--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_dnsupdate/README.markdown	Sat Nov 06 14:48:35 2021 +0100
@@ -0,0 +1,14 @@
+Generate a DNS UPDATE order in format suitable for `nsupdate` based on
+current port configuration.
+
+Example output:
+
+```
+$ prosodyctl mod_dnsupdate -d example.com -t xmpp.example.com example.com
+zone example.com
+server ns1.example.com
+ttl 3600
+add _xmpp-client._tcp.example.com IN SRV 1 1 5222
+add _xmpp-server._tcp.example.com IN SRV 1 1 5269
+```
+

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_dnsupdate/mod_dnsupdate.lua	Sat Nov 06 14:48:35 2021 +0100
@@ -0,0 +1,120 @@
+module:set_global();
+
+local config = require "core.configmanager";
+local argparse = require "util.argparse";
+local dns = require"net.adns".resolver();
+local async = require "util.async";
+local set = require "util.set";
+local nameprep = require"util.encodings".stringprep.nameprep;
+local idna_to_ascii = require"util.encodings".idna.to_ascii;
+
+local services = { "xmpp-client"; "xmpps-client"; "xmpp-server"; "xmpps-server" }
+
+local function validate_dnsname_option(options, option_name, default)
+	local host = options[option_name];
+	if host == nil then return default end
+	local normalized = nameprep(host);
+	if not normalized then
+		module:log("error", "--%s %q fails normalization");
+		return;
+	end
+	local alabel = idna_to_ascii(normalized);
+	if not alabel then
+		module:log("error", "--%s %q fails IDNA");
+		return;
+	end
+	return alabel;
+end
+
+function module.command(arg)
+	local opts = argparse.parse(arg, {
+		short_params = { d = "domain"; p = "primary"; t = "target"; l = "ttl"; h = "help"; ["?"] = "help" };
+		value_params = { domain = true; primary = true; target = true; ttl = true };
+	});
+
+	if not arg[1] or arg[2] or not opts or opts.help or not opts.domain then
+		local out = opts.help and io.stdout or io.stderr;
+		out:write("prosodyctl mod_dnsupdate [options] virtualhost\n");
+		out:write("\t-d --domain\tbase domain name *required*\n");
+		out:write("\t-p --primary\tprimary DNS name server\n");
+		out:write("\t-t --target\ttarget hostname for SRV\n");
+		out:write("\t-l --ttl\tTTL to use\n");
+		out:write("\t--each\tremove and replace individual SRV records\n");
+		out:write("\t--reset\tremove and replace all SRV records\n");
+		return opts and opts.help and 0 or 1;
+	end
+
+	local vhost = nameprep(arg[1]); -- TODO loop over arg[]?
+	if not vhost then
+		module:log("error", "Host %q fails normalization", arg[1]);
+		return 1;
+	end
+	local ihost = idna_to_ascii(vhost);
+	if not ihost then
+		module:log("error", "Host %q falis IDNA", vhost);
+		return 1;
+	end
+	if not config.get(vhost, "defined") then
+		module:log("error", "Host %q is not defined in the config", vhost);
+		return 1;
+	end
+
+	local domain = validate_dnsname_option(opts, "domain");
+	if not domain then
+		module:log("error", "--domain is required");
+		return 1;
+	end
+	local primary = validate_dnsname_option(opts, "primary")
+		or async.wait_for(dns:lookup_promise(domain, "SOA"):next(function(ret) return ret[1].soa.mname; end));
+	if not primary then
+		module:log("error", "Could not discover primary name server, specify it with --primary");
+		return 1;
+	end
+	local target = validate_dnsname_option(opts, "target", module:context(vhost):get_option_string("xmpp_host", ihost));
+	-- TODO validate that target has A/AAAA
+
+	local configured_ports = {
+		["xmpp-client"] = module:get_option_array("c2s_ports", { 5222 });
+		["xmpp-server"] = module:get_option_array("c2s_ports", { 5269 });
+		["xmpps-client"] = module:get_option_array("c2s_direct_tls_ports", {});
+		["xmpps-server"] = module:get_option_array("c2s_ports", {});
+	};
+
+	if opts.multiplex then
+		for opt, ports in pairs(configured_ports) do
+			ports:append(module:get_option_array(opt:sub(1, 5) == "xmpps" and "ssl_ports" or "ports", {}));
+		end
+	end
+
+	local existing_srv = {};
+	for _, service in ipairs(services) do
+		existing_srv[service] = dns:lookup_promise(("_%s._tcp.%s"):format(service, ihost), "SRV");
+	end
+
+	print("zone", domain);
+	print("server", primary);
+	print("ttl " .. tostring(opts.ttl or 60 * 60));
+
+	for _, service in ipairs(services) do
+		local ports = set.new(configured_ports[service]);
+		local records = (async.wait_for(existing_srv[service]));
+		local replace = opts.reset;
+		for _, rr in ipairs(records) do
+			if not ports:contains(rr.srv.port) or target ~= nameprep(rr.srv.target):gsub("%.$", "") then
+				if not opts.each then
+					replace = true;
+					break
+				end
+				print(("del _%s._tcp.%s IN SRV %s"):format(service, ihost, rr));
+			end
+		end
+		if replace then
+			print(("del _%s._tcp.%s IN SRV"):format(service, ihost));
+			for port in ports do print(("add _%s._tcp.%s IN SRV 1 1 %d %s"):format(service, ihost, port, target)); end
+		end
+	end
+
+	print("show");
+	print("send");
+	print("answer");
+end