The Exim configuration file

Exim uses a single runtime configuration file which it reads when it is starting up. The name of the file is compiled into the binary for security reasons, and is specified by the CONFIGURE_FILE compilation option.

Some sites may wish to use the same Exim binary on different machines that share a file system, but to use different configuration files on each machine. If CONFIGURE_FILE_USE_NODE is defined in `Local/Makefile', then Exim first looks for a file whose name is the configuration file name followed by a dot and the machine's node name, as obtained from the `uname()' function. If this file does not exist, the standard name is tried.

In some esoteric situations different versions of Exim may be run under different effective uids and the CONFIGURE_FILE_USE_EUID is defined to help with this. See the comments in `src/EDITME' for details.

The runtime configuration file must be owned by root or by the user that is specified at compile time by the EXIM_UID option, and it must not be world-writeable or group-writeable, unless its group is the one specified at compile time by the EXIM_GID option.

A one-off alternative configuration file can be specified by the `-C' command line option, but if this is done, Exim immediately gives up its root privilege, unless called by root or the Exim user, so this option is useful mainly for checking the syntax of configuration files before installing them. No owner or group checks are done on a configuration file specified by `-C'.

A default configuration file, which will work correctly in simple situations, is provided in the file `src/configure.default'. The installation process copies this into CONFIGURE_FILE if there is no previously-existing configuration file.

If a syntax error is detected while reading the configuration file, Exim writes a message on the standard error, and exists with a non-zero return code. The message is also written to the panic log.

Configuration file format

Exim's configuration file is in seven parts, which must appear in the correct order in the file, separated by lines containing just the word `end'. However, any parts at the end of the file that are not required may be omitted. The file contains:

• Main configuration settings.
• Configuration settings for the transport drivers. Transports define mechanisms for copying messages to destinations.
• Configuration settings for the director drivers. Directors process local addresses, that is, those with domains that match `local_domains'. These are typically (but not necessarily) delivered on the local host.
• Configuration settings for the router drivers. Routers process remote addresses, that is, those with domains that do not match `local_domains'.
• Retry rules, for use when a message cannot be immediately delivered.
• Address rewriting rules.
• Configuration settings for the authenticator drivers. These are concerned with the SMTP AUTH command (see chapter "SMTP authentication"), and this part of the configuration can be omitted when AUTH is not in use.

Blank lines in the file are ignored, and lines starting with a # character are treated as comments and are also ignored. Note that a # character other than at the beginning of a line is not treated specially, and does not introduce a comment. A convenient way to create a configuration file is to start from the default, which is supplied in `src/configure.default', and add, delete, or change settings as required.

The retry and rewriting rules have their own syntax which is described in chapters "Retry configuration" and "Address rewriting". The other parts of the configuration file (whose settings are described in chapters "Main configuration"--"The queryprogram router" and "SMTP authentication"--"The cram_md5 authenticator") have some syntactic items in common, and these are described in sections "Common option syntax" onwards. Before that, the simple macro facility is described.

Macros in the configuration file

If a line in the main part of the configuration (that is, before the first `end' line) begins with an upper-case letter, it is taken as a macro definition, of the form

<name> = <rest of line>

The name must consist of letters, digits, and underscores, and need not all be in upper-case, though that is recommended. The rest of the line is the replacement text, and has leading and trailing white space removed. Quotes are not removed. If the line ends with a backslash character after trailing space is removed, then the next line is concatenated with it, with the backslash character and any leading space on the following line omitted. This continues for as long as lines end in backslash. Thus a replacement text can never end with a backslash character, but this doesn't seem to be a serious limitation.

Once a macro is defined, all subsequent lines in the file are scanned for the macro name; if there are several macros, the line is scanned for each in turn, in the order in which they are defined. The replacement text is not re-scanned for the current macro, though it will be for subsequently defined macros. For this reason, a macro name may not contain the name of a previously defined macro as a substring. You could, for example, define

ABCD_XYZ = <<something>>
ABCD = <<something>>

but putting the definitions in the opposite order would provoke a configuration error.

As an example of macro usage, suppose you have lots of local domains, but they fall into three different categories. You could set up

LOCAL1 = domain1:\
         domain2
LOCAL2 = domain3:domain4
LOCAL3 = dbm;/list/of/other/domains

local_domains = LOCAL1:LOCAL2:LOCAL3

and then use the `domains' option on appropriate directors to handle each set of domains differently. This avoids having to list each domain in more than one place.

Common option syntax

For the main set of options and for driver options, each setting is on a line by itself, and starts with a name consisting of lower-case letters and underscores. Many options require a data value, and in these cases the name must be followed by an equals sign (with optional white space) and then the value. For example:

exim_user = exim

Options whose type is given as boolean are on/off switches that are not always followed by a data value. If the option name is specified on its own, the switch is turned on; if it is preceded by `no_' or `not_' then the switch is turned off. However, boolean options may be followed by an equals sign and one of the words `true', `false', `yes', or `no'. For example:

sender_verify
no_smtp_verify
queue_only = true

The types of data that may be required by non-boolean options are described in the following sections.

Integer

If a numerical data item starts with the characters `0x', the remainder of it is interpreted as a hexadecimal number. Otherwise, it is treated as octal if it starts with the digit 0, and decimal if not. If an integer value is followed by the letter K, it is multiplied by 1024; if it is followed by the letter M, it is multiplied by 1024x1024.

When the values of integer option settings are output, values which are an exact multiple of 1024 or 1024x1024 are printed using the letters K and M. The printing style is independent of the actual input format that was used.

Octal integer

The value of an option specified as an octal integer is always interpreted in octal, whether or not it starts with the digit zero. Such options are always output in octal.

Fixed point number

A fixed point number consists of a decimal integer, optionally followed by a decimal point and up to three further digits.

Time interval

A time interval is specified as a sequence of numbers, each followed by one of the following letters, with no intervening white space:

`s'    seconds
`m'    minutes
`h'    hours
`d'    days
`w'    weeks

For example, `3h50m' specifies 3 hours and 50 minutes. The values of time intervals are output in the same format.

String

If a string data item does not start with a double-quote character, then it is taken as consisting of the remainder of the line, starting at the first character after any white space, with trailing white space characters removed, and with no interpretation of the characters therein.

If a string does start with a double-quote, then it continues to a closing double-quote, with the backslash character being interpreted as an escape character. If a backslash occurs at the end of an input line, the string is continued on the following line, with any leading white space being removed. Because Exim removes comment lines (those beginning with #) at an early stage, they can appear in the middle of a multi-line string.

The following two settings are equivalent:

trusted_users = "uucp:\
                 mail"
trusted_users = uucp:mail

If a backslash occurs in the middle of a line in a quoted string, the following escapes are recognized:

\\               single backslash
\n               newline
\r               carriage return
\t               tab
\<octal digits>  up to 3 octal digits specify one character
\x<hex digits>   up to 2 hexadecimal digits specify one character

If a backslash is followed by some other character, including a double-quote character, then that character replaces the pair.

Expanded strings

Some strings in the configuration file are subjected to string expansion, by which means various parts of the string may be changed according to the circumstances (see chapter "String expansions"). The input syntax for such strings is as just described; in particular, the handling of backslashes in quoted strings is done as part of the input process, before expansion takes place. However, backslash is also an escape character for the expander, so any backslashes that are required for that reason must be doubled if they are within a quoted configuration string.

User and group names

User and group names are specified as strings, using the syntax described above, but the strings are interpreted specially. In the main section of the configuration file, a user or group name must either consist entirely of digits, or be a name that can be looked up using the `getpwnam()' or `getgrnam()' function, as appropriate.

When a user or group is specified as an option for a driver, it may alternatively be a string that gets expanded each time the user or group value is required. The presence of a `$' character in the string causes this action to happen. Each time the string is expanded, the result must either be a digit string, or a name that can be looked up using `getpwnam()' or `getgrnam()', as appropriate.

String lists

Some configuration settings accept a colon-separated list of strings. In these cases the entire list is treated as a single string as far as the input syntax is concerned. The `trusted_users' setting in section "String" above is an example. If a colon is actually needed in an item in a string list, it can be entered as two colons. This is unfortunately necessary for all colons appearing in IPv6 addresses.

Leading and trailing white space on each item in a string list is ignored. This makes it possible to include items that start with a colon, and in particular, certain forms of IPv6 address. For example:

local_interfaces = "127.0.0.1 : ::::1"

See `README.IPV6' for general information about IPv6 support in Exim.

Domain lists

Domain lists are colon-separated string lists containing a number of patterns that are to be matched against a mail domain. For example, the `local_domains' option is a domain list which must match all the domains that Exim is to treat as local.

Items in a domain list may be positive or negative. Negative items are indicated by a leading exclamation mark, which may be followed by optional white space. The list is scanned from left to right. If the domain matches a positive item, it is in the set of domains which the list defines; if it matches a negative item, it is not in the set. If the end of the list is reached without the domain having matched any of the patterns, it is accepted if the last item was a negative one, but not if it was a positive one. For example,

relay_domains = !a.b.c : *.b.c

matches any domain ending in `.b.c' except for `a.b.c'. Domains that match neither `a.b.c' nor `*.b.c' are not accepted, because the last item in the list is positive. However, if the setting were

relay_domains = !a.b.c

then all domains other than `a.b.c' would be accepted because the last item in the list is negative. In effect, a list that ends with a negative item behaves as if it had `: *' appended to it.

The following types of item may appear in domain lists:

• If an item in a domain list is a plain absolute file name (beginning with a slash character), then each line of the file is read and processed as if it were an independent item in the list, except that further plain file names are not allowed. This happens each time the list is searched. If a # character appears anywhere in a line of the file, it and all following characters are ignored. Blank lines are also ignored. Wild cards, negation, and regular expressions may be used in the lines of the file, just as in the main list. For example, if

  local_domains = /etc/local-domains
  

  then the file could contain lines like

  ^.*\d{3}\.mydomain\.com$
  

  If a plain file name is preceded by an exclamation mark, the sense of any match within the file is inverted. For example, if

  hold_domains = !/etc/nohold-domains
  

  and the file contains the lines

  !a.b.c
  *.b.c
  

  then `a.b.c' is in the set of domains defined by `hold_domains', whereas any domain matching `*.b.c' is not.
• If a pattern consists of a single @ character, it matches the local host name, as set in the `primary_hostname' option. This makes it possible to use the same configuration file on several different hosts that differ only in their names.
• If a pattern starts with an asterisk, then the remaining characters of the pattern are compared with the terminating characters of the domain. The use of `*' in domain lists differs from its use in partial matching lookups. In a domain list, the character following the asterisk need not be a dot, whereas partial matching works only in terms of dot-separated components. For example, a domain list item such as `*key.ex' matches `donkey.ex' as well as `cipher.key.ex'.
• If a pattern starts with a circumflex character, then it is treated as a regular expression, and matched against the domain using a regular expression matching function. The circumflex is treated as part of the regular expression. References to descriptions of the syntax of regular expressions are given in chapter "Regular expressions", but note that if a backslash is required in the regular expression, it must be given as two backslashes if the string is in quotes. There are some cases where a domain list is the result of string expansion, for example the `domains' option in routers and directors. In these cases you must escape any backslash and dollar characters in regular expressions, to prevent them from being interpreted by the string expander, and if the string is specified in quotes, the resulting backslashes must themselves also be escaped.
• If a pattern starts with the name of a single-key lookup type followed by a semicolon (for example, `dbm;' or `lsearch;') then the remainder of the pattern must be a file name in a suitable format for the lookup type. For example, for `lsearch;' it must be an absolute path. The appropriate type of lookup is done on the file using the domain name as the key. The data from the lookup is available in some cases via the expansion variable `$domain_data'. Note that this is not an `include' facility when the lookup type is `lsearch'. The keys in the file are not interpreted specially, as they would be if they appeared as individual items in the domain list, or as lines in a file referenced without a search type.
• Any of the single-key lookup type names may be preceded by `partial<n>-', where the <n> is optional, for example,

  partial-dbm;/partial/domains
  

  This causes partial matching logic to be invoked; a description of how this works is given in section "Partial matching in single-key lookups" in chapter "File and database lookups".
• Any of the single-key lookup types may be followed by an asterisk. This causes a default lookup for a key consisting of a single asterisk to be done if the original lookup fails. This is not a useful feature when using a domain list to select particular domains (because any domain would match), but it might have value if the result of the lookup is being used via the `$domain_data' expansion variable.
• If the pattern starts with the name of a query-style lookup type followed by a semicolon (for example, `nisplus;' or `ldap;') then the remainder of the pattern must be an appropriate query for the lookup type, as described in chapter "File and database lookups". The data returned by a successful query is available in some cases via the expansion variable `$domain_data'. The query is expanded before use, and the expansion substitution `$key' can be used to insert the domain that is being tested into the query. There are cases where a domain list is the result of an earlier string expansion, for example the `domains' option in routers and directors. In these cases you must use `\$key' to delay the substitution of the variable until the second expansion, and a double backslash is needed if the whole domain list is in quotes.
• If none of the above cases apply, a straight textual comparison is made between the pattern and the domain.

Here is an example which uses several different kinds of pattern:

local_domains = "@@:\
                 lib.unseen.edu:\
                 *.foundation.fict.book:\
                 ^[1-2]\\d{3}\\.fict\\.book$:\
                 partial-dbm;/opt/data/penguin/book:\
                 nis;domains.byname:\
                 nisplus;[name=$key,status=local],domains.org_dir"

Note the necessary doubling of the backslashes in the regular expression. There are obvious processing trade-offs among the various matching modes. Using an asterisk is faster than a regular expression, and listing a few names explicitly probably is too. The use of a file or database lookup is expensive, but may be the only option if hundreds of names are required. Because the patterns are tested in order, it makes sense to put the most commonly matched patterns earlier in the string.

Address lists

An address list is a string list in which each item is a pattern to be matched against a mail address. As in the case of domain lists, the list is searched from left to right, any item may be preceded by an exclamation mark to negate it, and a plain file name may appear as an entire item, causing each line of the file to be read and treated as a separate pattern. Because local parts may legitimately contain # characters, a comment in the file is recognized only if # is followed by white space or the end of the line.

The following kinds of pattern may appear inline or as lines in an included file:

• If a pattern starts with ^ then a regular expression match is done against the complete address, using the entire pattern as the regular expression.
• Otherwise, if there is no @ in the pattern, it is first matched against the domain part of the subject address, the local part being ignored. This match is done exactly as for an entry in a domain list, so, for example, the item may begin with * or it may be a (partial) lookup (see section "Domain lists"). If there is no match, and the pattern consists of a single lookup, then the entire subject address is looked up in the file, with partial matching disabled. This means that an item such as

  sender_reject_recipients = partial-dbm;/black/list
  

  can reference a single file whose keys are a mixture of complete domains, partial domains, and individual mail addresses. Note that this is not an `include' facility when the lookup type is `lsearch'. The keys in the file are not interpreted specially, as they would be if they appeared as individual items in the address list, or lines in a file given as a plain file name without a search type.
• If the pattern starts with `@@<lookup-item>' (for example, `@@lsearch;/some/file'), the address that is being checked is split into a local part and a domain. The domain is looked up in the file. If it is not found, there is no match. If it is found, the data that is looked up from the file is treated as a colon-separated list of local part patterns, each of which is matched against the subject local part in turn. The lookup may be a partial one, and/or one involving a search for a default keyed by `*'. The local part patterns that are looked up can be regular expressions or begin with `*', or even be further lookups. They may also be independently negated. For example, with

  sender_reject_recipients = @@dbm;/etc/reject-by-domain
  

  the data from which DBM file is built could contain lines like

  baddomain.com:  !postmaster : *
  

  If a local part that actually begins with an exclamation mark is required, it has to be specified using a regular expression. In `lsearch' files, an entry may be split over several lines by indenting the second and subsequent lines, but the separating colon must still be included at line breaks. White space surrounding the colons is ignored. For example:

  aol.com:  spammer1 : spammer2 : ^[0-9]+$ :
            spammer3 : spammer4
  

  As in all colon-separated lists in Exim, a colon can be included in an item by doubling. If the last item in the list starts with a right angle-bracket, then the remainder of the item is taken as a new key to look up in order to obtain a continuation list of local parts. The new key can be any sequence of characters. Thus one might have entries like

  aol.com: spammer1 : spammer 2 : >*
  xyz.com: spammer3 : >*
  *:       ^\d{8}$
  

  in a file that was searched with `@@dbm*', to specify a match for 8-digit local parts for all domains, in addition to the specific local parts listed for each domain. Of course, using this feature costs another lookup each time a chain is followed, but the effort needed to maintain the data is reduced. It is possible to construct loops using this facility, and in order to catch them, the chains may be no more than fifty items long.
• If none of the above cases apply, the local part of the subject address is compared with the local part of the pattern, which may start with an asterisk. If the local parts match, then the domains are compared in exactly the same way as entries in a domain list, except that a regular expression is not permitted for a domain only. However, file lookups are permitted. For example:

  sender_reject = "*@*.spamming.site:\
                   bozo@partial-lsearch;/list/of/dodgy/sites"
  

  The domain may be given as a single @ character, as in a domain list, standing for the local host name, leading to items of the form `user@@'. If a local part that actually begins with an exclamation mark is required, it has to be specified using a regular expression, as otherwise the exclamation mark is treated as a sign of negation.

Case of letters in address lists

Domains in email addresses are always handled caselessly, but for local parts case may be significant on some systems (see `locally_caseless' for how Exim deals with this when processing local addresses). However, RFC 2505 (Anti-Spam Recommendations for SMTP MTAs) suggests that matching of addresses to blocking lists should be done in a case-independent manner. Since most address lists in Exim are used for this kind of control, Exim attempts to do this by default.

The domain portion of an address is always lowercased before matching it to an address list. The local part is lowercased by default, and any string comparisons that take place are done caselessly. This means that the data in the address list itself, in files included as plain file names, and in any file that is looked up using the `@@' mechanism, can be in any case. However, the keys in files that are looked up by a search type other than `lsearch' (which works caselessly) must be in lower case, because these lookups are not case-independent.

To allow for the possibility of caseful address list matching, if an item in the list is the string `+caseful' then the original case of the local part is restored for any comparisons that follow, and string comparisons are no longer case-independent. This does not affect the domain.

Host lists

Host lists are used to control what remote hosts are allowed to do (for example, use the local host as a relay). A host list consists of host name and IP address patterns which define a set of hosts that the list matches. Items in the list may be positive or negative. Negation is indicated by preceding an item with an exclamation mark. A plain absolute file name (beginning with a slash) can be used to include items from a file. Negation and included files operate exactly as for domain lists -- see section "Domain lists" for examples.

The following types of pattern may appear in a host list:

• If the entire item is `*' it matches any host.
• If the item is in the form of an IP address, it is matched against the IP address of the subject host. The presence of a colon is taken as an indication that it is an IPv6 address (when IPv6 support is compiled into Exim); such colons have to be doubled, because colon is the item separator in the list.
• If the item is in the form of an IP address followed by a slash and a mask length (for example 10.11.0.0/16) then it is matched against the IP address of the subject host under the given mask, which specifies the number of address bits that must match, starting from the most significant end. Thus an entire network of hosts can be included (or excluded) by a single item. IPv4 addresses are given in the normal `dotted-quad' notation. IPv6 addresses are given in colon-separated format, but the colons have to be doubled so as not to be taken as item separators. This example shows both kinds of address:

  receiver_unqualified_hosts = "172.16.0.0/12: \
                                5f03::1200::836f::::/48"
  

  The doubling of colons in IPv6 addresses applies only when such addresses appear inline in a host list. It is not required when indirecting via a file. For example,

  receiver_unqualified_hosts = /opt/exim/unqualnets
  

  could make use of a file containing

  172.16.0.0/12
  5f03:1200:836f::/48
  

  to have exactly the same effect as the previous example. If an IPv4 host calls an IPv6 host, the incoming address actually appears in the IPv6 host as `::ffff:<v4address>'. When such an address is tested against a host list, it is converted into a traditional IPv4 address first.
• If the item is of the form

  net<number>-<search-type>;<search-data>
  

  for example:

  net24-dbm;/networks.db
  

  then the IP address of the subject host is masked using <number> as the mask length; a textual string is then constructed from the masked value, followed by the mask, and this is then used as the key for the lookup. For example, if the host's IP address is 192.168.34.6 then the key that is looked up for the above example is `192.168.34.0/24'. IPv6 addresses are converted to a text value using lower case letters and full stops (periods) as separators instead of the more usual colon, because colon is the key terminator in `lsearch' files. Full, unabbreviated IPv6 addresses are always used.
• If the item is of the form

  net-<search-type>;<search-data>
  

  then the text form of the IP address of the subject host is used unmasked as the lookup key. This is not the same as specifying `net32' for an IPv4 address or `net128' for an IPv6 address, because the mask value is not included in the key. However, IPv6 addresses are still converted to an unabbreviated form, using lower case letters and full stops as separators.
• If the entire item is `@' the primary host name is used as the match item, and the following applies:
• If the item is a plain domain name, then Exim calls `gethostbyname()' to find its IP address(es). This typically causes a forward DNS lookup of the name. The result is compared with the IP address of the subject host.

The remaining items are wildcarded patterns for matching against the host name. If the host name is not already known, Exim calls `gethostbyaddr()' to obtain it from the IP address. This typically causes a reverse DNS lookup to occur. If the lookup fails, Exim takes a hard line by default and access is not permitted. If the list is an `accept' list, Exim behaves as if the current host is not in the set defined by the list, whereas if it is a `reject' list, it behaves as if it is.

To change this behaviour, the special item `+allow_unknown' may appear in the list (at top level -- it is not recognized in an indirected file). If any subsequent items require a host name, and the reverse DNS lookup fails, Exim permits the access, that is, its behaviour is the opposite to the default. For example,

host_reject = +allow_unknown:*.enemy.ex

rejects connections from any host whose name matches `*.enemy.ex', but only if it can find a host name from the incoming IP address. If `+warn_unknown' is used instead of `+allow_unknown', the effect is the same, except that Exim writes an entry to its log when it accepts a host whose name it cannot look up.

As a result of aliasing, hosts may have more than one name. When processing any of the following items, all the host's names are checked.

• If the item starts with `*' then the remainder of the item must match the end of the host name. For example, `*.b.c' matches all hosts whose names end in `.b.c'. This special simple form is provided because this is a very common requirement. Other kinds of wildcarding require the use of a regular expression.
• If the item starts with `^' then it is taken to be a regular expression which is matched against the host name. For example,

  ^(a|b)\.c\.d$
  

  matches either of the two hosts `a.c.d' or `b.c.d'. If the option string in which this occurs is given in quotes, then the backslash characters must be doubled, because they are significant in quoted strings. The following two settings are exactly equivalent:

  host_reject = ^(a|b)\.c\.d$
  host_reject = "^(a|b)\\.c\\.d$"
  

• If the item is of the form

  <search-type>;<filename or query>
  

  for example

  dbm;/host/accept/list
  

  then the host name is looked up using the search type and file name or query (as appropriate). If the lookup succeeds, the host matches the item. The actual data that is looked up is not used.

Use of RFC 1413 identification in host lists

Any item in a host list (other than a plain file name or `+allow_unknown') can optionally be preceded by

<ident>@
or
!<ident>@

where <ident> is an RFC 1413 identification string. For example,

host_reject = !exim@my.mail.gate:192.168.111.111:!root@public.host

If an <ident> string is present, it must match the RFC 1413 identification sent by the remote host, unless it is preceded by an exclamation mark, in which case it must not match. The remainder of the item, following the @, may be either positive or negative.

Go to the first, previous, next, last section, table of contents.