X-Git-Url: https://vcs.fsf.org/?a=blobdiff_plain;ds=sidebyside;f=doc%2Fdoc-txt%2Fexperimental-spec.txt;h=5dd6832b1f6382cc71aa32f4f0736e99d5a161e8;hb=b5b871aca49fbf0fcc2c91997e70c3c57f77faa9;hp=682b60b24b189758374b9d0adfe570d267188de2;hpb=7bafa7d930a78939ca4914b2c0250524eafb7857;p=exim.git diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 682b60b24..5dd6832b1 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -1,5 +1,3 @@ -$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.2 2005/03/08 15:33:05 tom Exp $ - From time to time, experimental features may be added to Exim. While a feature is experimental, there will be a build-time option whose name starts "EXPERIMENTAL_" that must be set in @@ -8,256 +6,67 @@ about experimenatal features, all of which are unstable and liable to incompatibile change. - -1. Yahoo DomainKeys support +OCSP Stapling support -------------------------------------------------------------- -DomainKeys (DK) support is built into Exim using the -"libdomainkeys" reference library implementation. It is -available at - -http://domainkeys.sf.net +X509 PKI certificates expire and can be revoked; to handle this, the +clients need some way to determine if a particular certificate, from a +particular Certificate Authority (CA), is still valid. There are three +main ways to do so. -You must build this library on your system and compile Exim -against it. To build Exim with DK support, add these lines to -your Local/Makefile: +The simplest way is to serve up a Certificate Revocation List (CRL) with +an ordinary web-server, regenerating the CRL before it expires. The +downside is that clients have to periodically re-download a potentially +huge file from every certificate authority it knows of. -EXPERIMENTAL_DOMAINKEYS=yes -CFLAGS += -I/home/tom/exim-cvs/extra/libdomainkeys -LDFLAGS += -ldomainkeys -L/home/tom/exim-cvs/extra/libdomainkeys +The way with most moving parts at query time is Online Certificate +Status Protocol (OCSP), where the client verifies the certificate +against an OCSP server run by the CA. This lets the CA track all +usage of the certs. This requires running software with access to the +private key of the CA, to sign the responses to the OCSP queries. OCSP +is based on HTTP and can be proxied accordingly. -Remember to tweak the CFLAGS and LDFLAGS lines to match the -location of the libdomainkeys includes and lib on your system. +The only widespread OCSP server implementation (known to this writer) +comes as part of OpenSSL and aborts on an invalid request, such as +connecting to the port and then disconnecting. This requires +re-entering the passphrase each time some random client does this. -The current experimental implementation supports two -independent functions: +The third way is OCSP Stapling; in this, the server using a certificate +issued by the CA periodically requests an OCSP proof of validity from +the OCSP server, then serves it up inline as part of the TLS +negotiation. This approach adds no extra round trips, does not let the +CA track users, scales well with number of certs issued by the CA and is +resilient to temporary OCSP server failures, as long as the server +starts retrying to fetch an OCSP proof some time before its current +proof expires. The downside is that it requires server support. -o Validate incoming DK-signed email. -o Sign outgoing email with DK. +If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL, +then it gains one new option: "tls_ocsp_file". -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. +The file specified therein is expected to be in DER format, and contain +an OCSP proof. Exim will serve it as part of the TLS handshake. This +option will be re-expanded for SNI, if the tls_certificate option +contains $tls_sni, as per other TLS options. +Exim does not at this time implement any support for fetching a new OCSP +proof. The burden is on the administrator to handle this, outside of +Exim. The file specified should be replaced atomically, so that the +contents are always valid. Exim will expand the "tls_ocsp_file" option +on each connection, so a new file will be handled transparently on the +next connection. +Exim will check for a valid next update timestamp in the OCSP proof; +if not present, or if the proof has expired, it will be ignored. -1) Validate incoming email +At this point in time, we're gathering feedback on use, to determine if +it's worth adding complexity to the Exim daemon to periodically re-fetch +OCSP files and somehow handling multiple files. There is no client support +for OCSP in Exim, this is feature expected to be used by mail clients. -Incoming messages are fed to the DK 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 = dk_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 DK validator. - control = dk_verify - -You can check for the outcome of the DK check in the ACL after -data (acl_smtp_data), using a number of ACL conditions and/or -expansion variables. - - - -1.1.) DK ACL conditions - - dk_sender_domains = - - This condition takes a domainlist as argument and - succeeds if the domain that DK has been verifying for is - found in the list. - - - dk_senders =
- - This condition takes an addresslist as argument and - succeeds if the address that DK has been verifying for - is found in the list. - - - dk_sender_local_parts = - - This condition takes a local_part list as argument - and succeeds if the domain that DK has been - verifying for is found in the list. - - - dk_status = - - This condition takes a list of keywords as argument, and - succeeds if one of the listed keywords matches the outcome - of the DK check. The available keywords are: - - good DK check succeeded, mail is verified. - bad DK check failed. - no signature Mail is not signed with DK. - no key Public key missing in target domain DNS. - bad format Public key available, but unuseable. - non-participant Target domain states not to participate in DK. - revoked The signing key has been revoked by the domain. - - - dk_policy = - - This condition takes a list of keywords as argument, and - succeeds if one of the listed keywords matches the policy - announced by the target domain. The available keywords - are: - - signsall The target domain signs all outgoing email. - testing The target domain is currently testing DK. - - - dk_domain_source = - - This condition takes a list of keywords as argument, and - succeeds if one of the listed keywords matches the - location where DK found the sender domain it verified for. - The available keywords are: - - from The domain came from the "From:" header. - sender The domain came from the "Sender:" header. - none DK was unable to find the responsible domain. - - - -1.2.) DK verification expansion variables - - $dk_sender_domain - - Contains the domain that DK has verified for. - - - $dk_sender - - Contains the address that DK has verified for. - - - $dk_sender_local_part - - Contains the local part that DK has verified for. - - - $dk_sender_source - - Contains the "source" of the above three variables, one of - - "from" The address came from the "From:" header. - "sender" The address came from the "Sender:" header. - - When DK was unable to find a valid address, this variable - is "0". - - - $dk_signsall - - Is "1" if the target domain signs all outgoing email, - "0" otherwise. - - - $dk_testing - - Is "1" if the target domain is testing DK, "0" otherwise. - - - $dk_is_signed - - Is "1" if the message is signed, "0" otherwise. - - - $dk_status - - Contains the outcome of the DK check as a string, commonly - used to add a "DomainKey-Status:" header to messages. Will - contain one of: - - good DK check succeeded, mail is verified. - bad DK check failed. - no signature Mail is not signed with DK. - no key Public key missing in target domain DNS. - bad format Public key available, but unuseable. - non-participant Target domain states not to participate in DK. - revoked The signing key has been revoked by the domain. - - - $dk_result - - Contains a human-readable result of the DK check, more - verbose than $dk_status. Useful for logging purposes. - - - -2) Sign outgoing email with DK - -Outgoing messages are signed just before exim puts them "on -the wire". The only thing that happens after DK signing is -eventual TLS encryption. - -Signing is implemented by setting private options on the SMTP -transport. These options take (expandable) strings as -arguments. The most important variable to use in these -expansions is $dk_domain. It contains the domain that DK wants -to sign for. - - - dk_selector = [MANDATORY] - - This sets the key selector string. You can use the - $dk_domain expansion variable to look up a matching - selector. The result is put in the expansion variable - $dk_selector which should be used in the dk_private_key - option along with $dk_domain. - - - dk_private_key = [MANDATORY] - - This sets the private key to use. You SHOULD use the - $dk_domain and $dk_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 dk_strict is set. - - - dk_canon = [OPTIONAL] - - This option sets the canonicalization method used when - signing a message. The DK draft currently supports two - methods: "simple" and "nofws". The option defaults to - "simple" when unset. - - - dk_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 and should use the $dk_domain and $dk_selector - expansion variables here. - - - dk_domain = [NOT RECOMMENDED] - - This option overrides DKs autodetection of the signing - domain. You should only use this option if you know what - you are doing. The result of the string expansion is also - put in $dk_domain. - - - - -2. Brighmail AntiSpam (BMI) suppport +Brightmail AntiSpam (BMI) suppport -------------------------------------------------------------- Brightmail AntiSpam is a commercial package. Please see @@ -281,14 +90,14 @@ 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 of the config file). 5) (Optional) Set up per-recipient opt-in information. -These four steps are explained in more details below. +These four steps are explained in more details below. 1) Adding support for BMI at compile time @@ -300,16 +109,16 @@ 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 merge the content of these lines with them. Note for BMI6.x users: You'll also have to add -lxml2_single to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do this. - + You should also include the location of libbmiclient_single.so in your dynamic linker configuration file (usually /etc/ld.so.conf) and run "ldconfig" @@ -317,22 +126,22 @@ 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. Note for BMI6.x users: This file is in XML format in V6.xx and its name is /opt/brightmail/etc/bmiconfig.xml. So BMI 6.x users MUST set the bmi_config_file option. - + 3) Set up ACL control statement @@ -344,8 +153,8 @@ 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 endpass @@ -356,7 +165,7 @@ These four steps are explained in more details below. endpass verify = recipient control = bmi_run - + If bmi_run is not set in any ACL during reception of the message, it will NOT be passed to the BMI server. @@ -367,108 +176,108 @@ 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 o deliver the message to an alternate location o do not deliver the message - + To query the verdict for a recipient, the implementation offers the following tools: - - + + - Boolean router preconditions. These can be used in any router. For a simple implementation of BMI, these may be all that you need. The following preconditions are available: - + o bmi_deliver_default - + This precondition is TRUE if the verdict for the recipient is to deliver the message normally. If the message has not been processed by the BMI server, this variable defaults to TRUE. - + o bmi_deliver_alternate - + This precondition is TRUE if the verdict for the recipient is to deliver the message to an alternate location. You can get the location string from the $bmi_alt_location expansion variable if you need it. See further below. If the message has not been processed by the BMI server, this variable defaults to FALSE. - + o bmi_dont_deliver - + This precondition is TRUE if the verdict for the recipient is NOT to deliver the message to the recipient. You will typically use this precondition in a top-level blackhole router, like this: - + # don't deliver messages handled by the BMI server bmi_blackhole: driver = redirect bmi_dont_deliver data = :blackhole: - + This router should be on top of all others, so messages that should not be delivered do not reach other routers at all. If the message has not been processed by the BMI server, this variable defaults to FALSE. - - + + - A list router precondition to query if rules "fired" on the message for the recipient. Its name is "bmi_rule". You use it by passing it a colon-separated list of rule numbers. You can use this condition to route messages that matched specific rules. Here is an example: - + # special router for BMI rule #5, #8 and #11 bmi_rule_redirect: driver = redirect bmi_rule = 5:8:11 data = postmaster@mydomain.com - - + + - Expansion variables. Several expansion variables are set during routing. You can use them in custom router conditions, for example. The following variables are available: - + o $bmi_base64_verdict - + This variable will contain the BASE64 encoded verdict for the recipient being routed. You can use it to add a header to messages for tracking purposes, for example: - + localuser: driver = accept check_local_user headers_add = X-Brightmail-Verdict: $bmi_base64_verdict transport = local_delivery - + If there is no verdict available for the recipient being routed, this variable contains the empty string. - + o $bmi_base64_tracker_verdict - + This variable will contain a BASE64 encoded subset of the verdict information concerning the "rules" that fired on the message. You can add this string to a header, commonly named "X-Brightmail-Tracker". Example: - + localuser: driver = accept check_local_user headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict transport = local_delivery - + If there is no verdict available for the recipient being routed, this variable contains the empty string. - + o $bmi_alt_location - + If the verdict is to redirect the message to an alternate location, this variable will contain the alternate location string returned by the BMI server. In @@ -477,17 +286,17 @@ These four steps are explained in more details below. there is no verdict available for the recipient being routed, or if the message is to be delivered normally, this variable contains the empty string. - + o $bmi_deliver - + This is an additional integer variable that can be used to query if the message should be delivered at all. You should use router preconditions instead if possible. - + $bmi_deliver is '0': the message should NOT be delivered. $bmi_deliver is '1': the message should be delivered. - - + + IMPORTANT NOTE: Verdict inheritance. The message is passed to the BMI server during message reception, using the target addresses from the RCPT TO: @@ -496,8 +305,8 @@ These four steps are explained in more details below. inherit the verdict from the original address. This means that verdicts also apply to all "child" addresses generated from top-level addresses that were sent to the BMI server. - - + + 5) Using per-recipient opt-in information (Optional) The BMI server features multiple scanning "profiles" for @@ -505,7 +314,7 @@ 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 @@ -517,49 +326,49 @@ These four steps are explained in more details below. flag. Here is an example that will pull opt-in data for each recipient from a flat file called '/etc/exim/bmi_optin_data'. - + The file format: - + user1@mydomain.com: : user2@thatdomain.com: - - + + The example: - + accept domains = +relay_to_domains endpass verify = recipient bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}} - control = bmi_run - + 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. - + For a list of available opt-in strings, please contact your Brightmail representative. - - - -3. Sender Policy Framework (SPF) support + + + +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. -SPF support is added via the libspf2 library. Visit +SPF support is added via the libspf2 library. Visit http://www.libspf2.org/ - + 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 @@ -599,9 +408,9 @@ 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 is meaning, for example "!fail" will match all results but "fail". The string list is evaluated left-to-right, in a @@ -610,11 +419,12 @@ 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 --------------------- */ @@ -640,29 +450,63 @@ variables. This contains a human-readable string describing the outcome of the SPF check. You can add it to a custom header or use 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 err_temp. - + $spf_smtp_comment This contains a string that can be used in a SMTP response to the calling party. Useful for "fail". - - -4. SRS (Sender Rewriting Scheme) Support +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 = $sender_host_address doesn't look trustworthy to me + spf_guess = fail +--------------------- */ + +In case you decide to reject messages based on this check, you +should note that although it uses the same framework, "Best-guess" +is NOT SPF, and therefore you should not mention SPF at all in your +reject message. + +When the spf_guess condition has run, it sets up the same expansion +variables as when spf condition is run, described above. + +Additionally, since Best-guess is not standarized, you may redefine +what "Best-guess" means to you by redefining spf_guess variable in +global config. For example, the following: + +/* ----------------- +spf_guess = v=spf1 a/16 mx/16 ptr ?all +--------------------- */ + +would relax host matching rules to a broader network range. + + +SRS (Sender Rewriting Scheme) Support -------------------------------------------------------------- Exiscan currently includes SRS support via Miles Wilton's -libsrs_alt library. The current version of the supported +libsrs_alt library. The current version of the supported library is 0.5. In order to use SRS, you must get a copy of libsrs_alt from @@ -677,6 +521,60 @@ EXPERIMENTAL_SRS=yes in your Local/Makefile. +DCC Support +-------------------------------------------------------------- + +*) Building exim + +In order to build exim with DCC support add + +EXPERIMENTAL_DCC=yes + +to your Makefile. (Re-)build/install exim. exim -d should show +EXPERIMENTAL_DCC under "Support for". + + +*) Configuration + +In the main section of exim.cf add at least + dccifd_address = /usr/local/dcc/var/dccifd +or + dccifd_address = + +In the DATA ACL you can use the new condition + dcc = * + +After that "$dcc_header" contains the X-DCC-Header. + +Returnvalues are: + fail for overall "R", "G" from dccifd + defer for overall "T" from dccifd + accept for overall "A", "S" from dccifd + +dcc = */defer_ok works as for spamd. + +The "$dcc_result" variable contains the overall result from DCC +answer. There will an X-DCC: header added to the mail. + +Usually you'll use + defer !dcc = * +to greylist with DCC. + +If you set, in the main section, + dcc_direct_add_header = true +then the dcc header will be added "in deep" and if the spool +file was already written it gets removed. This forces Exim to +write it again if needed. This helps to get the DCC Header +through to eg. SpamAssassin. + +If you want to pass even more headers in the middle of the +DATA stage you can set + $acl_m_dcc_add_header +to tell the DCC routines add more information; eg, you might set +this to some results from ClamAV. Be careful. Header syntax is +not checked and is added "as is". + + -------------------------------------------------------------- End of file --------------------------------------------------------------