X-Git-Url: https://git.exim.org/users/jgh/exim.git/blobdiff_plain/8ff3788cdba63b1ef90132a3b62aafe1232ce438..89dec7b604c1d16d8762e99443ba7bf2a3086c21:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index fbc1dfb72..4175173c3 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -1,4 +1,4 @@ -$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.3 2005/06/27 18:32:20 tom Exp $ +$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.11 2008/02/12 12:52:51 nm4 Exp $ From time to time, experimental features may be added to Exim. While a feature is experimental, there will be a build-time @@ -8,6 +8,157 @@ about experimenatal features, all of which are unstable and liable to incompatibile change. +0. DKIM support +-------------------------------------------------------------- + +DKIM support is implemented via libdkim. A compatible version +is available here: + +http://duncanthrax.net/exim-experimental/libdkim-1.0.15-tk.tar.gz + +Build the lib according to the instructions in the enclosed +INSTALL file. + +To build Exim with DKIM support, specify this in Local/Makefile: + +EXPERIMENTAL_DKIM=yes +CFLAGS += -I/home/tom/libdkim/include +LDFLAGS += -ldkim -lssl -lstdc++ -L/home/tom/libdkim/lib + +Remember to tweak the CFLAGS and LDFLAGS lines to match the +location of the libdomainkeys includes and lib on your system. + +The current experimental implementation supports two independent +functions: + +o Validate incoming DKIM-signed email. +o Sign outgoing email with DKIM. + +The former is implemented in the ACLs for SMTP, the latter as +an extension to the SMTP transport. That means both facilities +are limited to SMTP I/O. + + +1) Validate incoming email + +Incoming messages are fed to the DKIM validation process as they +are received "on the wire". This happens synchronously to Exim's +buffering of the message in the spool. + +You must set "control = dkim_verify" in one of the ACLs preceding +DATA (you will typically use acl_smtp_rcpt), at a point where +non-local, non-relay, non-submission mail is processed. If that +control flag is not set, the message will NOT be verified. + +Example: + +warn log_message = Feeding message to DKIM validator. + control = dk_verify + +You can then check for DKIM signatures in the ACL after data +(acl_smtp_data), using the 'dkim' query-style lookup type. The +query string should be a domain or DKIM identity: + +${lookup dkim{domain.example}} + +Such a lookup will yield one of the following strings: + +unverified: Exim did not (yet) verify the eventual DKIM + signatures in this message. This may happen + if a) You did not use control=dkim_verify + or b) You are using the lookup before + the DATA ACL. + +unsigned: The message does not have a signature from + the specified domain. + +good: The message has a signature from the specified + domain, and it verified successfully. + +bad: The message has a signature from the specified + domain, but it did not verify. + +defer: A temporary DNS problem was encountered while + trying to verify the signature. + + + +2) Sign outgoing email with DKIM + +Outgoing messages are signed just before Exim puts them "on +the wire". The only thing that happens after DKIM signing is +eventual TLS encryption. + +Signing is implemented by setting private options on the SMTP +transport. These options take (expandable) strings as +arguments. + + dkim_domain = [MANDATORY] + + The domain you want to sign with. Should optimally match + the domain in the "From:" header of the message, but + does not necessarily have to. The result of this expanded + option is put into the $dkim_domain expansion variable. + + dkim_selector = [MANDATORY] + + This sets the key selector string. You can use the + $dkim_domain expansion variable to look up a matching + selector. The result is put in the expansion variable + $dkim_selector which should be used in the dkim_private_key + option along with $dkim_domain. + + dkim_private_key = [MANDATORY] + + This sets the private key to use. You can use the + $dkim_domain and $dkim_selector expansion variables to + determine the private key to use. The result can either + + o be a valid RSA private key in ASCII armor, including + line breaks. + o start with a slash, in which case it is treated as + a file that contains the private key. + o be "0", "false" or the empty string, in which case + the message will not be signed. This case will not + result in an error, even if dkim_strict is set. + + dkim_canon = [OPTIONAL] + + This option sets the canonicalization method used when + signing a message. The DKIM RFC currently supports two + methods: "simple" and "relaxed". The option defaults to + "relaxed" when unset. Note: the current implementation + only support using the same canonicalization method for + both headers and body. + + dkim_strict = [OPTIONAL] + + This option defines how Exim behaves when signing a + message that should be signed fails for some reason. When + the expansion evaluates to either "1" or "true", Exim will + defer. Otherwise Exim will send the message unsigned. You + can use the $dkim_domain and $dkim_selector expansion + variables here. + + dkim_sign_headers = [OPTIONAL] + + When set, this option must expand to (or be specified as) + a colon-separated list of header names. These headers will + be included in the message signature. When unspecified, + the recommended headers will be used. Currently, these + are: + + from:sender:reply-to:subject:date: + message-id:to:cc:mime-version:content-type: + content-transfer-encoding:content-id: + content-description:resent-date:resent-from: + resent-sender:resent-to:resent-cc:resent-message-id: + in-reply-to:references: + list-id:list-help:list-unsubscribe: + list-subscribe:list-post:list-owner:list-archive + + + 1. Yahoo DomainKeys support -------------------------------------------------------------- @@ -194,7 +345,7 @@ expansion variables. 2) Sign outgoing email with DK -Outgoing messages are signed just before exim puts them "on +Outgoing messages are signed just before Exim puts them "on the wire". The only thing that happens after DK signing is eventual TLS encryption. @@ -257,7 +408,7 @@ to sign for. -2. Brighmail AntiSpam (BMI) suppport +2. Brightmail AntiSpam (BMI) suppport -------------------------------------------------------------- Brightmail AntiSpam is a commercial package. Please see @@ -281,7 +432,7 @@ instead of per-message. To use BMI, you need to take the following steps: 1) Compile Exim with BMI support - 2) Set up main BMI options (top section of exim config file) + 2) Set up main BMI options (top section of Exim config file) 3) Set up ACL control statement (ACL section of the config file) 4) Set up your routers to use BMI verdicts (routers section @@ -300,7 +451,7 @@ These four steps are explained in more details below. with these lines in Local/Makefile: EXPERIMENTAL_BRIGHTMAIL=yes - CFLAGS=-DBRIGHTMAIL -I/path/to/the/dir/with/the/includefile + CFLAGS=-I/path/to/the/dir/with/the/includefile EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single If you use other CFLAGS or EXTRALIBS_EXIM settings then @@ -317,15 +468,15 @@ These four steps are explained in more details below. able to find the library file. -2) Setting up BMI support in the exim main configuration +2) Setting up BMI support in the Exim main configuration - To enable BMI support in the main exim configuration, you + To enable BMI support in the main Exim configuration, you should set the path to the main BMI configuration file with the "bmi_config_file" option, like this: bmi_config_file = /opt/brightmail/etc/brightmail.cfg - This must go into section 1 of exims configuration file (You + This must go into section 1 of Exim's configuration file (You can put it right on top). If you omit this option, it defaults to /opt/brightmail/etc/brightmail.cfg. @@ -344,7 +495,7 @@ These four steps are explained in more details below. an "accept" block in the "acl_check_rcpt" ACL. You should use the "accept" block(s) that accept messages from remote servers for your own domain(s). Here is an example that uses - the "accept" blocks from exims default configuration file: + the "accept" blocks from Exim's default configuration file: accept domains = +local_domains @@ -367,7 +518,7 @@ These four steps are explained in more details below. more "verdicts" are present. Different recipients can have different verdicts. Each recipient is treated individually during routing, so you can query the verdicts by recipient - at that stage. From Exims view, a verdict can have the + at that stage. From Exim's view, a verdict can have the following outcomes: o deliver the message normally @@ -505,7 +656,7 @@ These four steps are explained in more details below. server and are queried by the BMI server itself. However, you can also pass opt-in data for each recipient from the MTA to the BMI server. This is particularly useful if you - already look up recipient data in exim anyway (which can + already look up recipient data in Exim anyway (which can also be stored in a SQL database or other source). This implementation enables you to pass opt-in data to the BMI server in the RCPT ACL. This works by setting the @@ -533,7 +684,7 @@ These four steps are explained in more details below. control = bmi_run Of course, you can also use any other lookup method that - exim supports, including LDAP, Postgres, MySQL, Oracle etc., + Exim supports, including LDAP, Postgres, MySQL, Oracle etc., as long as the result is a list of colon-separated opt-in strings. @@ -546,7 +697,7 @@ These four steps are explained in more details below. 3. Sender Policy Framework (SPF) support -------------------------------------------------------------- -To learn more about SPF, visit http://spf.pobox.com. This +To learn more about SPF, visit http://www.openspf.org. This document does not explain the SPF fundamentals, you should read and understand the implications of deploying SPF on your system before doing so. @@ -559,7 +710,7 @@ to obtain a copy, then compile and install it. By default, this will put headers in /usr/local/include and the static library in /usr/local/lib. -To compile exim with SPF support, set these additional flags in +To compile Exim with SPF support, set these additional flags in Local/Makefile: EXPERIMENTAL_SPF=yes @@ -599,7 +750,7 @@ which the spf condition should succeed. Valid strings are: record of the queried domain. This should be treated like "none". o err_temp This indicates a temporary error during all - processing, including exim's SPF processing. + processing, including Exim's SPF processing. You may defer messages when this occurs. You can prefix each string with an exclamation mark to invert @@ -610,11 +761,12 @@ the SPF check, the condition succeeds. If none of the listed strings matches the outcome of the SPF check, the condition fails. -Here is a simple example to fail forgery attempts from domains -that publish SPF records: +Here is an example to fail forgery attempts from domains that +publish SPF records: /* ----------------- -deny message = $sender_host_address is not allowed to send mail from $sender_address_domain +deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}. \ + Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address spf = fail --------------------- */ @@ -642,11 +794,14 @@ variables. it for logging purposes. $spf_received - This contains a complete SPF-Received: header that can be + This contains a complete Received-SPF: header that can be added to the message. Please note that according to the SPF draft, this header must be added at the top of the header list. Please see section 10 on how you can do this. + Note: in case of "Best-guess" (see below), the convention is + to put this string in a header called X-SPF-Guess: instead. + $spf_result This contains the outcome of the SPF check in string form, one of pass, fail, softfail, none, neutral, err_perm or @@ -656,6 +811,37 @@ variables. This contains a string that can be used in a SMTP response to the calling party. Useful for "fail". +In addition to SPF, you can also perform checks for so-called +"Best-guess". Strictly speaking, "Best-guess" is not standard +SPF, but it is supported by the same framework that enables SPF +capability. Refer to http://www.openspf.org/FAQ/Best_guess_record +for a description of what it means. + +To access this feature, simply use the spf_guess condition in place +of the spf one. For example: + +/* ----------------- +deny message = $sender_host_address doesn't look trustworthy to me + spf_guess = fail +--------------------- */ + +In case you decide to reject messages based on this check, you +should note that although it uses the same framework, "Best-guess" +is NOT SPF, and therefore you should not mention SPF at all in your +reject message. + +When the spf_guess condition has run, it sets up the same expansion +variables as when spf condition is run, described above. + +Additionally, since Best-guess is not standarized, you may redefine +what "Best-guess" means to you by redefining spf_guess variable in +global config. For example, the following: + +/* ----------------- +spf_guess = v=spf1 a/16 mx/16 ptr ?all +--------------------- */ + +would relax host matching rules to a broader network range. 4. SRS (Sender Rewriting Scheme) Support