tls_dh_min_bits smtp transport option

Could not find an API for use with OpenSSL, so GnuTLS only

1 files changed, 20 insertions, 0 deletions

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 61cdc1ee1..78d5b0b18 100644
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -22451,6 +22451,19 @@ This option specifies a certificate revocation list. The expanded value must
 be the name of a file that contains a CRL in PEM format.
 
 
+.new
+.option tls_dh_min_bits smtp integer 1024
+.cindex "TLS" "Diffie-Hellman minimum acceptable size"
+When establishing a TLS session, if a ciphersuite which uses Diffie-Hellman
+key agreement is negotiated, the server will provide a large prime number
+for use.  This option establishes the minimum acceptable size of that number.
+If the parameter offered by the server is too small, then the TLS handshake
+will fail.
+
+Only supported when using GnuTLS.
+.wen
+
+
 .option tls_privatekey smtp string&!! unset
 .cindex "TLS" "client private key, location of"
 .vindex "&$host$&"
@@ -25004,6 +25017,13 @@ option).
 The &%tls_require_ciphers%& options operate differently, as described in the
 sections &<<SECTreqciphssl>>& and &<<SECTreqciphgnu>>&.
 .next
+.new
+The &%tls_dh_min_bits%& SMTP transport option is only honoured by GnuTLS.
+When using OpenSSL, this option is ignored.
+(If an API is found to let OpenSSL be configured in this way,
+let the Exim Maintainers know and we'll likely use it).
+.wen
+.next
 Some other recently added features may only be available in one or the other.
 This should be documented with the feature.  If the documentation does not
 explicitly state that the feature is infeasible in the other TLS