section &<<SECTnamedlists>>&.
+.new
+.section "Builtin macros" "SECTbuiltinmacros"
+Exim defines some macros depending on facilities available, which may
+differ due to build-time definitions and from one release to another.
+All of these macros start with an underscore.
+They can be used to conditionally include parts of a configuration
+(see below).
+
+The following classes of macros are defined:
+.display
+&` _HAVE_ `& build-time defines
+&` _DRVR_AUTH_ `& authehticator drivers
+&` _DRVR_RTR_ `& router drivers
+&` _DRVR_TPT_ `& transport drivers
+&` _OPT_ `& configuration option support
+.endd
+
+Use an &"exim -bP macros"& command to get the list of macros.
+.wen
+
+
.section "Conditional skips in the configuration file" "SECID46"
.cindex "configuration file" "conditional skips"
.cindex "&`.ifdef`&"
.cindex CHUNKING "enabling, in client"
.cindex BDAT "SMTP command"
.cindex "RFC 3030" "CHUNKING"
-This option provides a list of server to which, provided they announce
+This option provides a list of servers to which, provided they announce
CHUNKING support, Exim will attempt to use BDAT commands rather than DATA.
BDAT will not be used in conjuction with a transport filter.
.wen
+.new
+.option hosts_try_fastopen smtp "host list!!" unset
+.option "fast open, TCP" "enabling, in client"
+.option "TCP Fast Open" "enabling, in client"
+.option "RFC 7413" "TCP Fast Open"
+This option provides a list of servers to which, provided
+the facility is supported by this system, Exim will attempt to
+perform a TCP Fast Open.
+No data is sent on the SYN segment but, if the remote server also
+supports the facility, it can send its SMTP banner immediately after
+the SYN,ACK segment. This can save up to one round-trip time.
+
+The facility is only active for previously-contacted servers,
+as the initiator must present a cookie in the SYN segment.
+
+On (at least some) current Linux distributions the facility must be enabled
+in the kernel by the sysadmin before the support is usable.
+.wen
+
.option hosts_try_prdr smtp "host list&!!" *
.cindex "PRDR" "enabling, optional in client"
This option provides a list of servers to which, provided they announce
Great care should be taken to deal with matters of case, various injection
attacks in the string (&`../`& or SQL), and ensuring that a valid filename
-can always be referenced; it is important to remember that &$tls_sni$& is
+can always be referenced; it is important to remember that &$tls_in_sni$& is
arbitrary unverified data provided prior to authentication.
+.new
+Further, the initial cerificate is loaded before SNI is arrived, so
+an expansion for &%tls_certificate%& must have a default which is used
+when &$tls_in_sni$& is empty.
+.wen
The Exim developers are proceeding cautiously and so far no other TLS options
are re-expanded.