From abf05f332065a5cd05e9569945b0e3e12bd7ba92 Mon Sep 17 00:00:00 2001 From: Phil Pennock Date: Wed, 4 Sep 2013 10:58:51 -0700 Subject: tls_dhparam size constraint suggestions. Between NSS and Debian patching of older Exim releases, there's a narrow range of values likely to interoperate well. Document this. --- doc/doc-docbook/spec.xfpt | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt index ae6e33efe..371b28e43 100644 --- a/doc/doc-docbook/spec.xfpt +++ b/doc/doc-docbook/spec.xfpt @@ -16031,6 +16031,21 @@ The available primes are: Some of these will be too small to be accepted by clients. Some may be too large to be accepted by clients. +The TLS protocol does not negotiate an acceptable size for this; clients tend +to hard-drop connections if what is offered by the server is unacceptable, +whether too large or too small, and there's no provision for the client to +tell the server what these constraints are. Thus, as a server operator, you +need to make an educated guess as to what is most likely to work for your +userbase. + +Some known size constraints suggest that a bit-size in the range 2048 to 2236 +is most likely to maximise interoperability. The upper bound comes from +applications using the Mozilla Network Security Services (NSS) library, which +used to set its &`DH_MAX_P_BITS`& upper-bound to 2236. This affects many +mail user agents (MUAs). The lower bound comes from Debian installs of Exim4 +prior to the 4.80 release, as Debian used to patch Exim to raise the minimum +acceptable bound from 1024 to 2048. + .option tls_on_connect_ports main "string list" unset This option specifies a list of incoming SSMTP (aka SMTPS) ports that should @@ -25686,7 +25701,7 @@ tls_dhparam = none This may also be set to a string identifying a standard prime to be used for DH; if it is set to &`default`& or, for OpenSSL, is unset, then the prime used is &`ike23`&. There are a few standard primes available, see the -documetnation for &%tls_dhparam%& for the complete list. +documentation for &%tls_dhparam%& for the complete list. See the command .code -- cgit v1.2.3