Document new dane_require_tls_ciphers

Haven't written the code yet, but writing the docs first helped me
affirm that this makes sense and feels clean.  Code in next commit.

1 files changed, 12 insertions, 4 deletions

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 2dbe6d2d3..9722c0063 100644
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -28102,8 +28102,7 @@ that DNS lookups they do for the server have not been tampered with.  The domain
 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
+3) offer a server certificate, or certificate chain, in TLS connections which is is anchored by one of the TLSA records.
 
 There are no changes to Exim specific to server-side operation of DANE.
 Support for client-side operation of DANE can be included at compile time by defining SUPPORT_DANE=yes
@@ -28158,8 +28157,9 @@ This modification of hosts_request_ocsp is only done if it has the default value
 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%&.
-The latter variant will result in failure if the target host is not DNSSEC-secured.
+For client-side DANE there are three new smtp transport options, &%hosts_try_dane%&, &%hosts_require_dane%&
+and &%dane_require_tls_ciphers%&.
+The require variant 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.
 
@@ -28168,6 +28168,14 @@ 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 usable, then the TLS cipher list configuration
+prefers to use the option &%dane_require_tls_ciphers%& and falls
+back to &%tls_require_ciphers%& only if that is unset.
+This lets you configure "decent crypto" for DANE and "better than nothing
+crypto" as the default.  Note though that while GnuTLS lets the string control
+which versions of TLS/SSL will be negotiated, OpenSSL does not and you're
+limited to ciphersuite constraints.
+
 If DANE is requested and useable (see above) the following transport options are ignored:
 .code
   hosts_require_tls