libdns: Let kernel to decide the local port.

* dirmngr/dns.c (LEAVE_SELECTION_OF_PORT_TO_KERNEL): New.
(dns_socket): Don't select ephemeral port in user space.

--

There is no good reason to bind local port aggressively.  It might be
some reason to do so, then, a user can specify it in /etc/resolv.conf
by the second argument of "interface" directive.

At least, it causes a problem on Windows.  Binding a specified port in
user space can trigger the Firewall dialog on Windows.  Since it can
be considered valid question, it is better not to bind with an
ephemeral port which is selected in user space, by default.

GnuPG-bug-id: 3610
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

dirmngr/dns.c

@@ -7121,6 +7121,8 @@ static int dns_socket(struct sockaddr *local, int type, int *error_) {
 	if (type != SOCK_DGRAM)
 		return fd;
 
+#define LEAVE_SELECTION_OF_PORT_TO_KERNEL
+#if !defined(LEAVE_SELECTION_OF_PORT_TO_KERNEL)
 	/*
 	 * FreeBSD, Linux, OpenBSD, OS X, and Solaris use random ports by
 	 * default. Though the ephemeral range is quite small on OS X
@@ -7146,6 +7148,7 @@ static int dns_socket(struct sockaddr *local, int type, int *error_) {
 
 		/* NB: continue to next bind statement */
 	}
+#endif
 
 	if (0 == bind(fd, local, dns_sa_len(local)))
 		return fd;