dnsmasq: add handling of `dns-rr` to init script (add arbitrary resource records)

Add support for handling of DNS RR (Resource Records) requests, which
are needed for the HTTPS Type 65 records, introduced to support the
DNS-based Service Discovery (DNS-SD) mechanism for HTTPS services and
defined in the RFC 9460 (9.1. Query Names for HTTPS RRs).

Ref: https://forum.openwrt.org/t/resolving-query-type-65-to-local-address-for-ios-clients-in-dnsmasq/179504/11

uci config usage:

config dnsrr
    option rrname 'foo.example.com'
    option rrnumber '65'
    option hexdata '00'

hexdata is optional.

Available since dnsmasq 2.62 (for around 12 years at this point).

Note: dnsmasq dns-rr are not affected by filter-rr

Tested on 22.03.5

Signed-off-by: Paul Donald <[email protected]>
Tested-by: Vladimir Kochkovski <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/14975
Signed-off-by: Hauke Mehrtens <[email protected]>

package/network/services/dnsmasq/files/dnsmasq.init

@@ -788,6 +788,29 @@ dhcp_hostrecord_add() {
 	xappend "--host-record=$record"
 }
 
+dhcp_dnsrr_add() {
+	#This adds arbitrary resource record types (of IN class) whose optional data must be hex
+	local cfg="$1"
+	local rrname rrnumber hexdata
+
+	config_get rrname "$cfg" rrname
+	[ -n "$rrname" ] || return 0
+
+	config_get rrnumber "$cfg" rrnumber
+	[ -n "$rrnumber" ] && [ "$rrnumber" -gt 0 ] || return 0
+
+	config_get hexdata "$cfg" hexdata
+
+	# dnsmasq accepts colon XX:XX:.., space XX XX .., or contiguous XXXX.. hex forms or mixtures thereof
+	if [ -n "${hexdata//[0-9a-fA-F\:\ ]/}" ]; then
+		# is invalid hex literal
+		echo "dnsmasq: \"$hexdata\" is malformed hexadecimal (separate hex with colon, space or not at all)." >&2
+		return 1
+	fi
+
+	xappend "--dns-rr=${rrname},${rrnumber}${hexdata:+,$hexdata}"
+}
+
 dhcp_relay_add() {
 	local cfg="$1"
 	local local_addr server_addr interface
@@ -1161,6 +1184,7 @@ dnsmasq_start()
 	config_foreach filter_dnsmasq match dhcp_match_add "$cfg"
 	config_foreach filter_dnsmasq domain dhcp_domain_add "$cfg"
 	config_foreach filter_dnsmasq hostrecord dhcp_hostrecord_add "$cfg"
+	config_foreach filter_dnsmasq dnsrr dhcp_dnsrr_add "$cfg"
 	[ -n "$BOOT" ] || config_foreach filter_dnsmasq relay dhcp_relay_add "$cfg"
 
 	echo >> "$CONFIGFILE_TMP"