. 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)"
.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.
-.new
For the "-bP macro <name>" form, if no such macro is found
the exit status will be nonzero.
-.wen
.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%&
only by an admin user or by the user who originally caused the message to be
placed on the queue.
+. .new
+. .vitem &%-MS%&
+. .oindex "&%-MS%&"
+. .cindex REQUIRETLS
+. This option is used to request REQUIRETLS processing on the message.
+. It is used internally by Exim in conjunction with -E when generating
+. a bounce message.
+. .wen
+
.vitem &%-Mset%&&~<&'message&~id'&>
-.oindex "&%-Mset%&
+.oindex "&%-Mset%&"
.cindex "testing" "string expansion"
.cindex "expansion" "testing"
This option is useful only in conjunction with &%-be%& (that is, when testing
${lookup redis{get keyname}}
.endd
-.new
As of release 4.91, "lightweight" support for Redis Cluster is available.
Requires &%redis_servers%& list to contain all the servers in the cluster, all
of which must be reachable from the running exim instance. If the cluster has
master/slave replication, the list must contain all the master and slave
servers.
-When the Redis Cluster returns a "MOVED" response to a query, exim does not
+When the Redis Cluster returns a "MOVED" response to a query, Exim does not
immediately follow the redirection but treats the response as a DEFER, moving on
to the next server in the &%redis_servers%& list until the correct server is
reached.
-.wen
.ecindex IIDfidalo1
.ecindex IIDfidalo2
If the ACL returns defer the result is a forced-fail. Otherwise the expansion fails.
-.new
.vitem "&*${authresults{*&<&'authserv-id'&>&*}}*&"
.cindex authentication "results header"
.cindex headers "authentication-results:"
&'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:
+will be a domain name identifying the system performing the authentications.
+Methods that might be present in the result include:
.code
none
iprev
.code
add_header = :at_start:${authresults {$primary_hostname}}
.endd
-This is safe even if no authentication reselts are available.
-.wen
+This is safe even if no authentication results are available.
.vitem "&*${certextract{*&<&'field'&>&*}{*&<&'certificate'&>&*}&&&
&*$h_*&<&'header&~name'&>&*:*&" &&&
"&*$bheader_*&<&'header&~name'&>&*:*&&~or&~&&&
&*$bh_*&<&'header&~name'&>&*:*&" &&&
+ "&*$lheader_*&<&'header&~name'&>&*:*&&~or&~&&&
+ &*$lh_*&<&'header&~name'&>&*:*&"
"&*$rheader_*&<&'header&~name'&>&*:*&&~or&~&&&
&*$rh_*&<&'header&~name'&>&*:*&"
.cindex "expansion" "header insertion"
.vindex "&$header_$&"
.vindex "&$bheader_$&"
+.vindex "&$lheader_$&"
.vindex "&$rheader_$&"
.cindex "header lines" "in expansion strings"
.cindex "header lines" "character sets"
internal newlines (caused by splitting the header line over several physical
lines) may be present.
-The difference between &%rheader%&, &%bheader%&, and &%header%& is in the way
+The difference between the four pairs of expansions is in the way
the data in the header line is interpreted.
.ilist
&%rheader%& gives the original &"raw"& content of the header line, with no
processing at all, and without the removal of leading and trailing white space.
+.next
+.cindex "list" "of header lines"
+&%lheader%& gives a colon-separated list, one element per header when there
+are multiple headers with a given name.
+Any embedded colon characters within an element are doubled, so normal Exim
+list-processing facilities can be used.
+The terminating newline of each element is removed; in other respects
+the content is &"raw"&.
+
.next
.cindex "base64 encoding" "in header lines"
&%bheader%& removes leading and trailing white space, and then decodes base64
.code
${readsocket{/socket/name}{request string}{3s}}
.endd
+
The third argument is a list of options, of which the first element is the timeout
and must be present if the argument is given.
Further elements are options of form &'name=value'&.
-One option type is currently recognised, defining whether (the default)
+Two option types is currently recognised: shutdown and tls.
+The first defines whether (the default)
or not a shutdown is done on the connection after sending the request.
Example, to not do so (preferred, eg. by some webservers):
.code
${readsocket{/socket/name}{request string}{3s:shutdown=no}}
.endd
+.new
+The second, tls, controls the use of TLS on the connection. Example:
+.code
+${readsocket{/socket/name}{request string}{3s:tls=yes}}
+.endd
+The default is to not use TLS.
+If it is enabled, a shutdown as descripbed above is never done.
+.wen
+
A fourth argument allows you to change any newlines that are in the data
that is read, in the same way as for &%readfile%& (see above). This example
turns them into spaces:
.cindex "expansion" "hex to base64"
.cindex "&%hex2b64%& expansion item"
This operator converts a hex string into one that is base64 encoded. This can
-be useful for processing the output of the MD5 and SHA-1 hashing functions.
+be useful for processing the output of the various hashing functions.
The &%sha3%& expansion item is only supported if Exim has been
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 &*${stat:*&<&'string'&>&*}*&
the value of &$authenticated_id$& is normally the login name of the calling
process. However, a trusted user can override this by means of the &%-oMai%&
command line option.
-.new
This second case also sets up inforamtion used by the
&$authresults$& expansion item.
-.wen
.vitem &$authenticated_fail_id$&
.cindex "authentication" "fail" "id"
the result, the name is not accepted, and &$host_lookup_deferred$& is set to
&"1"&. See also &$sender_host_name$&.
-.new
.cindex authentication "expansion item"
Performing these checks sets up information used by the
&$authresults$& expansion item.
-.wen
.vitem &$host_lookup_failed$&
is compiled with the content-scanning extension. For details, see section
&<<SECTscanspamass>>&.
-.new
.vitem &$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$&"
&%certextract%& expansion item, &%md5%&, &%sha1%& or &%sha256%& operator,
or a &%def%& condition.
-&*Note*&: Under current versions of OpenSSL, when a list of more than one
+&*Note*&: Under versions of OpenSSL preceding 1.1.1,
+when a list of more than one
file is used for &%tls_certificate%&, this variable is not reliable.
.vitem &$tls_in_peercert$&
.row &%av_scanner%& "specify virus scanner"
.row &%check_rfc2047_length%& "check length of RFC 2047 &""encoded &&&
words""&"
+.row &%dns_cname_loops%& "follow CNAMEs returned by resolver"
.row &%dns_csa_search_limit%& "control CSA parent search depth"
.row &%dns_csa_use_reverse%& "en/disable CSA IP reverse search"
.row &%header_maxsize%& "total size of message header"
reversed and looked up in the reverse DNS, as described in more detail in
section &<<SECTverifyCSA>>&.
+.new
+.option dns_cname_loops main integer 1
+.cindex DNS "CNAME following"
+This option controls the following of CNAME chains, needed if the resolver does
+not do it internally.
+As of 2018 most should, and the default can be left.
+If you have an ancient one, a value of 10 is likely needed.
+
+The default value of one CNAME-follow is needed
+thanks to the observed return for an MX request,
+given no MX presence but a CNAME to an A, of the CNAME.
+.wen
+
.option dns_dnssec_ok main integer -1
.cindex "DNS" "resolver options"
implementations of TLS.
-option gnutls_allow_auto_pkcs11 main boolean unset
+.option gnutls_allow_auto_pkcs11 main boolean unset
This option will let GnuTLS (2.12.0 or later) autoload PKCS11 modules with
the p11-kit configuration files in &_/etc/pkcs11/modules/_&.
-.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 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 constructed for them).
&*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
+&*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
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
.cindex SSMTP
.cindex SMTPS
This option specifies a list of incoming SSMTP (aka SMTPS) ports that should
-operate the obsolete SSMTP (SMTPS) protocol, where a TLS session is immediately
+operate the SSMTP (SMTPS) protocol, where a TLS session is immediately
set up without waiting for the client to issue a STARTTLS command. For
further details, see section &<<SECTsupobssmt>>&.
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,
-.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"
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"
&%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:
option.
-.new
.option dane_require_tls_ciphers smtp string&!! unset
.cindex "TLS" "requiring specific ciphers for DANE"
.cindex "cipher" "requiring specific"
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.
-.wen
.option data_timeout smtp time 5m
of the message. Its value must not be zero. See also &%final_timeout%&.
+.option dkim_canon smtp string&!! unset
.option dkim_domain smtp string list&!! unset
-.option dkim_selector smtp string&!! unset
+.option dkim_hash smtp string&!! sha256
+.option dkim_identity smtp string&!! unset
.option dkim_private_key smtp string&!! unset
-.option dkim_canon smtp string&!! unset
+.option dkim_selector smtp string&!! unset
.option dkim_strict smtp string&!! unset
.option dkim_sign_headers smtp string&!! "per RFC"
-.option dkim_hash smtp string&!! sha256
-.option dkim_identity smtp string&!! unset
+.option dkim_timestamps smtp string&!! unset
DKIM signing options. For details see section &<<SECDKIMSIGN>>&.
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"
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"
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"
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"
If both this option and &%tls_try_verify_hosts%& are unset
operation is as if this option selected all hosts.
+.new
+.option utf8_downconvert smtp integer!! unset
+.cindex utf8 "address downconversion"
+.cindex i18n "utf8 address downconversion"
+If built with internationalization support,
+this option controls conversion of UTF-8 in message addresses
+to a-label form.
+For details see section &<<SECTi18nMTA>>&.
+.wen
+
client from which the message was received. This variable is empty if there was
no successful authentication.
-.new
.cindex authentication "expansion item"
Successful authentication sets up information used by the
&$authresults$& expansion item.
-.wen
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.
.option server_socket dovecot string unset
-This option must specify the socket that is the interface to Dovecot
+This option must specify the UNIX socket that is the interface to Dovecot
authentication. The &%public_name%& option must specify an authentication
mechanism that Dovecot is configured to support. You can have several
authenticators for different mechanisms. For example:
.option server_channelbinding gsasl boolean false
+Do not set this true without consulting a cryptographic engineer.
+
Some authentication mechanisms are able to use external context at both ends
of the session to bind the authentication to that context, and fail the
authentication process if that context differs. Specifically, some TLS
ciphersuites can provide identifying information about the cryptographic
context.
-This means that certificate identity and verification becomes a non-issue,
-as a man-in-the-middle attack will cause the correct client and server to
-see different identifiers and authentication will fail.
+This should have meant that certificate identity and verification becomes a
+non-issue, as a man-in-the-middle attack will cause the correct client and
+server to see different identifiers and authentication will fail.
This is currently only supported when using the GnuTLS library. This is
only usable by mechanisms which support "channel binding"; at time of
This defaults off to ensure smooth upgrade across Exim releases, in case
this option causes some clients to start failing. Some future release
-of Exim may switch the default to be true.
+of Exim might have switched the default to be true.
+
+However, Channel Binding in TLS has proven to be broken in current versions.
+Do not plan to rely upon this feature for security, ever, without consulting
+with a subject matter expert (a cryptographic engineer).
.option server_hostname gsasl string&!! "see below"
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.
+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
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.
+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%&
.oindex "&%tls_require_ciphers%&" "OpenSSL"
There is a function in the OpenSSL library that can be passed a list of cipher
suites before the cipher negotiation takes place. This specifies which ciphers
-are acceptable. The list is colon separated and may contain names like
+.new
+are acceptable for TLS versions prior to 1.3.
+.wen
+The list is colon separated and may contain names like
DES-CBC3-SHA. Exim passes the expanded value of &%tls_require_ciphers%&
directly to this function call.
Many systems will install the OpenSSL manual-pages, so you may have
tls_require_ciphers = ECDSA:RSA:!COMPLEMENTOFDEFAULT
.endd
+.new
+For TLS version 1.3 the control available is less fine-grained
+and Exim does not provide access to it at present.
+The value of the &%tls_require_ciphers%& option is ignored when
+TLS version 1.3 is negotiated.
+
+As of writing the library default cipher suite list for TLSv1.3 is
+.code
+TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256
+.endd
+.wen
+
.section "Requiring specific ciphers or other parameters in GnuTLS" &&&
"SECTreqciphgnu"
-.new
.section DANE "SECDANE"
.cindex DANE
DNS-based Authentication of Named Entities, as applied to SMTP over TLS, provides assurance to a client that
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.
+The TLSA record for the server may have "certificate usage" of DANE-TA(2) or DANE-EE(3).
+These are the "Trust Anchor" and "End Entity" variants.
+The latter specifies the End Entity directly, i.e. the certificate involved is that of the server
+(and if only DANE-EE is used then it 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.
+well-known one.
+A private CA at simplest is just a self-signed certificate (with certain
+attributes) which is used to sign cerver certificates, but running one securely
+does require careful arrangement.
+With DANE-TA, as implemented in Exim and commonly in other MTAs,
+the server TLS handshake must transmit the entire certificate chain from CA to server-certificate.
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.
+DANE-TA and DANE-EE can both be used together.
+
+.new
+Our recommendation is to use DANE with a certificate from a public CA,
+because this enables a variety of strategies for remote clients to verify
+your certificate.
+You can then publish information both via DANE and another technology,
+"MTA-STS", described below.
+
+When you use DANE-TA to publish trust anchor information, you ask entities
+outside your administrative control to trust the Certificate Authority for
+connections to you.
+If using a private CA then you should expect others to still apply the
+technical criteria they'd use for a public CA to your certificates.
+In particular, you should probably try to follow current best practices for CA
+operation around hash algorithms and key sizes.
+Do not expect other organizations to lower their security expectations just
+because a particular profile might be reasonable for your own internal use.
+
+When this text was last updated, this in practice means to avoid use of SHA-1
+and MD5; if using RSA to use key sizes of at least 2048 bits (and no larger
+than 4096, for interoperability); to use keyUsage fields correctly; to use
+random serial numbers.
+The list of requirements is subject to change as best practices evolve.
+If you're not already using a private CA, or it doesn't meet these
+requirements, then we encourage you to avoid all these issues and use a public
+CA such as &url(https://letsencrypt.org/,Let's Encrypt) instead.
+.wen
The TLSA record should have a Selector field of SPKI(1) and a Matching Type field of SHA2-512(2).
For use with the DANE-TA model, server certificates must have a correct name (SubjectName or SubjectAltName).
+.new
+The Certificate issued by the CA published in the DANE-TA model should be
+issued using a strong hash algorithm.
+Exim, and importantly various other MTAs sending to you, will not
+re-enable hash algorithms which have been disabled by default in TLS
+libraries.
+This means no MD5 and no SHA-1. SHA2-256 is the minimum for reliable
+interoperability (and probably the maximum too, in 2018).
+.wen
+
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:
Section 4.3 of that document.
Under GnuTLS, DANE is only supported from version 3.0.0 onwards.
-.wen
+
+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 of which CAs are trusted by others 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).
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.
warn message = Remove internal headers
remove_header = $acl_c_ihdrs
.endd
-Removed header lines are accumulated during the MAIL, RCPT, and predata ACLs.
-They are removed from the message before processing the DATA and MIME ACLs.
-There is no harm in attempting to remove the same header twice nor is removing
+Header names for removal are accumulated during the MAIL, RCPT, and predata ACLs.
+Matching header lines are removed from the message before processing the DATA and MIME ACLs.
+If multiple header lines match, all are removed.
+There is no harm in attempting to remove the same header twice nor in removing
a non-existent header. Further header lines to be removed may be accumulated
during the DATA and MIME ACLs, after which they are removed from the message,
if present. In the case of non-SMTP messages, headers to be removed are
a check that the IP being tested is indeed on the first list. The first
domain is the one that is put in &$dnslist_domain$&. For example:
.code
-reject message = \
+deny message = \
rejected because $sender_host_address is blacklisted \
at $dnslist_domain\n$dnslist_text
dnslists = \
given several times, but because the results of the DNS lookups are cached,
the DNS calls themselves are not repeated. For example:
.code
-reject dnslists = \
+deny dnslists = \
http.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.2 : \
socks.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.3 : \
misc.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.4 : \
The &%leaky%& (default) option means that the client's recorded rate is not
updated if it is above the limit. The effect of this is that Exim measures the
-client's average rate of successfully sent email, which cannot be greater than
-the maximum allowed. If the client is over the limit it may suffer some
-counter-measures (as specified in the ACL), but it will still be able to send
-email at the configured maximum rate, whatever the rate of its attempts. This
+client's average rate of successfully sent email,
+.new
+up to the given limit.
+This is appropriate if the countermeasure when the condition is true
+consists of refusing the message, and
is generally the better choice if you have clients that retry automatically.
-For example, it does not prevent a sender with an over-aggressive retry rate
-from getting any email through.
+If the action when true is anything more complex then this option is
+likely not what is wanted.
+.wen
The &%strict%& option means that the client's recorded rate is always
updated. The effect of this is that Exim measures the client's average rate
The main use of these variables is expected to be to distinguish between
rejections of MAIL and rejections of RCPT in callouts.
+.new
+The above variables may also be set after a &*successful*&
+address verification to:
+
+.ilist
+&%random%&: A random local-part callout succeeded
+.endlist
+.wen
+
Exim's side of the malware scanner, or are given on separate lines to
the daemon as options before the main scan command.
-.new
.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.
-.wen
For example:
.code
.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"
.section "Building Exim to use a local scan function" "SECID207"
.cindex "&[local_scan()]& function" "building Exim to use"
To make use of the local scan function feature, you must tell Exim where your
-function is before building Exim, by setting LOCAL_SCAN_SOURCE in your
+function is before building Exim, by setting
+.new
+both HAVE_LOCAL_SCAN and
+.wen
+LOCAL_SCAN_SOURCE in your
&_Local/Makefile_&. A recommended place to put it is in the &_Local_&
directory, so you might set
.code
+HAVE_LOCAL_SCAN=yes
LOCAL_SCAN_SOURCE=Local/local_scan.c
.endd
for example. The function must be called &[local_scan()]&. It is called by
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
&` 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
&%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
.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"
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
&%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
-.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"
be tracked on a per-domain basis, rather than merely upon source IP address.
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 (in the <= line) purposes.
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
.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
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.
The result can either
.ilist
be a valid RSA private key in ASCII armor (.pem file), including line breaks
-.new
.next
with GnuTLS 3.6.0 or OpenSSL 1.1.1 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
is set.
.endlist
-.new
To generate keys under OpenSSL:
.code
openssl genrsa -out dkim_rsa.private 2048
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
-.wen
+
+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.
+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
-.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
This option sets the canonicalization method used when signing a message.
The DKIM RFC currently supports two methods: "simple" and "relaxed".
The option defaults to "relaxed" when unset. Note: the current implementation
-only supports using the same canonicalization method for both headers and body.
+only supports signing with the same canonicalization method for both headers and body.
.option dkim_strict smtp string&!! unset
This option defines how Exim behaves when signing a message that
will be signed, and one signature added for a missing header with the
name will be appended.
+.new
+.option dkim_timestamps smtp integer&!! unset
+This option controls the inclusion of timestamp information in the signature.
+If not set, no such information will be included.
+Otherwise, must be an unsigned number giving an offset in seconds from the current time
+for the expiry tag
+(eg. 1209600 for two weeks);
+both creation (t=) and expiry (x=) tags will be included.
+
+RFC 6376 lists these tags as RECOMMENDED.
+.wen
+
.section "Verifying DKIM signatures in incoming mail" "SECDKIMVFY"
.cindex "DKIM" "verification"
-Verification of DKIM signatures in SMTP incoming email is implemented via the
-&%acl_smtp_dkim%& ACL. By default, this ACL is called once for each
+.new
+Verification of DKIM signatures in SMTP incoming email is done for all
+messages for which an ACL control &%dkim_disable_verify%& has not been set.
+.cindex authentication "expansion item"
+Performing verification sets up information used by the
+&$authresults$& expansion item.
+.wen
+
+.new The results of that verification are then made available to the
+&%acl_smtp_dkim%& ACL, &new(which can examine and modify them).
+By default, this ACL is called once for each
syntactically(!) correct signature in the incoming message.
A missing ACL definition defaults to accept.
If any ACL call does not accept, the message is not accepted.
If a cutthrough delivery was in progress for the message, that is
summarily dropped (having wasted the transmission effort).
-To evaluate the signature in the ACL a large number of expansion variables
+To evaluate the &new(verification result) in the ACL
+a large number of expansion variables
containing the signature status and its details are set up during the
runtime of the ACL.
-.new
-.cindex authentication "expansion item"
-Performing verification sets up information used by the
-&$authresults$& expansion item.
-.wen
-
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_verify_status}{pass}}
- condition = ${if eq {$len_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
+ warn condition = ${if eq {$dkim_verify_status}{pass}}
+ condition = ${if eq {${length_3:$dkim_algo}}{rsa}}
+ condition = ${if or {{eq {$dkim_algo}{rsa-sha1}} \
+ {< {$dkim_key_length}{1024}}}}
+ logwrite = NOTE: forcing DKIM verify fail (was pass)
+ set dkim_verify_status = fail
+ set dkim_verify_reason = hash too weak or key too short
.endd
-After all the DKIM ACL runs have completed, the value becomes a
+So long as a DKIM ACL is defined (it need do no more than accept),
+after all the DKIM ACL runs have completed, the value becomes a
colon-separated list of the values after each run.
+This is maintained for the mime, prdr and data ACLs.
.vitem &%$dkim_verify_reason%&
A string giving a little bit more detail when &%$dkim_verify_status%& is either
.vitem &%$dkim_algo%&
The algorithm used. One of 'rsa-sha1' or 'rsa-sha256'.
-.new
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.
-.wen
-.new
Note that RFC 8301 says:
.code
rsa-sha1 MUST NOT be used for signing or verifying.
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 &%$dkim_canon_body%&
The body canonicalization method. One of 'relaxed' or 'simple'.
The number of signed body bytes. If zero ("0"), the body is unsigned. If no
limit was set by the signer, "9999999999999" is returned. This makes sure
that this variable always expands to an integer value.
+.new
+&*Note:*& The presence of the signature tag specifying a signing body length
+is one possible route to spoofing of valid DKIM signatures.
+A paranoid implementation might wish to regard signature where this variable
+shows less than the "no limit" return as being invalid.
+.wen
.vitem &%$dkim_created%&
UNIX timestamp reflecting the date and time when the signature was created.
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.
-.new
Note that Exim does not check this value.
-.wen
.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.
-.new
Note that RFC 8301 says:
.code
Verifiers MUST NOT consider signatures using RSA keys of
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.
-.wen
.endlist
-.new
.section "SPF (Sender Policy Framework)" SECSPF
.cindex SPF verification
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.
+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.
-.new
.cindex authentication "expansion item"
Performing verification sets up information used by the
&$authresults$& expansion item.
-.wen
.cindex SPF "ACL condition"
${lookup {username@domain} spf {ip.ip.ip.ip}}
.endd
-The lookup will return the same result strings as they can appear in
+The lookup will return the same result strings as can appear in
&$spf_result$& (pass,fail,softfail,neutral,none,err_perm,err_temp).
Currently, only IPv4 addresses are supported.
-. wen-for SPF section
-.wen
. ////////////////////////////////////////////////////////////////////////////
.cindex utf8 "address downconversion"
.cindex i18n "utf8 address downconversion"
-.new
The RCPT ACL
-.wen
may use the following modifier:
.display
control = utf8_downconvert
If mua_wrapper is set, the utf8_downconvert control
is initially set to -1.
+.new
+The smtp transport has an option &%utf8_downconvert%&.
+If set it must expand to one of the three values described above,
+and it overrides any previously set value.
+.wen
+
There is no explicit support for VRFY and EXPN.
Configurations supporting these should inspect
git://git.exim.org / users / jgh / exim.git / blobdiff