-Certificate name checking
---------------------------------------------------------------
-The X509 certificates used for TLS are supposed be verified
-that they are owned by the expected host. The coding of TLS
-support to date has not made these checks.
-
-If built with EXPERIMENTAL_CERTNAMES defined, code is
-included to do so for server certificates, 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".
-
-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
-hte 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 the TLSA lookup succeeds, a TLS connection will be required
-for the host.
-
-(TODO: specify when fallback happens vs. when the host is not used)
-
-If dane is in use 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
-
-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).
+ Note that it would be wise to strip incoming messages of A-R headers
+ that claim to be from our own <admd-identifier>.
+
+There are four new variables:
+
+ $arc_state One of pass, fail, none
+ $arc_state_reason (if fail, why)
+ $arc_domains colon-sep list of ARC chain domains, in chain order.
+ problematic elements may have empty list elements
+ $arc_oldest_pass lowest passing instance number of chain
+
+Example:
+ logwrite = oldest-p-ams: <${reduce {$lh_ARC-Authentication-Results:} \
+ {} \
+ {${if = {$arc_oldest_pass} \
+ {${extract {i}{${extract {1}{;}{$item}}}}} \
+ {$item} {$value}}} \
+ }>
+
+Receive log lines for an ARC pass will be tagged "ARC".
+
+
+Signing
+--
+arc_sign = <admd-identifier> : <selector> : <privkey> [ : <options> ]
+An option on the smtp transport, which constructs and prepends to the message
+an ARC set of headers. The textually-first Authentication-Results: header
+is used as a basis (you must have added one on entry to the ADMD).
+Expanded as a whole; if unset, empty or forced-failure then no signing is done.
+If it is set, all of the first three elements must be non-empty.
+
+The fourth element is optional, and if present consists of a comma-separated list
+of options. The options implemented are
+
+ timestamps Add a t= tag to the generated AMS and AS headers, with the
+ current time.
+ expire[=<val>] Add an x= tag to the generated AMS header, with an expiry time.
+ If the value <val> is an plain number it is used unchanged.
+ If it starts with a '+' then the following number is added
+ to the current time, as an offset in seconds.
+ If a value is not given it defaults to a one month offset.
+
+[As of writing, gmail insist that a t= tag on the AS is mandatory]
+
+Caveats:
+ * There must be an Authentication-Results header, presumably added by an ACL
+ while receiving the message, for the same ADMD, for arc_sign to succeed.
+ This requires careful coordination between inbound and outbound logic.
+
+ Only one A-R header is taken account of. This is a limitation versus
+ the ARC spec (which says that all A-R headers from within the ADMD must
+ be used).
+
+ * If passing a message to another system, such as a mailing-list manager
+ (MLM), between receipt and sending, be wary of manipulations to headers made
+ by the MLM.
+ + For instance, Mailman with REMOVE_DKIM_HEADERS==3 might improve
+ deliverability in a pre-ARC world, but that option also renames the
+ Authentication-Results header, which breaks signing.
+
+ * Even if you use multiple DKIM keys for different domains, the ARC concept
+ should try to stick to one ADMD, so pick a primary domain and use that for
+ AR headers and outbound signing.
+
+Signing is not compatible with cutthrough delivery; any (before expansion)
+value set for the option will result in cutthrough delivery not being
+used via the transport in question.
+
+
+
+
+TLS Session Resumption
+----------------------
+TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
+in RFC 5077 for 1.2). The support for this can be included by building with
+EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1
+(or later).
+
+Session resumption (this is the "stateless" variant) involves the server sending
+a "session ticket" to the client on one connection, which can be stored by the
+client and used for a later session. The ticket contains sufficient state for
+the server to reconstruct the TLS session, avoiding some expensive crypto
+calculation and one full packet roundtrip time.
+
+Operational cost/benefit:
+ The extra data being transmitted costs a minor amount, and the client has
+ extra costs in storing and retrieving the data.
+
+ In the Exim/Gnutls implementation the extra cost on an initial connection
+ which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
+ The saved cost on a subsequent connection is about 4ms; three or more
+ connections become a net win. On longer network paths, two or more
+ connections will have an average lower startup time thanks to the one
+ saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
+ packet roundtrips.
+
+ Since a new hints DB is used, the hints DB maintenance should be updated
+ to additionally handle "tls".
+
+Security aspects:
+ The session ticket is encrypted, but is obviously an additional security
+ vulnarability surface. An attacker able to decrypt it would have access
+ all connections using the resumed session.
+ The session ticket encryption key is not committed to storage by the server
+ and is rotated regularly (OpenSSL: 1hr, and one previous key is used for
+ overlap; GnuTLS 6hr but does not specify any overlap).
+ Tickets have limited lifetime (2hr, and new ones issued after 1hr under
+ OpenSSL. GnuTLS 2hr, appears to not do overlap).
+
+ There is a question-mark over the security of the Diffie-Helman parameters
+ used for session negotiation. TBD. q-value; cf bug 1895
+
+Observability:
+ New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X="
+ element.
+
+ Variables $tls_{in,out}_resumption have bits 0-4 indicating respectively
+ support built, client requested ticket, client offered session,
+ server issued ticket, resume used. A suitable decode list is provided
+ in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
+
+Issues:
+ In a resumed session:
+ $tls_{in,out}_cipher will have values different to the original (under GnuTLS)
+ $tls_{in,out}_ocsp will be "not requested" or "no response", and
+ hosts_require_ocsp will fail
git://git.exim.org / users / jgh / exim.git / blobdiff