[Qemu-devel] [PATCH] slirp: add 'cmd:' target for guestfwd

[Alexander Graf <agraf@suse.de>]

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: Sascha Wilde <wilde@intevation.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
---
 net/slirp.c     |   44 ++++++++++++++++++++++++++++----------------
 qemu-options.hx |   20 +++++++++++++++++++-
 2 files changed, 47 insertions(+), 17 deletions(-)

diff --git a/net/slirp.c b/net/slirp.c
index e057a14..4e4e3c3 100644
--- a/net/slirp.c
+++ b/net/slirp.c
@@ -615,25 +615,37 @@ static int slirp_guestfwd(SlirpState *s, const char *config_str,
 
     fwd = qemu_malloc(sizeof(struct GuestFwd));
     snprintf(buf, sizeof(buf), "guestfwd.tcp.%d", port);
-    fwd->hd = qemu_chr_open(buf, p, NULL);
-    if (!fwd->hd) {
-        error_report("could not open guest forwarding device '%s'", buf);
-        qemu_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);
-        qemu_free(fwd);
-        return -1;
+    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);
+            qemu_free(fwd);
+            return -1;
+        }
+    } else {
+        fwd->hd = qemu_chr_open(buf, p, NULL);
+        if (!fwd->hd) {
+            error_report("could not open guest forwarding device '%s'", buf);
+            qemu_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);
+            qemu_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);
     }
-    fwd->server = server;
-    fwd->port = port;
-    fwd->slirp = s->slirp;
 
-    qemu_chr_add_handlers(fwd->hd, guestfwd_can_read, guestfwd_read,
-                          NULL, fwd);
     return 0;
 
  fail_syntax:
diff --git a/qemu-options.hx b/qemu-options.hx
index 82e085a..cc62d20 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1242,10 +1242,28 @@ telnet localhost 5555
 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{dev}|@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.
 
+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
 
 Note: Legacy stand-alone options -tftp, -bootp, -smb and -redir are still
-- 
1.6.0.2