Deal with GnuTLS DH generation overshoot

[exim.git] / doc / doc-docbook / spec.xfpt

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 2f06f126fc0d1a74f972bd21a2c89dd97b35876d..9c2bf199f7ede21fe2d9cbe8fb559afc27b84cc8 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -2991,6 +2991,26 @@ The specified sender is treated as if it were given as the argument to the
 preference to the address taken from the message. The caller of Exim must be a
 trusted user for the sender of a message to be set in this way.
 
 preference to the address taken from the message. The caller of Exim must be a
 trusted user for the sender of a message to be set in this way.
 

+.vitem &%-bmalware%&&~<&'filename'&>
+.oindex "&%-bmalware%&"
+.cindex "testing", "malware"
+.cindex "malware scan test"
+This debugging option causes Exim to scan the given file,
+using the malware scanning framework.  The option of &%av_scanner%& influences
+this option, so if &%av_scanner%&'s value is dependent upon an expansion then
+the expansion should have defaults which apply to this invocation.  ACLs are
+not invoked, so if &%av_scanner%& references an ACL variable then that variable
+will never be populated and &%-bmalware%& will fail.
+
+Exim will have changed working directory before resolving the filename, so
+using fully qualified pathnames is advisable.  Exim will be running as the Exim
+user when it tries to open the file, rather than as the invoking user.
+This option requires admin privileges.
+
+The &%-bmalware%& option will not be extended to be more generally useful,
+there are better tools for file-scanning.  This option exists to help
+administrators verify their Exim and AV scanner configuration.
+

 .vitem &%-bnq%&
 .oindex "&%-bnq%&"
 .cindex "address qualification, suppressing"
 .vitem &%-bnq%&
 .oindex "&%-bnq%&"
 .cindex "address qualification, suppressing"


@@ -3251,26 +3271,6 @@ above concerning senders and qualification do not apply. In this situation,
 Exim behaves in exactly the same way as it does when receiving a message via
 the listening daemon.
 
 Exim behaves in exactly the same way as it does when receiving a message via
 the listening daemon.
 

-.vitem &%-bmalware%&&~<&'filename'&>
-.oindex "&%-bmalware%&"
-.cindex "testing", "malware"
-.cindex "malware scan test"
-This debugging option causes Exim to scan the given file,
-using the malware scanning framework.  The option of &%av_scanner%& influences
-this option, so if &%av_scanner%&'s value is dependent upon an expansion then
-the expansion should have defaults which apply to this invocation.  ACLs are
-not invoked, so if &%av_scanner%& references an ACL variable then that variable
-will never be populated and &%-bmalware%& will fail.
-
-Exim will have changed working directory before resolving the filename, so
-using fully qualified pathnames is advisable.  Exim will be running as the Exim
-user when it tries to open the file, rather than as the invoking user.
-This option requires admin privileges.
-
-The &%-bmalware%& option will not be extended to be more generally useful,
-there are better tools for file-scanning.  This option exists to help
-administrators verify their Exim and AV scanner configuration.
-

 .vitem &%-bt%&
 .oindex "&%-bt%&"
 .cindex "testing" "addresses"
 .vitem &%-bt%&
 .oindex "&%-bt%&"
 .cindex "testing" "addresses"


@@ -6026,16 +6026,16 @@ that it implements the details of the specific authentication mechanism,
 i.e. PLAIN or LOGIN. The &%server_advertise_condition%& setting controls
 when Exim offers authentication to clients; in the examples, this is only
 when TLS or SSL has been started, so to enable the authenticators you also
 i.e. PLAIN or LOGIN. The &%server_advertise_condition%& setting controls
 when Exim offers authentication to clients; in the examples, this is only
 when TLS or SSL has been started, so to enable the authenticators you also

-need to add support for TLS as described in &<<SECTdefconfmain>>&.
+need to add support for TLS as described in section &<<SECTdefconfmain>>&.

 
 The &%server_condition%& setting defines how to verify that the username and
 password are correct. In the examples it just produces an error message.
 To make the authenticators work, you can use a string expansion
 
 The &%server_condition%& setting defines how to verify that the username and
 password are correct. In the examples it just produces an error message.
 To make the authenticators work, you can use a string expansion

-expression like one of the examples in &<<CHAPplaintext>>&.
+expression like one of the examples in chapter &<<CHAPplaintext>>&.

 
 Beware that the sequence of the parameters to PLAIN and LOGIN differ; the
 
 Beware that the sequence of the parameters to PLAIN and LOGIN differ; the

-usercode and password are in different positions.  &<<CHAPplaintext>>&
-covers both.
+usercode and password are in different positions.
+Chapter &<<CHAPplaintext>>& covers both.

 
 .ecindex IIDconfiwal
 
 
 .ecindex IIDconfiwal
 


@@ -15697,6 +15697,10 @@ by Thunderbird, while GnuTLS was suggesting 2432 bits as normal.
 
 If you prefer more security and are willing to break some clients, raise this
 number.
 
 If you prefer more security and are willing to break some clients, raise this
 number.

+
+Note that the value passed to GnuTLS for *generating* a new prime may be a
+little less than this figure, because GnuTLS is inexact and may produce a
+larger prime than requested.

 .wen
 
 
 .wen
 
 


@@ -15708,8 +15712,8 @@ This is used only for OpenSSL. When Exim is linked with GnuTLS, this option is
 ignored. See section &<<SECTopenvsgnu>>& for further details.
 
 .new
 ignored. See section &<<SECTopenvsgnu>>& for further details.
 
 .new

-If the DH bit-count from loading the file is greater than tls_dh_max_bits then
-it will be ignored.
+If the DH bit-count from loading the file is greater than &%tls_dh_max_bits$&
+then it will be ignored.

 .wen
 
 
 .wen
 
 


@@ -25070,6 +25074,10 @@ renaming. The relevant commands are something like this:
 # chown exim:exim new-params
 # chmod 0600 new-params
 # certtool --generate-dh-params --bits 2236 >>new-params
 # chown exim:exim new-params
 # chmod 0600 new-params
 # certtool --generate-dh-params --bits 2236 >>new-params

+# openssl dhparam -noout -text -in new-params | head
+[ check the first line, make sure it's not more than 2236;
+  if it is, then go back to the start ("rm") and repeat
+  until the size generated is at most the size requested ]

 # chmod 0400 new-params
 # mv new-params gnutls-params-2236
 .endd
 # chmod 0400 new-params
 # mv new-params gnutls-params-2236
 .endd


@@ -25092,6 +25100,12 @@ 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.
 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.

+
+In fact, the requested value will be *lower* than &%tls_dh_max_bits%&, to
+increase the chance of the generated prime actually being within acceptable
+bounds, as GnuTLS has been observed to overshoot.  Note the check step in the
+procedure above.  There is no sane procedure available to Exim to double-check
+the size of the generated prime, so it might still be too large.

 .wen
 
 
 .wen