DANE
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+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.
+
+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).
+
+
+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.
+
+(TODO: specify when fallback happens vs. when the host is not used)
+
If dane is in use the following transport options are ignored:
tls_verify_hosts
tls_try_verify_hosts
tls_verify_certificates
tls_crl
tls_verify_cert_hostnames
- hosts_require_ocsp
+ hosts_require_ocsp (might rethink those two)
hosts_request_ocsp
+Currently dnssec_request_domains must be active (need to think about that)
+and dnssec_require_domains is ignored.
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git://git.exim.org / users / jgh / exim.git / blobdiff