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 experimental features, all of which are unstable and liable to incompatible change. 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 dependent 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. This should be treated like "none". o permerror This indicates a syntax error in the SPF record of the queried domain. You may deny messages when this occurs. (Changed in 4.83) o temperror This indicates a temporary error during all processing, including Exim's SPF processing. You may defer messages when this occurs. (Changed in 4.83) o err_temp Same as permerror, deprecated in 4.83, will be removed in a future release. o err_perm Same as temperror, deprecated in 4.83, will be removed in a future release. You can prefix each string with an exclamation mark to invert its 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, permerror or temperror. $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 standardized, 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. A lookup expansion is also available. It takes an email address as the key and an IP address as the database: $lookup (username@domain} spf {ip.ip.ip.ip}} The lookup will return the same result strings as they can appear in $spf_result (pass,fail,softfail,neutral,none,err_perm,err_temp). Currently, only IPv4 addresses are supported. SRS (Sender Rewriting Scheme) Support -------------------------------------------------------------- Exiscan currently includes SRS support via Miles Wilton's libsrs_alt library. The current version of the supported library is 0.5. In order to use SRS, you must get a copy of libsrs_alt from http://srs.mirtol.com/ Unpack the tarball, then refer to MTAs/README.EXIM to proceed. You need to set EXPERIMENTAL_SRS=yes in your Local/Makefile. DCC Support -------------------------------------------------------------- Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/ *) Building exim In order to build exim with DCC support add EXPERIMENTAL_DCC=yes to your Makefile. (Re-)build/install exim. exim -d should show EXPERIMENTAL_DCC under "Support for". *) Configuration In the main section of exim.cf add at least dccifd_address = /usr/local/dcc/var/dccifd or dccifd_address = In the DATA ACL you can use the new condition dcc = * After that "$dcc_header" contains the X-DCC-Header. Return values 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 to 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". In case you've troubles with sites sending the same queue items from several hosts and fail to get through greylisting you can use $acl_m_dcc_override_client_ip Setting $acl_m_dcc_override_client_ip to an IP address overrides the default of $sender_host_address. eg. use the following ACL in DATA stage: warn set acl_m_dcc_override_client_ip = \ ${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}} condition = ${if def:acl_m_dcc_override_client_ip} log_message = dbg: acl_m_dcc_override_client_ip set to \ $acl_m_dcc_override_client_ip Then set something like # cat /etc/mail/multipleip_sites mout-xforward.gmx.net 82.165.159.12 mout.gmx.net 212.227.15.16 Use a reasonable IP. eg. one the sending cluster acutally uses. DMARC Support -------------------------------------------------------------- DMARC combines feedback from SPF, DKIM, and header From: in order to attempt to provide better indicators of the authenticity of an email. This document does not explain the fundamentals, you should read and understand how it works by visiting the website at http://www.dmarc.org/. DMARC support is added via the libopendmarc library. Visit: http://sourceforge.net/projects/opendmarc/ to obtain a copy, or find it in your favorite rpm package repository. If building from source, this description assumes that headers will be in /usr/local/include, and that the libraries are in /usr/local/lib. 1. To compile Exim with DMARC support, you must first enable SPF. Please read the above section on enabling the EXPERIMENTAL_SPF feature. You must also have DKIM support, so you cannot set the DISABLE_DKIM feature. Once both of those conditions have been met you can enable DMARC in Local/Makefile: EXPERIMENTAL_DMARC=yes LDFLAGS += -lopendmarc # CFLAGS += -I/usr/local/include # LDFLAGS += -L/usr/local/lib The first line sets the feature to include the correct code, and the second line says to link the libopendmarc libraries into the exim binary. The commented out lines should be uncommented if you built opendmarc from source and installed in the default location. Adjust the paths if you installed them elsewhere, but you do not need to uncomment them if an rpm (or you) installed them in the package controlled locations (/usr/include and /usr/lib). 2. Use the following global settings to configure DMARC: Required: dmarc_tld_file Defines the location of a text file of valid top level domains the opendmarc library uses during domain parsing. Maintained by Mozilla, the most current version can be downloaded from a link at http://publicsuffix.org/list/. Optional: dmarc_history_file Defines the location of a file to log results of dmarc verification on inbound emails. The contents are importable by the opendmarc tools which will manage the data, send out DMARC reports, and expire the data. Make sure the directory of this file is writable by the user exim runs as. dmarc_forensic_sender The email address to use when sending a forensic report detailing alignment failures if a sender domain's dmarc record specifies it and you have configured Exim to send them. Default: do-not-reply@$default_hostname 3. By default, the DMARC processing will run for any remote, non-authenticated user. It makes sense to only verify DMARC status of messages coming from remote, untrusted sources. You can use standard conditions such as hosts, senders, etc, to decide that DMARC verification should *not* be performed for them and disable DMARC with a control setting: control = dmarc_disable_verify A DMARC record can also specify a "forensic address", which gives exim an email address to submit reports about failed alignment. Exim does not do this by default because in certain conditions it results in unintended information leakage (what lists a user might be subscribed to, etc). You must configure exim to submit forensic reports to the owner of the domain. If the DMARC record contains a forensic address and you specify the control statement below, then exim will send these forensic emails. It's also advised that you configure a dmarc_forensic_sender because the default sender address construction might be inadequate. control = dmarc_enable_forensic (AGAIN: You can choose not to send these forensic reports by simply not putting the dmarc_enable_forensic control line at any point in your exim config. If you don't tell it to send them, it will not send them.) There are no options to either control. Both must appear before the DATA acl. 4. You can now run DMARC checks in incoming SMTP by using the "dmarc_status" ACL condition in the DATA ACL. You are required to call the spf condition first in the ACLs, then the "dmarc_status" condition. Putting this condition in the ACLs is required in order for a DMARC check to actually occur. All of the variables are set up before the DATA ACL, but there is no actual DMARC check that occurs until a "dmarc_status" condition is encountered in the ACLs. The dmarc_status condition takes a list of strings on its right-hand side. These strings describe recommended action based on the DMARC check. To understand what the policy recommendations mean, refer to the DMARC website above. Valid strings are: o accept The DMARC check passed and the library recommends accepting the email. o reject The DMARC check failed and the library recommends rejecting the email. o quarantine The DMARC check failed and the library recommends keeping it for further inspection. o none The DMARC check passed and the library recommends no specific action, neutral. o norecord No policy section in the DMARC record for this sender domain. o nofrom Unable to determine the domain of the sender. o temperror Library error or dns error. o off The DMARC check was disabled for this email. You can prefix each string with an exclamation mark to invert its meaning, for example "!accept" will match all results but "accept". The string list is evaluated left-to-right in a short-circuit fashion. When a string matches the outcome of the DMARC check, the condition succeeds. If none of the listed strings matches the outcome of the DMARC check, the condition fails. Of course, you can also use any other lookup method that Exim supports, including LDAP, Postgres, MySQL, etc, as long as the result is a list of colon-separated strings. Several expansion variables are set before the DATA ACL is processed, and you can use them in this ACL. The following expansion variables are available: o $dmarc_status This is a one word status indicating what the DMARC library thinks of the email. It is a combination of the results of DMARC record lookup and the SPF/DKIM/DMARC processing results (if a DMARC record was found). The actual policy declared in the DMARC record is in a separate expansion variable. o $dmarc_status_text This is a slightly longer, human readable status. o $dmarc_used_domain This is the domain which DMARC used to look up the DMARC policy record. o $dmarc_domain_policy This is the policy declared in the DMARC record. Valid values are "none", "reject" and "quarantine". It is blank when there is any error, including no DMARC record. o $dmarc_ar_header This is the entire Authentication-Results header which you can add using an add_header modifier. 5. How to enable DMARC advanced operation: By default, Exim's DMARC configuration is intended to be non-intrusive and conservative. To facilitate this, Exim will not create any type of logging files without explicit configuration by you, the admin. Nor will Exim send out any emails/reports about DMARC issues without explicit configuration by you, the admin (other than typical bounce messages that may come about due to ACL processing or failure delivery issues). In order to log statistics suitable to be imported by the opendmarc tools, you need to: a. Configure the global setting dmarc_history_file. b. Configure cron jobs to call the appropriate opendmarc history import scripts and truncating the dmarc_history_file. In order to send forensic reports, you need to: a. Configure the global setting dmarc_forensic_sender. b. Configure, somewhere before the DATA ACL, the control option to enable sending DMARC forensic reports. 6. Example usage: (RCPT ACL) warn domains = +local_domains hosts = +local_hosts control = dmarc_disable_verify warn !domains = +screwed_up_dmarc_records control = dmarc_enable_forensic warn condition = (lookup if destined to mailing list) set acl_m_mailing_list = 1 (DATA ACL) warn dmarc_status = accept : none : off !authenticated = * log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain add_header = $dmarc_ar_header warn dmarc_status = !accept !authenticated = * log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain warn dmarc_status = quarantine !authenticated = * set $acl_m_quarantine = 1 # Do something in a transport with this flag variable deny condition = ${if eq{$dmarc_domain_policy}{reject}} condition = ${if eq{$acl_m_mailing_list}{1}} message = Messages from $dmarc_used_domain break mailing lists deny dmarc_status = reject !authenticated = * message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT DANE ------------------------------------------------------------ DNS-based Authentication of Named Entities, as applied to SMTP over TLS, provides assurance to a client that it is actually talking to the server it wants to rather than some attacker operating a Man In The Middle (MITM) operation. The latter can terminate the TLS connection you make, and make another one to the server (so both you and the server still think you have an encrypted connection) and, if one of the "well known" set of Certificate Authorities has been suborned - something which *has* been seen already (2014), a verifiable certificate (if you're using normal root CAs, eg. the Mozilla set, as your trust anchors). What DANE does is replace the CAs with the DNS as the trust anchor. The assurance is limited to a) the possibility that the DNS has been suborned, b) mistakes made by the admins of the target server. The attack surface presented by (a) is thought to be smaller than that of the set of root CAs. It also allows the server to declare (implicitly) that connections to it should use TLS. An MITM could simply fail to pass on a server's STARTTLS. DANE scales better than having to maintain (and side-channel communicate) copies of server certificates for every possible target server. It also scales (slightly) better than having to maintain on an SMTP client a copy of the standard CAs bundle. It also means not having to pay a CA for certificates. DANE requires a server operator to do three things: 1) run DNSSEC. This provides assurance to clients that DNS lookups they do for the server have not been tampered with. The domain MX record applying to this server, its A record, its TLSA record and any associated CNAME records must all be covered by DNSSEC. 2) add TLSA DNS records. These say what the server certificate for a TLS connection should be. 3) offer a server certificate, or certificate chain, in TLS connections which is traceable to the one defined by (one of?) the TSLA records There are no changes to Exim specific to server-side operation of DANE. The TLSA record for the server may have "certificate usage" of DANE-TA(2) or DANE-EE(3). The latter specifies the End Entity directly, i.e. the certificate involved is that of the server (and should be the sole one transmitted during the TLS handshake); this is appropriate for a single system, using a self-signed certificate. DANE-TA usage is effectively declaring a specific CA to be used; this might be a private CA or a public, well-known one. A private CA at simplest is just a self-signed certificate which is used to sign cerver certificates, but running one securely does require careful arrangement. If a private CA is used then either all clients must be primed with it, or (probably simpler) the server TLS handshake must transmit the entire certificate chain from CA to server-certificate. If a public CA is used then all clients must be primed with it (losing one advantage of DANE) - but the attack surface is reduced from all public CAs to that single CA. DANE-TA is commonly used for several services and/or servers, each having a TLSA query-domain CNAME record, all of which point to a single TLSA record. The TLSA record should have a Selector field of SPKI(1) and a Matching Type field of SHA2-512(2). At the time of writing, https://www.huque.com/bin/gen_tlsa is useful for quickly generating TLSA records; and commands like openssl x509 -in -pubkey -noout /dev/null \ | openssl sha512 \ | awk '{print $2}' are workable for 4th-field hashes. For use with the DANE-TA model, server certificates must have a correct name (SubjectName or SubjectAltName). The use of OCSP-stapling should be considered, allowing for fast revocation of certificates (which would otherwise be limited by the DNS TTL on the TLSA records). However, this is likely to only be usable with DANE-TA. NOTE: the default of requesting OCSP for all hosts is modified iff DANE is in use, to: hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \ {= {4}{$tls_out_tlsa_usage}} } \ {*}{}} The (new) variable $tls_out_tlsa_usage is a bitfield with numbered bits set for TLSA record usage codes. The zero above means DANE was not in use, the four means that only DANE-TA usage TLSA records were found. If the definition of hosts_request_ocsp includes the string "tls_out_tlsa_usage", they are re-expanded in time to control the OCSP request. This modification of hosts_request_ocsp is only done if it has the default value of "*". Admins who change it, and those who use hosts_require_ocsp, should consider the interaction with DANE in their OCSP settings. For client-side DANE there are two new smtp transport options, hosts_try_dane and hosts_require_dane. They do the obvious thing. [ should they be domain-based rather than host-based? ] DANE will only be usable if the target host has DNSSEC-secured MX, A and TLSA records. A TLSA lookup will be done if either of the above options match and the host-lookup succeded using dnssec. If a TLSA lookup is done and succeeds, a DANE-verified TLS connection will be required for the host. (TODO: specify when fallback happens vs. when the host is not used) If DANE is requested and useable (see above) the following transport options are ignored: hosts_require_tls tls_verify_hosts tls_try_verify_hosts tls_verify_certificates tls_crl tls_verify_cert_hostnames If DANE is not usable, whether requested or not, and CA-anchored verification evaluation is wanted, the above variables should be set appropriately. Currently dnssec_request_domains must be active (need to think about that) and dnssec_require_domains is ignored. If verification was successful using DANE then the "CV" item in the delivery log line will show as "CV=dane". There is a new variable $tls_out_dane which will have "yes" if verification succeeded using DANE and "no" otherwise (only useful in combination with EXPERIMENTAL_EVENT), and a new variable $tls_out_tlsa_usage (detailed above). DSN extra information --------------------- If compiled with EXPERIMENTAL_DSN_INFO extra information will be added to DSN fail messages ("bounces"), when available. The intent is to aid tracing of specific failing messages, when presented with a "bounce" complaint and needing to search logs. The remote MTA IP address, with port number if nonstandard. Example: Remote-MTA: X-ip; [127.0.0.1]:587 Rationale: Several addresses may correspond to the (already available) dns name for the remote MTA. The remote MTA connect-time greeting. Example: X-Remote-MTA-smtp-greeting: X-str; 220 the.local.host.name ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000 Rationale: This string sometimes presents the remote MTA's idea of its own name, and sometimes identifies the MTA software. The remote MTA response to HELO or EHLO. Example: X-Remote-MTA-helo-response: X-str; 250-the.local.host.name Hello localhost [127.0.0.1] Limitations: Only the first line of a multiline response is recorded. Rationale: This string sometimes presents the remote MTA's view of the peer IP connecting to it. The reporting MTA detailed diagnostic. Example: X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:: 550 hard error Rationale: This string somtimes give extra information over the existing (already available) Diagnostic-Code field. Note that non-RFC-documented field names and data types are used. -------------------------------------------------------------- End of file --------------------------------------------------------------