Docs: more resumption notes

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

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 98ca33d23988c4af3e82b937ea3fc80aa87e4e25..9f10d17302f3cd19c46a9b7db9b78e471a376739 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -25515,6 +25515,38 @@ helo_data = ${lookup dnsdb{ptr=$sending_ip_address} \
 The use of &%helo_data%& applies both to sending messages and when doing
 callouts.
 
+.new
+.option host_name_extract smtp "string list&!!" "see below"
+.cindex "load balancer" "hosts behind"
+.cindex TLS resumption
+Some mail-accepting sites
+(notably Microsoft)
+operate many servers behind a network load-balancer.  When this is done,
+with separated TLS session caches, TLS session resuption becomes problematic.
+It will only succeed when the same server happens to be selected by the
+load-balancer, matching the session stored in the client's cache.
+
+Exim can pull out a server name, if there is one, from the response to the
+client's SMTP EHLO command.
+The default value of this option:
+.code
+    ${if and { {match {$host} {.outlook.com\$}} \
+               {match {$item} {\N^250-([\w.]+)\s\N}} \
+            } {$1}}
+.endd
+suffices for one known case.
+During the expansion of this option the &$item$& variable will have the
+server's EHLO response.
+The result of the option expansion is included in the key used to store and
+retrieve the TLS session, for session resumption.
+
+Operators of high-load sites may wish to evaluate their logs for indications
+of other destination sites operating load-balancers, and develop a suitable
+expression for this option.
+The smtp:ehlo event and the &$tls_out_resumption$& variable
+will be useful for such work.
+.wen
+
 .option hosts smtp "string list&!!" unset
 Hosts are associated with an address by a router such as &(dnslookup)&, which
 finds the hosts by looking up the address domain in the DNS, or by
@@ -39924,8 +39956,9 @@ The entire contents of a database are written to the standard output by the
 &'exim_dumpdb'& program,
 .new
 taking as arguments the spool and database names.
-An option &'-z'& may be given to regest times in UTC;
+An option &'-z'& may be given to request times in UTC;
 otherwise times are in the local timezone.
+An option &'-k'& may be given to dump only the record keys.
 .wen
 For example, to dump the retry database:
 .code