conf: Allow binding to ports on an interface without a specific address

Somebody might want to bind listening sockets to a specific
interface, but not a specific address, and there isn't really a
reason to prevent that. For example:

  -t %eth0/2022

Alternatively, we support options such as -t 0.0.0.0%eth0/2022 and
-t ::%eth0/2022, but not together, for the same port.

Enable this kind of syntax and add examples to the man page.

Reported-by: Paul Holzinger <[email protected]>
Link: containers/podman#14425 (comment)
Signed-off-by: Stefano Brivio <[email protected]>

conf.c

@@ -263,7 +263,9 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
 			ifname++;
 		}
 
-		if (inet_pton(AF_INET, buf, addr))
+		if (ifname == buf + 1)		/* Interface without address */
+			addr = NULL;
+		else if (inet_pton(AF_INET, buf, addr))
 			af = AF_INET;
 		else if (inet_pton(AF_INET6, buf, addr))
 			af = AF_INET6;

passt.1

@@ -385,6 +385,9 @@ Forward local port 22, bound to 192.0.2.1, to port 22 on the guest
 -t 192.0.2.1%eth0/22
 Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22
 .TP
+-t %eth0/22
+Forward local port 22, bound to any address on interface eth0, to port 22
+.TP
 -t 2000-5000,~3000-3010
 Forward local ports between 2000 and 5000, except for those between 3000 and
 3010
@@ -467,6 +470,9 @@ Forward local port 22, bound to 192.0.2.1, to port 22 in the target namespace
 -t 192.0.2.1%eth0/22
 Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22
 .TP
+-t %eth0/22
+Forward local port 22, bound to any address on interface eth0, to port 22
+.TP
 -t 2000-5000,~3000-3010
 Forward local ports between 2000 and 5000, except for those between 3000 and
 3010