slirp: Describe NAT limitations in the attach help

slirp_glue/sim_slirp.c

@@ -404,7 +404,30 @@ fprintf (st, "%s",
 "    NODHCP                              disables DHCP server\n"
 "Default NAT Options: GATEWAY=10.0.2.2, masklen=24(netmask is 255.255.255.0)\n"
 "                     DHCP=10.0.2.15, NAMESERVER=10.0.2.3\n"
-"    Nameserver defaults to proxy traffic to host system's active nameserver\n"
+"    Nameserver defaults to proxy traffic to host system's active nameserver\n\n"
+"NAT limitations\n\n"
+"There are four limitations of NAT mode which users should be aware of:\n\n"
+" 1) ICMP protocol limitations:\n"
+"    Some frequently used network debugging tools (e.g. ping or tracerouting)\n"
+"    rely on the ICMP protocol for sending/receiving messages. While some\n"
+"    ICMP support is available on some hosts (ping may or may not work),\n"
+"    some other tools may not work reliably.\n\n"
+" 2) Receiving of UDP broadcasts is not reliable:\n"
+"    The guest does not reliably receive broadcasts, since, in order to save\n"
+"    resources, it only listens for a certain amount of time after the guest\n"
+"    has sent UDP data on a particular port.\n\n"
+" 3) Protocols such as GRE, DECnet, LAT and Clustering are unsupported:\n"
+"    Protocols other than TCP and UDP are not supported.\n\n"
+" 4) Forwarding host ports < 1024 impossible:\n"
+"    On Unix-based hosts (e.g. Linux, Solaris, Mac OS X) it is not possible\n"
+"    to bind to ports below 1024 from applications that are not run by root.\n"
+"    As a result, if you try to configure such a port forwarding, the attach\n"
+"    will fail.\n\n"
+"These limitations normally don't affect standard network use. But the\n"
+"presence of NAT has also subtle effects that may interfere with protocols\n"
+"that are normally working. One example is NFS, where the server is often\n"
+"configured to refuse connections from non-privileged ports (i.e. ports not\n"
+" below 1024).\n"
 );
 return SCPE_OK;
 }