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
.code
${length_<n>:<string>}
.endd
-The result of this item is either the first <&'n'&> characters or the whole
+The result of this item is either the first <&'n'&> bytes or the whole
of <&'string2'&>, whichever is the shorter. Do not confuse &%length%& with
&%strlen%&, which gives the length of a string.
+All measurement is done in bytes and is not UTF-8 aware.
.vitem "&*${listextract{*&<&'number'&>&*}&&&
.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:
yields &"K1=A K4=D K3=C"&. Note the use of &`\N`& to protect the contents of
the regular expression from string expansion.
+The regular expression is compiled in 8-bit mode, working against bytes
+rather than any Unicode-aware character handling.
.vitem &*${sort{*&<&'string'&>&*}{*&<&'comparator'&>&*}{*&<&'extractor'&>&*}}*&
If the starting offset is greater than the string length the result is the
null string; if the length plus starting offset is greater than the string
length, the result is the right-hand part of the string, starting from the
-given offset. The first character in the string has offset zero.
+given offset. The first byte (character) in the string has offset zero.
The &%substr%& expansion item can take negative offset values to count
-from the right-hand end of its operand. The last character is offset -1, the
-second-last is offset -2, and so on. Thus, for example,
+from the right-hand end of its operand. The last byte (character) is offset -1,
+the second-last is offset -2, and so on. Thus, for example,
.code
${substr{-5}{2}{1234567}}
.endd
yields &"1"&.
When the second number is omitted from &%substr%&, the remainder of the string
-is taken if the offset is positive. If it is negative, all characters in the
+is taken if the offset is positive. If it is negative, all bytes (characters) in the
string preceding the offset point are taken. For example, an offset of -1 and
no length, as in these semantically identical examples:
.code
.endd
yields all but the last character of the string, that is, &"abcd"&.
+All measurement is done in bytes and is not UTF-8 aware.
+
.vitem "&*${tr{*&<&'subject'&>&*}{*&<&'characters'&>&*}&&&
{*&<&'replacements'&>&*}}*&"
.cindex "expansion" "character translation"
.cindex "&%tr%& expansion item"
-This item does single-character translation on its subject string. The second
+This item does single-character (in bytes) translation on its subject string. The second
argument is a list of characters to be translated in the subject string. Each
matching character is replaced by the corresponding character from the
replacement list. For example
last occurrence is used. If the third string is shorter than the second, its
last character is replicated. However, if it is empty, no translation takes
place.
+
+All character handling is done in bytes and is not UTF-8 aware.
+
.endlist
header line, and the effective address is extracted from it. If the string does
not parse successfully, the result is empty.
+The parsing correctly handles SMTPUTF8 Unicode in the string.
+
.vitem &*${addresses:*&<&'string'&>&*}*&
.cindex "expansion" "RFC 2822 address handling"
example below is passed the contents of &`$header_from:`&, meaning it gets
de-mimed. Exim sees the decoded "," so it treats it as &*two*& email addresses.
The third example shows that the presence of a comma is skipped when it is
-quoted.
+quoted. The fourth example shows SMTPUTF8 handling.
.code
# exim -be '${addresses:From: \
=?iso-8859-2?Q?Last=2C_First?= <user@example.com>}'
Last:user@example.com
# exim -be '${addresses:From: "Last, First" <user@example.com>}'
user@example.com
+# exim -be '${addresses:フィル <フィリップ@example.jp>}'
+フィリップ@example.jp
.endd
.vitem &*${base32:*&<&'digits'&>&*}*&
.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.
.code
${lc:$local_part}
.endd
+Case is defined per the system C locale.
.vitem &*${length_*&<&'number'&>&*:*&<&'string'&>&*}*&
.cindex "expansion" "string truncation"
See the description of the general &%length%& item above for details. Note that
&%length%& is not the same as &%strlen%&. The abbreviation &%l%& can be used
when &%length%& is used as an operator.
+All measurement is done in bytes and is not UTF-8 aware.
.vitem &*${listcount:*&<&'string'&>&*}*&
The string is interpreted as an RFC 2822 address and the local part is
extracted from it. If the string does not parse successfully, the result is
empty.
+The parsing correctly handles SMTPUTF8 Unicode in the string.
.vitem &*${mask:*&<&'IP&~address'&>&*/*&<&'bit&~count'&>&*}*&
If you are creating a new email address from the contents of &$local_part$&
(or any other unknown data), you should always use this operator.
+This quoting determination is not SMTPUTF8-aware, thus quoting non-ASCII data
+will likely use the quoting form.
+Thus &'${quote_local_part:フィル}'& will always become &'"フィル"'&.
+
.vitem &*${quote_*&<&'lookup-type'&>&*:*&<&'string'&>&*}*&
.cindex "quoting" "lookup-specific"
.cindex "&%strlen%& expansion item"
The item is replace by the length of the expanded string, expressed as a
decimal number. &*Note*&: Do not confuse &%strlen%& with &%length%&.
+All measurement is done in bytes and is not UTF-8 aware.
.vitem &*${substr_*&<&'start'&>&*_*&<&'length'&>&*:*&<&'string'&>&*}*&
.endd
See the description of the general &%substr%& item above for details. The
abbreviation &%s%& can be used when &%substr%& is used as an operator.
+All measurement is done in bytes and is not UTF-8 aware.
.vitem &*${time_eval:*&<&'string'&>&*}*&
.cindex "&%time_eval%& expansion item"
.cindex "expansion" "case forcing"
.cindex "&%uc%& expansion item"
This forces the letters in the string into upper-case.
+Case is defined per the system C locale.
.vitem &*${utf8clean:*&<&'string'&>&*}*&
.cindex "correction of invalid utf-8 sequences in strings"
.cindex "expansion" "utf-8 forcing"
.cindex "&%utf8clean%& expansion item"
This replaces any invalid utf-8 sequence in the string by the character &`?`&.
+.new
+In versions of Exim before 4.92, this did not correctly do so for a truncated
+final codepoint's encoding, and the character would be silently dropped.
+If you must handle detection of this scenario across both sets of Exim behavior,
+the complexity will depend upon the task.
+For instance, to detect if the first character is multibyte and a 1-byte
+extraction can be successfully used as a path component (as is common for
+dividing up delivery folders), you might use:
+.code
+condition = ${if inlist{${utf8clean:${length_1:$local_part}}}{:?}{yes}{no}}
+.endd
+(which will false-positive if the first character of the local part is a
+literal question mark).
+.wen
.vitem "&*${utf8_domain_to_alabel:*&<&'string'&>&*}*&" &&&
"&*${utf8_domain_from_alabel:*&<&'string'&>&*}*&" &&&
.cindex "&%eqi%& expansion condition"
The two substrings are first expanded. The condition is true if the two
resulting strings are identical. For &%eq%& the comparison includes the case of
-letters, whereas for &%eqi%& the comparison is case-independent.
+letters, whereas for &%eqi%& the comparison is case-independent, where
+case is defined per the system C locale.
.vitem &*exists&~{*&<&'file&~name'&>&*}*&
.cindex "expansion" "file existence test"
string is lexically greater than or equal to the second string. For &%ge%& the
comparison includes the case of letters, whereas for &%gei%& the comparison is
case-independent.
+Case and collation order are defined per the system C locale.
.vitem &*gt&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*& &&&
&*gti&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*&
string is lexically greater than the second string. For &%gt%& the comparison
includes the case of letters, whereas for &%gti%& the comparison is
case-independent.
+Case and collation order are defined per the system C locale.
.vitem &*inlist&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*& &&&
&*inlisti&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*&
Both strings are expanded; the second string is treated as a list of simple
strings; if the first string is a member of the second, then the condition
is true.
+For the case-independent &%inlisti%& condition, case is defined per the system C locale.
These are simpler to use versions of the more powerful &*forany*& condition.
Examples, and the &*forany*& equivalents:
string is lexically less than or equal to the second string. For &%le%& the
comparison includes the case of letters, whereas for &%lei%& the comparison is
case-independent.
+Case and collation order are defined per the system C locale.
.vitem &*lt&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*& &&&
&*lti&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*&
string is lexically less than the second string. For &%lt%& the comparison
includes the case of letters, whereas for &%lti%& the comparison is
case-independent.
+Case and collation order are defined per the system C locale.
.vitem &*match&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*&
and it may match anywhere in the subject, not just at the start. If you want
the pattern to match at the end of the subject, you must include the &`$`&
metacharacter at an appropriate point.
+All character handling is done in bytes and is not UTF-8 aware,
+but we might change this in a future Exim release.
.cindex "numerical variables (&$1$& &$2$& etc)" "in &%if%& expansion"
At the start of an &%if%& expansion the values of the numeric variable
.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"
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>>&.
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
+
.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"
.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"
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.
-Another approach which should be seriously considered is to use DANE with a certificate
-from a public CA, because of another technology, "MTA-STS", described below.
+.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:
information.
The MTA-STS design requires a certificate from a public Certificate Authority
-which is recognized by clients sending to you. That selection is outside your
-control.
+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
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
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
+
is insurance against disk crashes where the directory is lost but the files
themselves are recoverable.
+.new
+The file formats may be changed, or new formats added, at any release.
+Spool files are not intended as an interface to other programs
+and should not be used as such.
+.wen
+
Some people are tempted into editing -D files in order to modify messages. You
need to be extremely careful if you do this; it is not recommended and you are
on your own if you do it. Here are some of the pitfalls:
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"
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 &%$dkim_verify_status%& is either
The number of signed body bytes. If zero ("0"), the body is unsigned. If no
limit was set by the signer, "9999999999999" is returned. This makes sure
that this variable always expands to an integer value.
+.new
+&*Note:*& The presence of the signature tag specifying a signing body length
+is one possible route to spoofing of valid DKIM signatures.
+A paranoid implementation might wish to regard signature where this variable
+shows less than the "no limit" return as being invalid.
+.wen
.vitem &%$dkim_created%&
UNIX timestamp reflecting the date and time when the signature was created.
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
projects / exim.git / blobdiff