex "expansion" "extracting certificate fields"
separator used for the output list is the same as the one used for the
input, but a separator setting is not included in the output. For example:
.code
-${filter{a:b:c}{!eq{$item}{b}}
+${filter{a:b:c}{!eq{$item}{b}}}
.endd
yields &`a:c`&. At the end of the expansion, the value of &$item$& is restored
to what it was before. See also the &*map*& and &*reduce*& expansion items.
with 256 being the default.
The &%sha3%& expansion item is only supported if Exim has been
-compiled with GnuTLS 3.5.0 or later.
+compiled with GnuTLS 3.5.0 or later,
+or OpenSSL 1.1.1 or later.
+The macro "_CRYPTO_HASH_SHA3" will be defined if it is supported.
.vitem &*${stat:*&<&'string'&>&*}*&
user/password authenticator configuration might preserve the user name for use
in the routers. Note that this is not the same information that is saved in
&$sender_host_authenticated$&.
+
When a message is submitted locally (that is, not over a TCP connection)
the value of &$authenticated_id$& is normally the login name of the calling
process. However, a trusted user can override this by means of the &%-oMai%&
command line option.
+This second case also sets up inforamtion used by the
+&$authresults$& expansion item.
.vitem &$authenticated_fail_id$&
.cindex "authentication" "fail" "id"
.vindex "&$config_file$&"
The name of the main configuration file Exim is using.
-.vitem &$dkim_verify_status$& &&&
+.vitem &$dkim_verify_status$&
Results of DKIM verification.
-For details see chapter &<<CHAPdkim>>&.
+For details see section &<<SECDKIMVFY>>&.
.vitem &$dkim_cur_signer$& &&&
&$dkim_verify_reason$& &&&
&$dkim_key_notes$& &&&
&$dkim_key_length$&
These variables are only available within the DKIM ACL.
-For details see chapter &<<CHAPdkim>>&.
+For details see section &<<SECDKIMVFY>>&.
.vitem &$dkim_signers$&
.vindex &$dkim_signers$&
When a message has been received this variable contains
a colon-separated list of signer domains and identities for the message.
-For details see chapter &<<CHAPdkim>>&.
+For details see section &<<SECDKIMVFY>>&.
.vitem &$dnslist_domain$& &&&
&$dnslist_matched$& &&&
the result, the name is not accepted, and &$host_lookup_deferred$& is set to
&"1"&. See also &$sender_host_name$&.
+.cindex authentication "expansion item"
+Performing these checks sets up information used by the
+&$authresults$& expansion item.
+
+
.vitem &$host_lookup_failed$&
.vindex "&$host_lookup_failed$&"
See &$host_lookup_deferred$&.
is compiled with the content-scanning extension. For details, see section
&<<SECTscanspamass>>&.
+.vitem &$spf_header_comment$& &&&
+ &$spf_received$& &&&
+ &$spf_result$& &&&
+ &$spf_result_guessed$& &&&
+ &$spf_smtp_comment$&
+These variables are only available if Exim is built with SPF support.
+For details see section &<<SECSPF>>&.
.vitem &$spool_directory$&
.vindex "&$spool_directory$&"
&%certextract%& expansion item, &%md5%&, &%sha1%& or &%sha256%& operator,
or a &%def%& condition.
-&*Note*&: Under current versions of OpenSSL, when a list of more than one
+&*Note*&: Under versions of OpenSSL preceding 1.1.1,
+when a list of more than one
file is used for &%tls_certificate%&, this variable is not reliable.
.vitem &$tls_in_peercert$&
&<<CHAPTLS>>& for details of TLS support and chapter &<<CHAPsmtptrans>>& for
details of the &(smtp)& transport.
+.vitem &$tls_out_dane$&
+.vindex &$tls_out_dane$&
+DANE active status. See section &<<SECDANE>>&.
+
.vitem &$tls_in_ocsp$&
.vindex "&$tls_in_ocsp$&"
When a message is received from a remote client connection
SMTP deliveries, this variable reflects the value of the &%tls_sni%& option on
the transport.
+.vitem &$tls_out_tlsa_usage$&
+.vindex &$tls_out_tlsa_usage$&
+Bitfield of TLSA record types found. See section &<<SECDANE>>&.
+
.vitem &$tod_bsdinbox$&
.vindex "&$tod_bsdinbox$&"
The time of day and the date, in the format required for BSD-style mailbox
-.section "Support for the obsolete SSMTP (or SMTPS) protocol" "SECTsupobssmt"
+.section "Support for the submissions (aka SSMTP or SMTPS) protocol" "SECTsupobssmt"
+.cindex "submissions protocol"
.cindex "ssmtp protocol"
.cindex "smtps protocol"
.cindex "SMTP" "ssmtp protocol"
.cindex "SMTP" "smtps protocol"
-Exim supports the obsolete SSMTP protocol (also known as SMTPS) that was used
-before the STARTTLS command was standardized for SMTP. Some legacy clients
-still use this protocol. If the &%tls_on_connect_ports%& option is set to a
-list of port numbers or service names,
-connections to those ports must use SSMTP. The most
-common use of this option is expected to be
+Exim supports the use of TLS-on-connect, used by mail clients in the
+&"submissions"& protocol, historically also known as SMTPS or SSMTP.
+For some years, IETF Standards Track documents only blessed the
+STARTTLS-based Submission service (port 587) while common practice was to support
+the same feature set on port 465, but using TLS-on-connect.
+If your installation needs to provide service to mail clients
+(Mail User Agents, MUAs) then you should provide service on both the 587 and
+the 465 TCP ports.
+
+If the &%tls_on_connect_ports%& option is set to a list of port numbers or
+service names, connections to those ports must first establish TLS, before
+proceeding to the application layer use of the SMTP protocol.
+
+The common use of this option is expected to be
.code
tls_on_connect_ports = 465
.endd
-because 465 is the usual port number used by the legacy clients. There is also
-a command line option &%-tls-on-connect%&, which forces all ports to behave in
-this way when a daemon is started.
+per RFC 8314.
+There is also a command line option &%-tls-on-connect%&, which forces all ports
+to behave in this way when a daemon is started.
&*Warning*&: Setting &%tls_on_connect_ports%& does not of itself cause the
daemon to listen on those ports. You must still specify them in
This option defines the ACL that is run for each DKIM signature
(by default, or as specified in the dkim_verify_signers option)
of a received message.
-See chapter &<<CHAPdkim>>& for further details.
+See section &<<SECDKIMVFY>>& for further details.
.option acl_smtp_etrn main string&!! unset
.cindex "ETRN" "ACL for"
This option gives a list of DKIM domains for which the DKIM ACL is run.
It is expanded after the message is received; by default it runs
the ACL once for each signature in the message.
-See chapter &<<CHAPdkim>>&.
+See section &<<SECDKIMVFY>>&.
.option dns_again_means_nonexist main "domain list&!!" unset
.option dns_ipv4_lookup main "domain list&!!" unset
.cindex "IPv6" "DNS lookup for AAAA records"
.cindex "DNS" "IPv6 lookup for AAAA records"
+.cindex DNS "IPv6 disabling"
When Exim is compiled with IPv6 support and &%disable_ipv6%& is not set, it
looks for IPv6 address records (AAAA records) as well as IPv4 address records
(A records) when trying to find IP addresses for hosts, unless the host's
chapter &<<CHAPi18n>>& for details of Exim's support for internationalisation.
-.option spamd_address main string "see below"
+.option spamd_address main string "127.0.0.1 783"
This option is available when Exim is compiled with the content-scanning
extension. It specifies how Exim connects to SpamAssassin's &%spamd%& daemon.
-The default value is
-.code
-127.0.0.1 783
-.endd
See section &<<SECTscanspamass>>& for more details.
+.option spf_guess main string "v=spf1 a/24 mx/24 ptr ?all"
+This option is available when Exim is compiled with SPF support.
+See section &<<SECSPF>>& for more details.
+
+
+
.option split_spool_directory main boolean false
.cindex "multiple spool directories"
.cindex "spool directory" "split"
.option spool_wireformat main boolean false
.cindex "spool directory" "file formats"
-If this option is set, Exim may for some messages use an alternate format
+If this option is set, Exim may for some messages use an alternative format
for data-files in the spool which matches the wire format.
Doing this permits more efficient message reception and transmission.
-Currently it is only done for messages received using the EMSTP CHUNKING
+Currently it is only done for messages received using the ESMTP CHUNKING
option.
The following variables will not have useful values:
Users of the local_scan() API (see &<<CHAPlocalscan>>&),
and any external programs which are passed a reference to a message data file
(except via the &"regex"&, &"malware"& or &"spam"&) ACL conditions)
-will need to be aware of the potential different format.
+will need to be aware of the different formats potentially available.
Using any of the ACL conditions noted will negate the reception benefit
-(as a Unix-mbox-format file is contructed for them).
-The transimssion benefit is maintained.
+(as a Unix-mbox-format file is constructed for them).
+The transmission benefit is maintained.
.option sqlite_lock_timeout main time 5s
.cindex "sqlite lookup type" "lock timeout"
&*Note*&: If you use filenames based on IP addresses, change the list
separator in the usual way to avoid confusion under IPv6.
-&*Note*&: Under current versions of OpenSSL, when a list of more than one
-file is used, the &$tls_in_ourcert$& veriable is unreliable.
+&*Note*&: Under versions of OpenSSL preceding 1.1.1,
+when a list of more than one
+file is used, the &$tls_in_ourcert$& variable is unreliable.
&*Note*&: OCSP stapling is not usable under OpenSSL
when a list of more than one file is used.
This option specifies a certificate revocation list. The expanded value must
be the name of a file that contains CRLs in PEM format.
-.new
Under OpenSSL the option can specify a directory with CRL files.
&*Note:*& Under OpenSSL the option must, if given, supply a CRL
for each signing element of the certificate chain (i.e. all but the leaf).
For the file variant this can be multiple PEM blocks in the one file.
-.wen
See &<<SECTtlssni>>& for discussion of when this option might be re-expanded.
Usable for GnuTLS 3.4.4 or 3.3.17 or OpenSSL 1.1.0 (or later).
-.new
For GnuTLS 3.5.6 or later the expanded value of this option can be a list
of files, to match a list given for the &%tls_certificate%& option.
The ordering of the two lists must match.
-.wen
.option tls_on_connect_ports main "string list" unset
MX records of equal priority are sorted by Exim into a random order. Exim then
looks for address records for the host names obtained from MX or SRV records.
When a host has more than one IP address, they are sorted into a random order,
-except that IPv6 addresses are always sorted before IPv4 addresses. If all the
+except that IPv6 addresses are sorted before IPv4 addresses. If all the
IP addresses found are discarded by a setting of the &%ignore_target_hosts%&
generic option, the router declines.
also being queued.
+.option ipv4_only "string&!!" unset
+.cindex IPv6 disabling
+.cindex DNS "IPv6 disabling"
+The string is expanded, and if the result is anything but a forced failure,
+or an empty string, or one of the strings “0” or “no” or “false”
+(checked without regard to the case of the letters),
+only A records are used.
+
+.option ipv4_prefer "string&!!" unset
+.cindex IPv4 preference
+.cindex DNS "IPv4 preference"
+The string is expanded, and if the result is anything but a forced failure,
+or an empty string, or one of the strings “0” or “no” or “false”
+(checked without regard to the case of the letters),
+A records are sorted before AAAA records (inverting the default).
+
.option mx_domains dnslookup "domain list&!!" unset
.cindex "MX record" "required to exist"
.cindex "SRV record" "required to exist"
.section "How the options are used" "SECThowoptused"
-The options are a sequence of words; in practice no more than three are ever
-present. One of the words can be the name of a transport; this overrides the
+The options are a sequence of words, space-separated.
+One of the words can be the name of a transport; this overrides the
&%transport%& option on the router for this particular routing rule only. The
other words (if present) control randomization of the list of hosts on a
per-rule basis, and how the IP addresses of the hosts are to be found when
&%bydns%&: look up address records for the hosts directly in the DNS; fail if
no address records are found. If there is a temporary DNS error (such as a
timeout), delivery is deferred.
+.next
+&%ipv4_only%&: in direct DNS lookups, look up only A records.
+.next
+&%ipv4_prefer%&: in direct DNS lookups, sort A records before AAAA records.
.endlist
For example:
option.
+.option dane_require_tls_ciphers smtp string&!! unset
+.cindex "TLS" "requiring specific ciphers for DANE"
+.cindex "cipher" "requiring specific"
+.cindex DANE "TLS ciphers"
+This option may be used to override &%tls_require_ciphers%& for connections
+where DANE has been determined to be in effect.
+If not set, then &%tls_require_ciphers%& will be used.
+Normal SMTP delivery is not able to make strong demands of TLS cipher
+configuration, because delivery will fall back to plaintext. Once DANE has
+been determined to be in effect, there is no plaintext fallback and making the
+TLS cipherlist configuration stronger will increase security, rather than
+counter-intuitively decreasing it.
+If the option expands to be empty or is forced to fail, then it will
+be treated as unset and &%tls_require_ciphers%& will be used instead.
+
+
.option data_timeout smtp time 5m
This sets a timeout for the transmission of each block in the data portion of
the message. As a result, the overall timeout for a message depends on the size
TLS session for any host that matches this list.
&%tls_verify_certificates%& should also be set for the transport.
+.option hosts_require_dane smtp "host list&!!" unset
+.cindex DANE "transport options"
+.cindex DANE "requiring for certain servers"
+If built with DANE support, Exim will require that a DNSSEC-validated
+TLSA record is present for any host matching the list,
+and that a DANE-verified TLS connection is made.
+There will be no fallback to in-clear communication.
+See section &<<SECDANE>>&.
+
.option hosts_require_ocsp smtp "host list&!!" unset
.cindex "TLS" "requiring for certain servers"
Exim will request, and check for a valid Certificate Status being given, on a
CHUNKING support, Exim will attempt to use BDAT commands rather than DATA.
BDAT will not be used in conjunction with a transport filter.
+.option hosts_try_dane smtp "host list&!!" unset
+.cindex DANE "transport options"
+.cindex DANE "attempting for certain servers"
+If built with DANE support, Exim will lookup a
+TLSA record for any host matching the list.
+If found and verified by DNSSEC,
+a DANE-verified TLS connection is made to that host;
+there will be no fallback to in-clear communication.
+See section &<<SECDANE>>&.
+
.option hosts_try_fastopen smtp "host list&!!" unset
.cindex "fast open, TCP" "enabling, in client"
.cindex "TCP Fast Open" "enabling, in client"
On (at least some) current Linux distributions the facility must be enabled
in the kernel by the sysadmin before the support is usable.
There is no option for control of the server side; if the system supports
-it it is always enebled. Note that legthy operations in the connect ACL,
+it it is always enabled. Note that lengthy operations in the connect ACL,
such as DNSBL lookups, will still delay the emission of the SMTP banner.
.option hosts_try_prdr smtp "host list&!!" *
client from which the message was received. This variable is empty if there was
no successful authentication.
+.cindex authentication "expansion item"
+Successful authentication sets up information used by the
+&$authresults$& expansion item.
+
Note that the hostlist test for whether to do authentication can be
confused if name-IP lookups change between the time the peer is decided
-on and the transport running. For example, with a manualroute
-router given a host name, and DNS "round-robin" use by that name: if
+upon and the time that the transport runs. For example, with a manualroute
+router given a host name, and with DNS "round-robin" used by that name: if
the local resolver cache times out between the router and the transport
running, the transport may get an IP for the name for its authentication
check which does not match the connection peer IP.
-.section "Support for the legacy &""ssmtp""& (aka &""smtps""&) protocol" &&&
+.section "Support for the &""submissions""& (aka &""ssmtp""& and &""smtps""&) protocol" &&&
"SECID284"
+.cindex "submissions protocol"
.cindex "ssmtp protocol"
.cindex "smtps protocol"
+.cindex "SMTP" "submissions protocol"
.cindex "SMTP" "ssmtp protocol"
.cindex "SMTP" "smtps protocol"
-Early implementations of encrypted SMTP used a different TCP port from normal
-SMTP, and expected an encryption negotiation to start immediately, instead of
-waiting for a STARTTLS command from the client using the standard SMTP
-port. The protocol was called &"ssmtp"& or &"smtps"&, and port 465 was
-allocated for this purpose.
-
-This approach was abandoned when encrypted SMTP was standardized, but there are
-still some legacy clients that use it. Exim supports these clients by means of
-the &%tls_on_connect_ports%& global option. Its value must be a list of port
-numbers; the most common use is expected to be:
+The history of port numbers for TLS in SMTP is a little messy and has been
+contentious. As of RFC 8314, the common practice of using the historically
+allocated port 465 for "email submission but with TLS immediately upon connect
+instead of using STARTTLS" is officially blessed by the IETF, and recommended
+by them in preference to STARTTLS.
+
+The name originally assigned to the port was &"ssmtp"& or &"smtps"&, but as
+clarity emerged over the dual roles of SMTP, for MX delivery and Email
+Submission, nomenclature has shifted. The modern name is now &"submissions"&.
+
+This approach was, for a while, officially abandoned when encrypted SMTP was
+standardized, but many clients kept using it, even as the TCP port number was
+reassigned for other use.
+Thus you may encounter guidance claiming that you shouldn't enable use of
+this port.
+In practice, a number of mail-clients have only ever supported submissions,
+not submission with STARTTLS upgrade.
+Ideally, offer both submission (587) and submissions (465) service.
+
+Exim supports TLS-on-connect by means of the &%tls_on_connect_ports%&
+global option. Its value must be a list of port numbers;
+the most common use is expected to be:
.code
tls_on_connect_ports = 465
.endd
defined elsewhere.
There is also a &%-tls-on-connect%& command line option. This overrides
-&%tls_on_connect_ports%&; it forces the legacy behaviour for all ports.
+&%tls_on_connect_ports%&; it forces the TLS-only behaviour for all ports.
When Exim has been built with TLS support, it advertises the availability of
the STARTTLS command to client hosts that match &%tls_advertise_hosts%&,
but not to any others. The default value of this option is *, which means
-that STARTTLS is alway advertised. Set it to blank to never advertise;
-this is reasonble for systems that want to use TLS only as a client.
+that STARTTLS is always advertised. Set it to blank to never advertise;
+this is reasonable for systems that want to use TLS only as a client.
If STARTTLS is to be used you
need to set some other options in order to make TLS available.
during TLS session handshake, to permit alternative values to be chosen:
.ilist
-.vindex "&%tls_certificate%&"
&%tls_certificate%&
.next
-.vindex "&%tls_crl%&"
&%tls_crl%&
.next
-.vindex "&%tls_privatekey%&"
&%tls_privatekey%&
.next
-.vindex "&%tls_verify_certificates%&"
&%tls_verify_certificates%&
.next
-.vindex "&%tls_ocsp_file%&"
&%tls_ocsp_file%&
.endlist
An older mode of operation can be enabled on a per-host basis by the
&%hosts_noproxy_tls%& option on the &(smtp)& transport. If the host matches
-this list the proxy process descibed above is not used; instead Exim
+this list the proxy process described above is not used; instead Exim
shuts down an existing TLS session being run by the delivery process
before passing the socket to a new process. The new process may then
try to start a new TLS session, and if successful, may try to re-authenticate
+.section DANE "SECDANE"
+.cindex 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 is anchored by one of the TLSA records.
+
+There are no changes to Exim specific to server-side operation of DANE.
+Support for client-side operation of DANE can be included at compile time by defining SUPPORT_DANE=yes
+in &_Local/Makefile_&.
+If it has been included, the macro "_HAVE_DANE" will be defined.
+
+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.
+
+Another approach which should be seriously considered is to use DANE with a certificate
+from a public CA, because of another technology, "MTA-STS", described below.
+
+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, &url(https://www.huque.com/bin/gen_tlsa)
+is useful for quickly generating TLSA records; and commands like
+
+.code
+ openssl x509 -in -pubkey -noout <certificate.pem \
+ | openssl rsa -outform der -pubin 2>/dev/null \
+ | openssl sha512 \
+ | awk '{print $2}'
+.endd
+
+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:
+
+.code
+ hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \
+ {= {4}{$tls_out_tlsa_usage}} } \
+ {*}{}}
+.endd
+
+The (new) variable &$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 three new smtp transport options, &%hosts_try_dane%&, &%hosts_require_dane%&
+and &%dane_require_tls_ciphers%&.
+The require variant will result in failure if the target host is not DNSSEC-secured.
+
+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 succeeded using dnssec.
+If a TLSA lookup is done and succeeds, a DANE-verified TLS connection
+will be required for the host. If it does not, the host will not
+be used; there is no fallback to non-DANE or non-TLS.
+
+If DANE is requested and usable, then the TLS cipher list configuration
+prefers to use the option &%dane_require_tls_ciphers%& and falls
+back to &%tls_require_ciphers%& only if that is unset.
+This lets you configure "decent crypto" for DANE and "better than nothing
+crypto" as the default. Note though that while GnuTLS lets the string control
+which versions of TLS/SSL will be negotiated, OpenSSL does not and you're
+limited to ciphersuite constraints.
+
+If DANE is requested and useable (see above) the following transport options are ignored:
+.code
+ hosts_require_tls
+ tls_verify_hosts
+ tls_try_verify_hosts
+ tls_verify_certificates
+ tls_crl
+ tls_verify_cert_hostnames
+.endd
+
+If DANE is not usable, whether requested or not, and CA-anchored
+verification evaluation is wanted, the above variables should be set appropriately.
+
+Currently the &%dnssec_request_domains%& must be active 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$& which will have "yes" if
+verification succeeded using DANE and "no" otherwise (only useful
+in combination with events; see &<<CHAPevents>>&),
+and a new variable &$tls_out_tlsa_usage$& (detailed above).
+
+.cindex DANE reporting
+An event (see &<<CHAPevents>>&) of type "dane:fail" will be raised on failures
+to achieve DANE-verified connection, if one was either requested and offered, or
+required. This is intended to support TLS-reporting as defined in
+&url(https://tools.ietf.org/html/draft-ietf-uta-smtp-tlsrpt-17).
+The &$event_data$& will be one of the Result Types defined in
+Section 4.3 of that document.
+
+Under GnuTLS, DANE is only supported from version 3.0.0 onwards.
+
+DANE is specified in published RFCs and decouples certificate authority trust
+selection from a "race to the bottom" of "you must trust everything for mail
+to get through". There is an alternative technology called MTA-STS, which
+instead publishes MX trust anchor information on an HTTPS website. At the
+time this text was last updated, MTA-STS was still a draft, not yet an RFC.
+Exim has no support for MTA-STS as a client, but Exim mail server operators
+can choose to publish information describing their TLS configuration using
+MTA-STS to let those clients who do use that protocol derive trust
+information.
+
+The MTA-STS design requires a certificate from a public Certificate Authority
+which is recognized by clients sending to you. That selection is outside your
+control.
+
+The most interoperable course of action is probably to use
+&url(https://letsencrypt.org/,Let's Encrypt), with automated certificate
+renewal; to publish the anchor information in DNSSEC-secured DNS via TLSA
+records for DANE clients (such as Exim and Postfix) and to publish anchor
+information for MTA-STS as well. This is what is done for the &'exim.org'&
+domain itself (with caveats around occasionally broken MTA-STS because of
+incompatible specification changes prior to reaching RFC status).
+
+
+
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
This ACL is evaluated before &%acl_smtp_mime%& and &%acl_smtp_data%&.
-For details on the operation of DKIM, see chapter &<<CHAPdkim>>&.
+For details on the operation of DKIM, see section &<<SECDKIM>>&.
.section "The SMTP MIME ACL" "SECID194"
{acl_check_rcpt} {acl_check_rcpt_submit} }
.endd
In the default configuration file there are some example settings for
-providing an RFC 4409 message submission service on port 587 and a
-non-standard &"smtps"& service on port 465. You can use a string
+providing an RFC 4409 message &"submission"& service on port 587 and
+an RFC 8314 &"submissions"& service on port 465. You can use a string
expansion like this to choose an ACL for MUAs on these ports which is
more appropriate for this purpose than the default ACL on port 25.
Note also that headers cannot be
modified by any of the post-data ACLs (DATA, MIME and DKIM).
Headers may be modified by routers (subject to the above) and transports.
-The Received-By: header is generated as soon as the body reception starts,
+The &'Received-By:'& header is generated as soon as the body reception starts,
rather than the traditional time after the full message is received;
this will affect the timestamp.
.cindex "disable DKIM verify"
.cindex "DKIM" "disable verify"
This control turns off DKIM verification processing entirely. For details on
-the operation and configuration of DKIM, see chapter &<<CHAPdkim>>&.
+the operation and configuration of DKIM, see section &<<SECDKIM>>&.
.vitem &*control&~=&~dscp/*&<&'value'&>
.code
require verify = recipient/callout=use_sender,hold
.endd
-It causes the connection to be helod open and used for any further recipients
+It causes the connection to be held open and used for any further recipients
and for eventual delivery (should that be done quickly).
Doing this saves on TCP and SMTP startup costs, and TLS costs also
when that is used for the connections.
If the value of &%av_scanner%& starts with a dollar character, it is expanded
before use.
The usual list-parsing of the content (see &<<SECTlistconstruct>>&) applies.
-The following scanner types are supported in this release:
+The following scanner types are supported in this release,
+though individual ones can be included or not at build time:
.vlist
.vitem &%avast%&
.cindex "virus scanners" "avast"
This is the scanner daemon of Avast. It has been tested with Avast Core
-Security (currently at version 1.1.7).
-You can get a trial version at &url(http://www.avast.com) or for Linux
-at &url(http://www.avast.com/linux-server-antivirus).
+Security (currently at version 2.2.0).
+You can get a trial version at &url(https://www.avast.com) or for Linux
+at &url(https://www.avast.com/linux-server-antivirus).
This scanner type takes one option,
which can be either a full path to a UNIX socket,
or host and port specifiers separated by white space.
The host may be a name or an IP address; the port is either a
single number or a pair of numbers with a dash between.
-Any further options are given, on separate lines,
-to the daemon as options before the main scan command.
+A list of options may follow. These options are interpreted on the
+Exim's side of the malware scanner, or are given on separate lines to
+the daemon as options before the main scan command.
+
+.cindex &`pass_unscanned`& "avast"
+If &`pass_unscanned`&
+is set, any files the Avast scanner can't scan (e.g.
+decompression bombs, or invalid archives) are considered clean. Use with
+care.
+
For example:
.code
av_scanner = avast:/var/run/avast/scan.sock:FLAGS -fullfiles:SENSITIVITY -pup
+av_scanner = avast:/var/run/avast/scan.sock:pass_unscanned:FLAGS -fullfiles:SENSITIVITY -pup
av_scanner = avast:192.168.2.22 5036
.endd
If you omit the argument, the default path
PACK
.endd
+If the scanner returns a temporary failure (e.g. license issues, or
+permission problems), the message is deferred and a paniclog entry is
+written. The usual &`defer_ok`& option is available.
.vitem &%aveserver%&
.cindex "virus scanners" "Kaspersky"
If the value of av_scanner points to a UNIX socket file or contains the
&`local`&
option, then the ClamAV interface will pass a filename containing the data
-to be scanned, which will should normally result in less I/O happening and be
+to be scanned, which should normally result in less I/O happening and be
more efficient. Normally in the TCP case, the data is streamed to ClamAV as
Exim does not assume that there is a common filesystem with the remote host.
.code
av_scanner = f-protd:localhost 10200-10204
.endd
-If you omit the argument, the default values show above are used.
+If you omit the argument, the default values shown above are used.
.vitem &%f-prot6d%&
.cindex "virus scanners" "f-prot6d"
As an example, the following will ban &"HTML mail"& (including that sent with
alternative plain text), while allowing HTML files to be attached. HTML
-coverletter mail attached to non-HMTL coverletter mail will also be allowed:
+coverletter mail attached to non-HTML coverletter mail will also be allowed:
.code
deny message = HTML mail is not accepted here
!condition = $mime_is_rfc822
&` `& command list for &"no mail in SMTP session"&
&`CV `& certificate verification status
&`D `& duration of &"no mail in SMTP session"&
+&`DKIM`& domain verified in incoming message
&`DN `& distinguished name from peer certificate
&`DS `& DNSSEC secured lookups
&`DT `& on &`=>`& lines: time taken for a delivery
&` `& on &"Completed"& lines: time spent on queue
&`R `& on &`<=`& lines: reference for local bounce
&` `& on &`=>`& &`>>`& &`**`& and &`==`& lines: router name
+&`RT `& on &`<=`& lines: time taken for reception
&`S `& size of message in bytes
&`SNI `& server name indication from TLS client hello
&`ST `& shadow transport name
.endd
failed. The delivery was discarded.
.endlist olist
+.next
+.cindex DKIM "log line"
+&'DKIM: d='&&~&~Verbose results of a DKIM verification attempt, if enabled for
+logging and the message has a DKIM signature header.
.endlist ilist
&`*delay_delivery `& immediate delivery delayed
&` deliver_time `& time taken to perform delivery
&` delivery_size `& add &`S=`&&'nnn'& to => lines
+&`*dkim `& DKIM verified domain on <= lines
+&` dkim_verbose `& separate full DKIM verification result line, per signature
&`*dnslist_defer `& defers of DNS list (aka RBL) lookups
&` dnssec `& DNSSEC secured lookups
&`*etrn `& ETRN commands
&` incoming_interface `& local interface on <= and => lines
&` incoming_port `& remote port on <= lines
&`*lost_incoming_connection `& as it says (includes timeouts)
-&` millisec `& millisecond timestamps and QT,DT,D times
+&` millisec `& millisecond timestamps and RT,QT,DT,D times
&` outgoing_interface `& local interface on => lines
&` outgoing_port `& add remote port to => lines
&`*queue_run `& start and end queue runs
&` queue_time_overall `& time on queue for whole message
&` pid `& Exim process id
&` proxy `& proxy address on <= and => lines
+&` receive_time `& time taken to receive message
&` received_recipients `& recipients on <= lines
&` received_sender `& sender on <= lines
&`*rejected_header `& header contents on reject log
&%deliver_time%&: For each delivery, the amount of real time it has taken to
perform the actual delivery is logged as DT=<&'time'&>, for example, &`DT=1s`&.
If millisecond logging is enabled, short times will be shown with greater
-precision, eg. &`DT=0.304`&.
+precision, eg. &`DT=0.304s`&.
.next
.cindex "log" "message size on delivery"
.cindex "size" "of message"
&%delivery_size%&: For each delivery, the size of message delivered is added to
the &"=>"& line, tagged with S=.
.next
+.cindex log "DKIM verification"
+.cindex DKIM "verification logging"
+&%dkim%&: For message acceptance log lines, when an DKIM signature in the header
+verifies successfully a tag of DKIM is added, with one of the verified domains.
+.next
+.cindex log "DKIM verification"
+.cindex DKIM "verification logging"
+&%dkim_verbose%&: A log entry is written for each attempted DKIM verification.
+.next
.cindex "log" "dnslist defer"
.cindex "DNS list" "logging defer"
.cindex "black list (DNS)"
.next
.cindex "log" "millisecond timestamps"
.cindex millisecond logging
-.cindex timstamps "millisecond, in logs"
+.cindex timestamps "millisecond, in logs"
&%millisec%&: Timestamps have a period and three decimal places of finer granularity
appended to the seconds value.
.next
off the &%outgoing_interface%& option.
.next
.cindex "log" "outgoing remote port"
-.cindex "port" "logging outgoint remote"
+.cindex "port" "logging outgoing remote"
.cindex "TCP/IP" "logging outgoing remote port"
&%outgoing_port%&: The remote port number is added to delivery log lines (those
containing => tags) following the IP address.
example, &`QT=3m45s`&. The clock starts when Exim starts to receive the
message, so it includes reception time as well as the total delivery time.
.next
+.cindex "log" "receive duration"
+&%receive_time%&: For each message, the amount of real time it has taken to
+perform the reception is logged as RT=<&'time'&>, for example, &`RT=1s`&.
+If millisecond logging is enabled, short times will be shown with greater
+precision, eg. &`RT=0.204s`&.
+.next
.cindex "log" "recipients"
&%received_recipients%&: The recipients of a message are listed in the main log
as soon as the message is received. The list appears at the end of the log line
unchanged, or whether they should be rendered as escape sequences.
.next
.cindex "log" "certificate verification"
+.cindex log DANE
+.cindex DANE logging
&%tls_certificate_verified%&: An extra item is added to <= and => log lines
when TLS is in use. The item is &`CV=yes`& if the peer's certificate was
-verified, and &`CV=no`& if not.
+verified
+using a CA trust anchor,
+&`CA=dane`& if using a DNS trust anchor,
+and &`CV=no`& if not.
.next
.cindex "log" "TLS cipher"
.cindex "TLS" "logging cipher"
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
-.chapter "Support for DKIM (DomainKeys Identified Mail)" "CHAPdkim" &&&
- "DKIM Support"
+.chapter "DKIM and SPF" "CHAPdkim" &&&
+ "DKIM and SPF Support"
.cindex "DKIM"
+.section "DKIM (DomainKeys Identified Mail)" SECDKIM
+
DKIM is a mechanism by which messages sent by some entity can be provably
linked to a domain which that entity controls. It permits reputation to
be tracked on a per-domain basis, rather than merely upon source IP address.
-DKIM is documented in RFC 4871.
+DKIM is documented in RFC 6376.
+
+As DKIM relies on the message being unchanged in transit, messages handled
+by a mailing-list (which traditionally adds to the message) will not match
+any original DKIM signature.
DKIM support is compiled into Exim by default if TLS support is present.
It can be disabled by setting DISABLE_DKIM=yes in &_Local/Makefile_&.
Exim's standard controls.
Please note that verification of DKIM signatures in incoming mail is turned
-on by default for logging purposes. For each signature in incoming email,
+on by default for logging (in the <= line) purposes.
+
+Additional log detail can be enabled using the &%dkim_verbose%& log_selector.
+When set, for each signature in incoming email,
exim will log a line displaying the most important signature details, and the
signature status. Here is an example (with line-breaks added for clarity):
.code
c=relaxed/relaxed a=rsa-sha1
i=@facebookmail.com t=1252484542 [verification succeeded]
.endd
+
You might want to turn off DKIM verification processing entirely for internal
or relay mail sources. To do that, set the &%dkim_disable_verify%& ACL
control modifier. This should typically be done in the RCPT ACL, at points
.section "Signing outgoing messages" "SECDKIMSIGN"
.cindex "DKIM" "signing"
+For signing to be usable you must have published a DKIM record in DNS.
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+
+Signers MUST use RSA keys of at least 1024 bits for all keys.
+Signers SHOULD use RSA keys of at least 2048 bits.
+.endd
+
+Note also that the key content (the 'p=' field)
+in the DNS record is different between RSA and EC keys;
+for the former it is the base64 of the ASN.1 for the RSA public key
+(equivalent to the private-key .pem with the header/trailer stripped)
+but for EC keys it is the base64 of the pure key; no ASN.1 wrapping.
+
Signing is enabled by setting private options on the SMTP transport.
These options take (expandable) strings as arguments.
After expansion, this can be a list.
Each element in turn is put into the &%$dkim_domain%& expansion variable
while expanding the remaining signing options.
-If it is empty after expansion, DKIM signing is not done.
+If it is empty after expansion, DKIM signing is not done,
+and no error will result even if &%dkim_strict%& is set.
.option dkim_selector smtp string list&!! unset
This sets the key selector string.
Each element in turn is put in the expansion
variable &%$dkim_selector%& which may be used in the &%dkim_private_key%&
option along with &%$dkim_domain%&.
-If the option is empty after expansion, DKIM signing is not done for this domain.
+If the option is empty after expansion, DKIM signing is not done for this domain,
+and no error will result even if &%dkim_strict%& is set.
.option dkim_private_key smtp string&!! unset
This sets the private key to use.
&%$dkim_selector%& expansion variables to determine the private key to use.
The result can either
.ilist
-be a valid RSA private key in ASCII armor, including line breaks.
+be a valid RSA private key in ASCII armor (.pem file), including line breaks
+.next
+with GnuTLS 3.6.0 or OpenSSL 1.1.1 or later,
+be a valid Ed25519 private key (same format as above)
.next
start with a slash, in which case it is treated as a file that contains
-the private key.
+the private key
.next
be "0", "false" or the empty string, in which case the message will not
be signed. This case will not result in an error, even if &%dkim_strict%&
is set.
.endlist
-If the option is empty after expansion, DKIM signing is not done.
+
+To generate keys under OpenSSL:
+.code
+openssl genrsa -out dkim_rsa.private 2048
+openssl rsa -in dkim_rsa.private -out /dev/stdout -pubout -outform PEM
+.endd
+Take the base-64 lines from the output of the second command, concatenated,
+for the DNS TXT record.
+See section 3.6 of RFC6376 for the record specification.
+
+Under GnuTLS:
+.code
+certtool --generate-privkey --rsa --bits=2048 --password='' -8 --outfile=dkim_rsa.private
+certtool --load-privkey=dkim_rsa.private --pubkey-info
+.endd
+
+Note that RFC 8301 says:
+.code
+Signers MUST use RSA keys of at least 1024 bits for all keys.
+Signers SHOULD use RSA keys of at least 2048 bits.
+.endd
+
+Support for EC keys is being developed under
+&url(https://datatracker.ietf.org/doc/draft-ietf-dcrup-dkim-crypto/).
+They are considerably smaller than RSA keys for equivalent protection.
+As they are a recent development, users should consider dual-signing
+(by setting a list of selectors, and an expansion for this option)
+for some transition period.
+The "_CRYPTO_SIGN_ED25519" macro will be defined if support is present
+for EC keys.
+
+OpenSSL 1.1.1 and GnuTLS 3.6.0 can create Ed25519 private keys:
+.code
+openssl genpkey -algorithm ed25519 -out dkim_ed25519.private
+certtool --generate-privkey --key-type=ed25519 --outfile=dkim_ed25519.private
+.endd
+
+To produce the required public key value for a DNS record:
+.code
+openssl pkey -outform DER -pubout -in dkim_ed25519.private | tail -c +13 | base64
+certtool --load_privkey=dkim_ed25519.private --pubkey_info --outder | tail -c +13 | base64
+.endd
+
+Note that the format
+of Ed25519 keys in DNS has not yet been decided; this release supports
+both of the leading candidates at this time, a future release will
+probably drop support for whichever proposal loses.
.option dkim_hash smtp string&!! sha256
-Can be set alternatively to &"sha1"& to use an alternate hash
-method. Note that sha1 is now condidered insecure, and deprecated.
+Can be set to any one of the supported hash methods, which are:
+.ilist
+&`sha1`& &-- should not be used, is old and insecure
+.next
+&`sha256`& &-- the default
+.next
+&`sha512`& &-- possibly more secure but less well supported
+.endlist
+
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+.endd
.option dkim_identity smtp string&!! unset
If set after expansion, the value is used to set an "i=" tag in
The default list is available for the expansion in the macro
"_DKIM_SIGN_HEADERS".
-If a name is repeated, multiple headers by that name (or the absence therof)
+If a name is repeated, multiple headers by that name (or the absence thereof)
will be signed. The textually later headers in the headers part of the
message are signed first, if there are multiples.
If an '=' prefix is used, all headers that are present with this name
will be signed.
If a '+' prefix if used, all headers that are present with this name
-will be signed, and one signtature added for a missing header with the
+will be signed, and one signature added for a missing header with the
name will be appended.
-.section "Verifying DKIM signatures in incoming mail" "SECID514"
+.section "Verifying DKIM signatures in incoming mail" "SECDKIMVFY"
.cindex "DKIM" "verification"
Verification of DKIM signatures in SMTP incoming email is implemented via the
containing the signature status and its details are set up during the
runtime of the ACL.
+.cindex authentication "expansion item"
+Performing verification sets up information used by the
+&$authresults$& expansion item.
+
Calling the ACL only for existing signatures is not sufficient to build
more advanced policies. For that reason, the global option
&%dkim_verify_signers%&, and a global expansion variable
This might, for instance, be done to enforce a policy restriction on
hash-method or key-size:
.code
- warn condition = ${if eq {$dkim_algo}{rsa-sha1}}
- condition = ${if eq {$dkim_verify_status}{pass}}
- logwrite = NOTE: forcing dkim verify fail (was pass)
- set dkim_verify_status = fail
- set dkim_verify_reason = hash too weak
+ warn condition = ${if eq {$dkim_verify_status}{pass}}
+ condition = ${if eq {${length_3:$dkim_algo}}{rsa}}
+ condition = ${if or {{eq {$dkim_algo}{rsa-sha1}} \
+ {< {$dkim_key_length}{1024}}}}
+ logwrite = NOTE: forcing DKIM verify fail (was pass)
+ set dkim_verify_status = fail
+ set dkim_verify_reason = hash too weak or key too short
.endd
After all the DKIM ACL runs have completed, the value becomes a
DKIM verification. It may of course also mean that the signature is forged.
.endlist
-This variable can be overwritten using an ACL 'set' modifier.
+This variable can be overwritten, with any value, using an ACL 'set' modifier.
.vitem &%$dkim_domain%&
The signing domain. IMPORTANT: This variable is only populated if there is
.vitem &%$dkim_algo%&
The algorithm used. One of 'rsa-sha1' or 'rsa-sha256'.
+If running under GnuTLS 3.6.0 or OpenSSL 1.1.1 or later,
+may also be 'ed25519-sha256'.
+The "_CRYPTO_SIGN_ED25519" macro will be defined if support is present
+for EC keys.
+
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+
+DKIM signatures identified as having been signed with historic
+algorithms (currently, rsa-sha1) have permanently failed evaluation
+.endd
+
+To enforce this you must have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above.
.vitem &%$dkim_canon_body%&
The body canonicalization method. One of 'relaxed' or 'simple'.
signature to be treated as "expired". When this was not specified by the
signer, "9999999999999" is returned. This makes it possible to do useful
integer size comparisons against this value.
+Note that Exim does not check this value.
.vitem &%$dkim_headernames%&
A colon-separated list of names of headers included in the signature.
.vitem &%$dkim_key_length%&
Number of bits in the key.
+
+Note that RFC 8301 says:
+.code
+Verifiers MUST NOT consider signatures using RSA keys of
+less than 1024 bits as valid signatures.
+.endd
+
+To enforce this you must have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above.
+As EC keys are much smaller, the check should only do this for RSA keys.
+
.endlist
In addition, two ACL conditions are provided:
for more information of what they mean.
.endlist
+
+
+
+.section "SPF (Sender Policy Framework)" SECSPF
+.cindex SPF verification
+
+SPF is a mechanism whereby a domain may assert which IP addresses may transmit
+messages with its domain in the envelope from, documented by RFC 7208.
+For more information on SPF see &url(http://www.openspf.org).
+
+Messages sent by a system not authorised will fail checking of such assertions.
+This includes retransmissions done by traditional forwarders.
+
+SPF verification support is built into Exim if SUPPORT_SPF=yes is set in
+&_Local/Makefile_&. The support uses the &_libspf2_& library
+&url(http://www.libspf2.org/).
+There is no Exim involvement in the transmission of messages;
+publishing certain DNS records is all that is required.
+
+For verification, an ACL condition and an expansion lookup are provided.
+.cindex authentication "expansion item"
+Performing verification sets up information used by the
+&$authresults$& expansion item.
+
+
+.cindex SPF "ACL condition"
+.cindex ACL "spf condition"
+The ACL condition "spf" can be used at or after the MAIL ACL.
+It takes as an argument a list of strings giving the outcome of the SPF check,
+and will succeed for any matching outcome.
+Valid strings are:
+.vlist
+.vitem &%pass%&
+The SPF check passed, the sending host is positively verified by SPF.
+
+.vitem &%fail%&
+The SPF check failed, the sending host is NOT allowed to send mail for the
+domain in the envelope-from address.
+
+.vitem &%softfail%&
+The SPF check failed, but the queried domain can't absolutely confirm that this
+is a forgery.
+
+.vitem &%none%&
+The queried domain does not publish SPF records.
+
+.vitem &%neutral%&
+The SPF check returned a "neutral" state. This means the queried domain has
+published a SPF record, but wants to allow outside servers to send mail under
+its domain as well. This should be treated like "none".
+
+.vitem &%permerror%&
+This indicates a syntax error in the SPF record of the queried domain.
+You may deny messages when this occurs.
+
+.vitem &%temperror%&
+This indicates a temporary error during all processing, including Exim's
+SPF processing. You may defer messages when this occurs.
+.endlist
+
+You can prefix each string with an exclamation mark to invert
+its meaning, for example "!fail" will match all results but
+"fail". The string list is evaluated left-to-right, in a
+short-circuit fashion.
+
+Example:
+.code
+deny spf = fail
+ message = $sender_host_address is not allowed to send mail from \
+ ${if def:sender_address_domain \
+ {$sender_address_domain}{$sender_helo_name}}. \
+ Please see http://www.openspf.org/Why?scope=\
+ ${if def:sender_address_domain {mfrom}{helo}};\
+ identity=${if def:sender_address_domain \
+ {$sender_address}{$sender_helo_name}};\
+ ip=$sender_host_address
+.endd
+
+When the spf condition has run, it sets up several expansion
+variables:
+
+.cindex SPF "verification variables"
+.vlist
+.vitem &$spf_header_comment$&
+.vindex &$spf_header_comment$&
+ This contains a human-readable string describing the outcome
+ of the SPF check. You can add it to a custom header or use
+ it for logging purposes.
+
+.vitem &$spf_received$&
+.vindex &$spf_received$&
+ This contains a complete Received-SPF: header that can be
+ added to the message. Please note that according to the SPF
+ draft, this header must be added at the top of the header
+ list. Please see section 10 on how you can do this.
+
+ Note: in case of "Best-guess" (see below), the convention is
+ to put this string in a header called X-SPF-Guess: instead.
+
+.vitem &$spf_result$&
+.vindex &$spf_result$&
+ This contains the outcome of the SPF check in string form,
+ one of pass, fail, softfail, none, neutral, permerror or
+ temperror.
+
+.vitem &$spf_result_guessed$&
+.vindex &$spf_result_guessed$&
+ This boolean is true only if a best-guess operation was used
+ and required in order to obtain a result.
+
+.vitem &$spf_smtp_comment$&
+.vindex &$spf_smtp_comment$&
+ This contains a string that can be used in a SMTP response
+ to the calling party. Useful for "fail".
+.endlist
+
+
+.cindex SPF "ACL condition"
+.cindex ACL "spf_guess condition"
+.cindex SPF "best guess"
+In addition to SPF, you can also perform checks for so-called
+"Best-guess". Strictly speaking, "Best-guess" is not standard
+SPF, but it is supported by the same framework that enables SPF
+capability.
+Refer to &url(http://www.openspf.org/FAQ/Best_guess_record)
+for a description of what it means.
+
+To access this feature, simply use the spf_guess condition in place
+of the spf one. For example:
+
+.code
+deny spf_guess = fail
+ message = $sender_host_address doesn't look trustworthy to me
+.endd
+
+In case you decide to reject messages based on this check, you
+should note that although it uses the same framework, "Best-guess"
+is not SPF, and therefore you should not mention SPF at all in your
+reject message.
+
+When the spf_guess condition has run, it sets up the same expansion
+variables as when spf condition is run, described above.
+
+Additionally, since Best-guess is not standardized, you may redefine
+what "Best-guess" means to you by redefining the main configuration
+&%spf_guess%& option.
+For example, the following:
+
+.code
+spf_guess = v=spf1 a/16 mx/16 ptr ?all
+.endd
+
+would relax host matching rules to a broader network range.
+
+
+.cindex SPF "lookup expansion"
+.cindex lookup spf
+A lookup expansion is also available. It takes an email
+address as the key and an IP address as the database:
+
+.code
+ ${lookup {username@domain} spf {ip.ip.ip.ip}}
+.endd
+
+The lookup will return the same result strings as can appear in
+&$spf_result$& (pass,fail,softfail,neutral,none,err_perm,err_temp).
+Currently, only IPv4 addresses are supported.
+
+
+
+
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
.cindex internationalisation "email address"
.cindex EAI
.cindex i18n
-.cindex UTF-8 "mail name handling"
+.cindex utf8 "mail name handling"
Exim has support for Internationalised mail names.
To include this it must be built with SUPPORT_I18N and the libidn library.
.cindex log protocol
.cindex SMTPUTF8 logging
+.cindex i18n logging
Log lines and Received-by: header lines will acquire a "utf8"
prefix on the protocol element, eg. utf8esmtp.
${utf8_localpart_from_alabel:str}
.endd
-ACLs may use the following modifier:
+.cindex utf8 "address downconversion"
+.cindex i18n "utf8 address downconversion"
+The RCPT ACL
+may use the following modifier:
.display
control = utf8_downconvert
control = utf8_downconvert/<value>
or &%headers_charset%& main configuration option (otherwise),
to the
modified UTF-7 encoding specified by RFC 2060,
-with the following exception: All occurences of <sep>
+with the following exception: All occurrences of <sep>
(which has to be a single character)
are replaced with periods ("."), and all periods and slashes that are not
<sep> and are not in the <specials> string are BASE64 encoded.
The current list of events is:
.display
+&`dane:fail after transport `& per connection
&`msg:complete after main `& per message
&`msg:delivery after transport `& per recipient
&`msg:rcpt:host:defer after transport `& per recipient per host
before or after the action is associates with. Those which fire before
can be used to affect that action (more on this below).
-The third column in the table above says what section of the configumration
+The third column in the table above says what section of the configuration
should define the event action.
An additional variable, &$event_data$&, is filled with information varying
with the event type:
.display
+&`dane:fail `& failure reason
&`msg:delivery `& smtp confirmation message
&`msg:rcpt:host:defer `& error string
&`msg:rcpt:defer `& error string
return an empty string. Should it return anything else the
following will be forced:
.display
-&`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
.endd
-No other use is made of the result string.
+All other message types ignore the result string, and
+no other use is made of it.
For a tcp:connect event, if the connection is being made to a proxy
then the address and port variables will be that of the proxy and not
git://git.exim.org / users / heiko / exim.git / blobdiff