ppp: 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

usr.sbin/ppp/command.c

@@ -793,6 +793,10 @@ static struct cmdtab const NatCommands[] =
   {"use_sockets", NULL, NatOption, LOCAL_AUTH,
    "allocate host sockets", "nat use_sockets yes|no",
    (const void *) PKT_ALIAS_USE_SOCKETS},
+  {"udp_eim", NULL, NatOption, LOCAL_AUTH,
+   "UDP uses endpoint-independent mapping (\"full cone\" NAT)",
+   "nat udp_eim yes|no",
+   (const void *) PKT_ALIAS_UDP_EIM},
   {"help", "?", HelpCommand, LOCAL_AUTH | LOCAL_NO_AUTH,
    "Display this message", "nat help|? [command]", NatCommands},
   {NULL, NULL, NULL, 0, NULL, NULL, NULL},

usr.sbin/ppp/ppp.8

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd November 18, 2024
+.Dd December 6, 2024
 .Dt PPP 8
 .Os
 .Sh NAME
@@ -3579,6 +3579,26 @@ network.
 The target address may be set to
 .Dq MYADDR ,
 in which case libalias will redirect all packets to the interface address.
+.It nat udp_eim  yes|no
+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 nat use_sockets yes|no
 When enabled, this option tells the network address translation engine to
 create a socket so that it can guarantee a correct incoming ftp data or