.new
If the DH bit-count from loading the file is greater than tls_dh_max_bits then
it will be ignored.
-.end
+.wen
.option tls_on_connect_ports main "string list" unset
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 "&$tls_peerdn$&"
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
.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.
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.
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
suites before the cipher negotiation takes place. This specifies which ciphers
are acceptable. The list is colon separated and may contain names like
DES-CBC3-SHA. Exim passes the expanded value of &%tls_require_ciphers%&
-directly to this function call. The following quotation from the OpenSSL
+directly to this function call.
+Many systems will install the OpenSSL manual-pages, so you may have
+&'ciphers(1)'& available to you.
+The following quotation from the OpenSSL
documentation specifies what forms of item are allowed in the cipher string:
.ilist
not be moved to the end of the list.
.endlist
+.new
+The OpenSSL &'ciphers(1)'& command may be used to test the results of a given
+string:
+.code
+# note single-quotes to get ! past any shell history expansion
+$ openssl ciphers 'HIGH:!MD5:!SHA1'
+.endd
+
+This example will let the library defaults be permitted on the MX port, where
+there's probably no identity verification anyway, but ups the ante on the
+submission ports where the administrator might have some influence on the
+choice of clients used:
+.code
+# OpenSSL variant; see man ciphers(1)
+tls_require_ciphers = ${if =={$received_port}{25}\
+ {DEFAULT}\
+ {HIGH:!MD5:!SHA1}}
+.endd
+.wen
+
.new
Documentation of the strings accepted may be found in the GnuTLS manual, under
"Priority strings". This is online as
-&url(http://www.gnu.org/software/gnutls/manual/html_node/Priority-Strings.html).
+&url(http://www.gnu.org/software/gnutls/manual/html_node/Priority-Strings.html),
+but beware that this relates to GnuTLS 3, which may be newer than the version
+installed on your system. If you are using GnuTLS 3,
+&url(http://www.gnu.org/software/gnutls/manual/html_node/Listing-the-ciphersuites-in-a-priority-string.html, then the example code)
+on that site can be used to test a given string.
Prior to Exim 4.80, an older API of GnuTLS was used, and Exim supported three
additional options, "&%gnutls_require_kx%&", "&%gnutls_require_mac%&" and
"&%gnutls_require_protocols%&". &%tls_require_ciphers%& was an Exim list.
+
+This example will let the library defaults be permitted on the MX port, where
+there's probably no identity verification anyway, and lowers security further
+by increasing compatibility; but this ups the ante on the submission ports
+where the administrator might have some influence on the choice of clients
+used:
+.code
+# GnuTLS variant
+tls_require_ciphers = ${if =={$received_port}{25}\
+ {NORMAL:%COMPAT}\
+ {SECURE128}}
+.endd
.wen
git://git.exim.org / exim.git / blobdiff