slirp: add 'cmd:' target for guestfwd

When using guestfwd=, Qemu only connects the virtual server's TCP port
to a single chardev. This is useless in most cases, as we usually want
to have more than a single connection from the guest to the outside world.

This patch adds a new cmd: target to guestfwd= that allows for execution
of a command on every TCP connection. This leverages the same code as
the -smb parameter, just that here the command is user defined.
Reported-by: NSascha Wilde <wilde@intevation.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

net/slirp.c

@@ -626,25 +626,35 @@ static int slirp_guestfwd(SlirpState *s, const char *config_str,
 
     fwd = g_malloc(sizeof(struct GuestFwd));
     snprintf(buf, sizeof(buf), "guestfwd.tcp.%d", port);
-    fwd->hd = qemu_chr_new(buf, p, NULL);
-    if (!fwd->hd) {
-        error_report("could not open guest forwarding device '%s'", buf);
-        g_free(fwd);
-        return -1;
-    }
 
-    if (slirp_add_exec(s->slirp, 3, fwd->hd, &server, port) < 0) {
-        error_report("conflicting/invalid host:port in guest forwarding "
-                     "rule '%s'", config_str);
-        g_free(fwd);
-        return -1;
-    }
-    fwd->server = server;
-    fwd->port = port;
-    fwd->slirp = s->slirp;
+    if ((strlen(p) > 4) && !strncmp(p, "cmd:", 4)) {
+        if (slirp_add_exec(s->slirp, 0, &p[4], &server, port) < 0) {
+            error_report("conflicting/invalid host:port in guest forwarding "
+                         "rule '%s'", config_str);
+            g_free(fwd);
+            return -1;
+        }
+    } else {
+        fwd->hd = qemu_chr_new(buf, p, NULL);
+        if (!fwd->hd) {
+            error_report("could not open guest forwarding device '%s'", buf);
+            g_free(fwd);
+            return -1;
+        }
+
+        if (slirp_add_exec(s->slirp, 3, fwd->hd, &server, port) < 0) {
+            error_report("conflicting/invalid host:port in guest forwarding "
+                         "rule '%s'", config_str);
+            g_free(fwd);
+            return -1;
+        }
+        fwd->server = server;
+        fwd->port = port;
+        fwd->slirp = s->slirp;
 
-    qemu_chr_add_handlers(fwd->hd, guestfwd_can_read, guestfwd_read,
-                          NULL, fwd);
+        qemu_chr_add_handlers(fwd->hd, guestfwd_can_read, guestfwd_read,
+                              NULL, fwd);
+    }
     return 0;
 
  fail_syntax:

qemu-options.hx

@@ -1421,8 +1421,28 @@ Then when you use on the host @code{telnet localhost 5555}, you
 connect to the guest telnet server.
 
 @item guestfwd=[tcp]:@var{server}:@var{port}-@var{dev}
+@item guestfwd=[tcp]:@var{server}:@var{port}-@var{cmd:command}
 Forward guest TCP connections to the IP address @var{server} on port @var{port}
-to the character device @var{dev}. This option can be given multiple times.
+to the character device @var{dev} or to a program executed by @var{cmd:command}
+which gets spawned for each connection. This option can be given multiple times.
+
+You can either use a chardev directly and have that one used throughout Qemu's
+lifetime, like in the following example:
+
+@example
+# open 10.10.1.1:4321 on bootup, connect 10.0.2.100:1234 to it whenever
+# the guest accesses it
+qemu -net user,guestfwd=tcp:10.0.2.100:1234-tcp:10.10.1.1:4321 [...]
+@end example
+
+Or you can execute a command on every TCP connection established by the guest,
+so that Qemu behaves similar to an inetd process for that virtual server:
+
+@example
+# call "netcat 10.10.1.1 4321" on every TCP connection to 10.0.2.100:1234
+# and connect the TCP stream to its stdin/stdout
+qemu -net 'user,guestfwd=tcp:10.0.2.100:1234-cmd:netcat 10.10.1.1 4321'
+@end example
 
 @end table