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 order to include the feature. This file contains information about experimenatal features, all of which are unstable and liable to incompatibile change. OCSP Stapling support -------------------------------------------------------------- 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. 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. 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. 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 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. If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL, then it gains one new option: "tls_ocsp_file". 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. 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. Brightmail AntiSpam (BMI) suppport -------------------------------------------------------------- Brightmail AntiSpam is a commercial package. Please see http://www.brightmail.com for more information on the product. For the sake of clarity, we'll refer to it as "BMI" from now on. 0) BMI concept and implementation overview In contrast to how spam-scanning with SpamAssassin is implemented in exiscan-acl, BMI is more suited for per -recipient scanning of messages. However, each messages is scanned only once, but multiple "verdicts" for multiple recipients can be returned from the BMI server. The exiscan implementation passes the message to the BMI server just before accepting it. It then adds the retrieved verdicts to the messages header file in the spool. These verdicts can then be queried in routers, where operation is per-recipient 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) 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. 1) Adding support for BMI at compile time To compile with BMI support, you need to link Exim against the Brighmail client SDK, consisting of a library (libbmiclient_single.so) and a header file (bmi_api.h). You'll also need to explicitly set a flag in the Makefile to include BMI support in the Exim binary. Both can be achieved with these lines in Local/Makefile: EXPERIMENTAL_BRIGHTMAIL=yes 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" afterwards, or else the produced Exim binary will not be able to find the library file. 2) Setting up BMI support in the Exim main configuration 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 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 To optimize performance, it makes sense only to process messages coming from remote, untrusted sources with the BMI server. To set up a messages for processing by the BMI server, you MUST set the "bmi_run" control statement in any ACL for an incoming message. You will typically do this in 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 Exim's default configuration file: accept domains = +local_domains endpass verify = recipient control = bmi_run accept domains = +relay_to_domains 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. 4) Setting up routers to use BMI verdicts When a message has been run through the BMI server, one or 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 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 its default configuration, this is a header-like string that can be added to the message with "headers_add". If 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: commands in the SMTP transaction. If recipients get expanded or re-written (for example by aliasing), the new address(es) 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 individual recipients. These are usually stored in a LDAP 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 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 'bmi_optin' modifier in a block of that ACL. If should be set to a list of comma-separated strings that identify the features which the BMI server should use for that particular recipient. Ideally, you would use the 'bmi_optin' modifier in the same ACL block where you set the 'bmi_run' control 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 Of course, you can also use any other lookup method that 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. Sender Policy Framework (SPF) support -------------------------------------------------------------- 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 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 Local/Makefile: EXPERIMENTAL_SPF=yes CFLAGS=-DSPF -I/usr/local/include EXTRALIBS_EXIM=-L/usr/local/lib -lspf2 This assumes that the libspf2 files are installed in their default locations. You can now run SPF checks in incoming SMTP by using the "spf" ACL condition in either the MAIL, RCPT or DATA ACLs. When using it in the RCPT ACL, you can make the checks dependend on the RCPT address (or domain), so you can check SPF records only for certain target domains. This gives you the possibility to opt-out certain customers that do not want their mail to be subject to SPF checking. The spf condition takes a list of strings on its right-hand side. These strings describe the outcome of the SPF check for which the spf condition should succeed. Valid strings are: o pass The SPF check passed, the sending host is positively verified by SPF. o fail The SPF check failed, the sending host is NOT allowed to send mail for the domain in the envelope-from address. o softfail The SPF check failed, but the queried domain can't absolutely confirm that this is a forgery. o none The queried domain does not publish SPF records. o neutral The SPF check returned a "neutral" state. This means the queried domain has published a SPF record, but wants to allow outside servers to send mail under its domain as well. o err_perm This indicates a syntax error in the SPF 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. 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 short-circuit fashion. When a string matches the outcome of the SPF check, the condition succeeds. If none of the listed strings matches the outcome of the SPF check, the condition fails. 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 ${if def:sender_address_domain {$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 --------------------- */ You can also give special treatment to specific domains: /* ----------------- deny message = AOL sender, but not from AOL-approved relay. sender_domains = aol.com spf = fail:neutral --------------------- */ Explanation: AOL publishes SPF records, but is liberal and still allows non-approved relays to send mail from aol.com. This will result in a "neutral" state, while mail from genuine AOL servers will result in "pass". The example above takes this into account and treats "neutral" like "fail", but only for aol.com. Please note that this violates the SPF draft. When the spf condition has run, it sets up several expansion variables. $spf_header_comment 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 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". 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 = $sender_host_address doesn't look trustworthy to me spf_guess = fail --------------------- */ In case you decide to reject messages based on this check, you should note that although it uses the same framework, "Best-guess" is NOT SPF, and therefore you should not mention SPF at all in your reject message. When the spf_guess condition has run, it sets up the same expansion variables as when spf condition is run, described above. Additionally, since Best-guess is not standarized, you may redefine what "Best-guess" means to you by redefining spf_guess variable in global config. For example, the following: /* ----------------- spf_guess = v=spf1 a/16 mx/16 ptr ?all --------------------- */ would relax host matching rules to a broader network range. SRS (Sender Rewriting Scheme) Support -------------------------------------------------------------- Exiscan currently includes SRS support via Miles Wilton's libsrs_alt library. The current version of the supported library is 0.5. In order to use SRS, you must get a copy of libsrs_alt from http://srs.mirtol.com/ Unpack the tarball, then refer to MTAs/README.EXIM to proceed. You need to set EXPERIMENTAL_SRS=yes in your Local/Makefile. DCC Support -------------------------------------------------------------- *) Building exim In order to build exim with DCC support add EXPERIMENTAL_DCC=yes to your Makefile. (Re-)build/install exim. exim -d should show EXPERIMENTAL_DCC under "Support for". *) Configuration In the main section of exim.cf add at least dccifd_address = /usr/local/dcc/var/dccifd or dccifd_address = In the DATA ACL you can use the new condition dcc = * After that "$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 --------------------------------------------------------------