X-Git-Url: https://git.exim.org/users/heiko/exim.git/blobdiff_plain/d502442ac32f8964f6cf86469869cecb035d12c0..594706ea2e56fe8c972eab772bd3e58c7a0c89ab:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 6657f63c7..c060a6c5a 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -1130,6 +1130,7 @@ in a router. Exim will then send the success DSN himself if requested as if 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 @@ -1148,6 +1149,140 @@ a single wildcard being the initial component of a 3-or-more component FQDN). +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. + +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 is to request OCSP for all hosts; the certificate +chain in DANE_EE usage will be insufficient to validate +the OCSP proof and verification will fail. Either disable +OCSP completely or use the (new) variable $tls_out_tlsa_usage +like so: + + hosts_request_ocsp = ${if or { {= {4}{$tls_out_tlsa_usage}} \ + {= {0}{$tls_out_tlsa_usage}} } \ + {*}{}} +The variable 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_require_ocsp or +hosts_request_ocsp includes the string "tls_out_tlsa_usage", +they are re-expanded in time to control the OCSP request. + +[ All a bit complicated. Should we make that definition +the default? Should we override the user's definition? ] + + +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 + +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 $tls_out_dane which will have "yes" if +verification succeeded using DANE and "no" otherwise (only useful +in combination with EXPERIMENTAL_TPDA), and a new variable +$tls_out_tlsa_usage (detailed above). + -------------------------------------------------------------- End of file