. the <bookinfo> element must also be updated for each new edition.
. /////////////////////////////////////////////////////////////////////////////
-.set previousversion "4.75"
+.set previousversion "4.80"
.set version "4.80"
.set ACL "access control lists (ACLs)"
overridden if necessary.
-.new
.section "PCRE library" "SECTpcre"
.cindex "PCRE library"
Exim no longer has an embedded PCRE library as the vast majority of
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
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
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
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
if this is required. If the &%bi_command%& option is not set, calling Exim with
&%-bi%& is a no-op.
+.new
+. // Keep :help first, then the rest in alphabetical order
+.vitem &%-bI:help%&
+.oindex "&%-bI:help%&"
+.cindex "querying exim information"
+We shall provide various options starting &`-bI:`& for querying Exim for
+information. The output of many of these will be intended for machine
+consumption. This one is not. The &%-bI:help%& option asks Exim for a
+synopsis of supported options beginning &`-bI:`&. Use of any of these
+options shall cause Exim to exit after producing the requested output.
+
+.vitem &%-bI:dscp%&
+.oindex "&%-bI:dscp%&"
+.cindex "DSCP" "values"
+This option causes Exim to emit an alphabetically sorted list of all
+recognised DSCP names.
+
+.vitem &%-bI:sieve%&
+.oindex "&%-bI:sieve%&"
+.cindex "Sieve filter" "capabilities"
+This option causes Exim to emit an alphabetically sorted list of all supported
+Sieve protocol extensions on stdout, one per line. This is anticipated to be
+useful for ManageSieve (RFC 5804) implementations, in providing that protocol's
+&`SIEVE`& capability response line. As the precise list may depend upon
+compile-time build options, which this option will adapt to, this is the only
+way to guarantee a correct response.
+.wen
+
.vitem &%-bm%&
.oindex "&%-bm%&"
.cindex "local message reception"
If a list of configuration files was supplied, the value that is output here
is the name of the file that was actually used.
+.new
+.cindex "options" "hiding name of"
+If the &%-n%& flag is given, then for most modes of &%-bP%& operation the
+name will not be output.
+.wen
+
.cindex "daemon" "process id (pid)"
.cindex "pid (process id)" "of daemon"
If &%log_file_path%& or &%pid_file_path%& are given, the names of the
.vitem &%-n%&
.oindex "&%-n%&"
-.cindex "Sendmail compatibility" "&%-n%& option ignored"
-This option is interpreted by Sendmail to mean &"no aliasing"&. It is ignored
-by Exim.
+This option is interpreted by Sendmail to mean &"no aliasing"&.
+For normal modes of operation, it is ignored by Exim.
+When combined with &%-bP%& it suppresses the name of an option from being output.
.vitem &%-O%&&~<&'data'&>
.oindex "&%-O%&"
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"
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"
&`fail`& keyword causes a &'forced expansion failure'& &-- see section
&<<SECTforexpfai>>& for an explanation of what this means.
-.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
.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. 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}}
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().
includes the case of letters, whereas for &%gti%& the comparison is
case-independent.
-.new
.vitem &*inlist&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*& &&&
&*inlisti&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*&
.cindex "string" "comparison"
${if inlisti{Needle}{fOo:NeeDLE:bAr}}
${if forany{fOo:NeeDLE:bAr}{eqi{$item}{Needle}}}
.endd
-.wen
.vitem &*isip&~{*&<&'string'&>&*}*& &&&
&*isip4&~{*&<&'string'&>&*}*& &&&
.vitem &*match_ip&~{*&<&'string1'&>&*}{*&<&'string2'&>&*}*&
.cindex "&%match_ip%& expansion condition"
-.new
This condition matches an IP address to a list of IP address patterns. It must
be followed by two argument strings. The first (after expansion) must be an IP
address or an empty string. The second (not expanded) is a restricted host
list that can match only an IP address, not a host name. For example:
-.wen
.code
${if match_ip{$sender_host_address}{1.2.3.4:5.6.7.8}{...}{...}}
.endd
.endd
.endlist ilist
-.new
Note that <&'string2'&> is not itself subject to string expansion, unless
Exim was built with the EXPAND_LISTMATCH_RHS option.
-.wen
Consult section &<<SECThoslispatip>>& for further details of these patterns.
have their local parts matched casefully. Domains are always matched
caselessly.
-.new
Note that <&'string2'&> is not itself subject to string expansion, unless
Exim was built with the EXPAND_LISTMATCH_RHS option.
-.wen
&*Note*&: Host lists are &'not'& supported in this way. This is because
hosts have two identities: a name and an IP address, and it is not clear
negative response to an AUTH command, including (for example) an attempt to use
an undefined mechanism.
-.new
.vitem &$av_failed$&
.cindex "content scanning" "AV scanner failure"
This variable is available when Exim is compiled with the content-scanning
extension. It is set to &"0"& by default, but will be set to &"1"& if any
problem occurs with the virus scanner (specified by &%av_scanner%&) during
the ACL malware condition.
-.wen
.vitem &$body_linecount$&
.cindex "message body" "line count"
received. It is empty if there was no successful authentication. See also
&$authenticated_id$&.
+.new
+.vitem &$sender_host_dnssec$&
+.vindex "&$sender_host_dnssec$&"
+If &$sender_host_name$& has been populated (by reference, &%hosts_lookup%& or
+otherwise) then this boolean will have been set true if, and only if, the
+resolver library states that the reverse DNS was authenticated data. At all
+other times, this variable is false.
+
+It is likely that you will need to coerce DNSSEC support on in the resolver
+library, by setting:
+.code
+dns_use_dnssec = 1
+.endd
+
+Exim does not perform DNSSEC validation itself, instead leaving that to a
+validating resolver (eg, unbound, or bind with suitable configuration).
+
+Exim does not (currently) check to see if the forward DNS was also secured
+with DNSSEC, only the reverse DNS.
+
+If you have changed &%host_lookup_order%& so that &`bydns`& is not the first
+mechanism in the list, then this variable will be false.
+.wen
+
+
.vitem &$sender_host_name$&
.vindex "&$sender_host_name$&"
When a message is received from a remote host, this variable contains the
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
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 "&$tls_certificate_verified$&"
value is retained during message delivery, except during outbound SMTP
deliveries.
-.new
.vitem &$tls_sni$&
.vindex "&$tls_sni$&"
.cindex "TLS" "Server Name Indication"
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 "&$tod_bsdinbox$&"
.row &%dns_ipv4_lookup%& "only v4 lookup for these domains"
.row &%dns_retrans%& "parameter for resolver"
.row &%dns_retry%& "parameter for resolver"
+.new
+.row &%dns_use_dnssec%& "parameter for resolver"
+.wen
.row &%dns_use_edns0%& "parameter for resolver"
.row &%hold_domains%& "hold delivery for these domains"
.row &%local_interfaces%& "for routing checks"
Those options that undergo string expansion before use are marked with
†.
-.new
.option accept_8bitmime main boolean true
.cindex "8BITMIME"
.cindex "8-bit characters"
.display
&url(http://cr.yp.to/smtp/8bitmime.html)
.endd
-.wen
.option acl_not_smtp main string&!! unset
.cindex "&ACL;" "for non-SMTP messages"
.new
+.option dns_use_dnssec main integer -1
+.cindex "DNS" "resolver options"
+.cindex "DNS" "DNSSEC"
+If this option is set to a non-negative number then Exim will initialise the
+DNS resolver library to either use or not use DNSSEC, overriding the system
+default. A value of 0 coerces DNSSEC off, a value of 1 coerces DNSSEC on.
+
+If the resolver library does not support DNSSEC then this option has no effect.
+.wen
+
+
.option dns_use_edns0 main integer -1
.cindex "DNS" "resolver options"
.cindex "DNS" "EDNS0"
on.
If the resolver library does not support EDNS0 then this option has no effect.
-.wen
.option drop_cr main boolean false
transport driver.
-.option openssl_options main "string list" unset
+.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,
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
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
If you prefer more security and are willing to break some clients, raise this
number.
-.wen
+
+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.
.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.
+The value of this option is expanded and indicates the source of DH parameters
+to be used by Exim.
-.new
-If the DH bit-count from loading the file is greater than tls_dh_max_bits then
-it will be ignored.
-.wen
+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.
.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
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
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
&%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 &<<SECTmaildirdelivery>>& below.
-.new
.option maildir_use_size_file appendfile&!! boolean false
.cindex "maildir format" "&_maildirsize_& file"
The result of string expansion for this option must be a valid boolean value.
quota from the &%quota%& option of the transport. If &%quota%& is unset, the
value is zero. See &%maildir_quota_directory_regex%& above and section
&<<SECTmaildirdelivery>>& below for further details.
-.wen
.option maildirfolder_create_regex appendfile string unset
.cindex "maildir format" "&_maildirfolder_& file"
details.
+.new
+.option dscp smtp string&!! unset
+.cindex "DCSP"
+.cindex "DiffServ"
+This option causes the DSCP value associated with a socket to be set to one
+of a number of fixed strings or to numeric value.
+The &%-bI:dscp%& option may be used to ask Exim which names it knows of.
+Common values include &`throughput`&, &`mincost`&, and on newer systems
+&`ef`&, &`af41`&, etc. Numeric values may be in the range 0 to 0x3F.
+
+The outbound packets from Exim will be marked with this value in the header
+(for IPv4, the TOS field; for IPv6, the TCLASS field); there is no guarantee
+that these packets will have any effect, not be stripped by networking
+equipment, or do much of anything without cooperation with your Network
+Engineer and those of all network operators between the source and destination.
+.wen
+
.option fallback_hosts smtp "string list" unset
.cindex "fallback" "hosts specified on transport"
deliveries into closed message stores. Exim also has support for running LMTP
over a pipe to a local process &-- see chapter &<<CHAPLMTP>>&.
-.new
If this option is set to &"smtps"&, the default vaule for the &%port%& option
changes to &"smtps"&, and the transport initiates TLS immediately after
connecting, as an outbound SSL-on-connect, instead of using STARTTLS to upgrade.
The Internet standards bodies strongly discourage use of this mode.
-.wen
.option retry_include_ip_address smtp boolean true
be the name of a file that contains a CRL in PEM format.
+.new
+.option tls_dh_min_bits smtp integer 1024
+.cindex "TLS" "Diffie-Hellman minimum acceptable size"
+When establishing a TLS session, if a ciphersuite which uses Diffie-Hellman
+key agreement is negotiated, the server will provide a large prime number
+for use. This option establishes the minimum acceptable size of that number.
+If the parameter offered by the server is too small, then the TLS handshake
+will fail.
+
+Only supported when using GnuTLS.
+.wen
+
+
.option tls_privatekey smtp string&!! unset
.cindex "TLS" "client private key, location of"
.vindex "&$host$&"
-.new
.option tls_sni smtp string&!! unset
.cindex "TLS" "Server Name Indication"
.vindex "&$tls_sni$&"
See &<<SECTtlssni>>& for more information.
-OpenSSL only, also requiring a build of OpenSSL that supports TLS extensions.
-.wen
+Note that for OpenSSL, this feature requires a build of OpenSSL that supports
+TLS extensions.
+
.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.
-.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
the PLAIN authentication mechanism (RFC 2595) or the LOGIN mechanism, which is
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
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
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
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
dbmjz{/etc/sasldb2}}
server_set_id = $auth1
.endd
-.wen
.section "Using cram_md5 as a client" "SECID177"
.cindex "options" "&(cram_md5)& authenticator (client)"
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.
-.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
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 = $auth1
.endd
-.new
.option server_realm cyrus_sasl string&!! unset
This specifies the SASL realm that the server claims to be in.
-.wen
.option server_service cyrus_sasl string &`smtp`&
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
-.new
.chapter "The gsasl authenticator" "CHAPgsasl"
.scindex IIDgsaslauth1 "&(gsasl)& authenticator"
.scindex IIDgsaslauth2 "authenticators" "&(gsasl)&"
server_condition = yes
.endd
-.wen
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
-.new
.chapter "The heimdal_gssapi authenticator" "CHAPheimdalgss"
.scindex IIDheimdalgssauth1 "&(heimdal_gssapi)& authenticator"
.scindex IIDheimdalgssauth2 "authenticators" "&(heimdal_gssapi)&"
GSS Display Name.
.endlist
-.wen
. ////////////////////////////////////////////////////////////////////////////
. ////////////////////////////////////////////////////////////////////////////
The &%tls_verify_certificates%& option must contain the name of a file, not the
name of a directory (for OpenSSL it can be either).
.next
-The &%tls_dhparam%& option is ignored, because early versions of GnuTLS had no
-facility for varying its Diffie-Hellman parameters.
-.new
-Since then, the GnuTLS support has been updated to generate parameters upon
-demand, keeping them in the spool directory. See &<<SECTgnutlsparam>>& for
-details.
-.wen
+The default value for &%tls_dhparam%& differs for historical reasons.
.next
.vindex "&$tls_peerdn$&"
Distinguished Name (DN) strings reported by the OpenSSL library use a slash for
.next
The &%tls_require_ciphers%& options operate differently, as described in the
sections &<<SECTreqciphssl>>& and &<<SECTreqciphgnu>>&.
+.next
.new
+The &%tls_dh_min_bits%& SMTP transport option is only honoured by GnuTLS.
+When using OpenSSL, this option is ignored.
+(If an API is found to let OpenSSL be configured in this way,
+let the Exim Maintainers know and we'll likely use it).
+.wen
.next
Some other recently added features may only be available in one or the other.
This should be documented with the feature. If the documentation does not
explicitly state that the feature is infeasible in the other TLS
implementation, then patches are welcome.
-.wen
.endlist
.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
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_&.
# 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
# mv new-params gnutls-params-2236
.endd
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.
-.wen
+
+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.
.section "Requiring specific ciphers in OpenSSL" "SECTreqciphssl"
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
{DEFAULT}\
{HIGH:!MD5:!SHA1}}
.endd
-.wen
-.new
.section "Requiring specific ciphers or other parameters in GnuTLS" &&&
"SECTreqciphgnu"
.cindex "GnuTLS" "specifying parameters for"
{NORMAL:%COMPAT}\
{SECURE128}}
.endd
-.wen
.section "Configuring an Exim server to use TLS" "SECID182"
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.
+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.
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
-.new
.section "Use of TLS Server Name Indication" "SECTtlssni"
.cindex "TLS" "Server Name Indication"
.vindex "&$tls_sni$&"
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
git://git.exim.org / exim.git / blobdiff