. /////////////////////////////////////////////////////////////////////////////
.set previousversion "4.75"
-.set version "4.77"
+.set version "4.80"
.set ACL "access control lists (ACLs)"
.set I " "
<bookinfo>
<title>Specification of the Exim Mail Transfer Agent</title>
<titleabbrev>The Exim MTA</titleabbrev>
-<date>06 May 2011</date>
+<date>17 May 2012</date>
<author><firstname>Exim</firstname><surname>Maintainers</surname></author>
<authorinitials>EM</authorinitials>
<revhistory><revision>
- <revnumber>4.77</revnumber>
- <date>10 Oct 2011</date>
+ <revnumber>4.80</revnumber>
+ <date>17 May 2012</date>
<authorinitials>EM</authorinitials>
</revision></revhistory>
-<copyright><year>2011</year><holder>University of Cambridge</holder></copyright>
+<copyright><year>2012</year><holder>University of Cambridge</holder></copyright>
</bookinfo>
.literal off
overridden if necessary.
+.new
.section "PCRE library" "SECTpcre"
.cindex "PCRE library"
Exim no longer has an embedded PCRE library as the vast majority of
to install the PCRE or PCRE development package for your operating
system. If your system has a normal PCRE installation the Exim build
process will need no further configuration. If the library or the
-headers are in an unusual location you will need to set the PCRE_LIBS
-and INCLUDE directives appropriately. If your operating system has no
+headers are in an unusual location you will need to either set the PCRE_LIBS
+and INCLUDE directives appropriately,
+or set PCRE_CONFIG=yes to use the installed &(pcre-config)& command.
+If your operating system has no
PCRE support then you will need to obtain and build the current PCRE
from &url(ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/).
+More information on PCRE is available at &url(http://www.pcre.org/).
+.wen
.section "DBM libraries" "SECTdb"
.cindex "DBM libraries" "discussion of"
TLS_LIBS=-L/usr/local/openssl/lib -lssl -lcrypto
TLS_INCLUDE=-I/usr/local/openssl/include/
.endd
+.new
+.cindex "pkg-config" "OpenSSL"
+If you have &'pkg-config'& available, then instead you can just use:
+.code
+SUPPORT_TLS=yes
+USE_OPENSSL_PC=openssl
+.endd
+.wen
.cindex "USE_GNUTLS"
If GnuTLS is installed, you should set
.code
TLS_LIBS=-L/usr/gnu/lib -lgnutls -ltasn1 -lgcrypt
TLS_INCLUDE=-I/usr/gnu/include
.endd
+.new
+.cindex "pkg-config" "GnuTLS"
+If you have &'pkg-config'& available, then instead you can just use:
+.code
+SUPPORT_TLS=yes
+USE_GNUTLS=yes
+USE_GNUTLS_PC=gnutls
+.endd
+.wen
+
You do not need to set TLS_INCLUDE if the relevant directory is already
specified in INCLUDE. Details of how to configure Exim to make use of TLS are
given in chapter &<<CHAPTLS>>&.
binary, attempts to configure Exim to use it cause run time configuration
errors.
+.new
+.cindex "pkg-config" "lookups"
+.cindex "pkg-config" "authenticators"
+Many systems now use a tool called &'pkg-config'& to encapsulate information
+about how to compile against a library; Exim has some initial support for
+being able to use pkg-config for lookups and authenticators. For any given
+makefile variable which starts &`LOOKUP_`& or &`AUTH_`&, you can add a new
+variable with the &`_PC`& suffix in the name and assign as the value the
+name of the package to be queried. The results of querying via the
+&'pkg-config'& command will be added to the appropriate Makefile variables
+with &`+=`& directives, so your version of &'make'& will need to support that
+syntax. For instance:
+.code
+LOOKUP_SQLITE=yes
+LOOKUP_SQLITE_PC=sqlite3
+AUTH_GSASL=yes
+AUTH_GSASL_PC=libgsasl
+AUTH_HEIMDAL_GSSAPI=yes
+AUTH_HEIMDAL_GSSAPI_PC=heimdal-gssapi
+.endd
+.wen
+
.cindex "Perl" "including support for"
Exim can be linked with an embedded Perl interpreter, allowing Perl
subroutines to be called during string expansion. To enable this facility,
preference to the address taken from the message. The caller of Exim must be a
trusted user for the sender of a message to be set in this way.
+.vitem &%-bmalware%&&~<&'filename'&>
+.oindex "&%-bmalware%&"
+.cindex "testing", "malware"
+.cindex "malware scan test"
+This debugging option causes Exim to scan the given file,
+using the malware scanning framework. The option of &%av_scanner%& influences
+this option, so if &%av_scanner%&'s value is dependent upon an expansion then
+the expansion should have defaults which apply to this invocation. ACLs are
+not invoked, so if &%av_scanner%& references an ACL variable then that variable
+will never be populated and &%-bmalware%& will fail.
+
+Exim will have changed working directory before resolving the filename, so
+using fully qualified pathnames is advisable. Exim will be running as the Exim
+user when it tries to open the file, rather than as the invoking user.
+This option requires admin privileges.
+
+The &%-bmalware%& option will not be extended to be more generally useful,
+there are better tools for file-scanning. This option exists to help
+administrators verify their Exim and AV scanner configuration.
+
.vitem &%-bnq%&
.oindex "&%-bnq%&"
.cindex "address qualification, suppressing"
Exim behaves in exactly the same way as it does when receiving a message via
the listening daemon.
-.vitem &%-bmalware%&&~<&'filename'&>
-.oindex "&%-bmalware%&"
-.cindex "testing", "malware"
-.cindex "malware scan test"
-This debugging option causes Exim to scan the given file,
-using the malware scanning framework. The option of &%av_scanner%& influences
-this option, so if &%av_scanner%&'s value is dependent upon an expansion then
-the expansion should have defaults which apply to this invocation. ACLs are
-not invoked, so if &%av_scanner%& references an ACL variable then that variable
-will never be populated and &%-bmalware%& will fail.
-
-Exim will have changed working directory before resolving the filename, so
-using fully qualified pathnames is advisable. Exim will be running as the Exim
-user when it tries to open the file, rather than as the invoking user.
-This option requires admin privileges.
-
-The &%-bmalware%& option will not be extended to be more generally useful,
-there are better tools for file-scanning. This option exists to help
-administrators verify their Exim and AV scanner configuration.
-
.vitem &%-bt%&
.oindex "&%-bt%&"
.cindex "testing" "addresses"
than a recipient address. This affects any rewriting and qualification that
might happen.
+.vitem &%-bw%&
+.oindex "&%-bw%&"
+.cindex "daemon"
+.cindex "inetd"
+.cindex "inetd" "wait mode"
+This option runs Exim as a daemon, awaiting incoming SMTP connections,
+similarly to the &%-bd%& option. All port specifications on the command-line
+and in the configuration file are ignored. Queue-running may not be specified.
+
+In this mode, Exim expects to be passed a socket as fd 0 (stdin) which is
+listening for connections. This permits the system to start up and have
+inetd (or equivalent) listen on the SMTP ports, starting an Exim daemon for
+each port only when the first connection is received.
+
+If the option is given as &%-bw%&<&'time'&> then the time is a timeout, after
+which the daemon will exit, which should cause inetd to listen once more.
+
.vitem &%-C%&&~<&'filelist'&>
.oindex "&%-C%&"
.cindex "configuration file" "alternate"
i.e. PLAIN or LOGIN. The &%server_advertise_condition%& setting controls
when Exim offers authentication to clients; in the examples, this is only
when TLS or SSL has been started, so to enable the authenticators you also
-need to add support for TLS as described in &<<SECTdefconfmain>>&.
+need to add support for TLS as described in section &<<SECTdefconfmain>>&.
The &%server_condition%& setting defines how to verify that the username and
password are correct. In the examples it just produces an error message.
To make the authenticators work, you can use a string expansion
-expression like one of the examples in &<<CHAPplaintext>>&.
+expression like one of the examples in chapter &<<CHAPplaintext>>&.
Beware that the sequence of the parameters to PLAIN and LOGIN differ; the
-usercode and password are in different positions. &<<CHAPplaintext>>&
-covers both.
+usercode and password are in different positions.
+Chapter &<<CHAPplaintext>>& covers both.
.ecindex IIDconfiwal
the DB_UNKNOWN option. This enables it to handle any of the types of database
that the library supports, and can be useful for accessing DBM files created by
other applications. (For earlier DB versions, DB_HASH is always used.)
+.new
+.next
+.cindex "lookup" "dbmjz"
+.cindex "lookup" "dbm &-- embedded NULs"
+.cindex "sasldb2"
+.cindex "dbmjz lookup type"
+&(dbmjz)&: This is the same as &(dbm)&, except that the lookup key is
+interpreted as an Exim list; the elements of the list are joined together with
+ASCII NUL characters to form the lookup key. An example usage would be to
+authenticate incoming SMTP calls using the passwords from Cyrus SASL's
+&_/etc/sasldb2_& file with the &(gsasl)& authenticator or Exim's own
+&(cram_md5)& authenticator.
+.wen
.next
.cindex "lookup" "dbmnz"
.cindex "lookup" "dbm &-- terminating zero"
.cindex "binary zero" "in lookup key"
.cindex "Courier"
.cindex "&_/etc/userdbshadow.dat_&"
-.cindex "dmbnz lookup type"
+.cindex "dbmnz lookup type"
&(dbmnz)&: This is the same as &(dbm)&, except that a terminating binary zero
is not included in the key that is passed to the DBM library. You may need this
if you want to look up data in files that are created by or shared with some
&`fail`& keyword causes a &'forced expansion failure'& &-- see section
&<<SECTforexpfai>>& for an explanation of what this means.
-The supported DNS record types are A, CNAME, MX, NS, PTR, SRV, and TXT, and,
-when Exim is compiled with IPv6 support, AAAA (and A6 if that is also
+.new
+The supported DNS record types are A, CNAME, MX, NS, PTR, SPF, SRV, and TXT,
+and, when Exim is compiled with IPv6 support, AAAA (and A6 if that is also
configured). If no type is given, TXT is assumed. When the type is PTR,
the data can be an IP address, written as normal; inversion and the addition of
&%in-addr.arpa%& or &%ip6.arpa%& happens automatically. For example:
+.wen
.code
${lookup dnsdb{ptr=192.168.4.5}{$value}fail}
.endd
white space is ignored.
.cindex "TXT record" "in &(dnsdb)& lookup"
+.cindex "SPF record" "in &(dnsdb)& lookup"
+.new
For TXT records with multiple items of data, only the first item is returned,
unless a separator for them is specified using a comma after the separator
character followed immediately by the TXT record item separator. To concatenate
-items without a separator, use a semicolon instead.
+items without a separator, use a semicolon instead. For SPF records the
+default behaviour is to concatenate multiple items without using a separator.
+.wen
.code
${lookup dnsdb{>\n,: txt=a.b.example}}
${lookup dnsdb{>\n; txt=a.b.example}}
+${lookup dnsdb{spf=example.org}}
.endd
It is permitted to specify a space as the separator character. Further
white space is ignored.
below in section &<<SECTexpansionitems>>& onwards. Backslash is used as an
escape character, as described in the following section.
+Whether a string is expanded depends upon the context. Usually this is solely
+dependent upon the option for which a value is sought; in this documentation,
+options for which string expansion is performed are marked with † after
+the data type. ACL rules always expand strings. A couple of expansion
+conditions do not expand some of the brace-delimited branches, for security
+reasons.
+
.section "Literal text in expanded strings" "SECTlittext"
permitted. This can be useful when processing numbers extracted from dates or
times, which often do have leading zeros.
-A number may be followed by &"K"& or &"M"& to multiply it by 1024 or 1024*1024,
+A number may be followed by &"K"&, &"M"& or &"G"& to multiply it by 1024, 1024*1024
+or 1024*1024*1024,
respectively. Negative numbers are supported. The result of the computation is
-a decimal representation of the answer (without &"K"& or &"M"&). For example:
+a decimal representation of the answer (without &"K"&, &"M"& or &"G"&). For example:
.display
&`${eval:1+1} `& yields 2
supplied number and is at least 0. The quality of this randomness depends
on how Exim was built; the values are not suitable for keying material.
If Exim is linked against OpenSSL then RAND_pseudo_bytes() is used.
+.new
+If Exim is linked against GnuTLS then gnutls_rnd(GNUTLS_RND_NONCE) is used,
+for versions of GnuTLS with that function.
+.wen
Otherwise, the implementation may be arc4random(), random() seeded by
srandomdev() or srandom(), or a custom implementation even weaker than
random().
dotted-nibble hexadecimal form. In both cases, this is the "natural" form
for DNS. For example,
.code
-${reverse_ip:192.0.2.4} and ${reverse_ip:2001:0db8:c42:9:1:abcd:192.0.2.3}
+${reverse_ip:192.0.2.4}
+${reverse_ip:2001:0db8:c42:9:1:abcd:192.0.2.3}
.endd
returns
.code
-4.2.0.192 and 3.0.2.0.0.0.0.c.d.c.b.a.1.0.0.0.9.0.0.0.2.4.c.0.8.b.d.0.1.0.0.2
+4.2.0.192
+3.0.2.0.0.0.0.c.d.c.b.a.1.0.0.0.9.0.0.0.2.4.c.0.8.b.d.0.1.0.0.2
.endd
As a special case, the numerical value of an empty string is taken as
zero.
+In all cases, a relative comparator OP is testing if <&'string1'&> OP
+<&'string2'&>; the above example is checking if &$message_size$& is larger than
+10M, not if 10M is larger than &$message_size$&.
+
.vitem &*bool&~{*&<&'string'&>&*}*&
.cindex "expansion" "boolean parsing"
This condition turns a string holding a true or false representation into
a boolean state. It parses &"true"&, &"false"&, &"yes"& and &"no"&
(case-insensitively); also positive integer numbers map to true if non-zero,
-false if zero. Leading and trailing whitespace is ignored.
+false if zero.
+An empty string is treated as false.
+Leading and trailing whitespace is ignored;
+thus a string consisting only of whitespace is false.
All other string values will result in expansion failure.
When combined with ACL variables, this expansion condition will let you
command, which can be found in the separate document entitled &'Exim's
interfaces to mail filtering'&.
+.new
+.vitem &$tls_bits$&
+.vindex "&$tls_bits$&"
+Contains an approximation of the TLS cipher's bit-strength; the meaning of
+this depends upon the TLS implementation used.
+If TLS has not been negotiated, the value will be 0.
+The value of this is automatically fed into the Cyrus SASL authenticator
+when acting as a server, to specify the "external SSF" (a SASL term).
+.wen
+
.vitem &$tls_certificate_verified$&
.vindex "&$tls_certificate_verified$&"
This variable is set to &"1"& if a TLS certificate was verified when the
value is retained during message delivery, except during outbound SMTP
deliveries.
+.new
+.vitem &$tls_sni$&
+.vindex "&$tls_sni$&"
+.cindex "TLS" "Server Name Indication"
+When a TLS session is being established, if the client sends the Server
+Name Indication extension, the value will be placed in this variable.
+If the variable appears in &%tls_certificate%& then this option and
+some others, described in &<<SECTtlssni>>&,
+will be re-expanded early in the TLS session, to permit
+a different certificate to be presented (and optionally a different key to be
+used) to the client, based upon the value of the SNI extension.
+
+The value will be retained for the lifetime of the message. During outbound
+SMTP deliveries, it reflects the value of the &%tls_sni%& option on
+the transport.
+.wen
+
.vitem &$tod_bsdinbox$&
.vindex "&$tod_bsdinbox$&"
The time of day and the date, in the format required for BSD-style mailbox
.vindex "&$tod_epoch$&"
The time and date as a number of seconds since the start of the Unix epoch.
+.vitem &$tod_epoch_l$&
+.vindex "&$tod_epoch_l$&"
+The time and date as a number of microseconds since the start of the Unix epoch.
+
.vitem &$tod_full$&
.vindex "&$tod_full$&"
A full version of the time and date, for example: Wed, 16 Oct 1995 09:51:40
.section "TLS" "SECID108"
.table2
-.row &%gnutls_require_kx%& "control GnuTLS key exchanges"
-.row &%gnutls_require_mac%& "control GnuTLS MAC algorithms"
-.row &%gnutls_require_protocols%& "control GnuTLS protocols"
.row &%gnutls_compat_mode%& "use GnuTLS compatibility mode"
.row &%openssl_options%& "adjust OpenSSL compatibility options"
.row &%tls_advertise_hosts%& "advertise TLS to these hosts"
.row &%tls_certificate%& "location of server certificate"
.row &%tls_crl%& "certificate revocation list"
+.row &%tls_dh_max_bits%& "clamp D-H bit count suggestion"
.row &%tls_dhparam%& "DH parameters for server"
.row &%tls_on_connect_ports%& "specify SSMTP (SMTPS) ports"
.row &%tls_privatekey%& "location of server private key"
Those options that undergo string expansion before use are marked with
†.
-.option accept_8bitmime main boolean false
+.new
+.option accept_8bitmime main boolean true
.cindex "8BITMIME"
.cindex "8-bit characters"
This option causes Exim to send 8BITMIME in its response to an SMTP
EHLO command, and to accept the BODY= parameter on MAIL commands.
However, though Exim is 8-bit clean, it is not a protocol converter, and it
takes no steps to do anything special with messages received by this route.
-Consequently, this option is turned off by default.
+
+Historically Exim kept this option off by default, but the maintainers
+feel that in today's Internet, this causes more problems than it solves.
+It now defaults to true.
+A more detailed analysis of the issues is provided by Dan Bernstein:
+.display
+&url(http://cr.yp.to/smtp/8bitmime.html)
+.endd
+.wen
.option acl_not_smtp main string&!! unset
.cindex "&ACL;" "for non-SMTP messages"
See &%gecos_name%& above.
-.option gnutls_require_kx main string unset
-This option controls the key exchange mechanisms when GnuTLS is used in an Exim
-server. For details, see section &<<SECTreqciphgnu>>&.
-
-.option gnutls_require_mac main string unset
-This option controls the MAC algorithms when GnuTLS is used in an Exim
-server. For details, see section &<<SECTreqciphgnu>>&.
-
-.option gnutls_require_protocols main string unset
-This option controls the protocols when GnuTLS is used in an Exim
-server. For details, see section &<<SECTreqciphgnu>>&.
-
.option gnutls_compat_mode main boolean unset
This option controls whether GnuTLS is used in compatibility mode in an Exim
server. This reduces security slightly, but improves interworking with older
transport driver.
-.option openssl_options main "string list" +dont_insert_empty_fragments
+.option openssl_options main "string list" "+no_sslv2"
.cindex "OpenSSL "compatibility options"
This option allows an administrator to adjust the SSL options applied
by OpenSSL to connections. It is given as a space-separated list of items,
-each one to be +added or -subtracted from the current value. The default
-value is one option which happens to have been set historically. You can
-remove all options with:
-.code
-openssl_options = -all
-.endd
+each one to be +added or -subtracted from the current value.
+
This option is only available if Exim is built against OpenSSL. The values
available for this option vary according to the age of your OpenSSL install.
The &"all"& value controls a subset of flags which are available, typically
Note that adjusting the options can have severe impact upon the security of
SSL as used by Exim. It is possible to disable safety checks and shoot
yourself in the foot in various unpleasant ways. This option should not be
-adjusted lightly. An unrecognised item will be detected at by invoking Exim
-with the &%-bV%& flag.
+adjusted lightly. An unrecognised item will be detected at startup, by
+invoking Exim with the &%-bV%& flag.
+
+.new
+Historical note: prior to release 4.80, Exim defaulted this value to
+"+dont_insert_empty_fragments", which may still be needed for compatibility
+with some clients, but which lowers security by increasing exposure to
+some now infamous attacks.
+.wen
An example:
.code
-openssl_options = -all +microsoft_big_sslv3_buffer
+# Make both old MS and old Eudora happy:
+openssl_options = -all +microsoft_big_sslv3_buffer \
+ +dont_insert_empty_fragments
.endd
+Possible options may include:
+.ilist
+&`all`&
+.next
+&`allow_unsafe_legacy_renegotiation`&
+.next
+&`cipher_server_preference`&
+.next
+&`dont_insert_empty_fragments`&
+.next
+&`ephemeral_rsa`&
+.next
+&`legacy_server_connect`&
+.next
+&`microsoft_big_sslv3_buffer`&
+.next
+&`microsoft_sess_id_bug`&
+.next
+&`msie_sslv2_rsa_padding`&
+.next
+&`netscape_challenge_bug`&
+.next
+&`netscape_reuse_cipher_change_bug`&
+.next
+&`no_compression`&
+.next
+&`no_session_resumption_on_renegotiation`&
+.next
+&`no_sslv2`&
+.next
+&`no_sslv3`&
+.next
+&`no_ticket`&
+.next
+&`no_tlsv1`&
+.next
+&`no_tlsv1_1`&
+.next
+&`no_tlsv1_2`&
+.next
+&`single_dh_use`&
+.next
+&`single_ecdh_use`&
+.next
+&`ssleay_080_client_dh_bug`&
+.next
+&`sslref2_reuse_cert_type_bug`&
+.next
+&`tls_block_padding_bug`&
+.next
+&`tls_d5_bug`&
+.next
+&`tls_rollback_bug`&
+.endlist
+
.option oracle_servers main "string list" unset
.cindex "Oracle" "server list"
use when sending messages as a client, you must set the &%tls_certificate%&
option in the relevant &(smtp)& transport.
+.new
+If the option contains &$tls_sni$& and Exim is built against OpenSSL, then
+if the OpenSSL build supports TLS extensions and the TLS client sends the
+Server Name Indication extension, then this option and others documented in
+&<<SECTtlssni>>& will be re-expanded.
+.wen
.option tls_crl main string&!! unset
.cindex "TLS" "server certificate revocation list"
This option specifies a certificate revocation list. The expanded value must
be the name of a file that contains a CRL in PEM format.
+.new
+See &<<SECTtlssni>>& for discussion of when this option might be re-expanded.
+.wen
+
+
+.new
+.option tls_dh_max_bits main integer 2236
+.cindex "TLS" "D-H bit count"
+The number of bits used for Diffie-Hellman key-exchange may be suggested by
+the chosen TLS library. That value might prove to be too high for
+interoperability. This option provides a maximum clamp on the value
+suggested, trading off security for interoperability.
+
+The value must be at least 1024.
+
+The value 2236 was chosen because, at time of adding the option, it was the
+hard-coded maximum value supported by the NSS cryptographic library, as used
+by Thunderbird, while GnuTLS was suggesting 2432 bits as normal.
+
+If you prefer more security and are willing to break some clients, raise this
+number.
+
+Note that the value passed to GnuTLS for *generating* a new prime may be a
+little less than this figure, because GnuTLS is inexact and may produce a
+larger prime than requested.
+.wen
+
.option tls_dhparam main string&!! unset
.cindex "TLS" "D-H parameters for server"
-The value of this option is expanded, and must then be the absolute path to
-a file which contains the server's DH parameter values.
-This is used only for OpenSSL. When Exim is linked with GnuTLS, this option is
-ignored. See section &<<SECTopenvsgnu>>& for further details.
+.new
+The value of this option is expanded and indicates the source of DH parameters
+to be used by Exim.
+
+If it is a filename starting with a &`/`&, then it names a file from which DH
+parameters should be loaded. If the file exists, it should hold a PEM-encoded
+PKCS#3 representation of the DH prime. If the file does not exist, for
+OpenSSL it is an error. For GnuTLS, Exim will attempt to create the file and
+fill it with a generated DH prime. For OpenSSL, if the DH bit-count from
+loading the file is greater than &%tls_dh_max_bits%& then it will be ignored,
+and treated as though the &%tls_dhparam%& were set to "none".
+
+If this option expands to the string "none", then no DH parameters will be
+loaded by Exim.
+
+If this option expands to the string "historic" and Exim is using GnuTLS, then
+Exim will attempt to load a file from inside the spool directory. If the file
+does not exist, Exim will attempt to create it.
+See section &<<SECTgnutlsparam>>& for further details.
+
+If Exim is using OpenSSL and this option is empty or unset, then Exim will load
+a default DH prime; the default is the 2048 bit prime described in section
+2.2 of RFC 5114, "2048-bit MODP Group with 224-bit Prime Order Subgroup", which
+in IKE is assigned number 23.
+
+Otherwise, the option must expand to the name used by Exim for any of a number
+of DH primes specified in RFC 2409, RFC 3526 and RFC 5114. As names, Exim uses
+"ike" followed by the number used by IKE, of "default" which corresponds to
+"ike23".
+
+The available primes are:
+&`ike1`&, &`ike2`&, &`ike5`&,
+&`ike14`&, &`ike15`&, &`ike16`&, &`ike17`&, &`ike18`&,
+&`ike22`&, &`ike23`& (aka &`default`&) and &`ike24`&.
+
+Some of these will be too small to be accepted by clients.
+Some may be too large to be accepted by clients.
+.wen
.option tls_on_connect_ports main "string list" unset
key is assumed to be in the same file as the server's certificates. See chapter
&<<CHAPTLS>>& for further details.
+.new
+See &<<SECTtlssni>>& for discussion of when this option might be re-expanded.
+.wen
+
.option tls_remember_esmtp main boolean false
.cindex "TLS" "esmtp state; remembering"
Thus the values defined should be considered public data. To avoid this,
use OpenSSL with a directory.
+.new
+See &<<SECTtlssni>>& for discussion of when this option might be re-expanded.
+.wen
+
.option tls_verify_hosts main "host list&!!" unset
.cindex "TLS" "client certificate verification"
case routing fails.
+.new
+.section "Declining addresses by dnslookup" "SECTdnslookupdecline"
+.cindex "&(dnslookup)& router" "declines"
+There are a few cases where a &(dnslookup)& router will decline to accept
+an address; if such a router is expected to handle "all remaining non-local
+domains", then it is important to set &%no_more%&.
+
+Reasons for a &(dnslookup)& router to decline currently include:
+.ilist
+The domain does not exist in DNS
+.next
+The domain exists but the MX record's host part is just "."; this is a common
+convention (borrowed from SRV) used to indicate that there is no such service
+for this domain and to not fall back to trying A/AAAA records.
+.next
+Ditto, but for SRV records, when &%check_srv%& is set on this router.
+.next
+MX record points to a non-existent host.
+.next
+MX record points to an IP address and the main section option
+&%allow_mx_to_ip%& is not set.
+.next
+MX records exist and point to valid hosts, but all hosts resolve only to
+addresses blocked by the &%ignore_target_hosts%& generic option on this router.
+.next
+The domain is not syntactically valid (see also &%allow_utf8_domains%& and
+&%dns_check_names_pattern%& for handling one variant of this)
+.next
+&%check_secondary_mx%& is set on this router but the local host can
+not be found in the MX records (see below)
+.endlist
+.wen
+
+
.section "Private options for dnslookup" "SECID118"
.section "Use of $tls_cipher and $tls_peerdn" "usecippeer"
+.vindex &$tls_bits$&
.vindex &$tls_cipher$&
.vindex &$tls_peerdn$&
-At the start of a run of the &(smtp)& transport, the values of &$tls_cipher$&
-and &$tls_peerdn$& are the values that were set when the message was received.
+.vindex &$tls_sni$&
+At the start of a run of the &(smtp)& transport, the values of &$tls_bits$&,
+&$tls_cipher$&, &$tls_peerdn$& and &$tls_sni$&
+are the values that were set when the message was received.
These are the values that are used for options that are expanded before any
-SMTP connections are made. Just before each connection is made, these two
+SMTP connections are made. Just before each connection is made, these four
variables are emptied. If TLS is subsequently started, they are set to the
appropriate values for the outgoing connection, and these are the values that
are in force when any authenticators are run and when the
instead of using the DNS. Of course, that function may in fact use the DNS, but
it may also consult other sources of information such as &_/etc/hosts_&.
-.option gnutls_require_kx smtp string unset
-This option controls the key exchange mechanisms when GnuTLS is used in an Exim
-client. For details, see section &<<SECTreqciphgnu>>&.
-
-.option gnutls_require_mac smtp string unset
-This option controls the MAC algorithms when GnuTLS is used in an Exim
-client. For details, see section &<<SECTreqciphgnu>>&.
-
-.option gnutls_require_protocols smtp string unset
-This option controls the protocols when GnuTLS is used in an Exim
-client. For details, see section &<<SECTreqciphgnu>>&.
-
.option gnutls_compat_mode smtp boolean unset
This option controls whether GnuTLS is used in compatibility mode in an Exim
server. This reduces security slightly, but improves interworking with older
+.new
+.option tls_sni smtp string&!! unset
+.cindex "TLS" "Server Name Indication"
+.vindex "&$tls_sni$&"
+If this option is set then it sets the $tls_sni variable and causes any
+TLS session to pass this value as the Server Name Indication extension to
+the remote side, which can be used by the remote side to select an appropriate
+certificate and private key for the session.
+
+See &<<SECTtlssni>>& for more information.
+
+OpenSSL only, also requiring a build of OpenSSL that supports TLS extensions.
+.wen
+
+
+
.option tls_tempfail_tryclear smtp boolean true
.cindex "4&'xx'& responses" "to STARTTLS"
When the server host is not in &%hosts_require_tls%&, and there is a problem in
.code
AUTH_CRAM_MD5=yes
AUTH_CYRUS_SASL=yes
+.new
+AUTH_DOVECOT=yes
+AUTH_GSASL=yes
+AUTH_HEIMDAL_GSSAPI=yes
+.wen
AUTH_PLAINTEXT=yes
AUTH_SPA=yes
.endd
in &_Local/Makefile_&, respectively. The first of these supports the CRAM-MD5
authentication mechanism (RFC 2195), and the second provides an interface to
-the Cyrus SASL authentication library. The third can be configured to support
+the Cyrus SASL authentication library.
+.new
+The third is an interface to Dovecot's authentication system, delegating the
+work via a socket interface.
+The fourth provides an interface to the GNU SASL authentication library, which
+provides mechanisms but typically not data sources.
+The fifth provides direct access to Heimdal GSSAPI, geared for Kerberos, but
+supporting setting a server keytab.
+The sixth can be configured to support
the PLAIN authentication mechanism (RFC 2595) or the LOGIN mechanism, which is
-not formally documented, but used by several MUAs. The fourth authenticator
+not formally documented, but used by several MUAs. The seventh authenticator
supports Microsoft's &'Secure Password Authentication'& mechanism.
+.wen
The authenticators are configured using the same syntax as other drivers (see
section &<<SECTfordricon>>&). If no authenticators are required, no
authenticators, followed by general discussion of the way authentication works
in Exim.
+.new
+&*Beware:*& the meaning of &$auth1$&, &$auth2$&, ... varies on a per-driver and
+per-mechanism basis. Please read carefully to determine which variables hold
+account labels such as usercodes and which hold passwords or other
+authenticating data.
+
+Note that some mechanisms support two different identifiers for accounts: the
+&'authentication id'& and the &'authorization id'&. The contractions &'authn'&
+and &'authz'& are commonly encountered. The American spelling is standard here.
+Conceptually, authentication data such as passwords are tied to the identifier
+used to authenticate; servers may have rules to permit one user to act as a
+second user, so that after login the session is treated as though that second
+user had logged in. That second user is the &'authorization id'&. A robust
+configuration might confirm that the &'authz'& field is empty or matches the
+&'authn'& field. Often this is just ignored. The &'authn'& can be considered
+as verified data, the &'authz'& as an unverified request which the server might
+choose to honour.
+
+A &'realm'& is a text string, typically a domain name, presented by a server
+to a client to help it select an account and credentials to use. In some
+mechanisms, the client and server provably agree on the realm, but clients
+typically can not treat the realm as secure data to be blindly trusted.
+.wen
+
.section "Generic options for authenticators" "SECID168"
is used directly to control authentication. See section &<<SECTplainserver>>&
for details.
+.new
+For the &(gsasl)& authenticator, this option is required for various
+mechanisms; see chapter &<<CHAPgsasl>>& for details.
+.wen
+
For the other authenticators, &%server_condition%& can be used as an additional
authentication or authorization mechanism that is applied after the other
authenticator conditions succeed. If it is set, it is expanded when the
server_prompts = Username:: : Password::
server_condition = ${if and{{ \
!eq{}{$auth1} }{ \
- ldapauth{user="cn=${quote_ldap_dn:$auth1},ou=people,o=example.org" \
- pass=${quote:$auth2} \
- ldap://ldap.example.org/} }} }
+ ldapauth{\
+ user="uid=${quote_ldap_dn:$auth1},ou=people,o=example.org" \
+ pass=${quote:$auth2} \
+ ldap://ldap.example.org/} }} }
server_set_id = uid=$auth1,ou=people,o=example.org
.endd
We have to check that the username is not empty before using it, because LDAP
Note that this expansion explicitly forces failure if the lookup fails
because &$auth1$& contains an unknown user name.
+.new
+As another example, if you wish to re-use a Cyrus SASL sasldb2 file without
+using the relevant libraries, you need to know the realm to specify in the
+lookup and then ask for the &"userPassword"& attribute for that user in that
+realm, with:
+.code
+cyrusless_crammd5:
+ driver = cram_md5
+ public_name = CRAM-MD5
+ server_secret = ${lookup{$auth1:mail.example.org:userPassword}\
+ dbmjz{/etc/sasldb2}}
+ server_set_id = $auth1
+.endd
+.wen
.section "Using cram_md5 as a client" "SECID177"
.cindex "options" "&(cram_md5)& authenticator (client)"
Kerberos, note that because of limitations in the GSSAPI interface,
changing the server keytab might need to be communicated down to the Kerberos
layer independently. The mechanism for doing so is dependent upon the Kerberos
-implementation. For example, for Heimdal, the environment variable KRB5_KTNAME
+implementation.
+.new
+For example, for older releases of Heimdal, the environment variable KRB5_KTNAME
may be set to point to an alternative keytab file. Exim will pass this
variable through from its own inherited environment when started as root or the
Exim user. The keytab file needs to be readable by the Exim user.
+With newer releases of Heimdal, a setuid Exim may cause Heimdal to discard the
+environment variable. In practice, for those releases, the Cyrus authenticator
+is not a suitable interface for GSSAPI (Kerberos) support. Instead, consider
+the &(heimdal_gssapi)& authenticator, described in chapter &<<CHAPheimdalgss>>&
+.wen
.section "Using cyrus_sasl as a server" "SECID178"
server_set_id = $auth1
.endd
-.option server_realm cyrus_sasl string unset
+.new
+.option server_realm cyrus_sasl string&!! unset
This specifies the SASL realm that the server claims to be in.
+.wen
.option server_service cyrus_sasl string &`smtp`&
.ecindex IIDdcotauth2
+. ////////////////////////////////////////////////////////////////////////////
+. ////////////////////////////////////////////////////////////////////////////
+.new
+.chapter "The gsasl authenticator" "CHAPgsasl"
+.scindex IIDgsaslauth1 "&(gsasl)& authenticator"
+.scindex IIDgsaslauth2 "authenticators" "&(gsasl)&"
+.cindex "authentication" "GNU SASL"
+.cindex "authentication" "SASL"
+.cindex "authentication" "EXTERNAL"
+.cindex "authentication" "ANONYMOUS"
+.cindex "authentication" "PLAIN"
+.cindex "authentication" "LOGIN"
+.cindex "authentication" "DIGEST-MD5"
+.cindex "authentication" "CRAM-MD5"
+.cindex "authentication" "SCRAM-SHA-1"
+The &(gsasl)& authenticator provides server integration for the GNU SASL
+library and the mechanisms it provides. This is new as of the 4.80 release
+and there are a few areas where the library does not let Exim smoothly
+scale to handle future authentication mechanisms, so no guarantee can be
+made that any particular new authentication mechanism will be supported
+without code changes in Exim.
+
+
+.option server_channelbinding gsasl boolean false
+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 is currently only supported when using the GnuTLS library. This is
+only usable by mechanisms which support "channel binding"; at time of
+writing, that's the SCRAM family.
+
+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.
+
+
+.option server_hostname gsasl string&!! "see below"
+This option selects the hostname that is used when communicating with the
+library. The default value is &`$primary_hostname`&.
+Some mechanisms will use this data.
+
+
+.option server_mech gsasl string "see below"
+This option selects the authentication mechanism this driver should use. The
+default is the value of the generic &%public_name%& option. This option allows
+you to use a different underlying mechanism from the advertised name. For
+example:
+.code
+sasl:
+ driver = gsasl
+ public_name = X-ANYTHING
+ server_mech = CRAM-MD5
+ server_set_id = $auth1
+.endd
+
+
+.option server_password gsasl string&!! unset
+Various mechanisms need access to the cleartext password on the server, so
+that proof-of-possession can be demonstrated on the wire, without sending
+the password itself.
+
+The data available for lookup varies per mechanism.
+In all cases, &$auth1$& is set to the &'authentication id'&.
+The &$auth2$& variable will always be the &'authorization id'& (&'authz'&)
+if available, else the empty string.
+The &$auth3$& variable will always be the &'realm'& if available,
+else the empty string.
+
+A forced failure will cause authentication to defer.
+
+If using this option, it may make sense to set the &%server_condition%&
+option to be simply "true".
+
+
+.option server_realm gsasl string&!! unset
+This specifies the SASL realm that the server claims to be in.
+Some mechanisms will use this data.
+
+
+.option server_scram_iter gsasl string&!! unset
+This option provides data for the SCRAM family of mechanisms.
+&$auth1$& is not available at evaluation time.
+(This may change, as we receive feedback on use)
+
+
+.option server_scram_salt gsasl string&!! unset
+This option provides data for the SCRAM family of mechanisms.
+&$auth1$& is not available at evaluation time.
+(This may change, as we receive feedback on use)
+
+
+.option server_service gsasl string &`smtp`&
+This is the SASL service that the server claims to implement.
+Some mechanisms will use this data.
+
+
+.section "&(gsasl)& auth variables" "SECTgsaslauthvar"
+.vindex "&$auth1$&, &$auth2$&, etc"
+These may be set when evaluating specific options, as detailed above.
+They will also be set when evaluating &%server_condition%&.
+
+Unless otherwise stated below, the &(gsasl)& integration will use the following
+meanings for these variables:
+
+.ilist
+.vindex "&$auth1$&"
+&$auth1$&: the &'authentication id'&
+.next
+.vindex "&$auth2$&"
+&$auth2$&: the &'authorization id'&
+.next
+.vindex "&$auth3$&"
+&$auth3$&: the &'realm'&
+.endlist
+
+On a per-mechanism basis:
+
+.ilist
+.cindex "authentication" "EXTERNAL"
+EXTERNAL: only &$auth1$& is set, to the possibly empty &'authorization id'&;
+the &%server_condition%& option must be present.
+.next
+.cindex "authentication" "ANONYMOUS"
+ANONYMOUS: only &$auth1$& is set, to the possibly empty &'anonymous token'&;
+the &%server_condition%& option must be present.
+.next
+.cindex "authentication" "GSSAPI"
+GSSAPI: &$auth1$& will be set to the &'GSSAPI Display Name'&;
+&$auth2$& will be set to the &'authorization id'&,
+the &%server_condition%& option must be present.
+.endlist
+
+An &'anonymous token'& is something passed along as an unauthenticated
+identifier; this is analogous to FTP anonymous authentication passing an
+email address, or software-identifier@, as the "password".
+
+
+An example showing the password having the realm specified in the callback
+and demonstrating a Cyrus SASL to GSASL migration approach is:
+.code
+gsasl_cyrusless_crammd5:
+ driver = gsasl
+ public_name = CRAM-MD5
+ server_realm = imap.example.org
+ server_password = ${lookup{$auth1:$auth3:userPassword}\
+ dbmjz{/etc/sasldb2}{$value}fail}
+ server_set_id = ${quote:$auth1}
+ server_condition = yes
+.endd
+
+.wen
+
+. ////////////////////////////////////////////////////////////////////////////
+. ////////////////////////////////////////////////////////////////////////////
+
+.new
+.chapter "The heimdal_gssapi authenticator" "CHAPheimdalgss"
+.scindex IIDheimdalgssauth1 "&(heimdal_gssapi)& authenticator"
+.scindex IIDheimdalgssauth2 "authenticators" "&(heimdal_gssapi)&"
+.cindex "authentication" "GSSAPI"
+.cindex "authentication" "Kerberos"
+The &(heimdal_gssapi)& authenticator provides server integration for the
+Heimdal GSSAPI/Kerberos library, permitting Exim to set a keytab pathname
+reliably.
+
+.option server_hostname heimdal_gssapi string&!! "see below"
+This option selects the hostname that is used, with &%server_service%&,
+for constructing the GSS server name, as a &'GSS_C_NT_HOSTBASED_SERVICE'&
+identifier. The default value is &`$primary_hostname`&.
+
+.option server_keytab heimdal_gssapi string&!! unset
+If set, then Heimdal will not use the system default keytab (typically
+&_/etc/krb5.keytab_&) but instead the pathname given in this option.
+The value should be a pathname, with no &"file:"& prefix.
+
+.option server_service heimdal_gssapi string&!! "smtp"
+This option specifies the service identifier used, in conjunction with
+&%server_hostname%&, for building the identifer for finding credentials
+from the keytab.
+
+
+.section "&(heimdal_gssapi)& auth variables" "SECTheimdalgssauthvar"
+Beware that these variables will typically include a realm, thus will appear
+to be roughly like an email address already. The &'authzid'& in &$auth2$& is
+not verified, so a malicious client can set it to anything.
+
+The &$auth1$& field should be safely trustable as a value from the Key
+Distribution Center. Note that these are not quite email addresses.
+Each identifier is for a role, and so the left-hand-side may include a
+role suffix. For instance, &"joe/admin@EXAMPLE.ORG"&.
+
+.vindex "&$auth1$&, &$auth2$&, etc"
+.ilist
+.vindex "&$auth1$&"
+&$auth1$&: the &'authentication id'&, set to the GSS Display Name.
+.next
+.vindex "&$auth2$&"
+&$auth2$&: the &'authorization id'&, sent within SASL encapsulation after
+authentication. If that was empty, this will also be set to the
+GSS Display Name.
+.endlist
+
+.wen
+
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
The &%tls_verify_certificates%& option must contain the name of a file, not the
name of a directory (for OpenSSL it can be either).
.next
-The &%tls_dhparam%& option is ignored, because early versions of GnuTLS had no
-facility for varying its Diffie-Hellman parameters. I understand that this has
-changed, but Exim has not been updated to provide this facility.
+.new
+The default value for &%tls_dhparam%& differs for historical reasons.
+.wen
.next
.vindex "&$tls_peerdn$&"
Distinguished Name (DN) strings reported by the OpenSSL library use a slash for
affects the value of the &$tls_peerdn$& variable.
.next
OpenSSL identifies cipher suites using hyphens as separators, for example:
-DES-CBC3-SHA. GnuTLS uses underscores, for example: RSA_ARCFOUR_SHA. What is
-more, OpenSSL complains if underscores are present in a cipher list. To make
-life simpler, Exim changes underscores to hyphens for OpenSSL and hyphens to
-underscores for GnuTLS when processing lists of cipher suites in the
+DES-CBC3-SHA. GnuTLS historically used underscores, for example:
+RSA_ARCFOUR_SHA. What is more, OpenSSL complains if underscores are present
+in a cipher list. To make life simpler, Exim changes underscores to hyphens
+for OpenSSL and passes the string unchanged to GnuTLS (expecting the library
+to handle its own older variants) when processing lists of cipher suites in the
&%tls_require_ciphers%& options (the global option and the &(smtp)& transport
option).
.next
The &%tls_require_ciphers%& options operate differently, as described in the
sections &<<SECTreqciphssl>>& and &<<SECTreqciphgnu>>&.
+.new
+.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
+explicitly state that the feature is infeasible in the other TLS
+implementation, then patches are welcome.
+.wen
.endlist
-.section "GnuTLS parameter computation" "SECID181"
+.section "GnuTLS parameter computation" "SECTgnutlsparam"
+.new
+This section only applies if &%tls_dhparam%& is set to &`historic`& or to
+an explicit path; if the latter, then the text about generation still applies,
+but not the chosen filename.
+By default, as of Exim 4.80 a hard-coded D-H prime is used.
+See the documentation of &%tls_dhparam%& for more information.
+
GnuTLS uses D-H parameters that may take a substantial amount of time
to compute. It is unreasonable to re-compute them for every TLS session.
Therefore, Exim keeps this data in a file in its spool directory, called
-&_gnutls-params_&. The file is owned by the Exim user and is readable only by
+&_gnutls-params-NNNN_& for some value of NNNN, corresponding to the number
+of bits requested.
+The file is owned by the Exim user and is readable only by
its owner. Every Exim process that start up GnuTLS reads the D-H
parameters from this file. If the file does not exist, the first Exim process
that needs it computes the data and writes it to a temporary file which is
For maximum security, the parameters that are stored in this file should be
recalculated periodically, the frequency depending on your paranoia level.
+If you are avoiding using the fixed D-H primes published in RFCs, then you
+are concerned about some advanced attacks and will wish to do this; if you do
+not regenerate then you might as well stick to the standard primes.
+
Arranging this is easy in principle; just delete the file when you want new
values to be computed. However, there may be a problem. The calculation of new
parameters needs random numbers, and these are obtained from &_/dev/random_&.
a substantial amount of time, causing timeouts on incoming connections.
The solution is to generate the parameters externally to Exim. They are stored
-in &_gnutls-params_& in PEM format, which means that they can be generated
-externally using the &(certtool)& command that is part of GnuTLS.
+in &_gnutls-params-N_& in PEM format, which means that they can be
+generated externally using the &(certtool)& command that is part of GnuTLS.
To replace the parameters with new ones, instead of deleting the file
and letting Exim re-create it, you can generate new parameters using
&(certtool)& and, when this has been done, replace Exim's cache file by
renaming. The relevant commands are something like this:
.code
+# ls
+[ look for file; assume gnutls-params-2236 is the most recent ]
# rm -f new-params
# touch new-params
# chown exim:exim new-params
+# chmod 0600 new-params
+# certtool --generate-dh-params --bits 2236 >>new-params
+# openssl dhparam -noout -text -in new-params | head
+[ check the first line, make sure it's not more than 2236;
+ if it is, then go back to the start ("rm") and repeat
+ until the size generated is at most the size requested ]
# chmod 0400 new-params
-# certtool --generate-privkey --bits 512 >new-params
-# echo "" >>new-params
-# certtool --generate-dh-params --bits 1024 >> new-params
-# mv new-params gnutls-params
+# mv new-params gnutls-params-2236
.endd
If Exim never has to generate the parameters itself, the possibility of
stalling is removed.
+The filename changed in Exim 4.80, to gain the -bits suffix. The value which
+Exim will choose depends upon the version of GnuTLS in use. For older GnuTLS,
+the value remains hard-coded in Exim as 1024. As of GnuTLS 2.12.x, there is
+a way for Exim to ask for the "normal" number of bits for D-H public-key usage,
+and Exim does so. This attempt to remove Exim from TLS policy decisions
+failed, as GnuTLS 2.12 returns a value higher than the current hard-coded limit
+of the NSS library. Thus Exim gains the &%tls_dh_max_bits%& global option,
+which applies to all D-H usage, client or server. If the value returned by
+GnuTLS is greater than &%tls_dh_max_bits%& then the value will be clamped down
+to &%tls_dh_max_bits%&. The default value has been set at the current NSS
+limit, which is still much higher than Exim historically used.
+
+The filename and bits used will change as the GnuTLS maintainers change the
+value for their parameter &`GNUTLS_SEC_PARAM_NORMAL`&, as clamped by
+&%tls_dh_max_bits%&. At the time of writing (mid 2012), GnuTLS 2.12 recommends
+2432 bits, while NSS is limited to 2236 bits.
+
+In fact, the requested value will be *lower* than &%tls_dh_max_bits%&, to
+increase the chance of the generated prime actually being within acceptable
+bounds, as GnuTLS has been observed to overshoot. Note the check step in the
+procedure above. There is no sane procedure available to Exim to double-check
+the size of the generated prime, so it might still be too large.
+.wen
+
.section "Requiring specific ciphers in OpenSSL" "SECTreqciphssl"
.cindex "TLS" "requiring specific ciphers (OpenSSL)"
suites before the cipher negotiation takes place. This specifies which ciphers
are acceptable. 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. The following quotation from the OpenSSL
+directly to this function call.
+Many systems will install the OpenSSL manual-pages, so you may have
+&'ciphers(1)'& available to you.
+The following quotation from the OpenSSL
documentation specifies what forms of item are allowed in the cipher string:
.ilist
not be moved to the end of the list.
.endlist
+.new
+The OpenSSL &'ciphers(1)'& command may be used to test the results of a given
+string:
+.code
+# note single-quotes to get ! past any shell history expansion
+$ openssl ciphers 'HIGH:!MD5:!SHA1'
+.endd
+
+This example will let the library defaults be permitted on the MX port, where
+there's probably no identity verification anyway, but ups the ante on the
+submission ports where the administrator might have some influence on the
+choice of clients used:
+.code
+# OpenSSL variant; see man ciphers(1)
+tls_require_ciphers = ${if =={$received_port}{25}\
+ {DEFAULT}\
+ {HIGH:!MD5:!SHA1}}
+.endd
+.wen
+
+.new
.section "Requiring specific ciphers or other parameters in GnuTLS" &&&
"SECTreqciphgnu"
.cindex "GnuTLS" "specifying parameters for"
.cindex "TLS" "specifying key exchange methods (GnuTLS)"
.cindex "TLS" "specifying MAC algorithms (GnuTLS)"
.cindex "TLS" "specifying protocols (GnuTLS)"
+.cindex "TLS" "specifying priority string (GnuTLS)"
.oindex "&%tls_require_ciphers%&" "GnuTLS"
-The GnuTLS library allows the caller to specify separate lists of permitted key
-exchange methods, main cipher algorithms, MAC algorithms, and protocols.
-Unfortunately, these lists are numerical, and the library does not have a
-function for turning names into numbers. Consequently, lists of recognized
-names have to be built into the application. The permitted key exchange
-methods, ciphers, and MAC algorithms may be used in any combination to form a
-cipher suite. This is unlike OpenSSL, where complete cipher suite names are
-passed to its control function.
-
-For compatibility with OpenSSL, the &%tls_require_ciphers%& option can be set
-to complete cipher suite names such as RSA_ARCFOUR_SHA, but for GnuTLS this
-option controls only the cipher algorithms. Exim searches each item in the
-list for the name of an available algorithm. For example, if the list
-contains RSA_AES_SHA, then AES is recognized, and the behaviour is exactly
-the same as if just AES were given.
-
-.oindex "&%gnutls_require_kx%&"
-.oindex "&%gnutls_require_mac%&"
-.oindex "&%gnutls_require_protocols%&"
-There are additional options called &%gnutls_require_kx%&,
-&%gnutls_require_mac%&, and &%gnutls_require_protocols%& that can be used to
-restrict the key exchange methods, MAC algorithms, and protocols, respectively.
-These options are ignored if OpenSSL is in use.
-
-All four options are available as global options, controlling how Exim
-behaves as a server, and also as options of the &(smtp)& transport, controlling
-how Exim behaves as a client. All the values are string expanded. After
-expansion, the values must be colon-separated lists, though the separator
-can be changed in the usual way.
-
-Each of the four lists starts out with a default set of algorithms. If the
-first item in a list does &'not'& start with an exclamation mark, all the
-default items are deleted. In this case, only those that are explicitly
-specified can be used. If the first item in a list &'does'& start with an
-exclamation mark, the defaults are left on the list.
-
-Then, any item that starts with an exclamation mark causes the relevant
-entry to be removed from the list, and any item that does not start with an
-exclamation mark causes a new entry to be added to the list. Unrecognized
-items in the list are ignored. Thus:
-.code
-tls_require_ciphers = !ARCFOUR
-.endd
-allows all the defaults except ARCFOUR, whereas
-.code
-tls_require_ciphers = AES : 3DES
-.endd
-allows only cipher suites that use AES or 3DES.
-
-For &%tls_require_ciphers%& the recognized names are AES_256, AES_128, AES
-(both of the preceding), 3DES, ARCFOUR_128, ARCFOUR_40, and ARCFOUR (both of
-the preceding). The default list does not contain all of these; it just has
-AES_256, AES_128, 3DES, and ARCFOUR_128.
-
-For &%gnutls_require_kx%&, the recognized names are DHE_RSA, RSA (which
-includes DHE_RSA), DHE_DSS, and DHE (which includes both DHE_RSA and
-DHE_DSS). The default list contains RSA, DHE_DSS, DHE_RSA.
-
-For &%gnutls_require_mac%&, the recognized names are SHA (synonym SHA1), and
-MD5. The default list contains SHA, MD5.
-
-.new
-For &%gnutls_require_protocols%&, the recognized names are TLS1.2, TLS1.1,
-TLS1.0, (TLS1) and SSL3.
-The default list contains TLS1.2, TLS1.1, TLS1.0, SSL3.
-TLS1 is an alias for TLS1.0, for backwards compatibility.
-For sufficiently old versions of the GnuTLS library, TLS1.2 or TLS1.1 might
-not be supported and will not be recognised by Exim.
+The GnuTLS library allows the caller to provide a "priority string", documented
+as part of the &[gnutls_priority_init]& function. This is very similar to the
+ciphersuite specification in OpenSSL.
+
+The &%tls_require_ciphers%& option is treated as the GnuTLS priority string.
+
+The &%tls_require_ciphers%& option is available both as an global option,
+controlling how Exim behaves as a server, and also as an option of the
+&(smtp)& transport, controlling how Exim behaves as a client. In both cases
+the value is string expanded. The resulting string is not an Exim list and
+the string is given to the GnuTLS library, so that Exim does not need to be
+aware of future feature enhancements of GnuTLS.
+
+Documentation of the strings accepted may be found in the GnuTLS manual, under
+"Priority strings". This is online as
+&url(http://www.gnu.org/software/gnutls/manual/html_node/Priority-Strings.html),
+but beware that this relates to GnuTLS 3, which may be newer than the version
+installed on your system. If you are using GnuTLS 3,
+&url(http://www.gnu.org/software/gnutls/manual/html_node/Listing-the-ciphersuites-in-a-priority-string.html, then the example code)
+on that site can be used to test a given string.
+
+Prior to Exim 4.80, an older API of GnuTLS was used, and Exim supported three
+additional options, "&%gnutls_require_kx%&", "&%gnutls_require_mac%&" and
+"&%gnutls_require_protocols%&". &%tls_require_ciphers%& was an Exim list.
+
+This example will let the library defaults be permitted on the MX port, where
+there's probably no identity verification anyway, and lowers security further
+by increasing compatibility; but this ups the ante on the submission ports
+where the administrator might have some influence on the choice of clients
+used:
+.code
+# GnuTLS variant
+tls_require_ciphers = ${if =={$received_port}{25}\
+ {NORMAL:%COMPAT}\
+ {SECURE128}}
+.endd
.wen
-In a server, the order of items in these lists is unimportant. The server
-advertises the availability of all the relevant cipher suites. However, in a
-client, the order in the &%tls_require_ciphers%& list specifies a preference
-order for the cipher algorithms. The first one in the client's list that is
-also advertised by the server is tried first. The default order is as listed
-above.
-
-
.section "Configuring an Exim server to use TLS" "SECID182"
.cindex "TLS" "configuring an Exim server"
tls_dhparam = /some/file/name
.endd
is set, the SSL library is initialized for the use of Diffie-Hellman ciphers
-with the parameters contained in the file. This increases the set of cipher
-suites that the server supports. See the command
+with the parameters contained in the file.
+.new
+Set this to &`none`& to disable use of DH entirely, by making no prime
+available:
+.code
+tls_dhparam = none
+.endd
+This may also be set to a string identifying a standard prime to be used for
+DH; if it is set to &`default`& or, for OpenSSL, is unset, then the prime
+used is &`ike23`&. There are a few standard primes available, see the
+documetnation for &%tls_dhparam%& for the complete list.
+
+See the command
.code
openssl dhparam
.endd
-for a way of generating this data. At present, &%tls_dhparam%& is used only
-when Exim is linked with OpenSSL. It is ignored if GnuTLS is being used.
+for a way of generating file data.
+.wen
The strings supplied for these three options are expanded every time a client
host connects. It is therefore possible to use different certificates and keys
which the client is connected. Forced failure of an expansion causes Exim to
behave as if the relevant option were unset.
+.vindex &$tls_bits$&
.vindex &$tls_cipher$&
.vindex &$tls_peerdn$&
-Before an SMTP connection is established, the &$tls_cipher$& and &$tls_peerdn$&
+.vindex &$tls_sni$&
+Before an SMTP connection is established, the
+&$tls_bits$&, &$tls_cipher$&, &$tls_peerdn$& and &$tls_sni$&
variables are emptied. (Until the first connection, they contain the values
that were set when the message was received.) If STARTTLS is subsequently
successfully obeyed, these variables are set to the relevant values for the
+.new
+.section "Use of TLS Server Name Indication" "SECTtlssni"
+.cindex "TLS" "Server Name Indication"
+.vindex "&$tls_sni$&"
+.oindex "&%tls_sni%&"
+With TLS1.0 or above, there is an extension mechanism by which extra
+information can be included at various points in the protocol. One of these
+extensions, documented in RFC 6066 (and before that RFC 4366) is
+&"Server Name Indication"&, commonly &"SNI"&. This extension is sent by the
+client in the initial handshake, so that the server can examine the servername
+within and possibly choose to use different certificates and keys (and more)
+for this session.
+
+This is analagous to HTTP's &"Host:"& header, and is the main mechanism by
+which HTTPS-enabled web-sites can be virtual-hosted, many sites to one IP
+address.
+
+With SMTP to MX, there are the same problems here as in choosing the identity
+against which to validate a certificate: you can't rely on insecure DNS to
+provide the identity which you then cryptographically verify. So this will
+be of limited use in that environment.
+
+With SMTP to Submission, there is a well-defined hostname which clients are
+connecting to and can validate certificates against. Thus clients &*can*&
+choose to include this information in the TLS negotiation. If this becomes
+wide-spread, then hosters can choose to present different certificates to
+different clients. Or even negotiate different cipher suites.
+
+The &%tls_sni%& option on an SMTP transport is an expanded string; the result,
+if not empty, will be sent on a TLS session as part of the handshake. There's
+nothing more to it. Choosing a sensible value not derived insecurely is the
+only point of caution. The &$tls_sni$& variable will be set to this string
+for the lifetime of the client connection (including during authentication).
+
+Except during SMTP client sessions, if &$tls_sni$& is set then it is a string
+received from a client.
+It can be logged with the &%log_selector%& item &`+tls_sni`&.
+
+If the string &`tls_sni`& appears in the main section's &%tls_certificate%&
+option (prior to expansion) then the following options will be re-expanded
+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%&
+.endlist
+
+Great care should be taken to deal with matters of case, various injection
+attacks in the string (&`../`& or SQL), and ensuring that a valid filename
+can always be referenced; it is important to remember that &$tls_sni$& is
+arbitrary unverified data provided prior to authentication.
+
+The Exim developers are proceeding cautiously and so far no other TLS options
+are re-expanded.
+
+When Exim is built againt OpenSSL, OpenSSL must have been built with support
+for TLS Extensions. This holds true for OpenSSL 1.0.0+ and 0.9.8+ with
+enable-tlsext in EXTRACONFIGURE. If you invoke &(openssl s_client -h)& and
+see &`-servername`& in the output, then OpenSSL has support.
+
+When Exim is built against GnuTLS, SNI support is available as of GnuTLS
+0.5.10. (Its presence predates the current API which Exim uses, so if Exim
+built, then you have SNI support).
+.wen
+
+
+
.section "Multiple messages on the same encrypted TCP/IP connection" &&&
"SECTmulmessam"
.cindex "multiple SMTP deliveries with TLS"
new rate.
.code
acl_check_connect:
- deny ratelimit = 100 / 5m / readonly
- log_message = RATE CHECK: $sender_rate/$sender_rate_period \
- (max $sender_rate_limit)
+ deny ratelimit = 100 / 5m / readonly
+ log_message = RATE CHECK: $sender_rate/$sender_rate_period \
+ (max $sender_rate_limit)
# ...
acl_check_mail:
- warn ratelimit = 100 / 5m / strict
- log_message = RATE UPDATE: $sender_rate/$sender_rate_period \
- (max $sender_rate_limit)
+ warn ratelimit = 100 / 5m / strict
+ log_message = RATE UPDATE: $sender_rate/$sender_rate_period \
+ (max $sender_rate_limit)
.endd
If Exim encounters multiple &%ratelimit%& conditions with the same key when
&` tls_certificate_verified `& certificate verification status
&`*tls_cipher `& TLS cipher suite on <= and => lines
&` tls_peerdn `& TLS peer DN on <= and => lines
+&` tls_sni `& TLS SNI on <= lines
&` unknown_in_list `& DNS lookup failed in list match
&` all `& all of the above
connection, and a certificate is supplied by the remote host, the peer DN is
added to the log line, preceded by DN=.
.next
+.cindex "log" "TLS SNI"
+.cindex "TLS" "logging SNI"
+&%tls_sni%&: When a message is received over an encrypted connection, and
+the remote host provided the Server Name Indication extension, the SNI is
+added to the log line, preceded by SNI=.
+.next
.cindex "log" "DNS failure in list"
&%unknown_in_list%&: This setting causes a log entry to be written when the
result of a list match is failure because a DNS lookup failed.
In order to see the contents of messages on the queue, and to operate on them,
&'eximon'& must either be run as root or by an admin user.
+The command-line parameters of &'eximon'& are passed to &_eximon.bin_& and may
+contain X11 resource parameters interpreted by the X11 library. In addition,
+if the first parameter starts with the string "gdb" then it is removed and the
+binary is invoked under gdb (the parameter is used as the gdb command-name, so
+versioned variants of gdb can be invoked).
+
The monitor's window is divided into three parts. The first contains one or
more stripcharts and two action buttons, the second contains a &"tail"& of the
main log file, and the third is a display of the queue of messages awaiting
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
-.chapter "Support for DKIM (DomainKeys Identified Mail) - RFC4871" "CHID12" &&&
+.chapter "Support for DKIM (DomainKeys Identified Mail)" "CHID12" &&&
"DKIM Support"
.cindex "DKIM"
+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.
+
Since version 4.70, DKIM support is compiled into Exim by default. It can be
disabled by setting DISABLE_DKIM=yes in Local/Makefile.
Please note that verification of DKIM signatures in incoming mail is turned
on by default for logging purposes. 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:
+signature status. Here is an example (with line-breaks added for clarity):
.code
-2009-09-09 10:22:28 1MlIRf-0003LU-U3 DKIM: d=facebookmail.com s=q1-2009b c=relaxed/relaxed a=rsa-sha1 i=@facebookmail.com t=1252484542 [verification succeeded]
+2009-09-09 10:22:28 1MlIRf-0003LU-U3 DKIM:
+ d=facebookmail.com s=q1-2009b
+ c=relaxed/relaxed a=rsa-sha1
+ i=@facebookmail.com t=1252484542 [verification succeeded]
.endd
You might want to turn off DKIM verification processing entirely for internal
or relay mail sources. To do that, set the &%dkim_disable_verify%& ACL
verb to a group of domains or identities. For example:
.code
-# Warn when message apparently from GMail has no signature at all
+# Warn when Mail purportedly from GMail has no signature at all
warn log_message = GMail sender without DKIM signature
sender_domains = gmail.com
dkim_signers = gmail.com
.vitem &%dkim_status%&
ACL condition that checks a colon-separated list of possible DKIM verification
results agains the actual result of verification. This is typically used
-to restrict an ACL verb to a list of verification outcomes, like:
+to restrict an ACL verb to a list of verification outcomes, for example:
.code
-deny message = Message from Paypal with invalid or missing signature
+deny message = Mail from Paypal with invalid/missing signature
sender_domains = paypal.com:paypal.de
dkim_signers = paypal.com:paypal.de
dkim_status = none:invalid:fail
git://git.exim.org / exim.git / blobdiff