diff options
| -rw-r--r-- | doc/doc-docbook/spec.xfpt | 40 | ||||
| -rw-r--r-- | doc/doc-txt/ChangeLog | 3 |
2 files changed, 27 insertions, 16 deletions
diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt index da97d4082..3e73de6c3 100644 --- a/doc/doc-docbook/spec.xfpt +++ b/doc/doc-docbook/spec.xfpt @@ -24963,10 +24963,11 @@ The &%tls_verify_certificates%& option must contain the name of a file, not the name of a directory (for OpenSSL it can be either). .next The &%tls_dhparam%& option is ignored, because early versions of GnuTLS had no -facility for varying its Diffie-Hellman parameters. I understand that this has -changed, but Exim has not been updated to provide this facility. +facility for varying its Diffie-Hellman parameters. .new -Instead, the GnuTLS support will use a file from the spool directory. +Since then, the GnuTLS support has been updated to generate parameters upon +demand, keeping them in the spool directory. See &<<SECTgnutlsparam>>& for +details. .wen .next .vindex "&$tls_peerdn$&" @@ -24975,10 +24976,11 @@ separating fields; GnuTLS uses commas, in accordance with RFC 2253. This affects the value of the &$tls_peerdn$& variable. .next OpenSSL identifies cipher suites using hyphens as separators, for example: -DES-CBC3-SHA. GnuTLS uses underscores, for example: RSA_ARCFOUR_SHA. What is -more, OpenSSL complains if underscores are present in a cipher list. To make -life simpler, Exim changes underscores to hyphens for OpenSSL and hyphens to -underscores for GnuTLS when processing lists of cipher suites in the +DES-CBC3-SHA. GnuTLS historically used underscores, for example: +RSA_ARCFOUR_SHA. What is more, OpenSSL complains if underscores are present +in a cipher list. To make life simpler, Exim changes underscores to hyphens +for OpenSSL and passes the string unchanged to GnuTLS (expecting the library +to handle its own older variants) when processing lists of cipher suites in the &%tls_require_ciphers%& options (the global option and the &(smtp)& transport option). .next @@ -24994,7 +24996,7 @@ implementation, then patches are welcome. .endlist -.section "GnuTLS parameter computation" "SECID181" +.section "GnuTLS parameter computation" "SECTgnutlsparam" .new GnuTLS uses D-H parameters that may take a substantial amount of time to compute. It is unreasonable to re-compute them for every TLS session. @@ -25028,14 +25030,14 @@ and letting Exim re-create it, you can generate new parameters using renaming. The relevant commands are something like this: .code # ls -[ look for file; assume gnutls-params-1024 is the most recent ] +[ look for file; assume gnutls-params-2236 is the most recent ] # rm -f new-params # touch new-params # chown exim:exim new-params # chmod 0600 new-params -# certtool --generate-dh-params --bits 1024 >>new-params +# certtool --generate-dh-params --bits 2236 >>new-params # chmod 0400 new-params -# mv new-params gnutls-params-1024 +# mv new-params gnutls-params-2236 .endd If Exim never has to generate the parameters itself, the possibility of stalling is removed. @@ -25044,10 +25046,18 @@ The filename changed in Exim 4.80, to gain the -bits suffix. The value which Exim will choose depends upon the version of GnuTLS in use. For older GnuTLS, the value remains hard-coded in Exim as 1024. As of GnuTLS 2.12.x, there is a way for Exim to ask for the "normal" number of bits for D-H public-key usage, -and Exim does so. Exim thus removes itself from the policy decision, and the -filename and bits used change as the GnuTLS maintainers change the value for -their parameter &`GNUTLS_SEC_PARAM_NORMAL`&. At the time of writing, this -gives 2432 bits. +and Exim does so. This attempt to remove Exim from TLS policy decisions +failed, as GnuTLS 2.12 returns a value higher than the current hard-coded limit +of the NSS library. Thus Exim gains the &%tls_dh_max_bits%& global option, +which applies to all D-H usage, client or server. If the value returned by +GnuTLS is greater than &%tls_dh_max_bits%& then the value will be clamped down +to &%tls_dh_max_bits%&. The default value has been set at the current NSS +limit, which is still much higher than Exim historically used. + +The filename and bits used will change as the GnuTLS maintainers change the +value for their parameter &`GNUTLS_SEC_PARAM_NORMAL`&, as clamped by +&%tls_dh_max_bits%&. At the time of writing (mid 2012), GnuTLS 2.12 recommends +2432 bits, while NSS is limited to 2236 bits. .wen diff --git a/doc/doc-txt/ChangeLog b/doc/doc-txt/ChangeLog index 9db1c3823..23c727c92 100644 --- a/doc/doc-txt/ChangeLog +++ b/doc/doc-txt/ChangeLog @@ -143,7 +143,8 @@ PP/33 Added tls_dh_max_bits option, defaulting to current hard-coded limit PP/34 Validate tls_require_ciphers on startup, since debugging an invalid string otherwise requires a connection and a bunch more work and it's - relatively easy to get wrong. + relatively easy to get wrong. Should also expose TLS library linkage + problems. Exim version 4.77 |