. Update the Copyright year (only) when changing content.
. /////////////////////////////////////////////////////////////////////////////
-.set previousversion "4.89"
+.set previousversion "4.90"
.include ./local_params
.set ACL "access control lists (ACLs)"
the same lookup again. Otherwise, because each Exim process caches the results
of lookups, you will just get the same result as before.
-.new
Macro processing is done on lines before string-expansion: new macros can be
defined and macros will be expanded.
Because macros in the config file are often used for secrets, those are only
available to admin users.
-.wen
.vitem &%-bem%&&~<&'filename'&>
.oindex "&%-bem%&"
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.
+.new
+For the "-bP macro <name>" form, if no such macro is found
+the exit status will be nonzero.
+.wen
.vitem &%-bp%&
.oindex "&%-bp%&"
by Exim in conjunction with the &%-MC%& option, and passes on the fact that the
host to which Exim is connected supports TLS encryption.
-.new
.vitem &%-MCt%&&~<&'IP&~address'&>&~<&'port'&>&~<&'cipher'&>
.oindex "&%-MCt%&"
This option is not intended for use by external callers. It is used internally
by Exim in conjunction with the &%-MC%& option, and passes on the fact that the
connection is being proxied by a parent process for handling TLS encryption.
The arguments give the local address and port being proxied, and the TLS cipher.
-.wen
.vitem &%-Mc%&&~<&'message&~id'&>&~<&'message&~id'&>&~...
.oindex "&%-Mc%&"
.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
If the ACL returns defer the result is a forced-fail. Otherwise the expansion fails.
+.new
+.vitem "&*${authresults{*&<&'authserv-id'&>&*}}*&"
+.cindex authentication "results header"
+.cindex headers "authentication-results:"
+This item returns a string suitable for insertion as an
+&'Authentication-Results"'&
+header line.
+The given <&'authserv-id'&> is included in the result; typically this
+will ba a domain name identifying the system performing the authentications.
+Methods that may be present in the result include:
+.code
+none
+iprev
+auth
+spf
+dkim
+.endd
+
+Example use (as an ACL modifier):
+.code
+ add_header = :at_start:${authresults {$primary_hostname}}
+.endd
+.wen
+
+
.vitem "&*${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,
+.new
+or OpenSSL 1.1.1 or later.
+The macro "_CRYPTO_HASH_SHA3" will be defined if it is supported.
+.wen
.vitem &*${stat:*&<&'string'&>&*}*&
hexadecimal digits. There may be fewer than eight components if an empty
component (adjacent colons) is present. Only one empty component is permitted.
-.new
&*Note*&: The checks used to be just on the form of the address; actual numerical
values were not considered. Thus, for example, 999.999.999.999 passed the IPv4
check.
This is no longer the case.
-.wen
The main use of these tests is to distinguish between IP addresses and
host names, or between IPv4 and IPv6 addresses. For example, you could use
.vindex "&$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$& &&&
This variable contains the number of bytes in the longest line that was
received as part of the message, not counting the line termination
character(s).
-.new
It is not valid if the &%spool_files_wireformat%& option is used.
-.wen
.vitem &$message_age$&
.cindex "message" "age of"
separates the body from the header. Newlines are included in the count. See
also &$message_size$&, &$body_linecount$&, and &$body_zerocount$&.
-.new
If the spool file is wireformat
(see the &%spool_files_wireformat%& main option)
the CRLF line-terminators are included in the count.
-.wen
.vitem &$message_exim_id$&
.vindex "&$message_exim_id$&"
In the MAIL and RCPT ACLs, the value is zero because at that stage the
message has not yet been received.
-.new
This variable is not valid if the &%spool_files_wireformat%& option is used.
-.wen
.vitem &$message_size$&
.cindex "size" "of message"
space removed. Following the introduction of &$smtp_command$&, this variable is
somewhat redundant, but is retained for backwards compatibility.
-.new
.vitem &$smtp_command_history$&
.cindex SMTP "command history"
.vindex "&$smtp_command_history$&"
A comma-separated list (with no whitespace) of the most-recent SMTP commands
received, in time-order left to right. Only a limited number of commands
are remembered.
-.wen
.vitem &$smtp_count_at_connection_start$&
.vindex "&$smtp_count_at_connection_start$&"
is compiled with the content-scanning extension. For details, see section
&<<SECTscanspamass>>&.
+.new
+.vitem &$spf_header_comment$& &&&
+ &$spf_received$& &&&
+ &$spf_result$& &&&
+ &$spf_smtp_comment$&
+These variables are only available if Exim is built with SPF support.
+For details see section &<<SECSPF>>&.
+.wen
.vitem &$spool_directory$&
.vindex "&$spool_directory$&"
&%certextract%& expansion item, &%md5%&, &%sha1%& or &%sha256%& operator,
or a &%def%& condition.
-.new
&*Note*&: Under current versions of OpenSSL, when a list of more than one
file is used for &%tls_certificate%&, this variable is not reliable.
-.wen
.vitem &$tls_in_peercert$&
.vindex "&$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"
these hosts.
Hosts may use the BDAT command as an alternate to DATA.
-.new
.option commandline_checks_require_admin main boolean &`false`&
.cindex "restricting access to features"
This option restricts various basic checking features to require an
administrative user.
This affects most of the &%-b*%& options, such as &%-be%&.
-.wen
.option debug_store main boolean &`false`&
.cindex debugging "memory corruption"
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.
+.new
+.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.
+.wen
+
+
+
.option split_spool_directory main boolean false
.cindex "multiple spool directories"
.cindex "spool directory" "split"
By using this option to override the compiled-in path, it is possible to run
tests of Exim without using the standard spool.
-.new
.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
will need to be aware of the potential different format.
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.
-.wen
+(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"
.option tls_certificate main string list&!! unset
.cindex "TLS" "server certificate; location of"
.cindex "certificate" "server, location of"
-.new
The value of this option is expanded, and must then be a list of absolute paths to
files which contains the server's certificates. Commonly only one file is
needed.
-.wen
The server's private key is also
assumed to be in this file if &%tls_privatekey%& is unset. See chapter
&<<CHAPTLS>>& for further details.
use when sending messages as a client, you must set the &%tls_certificate%&
option in the relevant &(smtp)& transport.
-.new
&*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$& veriable is unreliable.
+file is used, the &$tls_in_ourcert$& variable is unreliable.
-&*Note*&: OCSP stapling is not usable when a list of more than one file is used.
-.wen
+&*Note*&: OCSP stapling is not usable under OpenSSL
+when a list of more than one file is used.
If the option contains &$tls_out_sni$& and Exim is built against OpenSSL, then
if the OpenSSL build supports TLS extensions and the TLS client sends the
.cindex "TLS" "server certificate revocation list"
.cindex "certificate" "revocation list for server"
This option specifies a certificate revocation list. The expanded value must
-be the name of a file that contains a CRL in PEM format.
+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
-&*Note*&: There is currently no support for multiple OCSP proofs to match the
-multiple certificates facility.
+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_privatekey main string list&!! unset
.cindex "TLS" "server private key; location of"
-.new
The value of this option is expanded, and must then be a list of absolute paths to
files which contains the server's private keys.
-.wen
If this option is unset, or if
the expansion is forced to fail, or the result is an empty string, the private
key is assumed to be in the same file as the server's certificates. See chapter
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
+.new
+except that IPv6 addresses are sorted before IPv4 addresses. If all the
+.wen
IP addresses found are discarded by a setting of the &%ignore_target_hosts%&
generic option, the router declines.
also being queued.
+.new
+.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).
+.wen
+
.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.
+.new
+.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.
+.wen
.endlist
For example:
message on the same connection. See section &<<SECTmulmessam>>& for an
explanation of when this might be needed.
-.new
.option hosts_noproxy_tls smtp "host list&!!" *
.cindex "TLS" "passing connection"
.cindex "multiple SMTP deliveries"
the SMTP connection from and to the new process and any subsequents.
The new process has no access to TLS information, so cannot include it in
logging.
-.wen
TLS session for any host that matches this list.
&%tls_verify_certificates%& should also be set for the transport.
+.new
+.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>>&.
+.wen
+
.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.
+.new
+.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>>&.
+.wen
+
.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&!!" *
-.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
+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 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.
(If an API is found to let OpenSSL be configured in this way,
let the Exim Maintainers know and we'll likely use it).
.next
-.new
With GnuTLS, if an explicit list is used for the &%tls_privatekey%& main option
main option, it must be ordered to match the &%tls_certificate%& list.
-.wen
.next
Some other recently added features may only be available in one or the other.
This should be documented with the feature. If the documentation does not
{HIGH:!MD5:!SHA1}}
.endd
-.new
This example will prefer ECDSA-authenticated ciphers over RSA ones:
.code
tls_require_ciphers = ECDSA:RSA:!COMPLEMENTOFDEFAULT
.endd
-.wen
.section "Requiring specific ciphers or other parameters in GnuTLS" &&&
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.
certificates that need to be sent to the client to enable it to authenticate
the server's certificate.
-.new
For dual-stack (eg. RSA and ECDSA) configurations, these options can be
colon-separated lists of file paths. Ciphers using given authentication
algorithms require the presence of a suitable certificate to supply the
public-key. The server selects among the certificates to present to the
client depending on the selected cipher, hence the priority ordering for
ciphers will affect which certificate is used.
-.wen
If you do not understand about certificates and keys, please try to find a
source of this background information, which is not Exim-specific. (There are a
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
"SECTmulmessam"
.cindex "multiple SMTP deliveries with TLS"
.cindex "TLS" "multiple message deliveries"
-.new
Exim sends multiple messages down the same TCP/IP connection by starting up
an entirely new delivery process for each message, passing the socket from
one process to the next. This implementation does not fit well with the use
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
-.wen
+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
+.new
+.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 traceable to the one
+defined by (one of?) the TSLA 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.
+
+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$& is a bitfield with numbered bits set for TLSA record usage codes.
+The zero above means DANE was not in use, the four means that only DANE-TA usage TLSA records were
+found. If the definition of &%hosts_request_ocsp%& includes the
+string "tls_out_tlsa_usage", they are re-expanded in time to
+control the OCSP request.
+
+This modification of hosts_request_ocsp is only done if it has the default value of "*". Admins who change it, and
+those who use &%hosts_require_ocsp%&, should consider the interaction with DANE in their OCSP settings.
+
+
+For client-side DANE there are two new smtp transport options, &%hosts_try_dane%& and &%hosts_require_dane%&.
+The latter variant will result in failure if the target host is not DNSSEC-secured.
+
+DANE will only be usable if the target host has DNSSEC-secured MX, A and TLSA records.
+
+A TLSA lookup will be done if either of the above options match and the host-lookup succeeded using dnssec.
+If a TLSA lookup is done and succeeds, a DANE-verified TLS connection
+will be required for the host. If it does not, the host will not
+be used; there is no fallback to non-DANE or non-TLS.
+
+If DANE is requested and useable (see above) the following transport options are ignored:
+.code
+ hosts_require_tls
+ tls_verify_hosts
+ tls_try_verify_hosts
+ tls_verify_certificates
+ tls_crl
+ tls_verify_cert_hostnames
+.endd
+
+If DANE is not usable, whether requested or not, and CA-anchored
+verification evaluation is wanted, the above variables should be set appropriately.
+
+Currently the &%dnssec_request_domains%& must be active and &%dnssec_require_domains%& is ignored.
+
+If verification was successful using DANE then the "CV" item in the delivery log line will show as "CV=dane".
+
+There is a new variable &$tls_out_dane$& which will have "yes" if
+verification succeeded using DANE and "no" otherwise (only useful
+in combination with EXPERIMENTAL_EVENT), and a new variable &$tls_out_tlsa_usage$& (detailed above).
+
+Under GnuTLS, DANE is only supported from version 3.0.0 onwards.
+.wen
+
+
+
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
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.
.vitem &*queue*&&~=&~<&'text'&>
+.cindex "&%queue%& ACL modifier"
+.cindex "named queues" "selecting in ACL"
This modifier specifies the use of a named queue for spool files
for the message.
It can only be used before the message is received (i.e. not in
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.
-.new
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.
-.wen
All the usual ACLs are called; if one results in the message being
rejected, all effort spent in delivery (including the costs on
.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'&>
sender when checking recipients. If used indiscriminately, it reduces the
usefulness of callout caching.
-.new
.vitem &*hold*&
This option applies to recipient callouts only. For example:
.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 use_sender option is used,
if neither the random nor the use_postmaster option is used,
and if no other callouts intervene.
-.wen
.endlist
If you use any of the parameters that set a non-empty sender for the MAIL
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,
+.new
+though individual ones can be included or not at build time:
+.wen
.vlist
.vitem &%avast%&
to be scanned, which will 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.
-There is an option WITH_OLD_CLAMAV_STREAM in &_src/EDITME_& available, should
-you be running a version of ClamAV prior to 0.95.
The final example shows that multiple TCP targets can be specified. Exim will
randomly use one for each incoming email (i.e. it load balances them). Note
.endd
If you omit the argument, the default values show above are used.
-.new
.vitem &%f-prot6d%&
.cindex "virus scanners" "f-prot6d"
The f-prot6d scanner is accessed using the FPSCAND protocol over TCP.
av_scanner = f-prot6d:localhost 10200
.endd
If you omit the argument, the default values show above are used.
-.wen
.vitem &%fsecure%&
.cindex "virus scanners" "F-Secure"
.code
av_scanner = sock:127.0.0.1 6001:%s:(SPAM|VIRUS):(.*)$
.endd
-.new
Note that surrounding whitespace is stripped from each option, meaning
there is no way to specify a trailing newline.
The socket specifier and both regular-expressions are required.
Default for the commandline is &_%s\n_& (note this does have a trailing newline);
specify an empty element to get this.
-.wen
.vitem &%sophie%&
.cindex "virus scanners" "Sophos and Sophie"
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
.vlist
.vitem &*int&~body_linecount*&
This variable contains the number of lines in the message's body.
-.new
It is not valid if the &%spool_files_wireformat%& option is used.
-.wen
.vitem &*int&~body_zerocount*&
This variable contains the number of binary zero bytes in the message's body.
-.new
It is not valid if the &%spool_files_wireformat%& option is used.
-.wen
.vitem &*unsigned&~int&~debug_selector*&
This variable is set to zero when no debugging is taking place. Otherwise, it
flagged with &`->`& instead of &`=>`&. When two or more messages are delivered
down a single SMTP connection, an asterisk follows the IP address in the log
lines for the second and subsequent messages.
-.new
When two or more messages are delivered down a single TLS connection, the
DNS and some TLS-related information logged for the first message delivered
will not be present in the log lines for the second and subsequent messages.
TLS cipher information is still available.
-.wen
.cindex "delivery" "cutthrough; logging"
.cindex "cutthrough" "logging"
&` `& 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
+.new
+.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.
+.wen
.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
&%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
+.new
+.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.
+.wen
+.next
.cindex "log" "dnslist defer"
.cindex "DNS list" "logging defer"
.cindex "black list (DNS)"
&%lost_incoming_connection%&: A log line is written when an incoming SMTP
connection is unexpectedly dropped.
.next
-.new
.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.
-.wen
.next
.cindex "log" "outgoing interface"
.cindex "log" "local interface"
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.
the local host is logged as QT=<&'time'&> on &"Completed"& lines, for
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.
+.new
+.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`&.
+.wen
.next
.cindex "log" "recipients"
&%received_recipients%&: The recipients of a message are listed in the main log
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
+.new
+using a CA trust anchor,
+&`CA=dane`& if using a DNS trust anchor,
+.wen
+and &`CV=no`& if not.
.next
.cindex "log" "TLS cipher"
.cindex "TLS" "logging cipher"
unprivileged), Exim must be built to allow group read access to its spool
files.
-.new
By default, regular users are trusted to perform basic testing and
introspection commands, as themselves. This setting can be tightened by
setting the &%commandline_checks_require_admin%& option.
This affects most of the checking options,
such as &%-be%& and anything else &%-b*%&.
-.wen
.section "Spool files" "SECID275"
-J file and uses it to update the -H file before starting the next delivery
attempt.
-.new
Files whose names end with -K or .eml may also be seen in the spool.
These are temporaries used for DKIM or malware processing, when that is used.
They should be tidied up by normal operations; any old ones are probably
relics of crashes and can be removed.
-.wen
.section "Format of the -H file" "SECID282"
.cindex "uid (user id)" "in spool file"
If a message was scanned by SpamAssassin, this is present. It records the value
of &$spam_score_int$&.
-.new
.vitem &%-spool_file_wireformat%&
The -D file for this message is in wire-format (for ESMTP CHUNKING)
rather than Unix-format.
The line-ending is CRLF rather than newline.
There is still, however, no leading-dot-stuffing.
-.wen
.vitem &%-tls_certificate_verified%&
A TLS certificate was received from the client that sent this message, and the
.ecindex IIDforspo2
.ecindex IIDforspo3
-.new
.section "Format of the -D file" "SECID282a"
The data file is traditionally in Unix-standard format: lines are ended with
an ASCII newline character.
ESMTP CHUNKING option, meaning lower processing overhead.
Lines are terminated with an ASCII CRLF pair.
There is no dot-stuffing (and no dot-termination).
-.wen
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
-.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.
+
+.new
+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.
+.wen
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_&.
default "policy". Instead it enables you to build your own policy using
Exim's standard controls.
+.new
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
+.wen
+
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"
+.new
+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.
+.wen
+
Signing is enabled by setting private options on the SMTP transport.
These options take (expandable) strings as arguments.
.option dkim_domain smtp string list&!! unset
The domain(s) you want to sign with.
-.new
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.
-.wen
-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.
-.new
After expansion, which can use &$dkim_domain$&, this can be a list.
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.
-.wen
+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
+.new
+.next
+with GnuTLS 3.6.0 or later, be a valid Ed25519 private key (same format as above)
+.wen
.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.
.new
+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.
+.wen
+
.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.
+method.
+
+.new
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+.endd
+.wen
.option dkim_identity smtp string&!! unset
If set after expansion, the value is used to set an "i=" tag in
syntax of this optional tag to a mail address, with possibly-empty
local part, an @, and a domain identical to or subdomain of the "d="
tag value. Note that Exim does not check the value.
-.wen
.option dkim_canon smtp string&!! unset
This option sets the canonicalization method used when signing a message.
.option dkim_sign_headers smtp string&!! "see below"
If set, this option must expand to a colon-separated
list of header names.
-.new
Headers with these names, or the absence or such a header, will be included
in the message signature.
When unspecified, the header names listed in RFC4871 will be used,
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.
-.wen
-.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
If a domain or identity is listed several times in the (expanded) value of
&%dkim_verify_signers%&, the ACL is only called once for that domain or identity.
-.new
If multiple signatures match a domain (or identity), the ACL is called once
for each matching signature.
-.wen
Inside the &%acl_smtp_dkim%&, the following expansion variables are
&%pass%&: The signature passed verification. It is valid.
.endlist
-.new
This variable can be overwritten using an ACL 'set' modifier.
This might, for instance, be done to enforce a policy restriction on
hash-method or key-size:
After all the DKIM ACL runs have completed, the value becomes a
colon-separated list of the values after each run.
-.wen
.vitem &%$dkim_verify_reason%&
A string giving a little bit more detail when &%$dkim_verify_status%& is either
DKIM verification. It may of course also mean that the signature is forged.
.endlist
-.new
-This variable can be overwritten using an ACL 'set' modifier.
-.wen
+This variable can be overwritten, with any value, using an ACL 'set' modifier.
.vitem &%$dkim_domain%&
The signing domain. IMPORTANT: This variable is only populated if there is
.vitem &%$dkim_algo%&
The algorithm used. One of 'rsa-sha1' or 'rsa-sha256'.
+.new
+If running under GnuTLS 3.6.0 or later, may also be 'ed25519-sha256'.
+The "_CRYPTO_SIGN_ED25519" macro will be defined if support is present
+for EC keys.
+.wen
+
+.new
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+
+DKIM signatures identified as having been signed with historic
+algorithms (currently, rsa-sha1) have permanently failed evaluation
+.endd
+
+To enforce this you must have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above.
+.wen
.vitem &%$dkim_canon_body%&
The body canonicalization method. One of 'relaxed' or 'simple'.
.vitem &%$dkim_key_length%&
Number of bits in the key.
+
+.new
+Note that RFC 8301 says:
+.code
+Verifiers MUST NOT consider signatures using RSA keys of
+less than 1024 bits as valid signatures.
+.endd
+
+To enforce this you must have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above.
+.wen
+
.endlist
In addition, two ACL conditions are provided:
for more information of what they mean.
.endlist
+
+
+
+.new
+.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 on the trasmission of messages; publishing certain
+DNS records is all that is required.
+
+For verification, an ACL condition and an expansion lookup are provided.
+
+.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. (Changed in 4.83)
+
+.vitem &%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)
+
+.vitem &%err_temp%&
+Same as permerror, deprecated in 4.83, will be removed in a future release.
+
+.vitem &%err_perm%&
+Same as temperror, deprecated in 4.83, will be removed in a future release.
+.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_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
+ ${lookup {username@domain} spf {ip.ip.ip.ip}}
+.endd
+
+The lookup will return the same result strings as they can appear in
+&$spf_result$& (pass,fail,softfail,neutral,none,err_perm,err_temp).
+Currently, only IPv4 addresses are supported.
+
+
+. wen-for SPF section
+.wen
+
+
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
.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.
${utf8_localpart_from_alabel:str}
.endd
-ACLs may use the following modifier:
+.cindex utf8 "address downconversion"
+.cindex i18n "utf8 address downconversion"
+.new
+The RCPT ACL
+.wen
+may use the following modifier:
.display
control = utf8_downconvert
control = utf8_downconvert/<value>
or &%headers_charset%& main configuration option (otherwise),
to the
modified UTF-7 encoding specified by RFC 2060,
-with the following exception: All occurences of <sep>
+with the following exception: All occurrences of <sep>
(which has to be a single character)
are replaced with periods ("."), and all periods and slashes that are not
<sep> and are not in the <specials> string are BASE64 encoded.
before or after the action is associates with. Those which fire before
can be used to affect that action (more on this below).
-.new
-The third column in the table above says what section of the configumration
+The third column in the table above says what section of the configuration
should define the event action.
-.wen
An additional variable, &$event_data$&, is filled with information varying
with the event type:
git://git.exim.org / exim.git / blobdiff