natd: Enable support for EIM NAT

Enable support for endpoint-independent mapping ("full cone NAT") via
Libalias's UDP NAT.

Reviewed by:    igoro, thj
Differential Revision:  https://reviews.freebsd.org/D46689

sbin/natd/natd.8

@@ -1,4 +1,4 @@
-.Dd October 5, 2016
+.Dd December 6, 2024
 .Dt NATD 8
 .Os
 .Sh NAME
@@ -14,6 +14,7 @@
 .Op Fl deny_incoming | d
 .Op Fl use_sockets | s
 .Op Fl same_ports | m
+.Op Fl udp_eim
 .Op Fl verbose | v
 .Op Fl dynamic
 .Op Fl in_port | i Ar port
@@ -114,6 +115,26 @@ With this option, protocols such as RPC will have a better chance
 of working.
 If it is not possible to maintain the port number, it will be silently
 changed as per normal.
+.It Fl udp_eim
+When enabled, UDP packets use endpoint-independent mapping (EIM) from RFC 4787
+("full cone" NAT of RFC 3489).
+All packets from the same internal address:port are mapped to the same NAT
+address:port, regardless of their destination address:port.
+If filtering rules allow, and if
+.Em deny_incoming
+is disabled, any other external address:port can
+also send to the internal address:port through its mapped NAT address:port.
+This is more compatible with applications, and can reduce the need for port
+forwarding, but less scalable as each NAT address:port can only be
+concurrently used by at most one internal address:port.
+.Pp
+When disabled, UDP packets use endpoint-dependent mapping (EDM) ("symmetric"
+NAT).
+Each connection from a particular internal address:port to different
+external addresses:ports is mapped to a random and unpredictable NAT
+address:port.
+Two appplications behind EDM NATs can only connect to each other
+by port forwarding on the NAT, or tunnelling through an in-between server.
 .It Fl verbose | v
 Do not call
 .Xr daemon 3

sbin/natd/natd.c

@@ -1138,6 +1138,14 @@ static struct OptionInfo optionTable[] = {
 		"same_ports",
 		"m" },
 
+	{ LibAliasOption,
+		PKT_ALIAS_UDP_EIM,
+		YesNo,
+		"[yes|no]",
+		"UDP traffic uses endpoint-independent mapping (\"full cone\" NAT)",
+		"udp_eim",
+		NULL },
+
 	{ Verbose,
 		0,
 		YesNo,