-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.5 2005/07/23 20:16:32 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
order to include the feature. This file contains information
-about experimenatal features, all of which are unstable and
-liable to incompatibile change.
-
+about experimental features, all of which are unstable and
+liable to incompatible change.
-1. Yahoo DomainKeys support
---------------------------------------------------------------
-
-DomainKeys (DK) support is built into Exim using the
-"libdomainkeys" reference library implementation. It is
-available at
-
-http://domainkeys.sf.net
-
-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:
-
-EXPERIMENTAL_DOMAINKEYS=yes
-CFLAGS += -I/home/tom/exim-cvs/extra/libdomainkeys
-LDFLAGS += -ldomainkeys -L/home/tom/exim-cvs/extra/libdomainkeys
-
-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 DK-signed email.
-o Sign outgoing email with DK.
-
-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 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 = <domain list>
-
- 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 = <address list>
-
- 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 = <local part list>
-
- 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 = <colon separated list of keywords>
-
- 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 = <colon separated list of keywords>
-
- 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 = <colon separated list of keywords>
-
- 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 = <expanded string> [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 = <expanded string> [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 = <expanded string> [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 = <expanded string> [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 = <expanded string> [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. Brightmail AntiSpam (BMI) suppport
+Brightmail AntiSpam (BMI) suppport
--------------------------------------------------------------
Brightmail AntiSpam is a commercial package. Please see
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
+ 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.
-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.
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
+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
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
+ 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
-is meaning, for example "!fail" will match all results but
+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 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
--------------------- */
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.
+ 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.
+
-4. SRS (Sender Rewriting Scheme) Support
+SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------
Exiscan currently includes SRS support via Miles Wilton's
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 = <ip> <port>
+
+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 <certificate.pem \
+ | openssl rsa -outform der -pubin 2>/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.
+[ should they be domain-based rather than host-based? ]
+
+Hosts_require_dane will result in failure if the target host
+is not DNSSEC-secured.
+
+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. If it does not, the host will not
+be used; there is no fallback to non-DANE or non-TLS.
+
+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:<d3@myhost.test.ex>: 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.
+
+
+
+
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git://git.exim.org / users / jgh / exim.git / blobdiff