Docs: add note on system_filter forced expansion fail

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

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 7d8b908f81a18c638d71408d9c85179518c65082..1bc2028d50637596802ff2da4a494154ec50e838 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -12607,6 +12607,13 @@ validating resolver (e.g. unbound, or bind with suitable configuration).
 If you have changed &%host_lookup_order%& so that &`bydns`& is not the first
 mechanism in the list, then this variable will be false.
 
+.new
+This requires that your system resolver library support EDNS0 (and that
+DNSSEC flags exist in the system headers).  If the resolver silently drops
+all EDNS0 options, then this will have no effect.  OpenBSD's asr resolver
+is known to currently ignore EDNS0, documented in CAVEATS of asr_run(3).
+.wen
+
 
 .vitem &$sender_host_name$&
 .vindex "&$sender_host_name$&"
@@ -14665,6 +14672,7 @@ record in the authoritative section is used instead.
 .option dns_use_edns0 main integer -1
 .cindex "DNS" "resolver options"
 .cindex "DNS" "EDNS0"
+.cindex "DNS" "OpenBSD
 If this option is set to a non-negative number then Exim will initialise the
 DNS resolver library to either use or not use EDNS0 extensions, overriding
 the system default. A value of 0 coerces EDNS0 off, a value of 1 coerces EDNS0
@@ -14672,6 +14680,12 @@ on.
 
 If the resolver library does not support EDNS0 then this option has no effect.
 
+.new
+OpenBSD's asr resolver routines are known to ignore the EDNS0 option; this
+means that DNSSEC will not work with Exim on that platform either, unless Exim
+is linked against an alternative DNS client library.
+.wen
+
 
 .option drop_cr main boolean false
 This is an obsolete option that is now a no-op. It used to affect the way Exim
@@ -16871,6 +16885,9 @@ generates any deliveries to files or pipes, or any new mail messages, the
 appropriate &%system_filter_..._transport%& option(s) must be set, to define
 which transports are to be used. Details of this facility are given in chapter
 &<<CHAPsystemfilter>>&.
+.new
+A forced expansion failure results in no filter operation.
+.wen
 
 
 .option system_filter_directory_transport main string&!! unset