-Transport post-delivery actions
+Event Actions
--------------------------------------------------------------
-An arbitrary per-transport string can be expanded upon various transport events
-and (for SMTP transports) a second string on deferrals caused by a host error.
+(Renamed from TPDA, Transport post-delivery actions)
+
+An arbitrary per-transport string can be expanded upon various transport events.
Additionally a main-section configuration option can be expanded on some
per-message events.
This feature may be used, for example, to write exim internal log information
In order to use the feature, you must compile with
-EXPERIMENTAL_TPDA=yes
+EXPERIMENTAL_EVENT=yes
in your Local/Makefile
and define one or both of
-- the tpda_event_action option in the transport
-- the delivery_event_action
+- the event_action option in the transport
+- the event_action main option
to be expanded when the event fires.
-A new variable, $tpda_event, is set to the event type when the
+A new variable, $event_name, is set to the event type when the
expansion is done. The current list of events is:
- msg:complete main per message
- msg:delivery transport per recipient
- msg:host:defer transport per attempt
- msg:fail:delivery main per recipient
- msg:fail:internal main per recipient
- tcp:connect transport per connection
- tcp:close transport per connection
- tls:cert transport per certificate in verification chain
- smtp:connect transport per connection
-
-The expansion is called for all event types, and should use the $tpda_event
+ msg:complete after main per message
+ msg:delivery after transport per recipient
+ msg:host:defer after transport per attempt
+ msg:fail:delivery after main per recipient
+ msg:fail:internal after main per recipient
+ tcp:connect before transport per connection
+ tcp:close after transport per connection
+ tls:cert before both per certificate in verification chain
+ smtp:connect after transport per connection
+
+The expansion is called for all event types, and should use the $event_name
value to decide when to act. The variable data is a colon-separated
list, describing an event tree.
-There is an auxilary variable, $tpda_data, for which the
+There is an auxilary variable, $event_data, for which the
content is event_dependent:
msg:delivery smtp confirmation mssage
tls:cert verification chain depth
smtp:connect smtp banner
-The msg:host:defer event populates one extra variable, $tpda_defer_errno.
+The msg:host:defer event populates one extra variable, $event_defer_errno.
The following variables are likely to be useful depending on the event type:
tls_out_peercert
lookup_dnssec_authenticated, tls_out_dane
sending_ip_address, sending_port
- message_exim_id
+ message_exim_id, verify_mode
An example might look like:
-tpda_event_action = ${if = {msg:delivery}{$tpda_event} \
+event_action = ${if eq {msg:delivery}{$event_name} \
{${lookup pgsql {SELECT * FROM record_Delivery( \
'${quote_pgsql:$sender_address_domain}',\
'${quote_pgsql:${lc:$sender_address_local_part}}', \
'${quote_pgsql:$message_exim_id}')}} \
} {}}
-The string is expanded for each of the supported events and any
-side-effects will happen. The result is then discarded.
+The string is expanded when each of the supported events occur
+and any side-effects of the expansion will happen.
Note that for complex operations an ACL expansion can be used.
-The expansion of the tpda_event_action option should normally
+The expansion of the event_action option should normally
return an empty string. Should it return anything else the
following will be forced:
tls:cert refuse verification
smtp:connect close connection
+No other use is made of the result string.
+Known issues:
+- the tls:cert event is only called for the cert chain elements
+ received over the wire, with GnuTLS. OpenSSL gives the entire
+ chain including thse loaded locally.
Redis Lookup
the next hop does not support DSN.
Adding it to a redirect router makes no difference.
+
+
+
Certificate name checking
--------------------------------------------------------------
The X509 certificates used for TLS are supposed be verified
support to date has not made these checks.
If built with EXPERIMENTAL_CERTNAMES defined, code is
-included to do so, and a new smtp transport option
-"tls_verify_cert_hostname" supported which takes a list of
-names for which the checks must be made. The host must
-also be in "tls_verify_hosts".
+included to do so for server certificates, and a new smtp transport option
+"tls_verify_cert_hostnames" supported which takes a hostlist
+which must match the target host for the additional checks must be made.
+The option currently defaults to empty, but this may change in
+the future. "*" is probably a suitable value.
+Whether certificate verification is done at all, and the result of
+it failing, is stll under the control of "tls_verify_hosts" nad
+"tls_try_verify_hosts".
+
+The name being checked is that for the host, generally
+the result of an MX lookup.
Both Subject and Subject-Alternate-Name certificate fields
are supported, as are wildcard certificates (limited to
a single wildcard being the initial component of a 3-or-more
component FQDN).
+The equivalent check on the server for client certificates is not
+implemented. At least one major email provider is using a client
+certificate which fails this check. They do not retry either without
+the client certificate or in clear.
+
+It is possible to duplicate the effect of this checking by
+creative use of Events.
+
+
+
+
+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. 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).
--------------------------------------------------------------
projects / exim.git / blobdiff