MINOR: resolvers: add command-line argument -4 to force IPv4-only DNS

In order to ease troubleshooting and testing, the new "-4" command line
argument enforces queries and processing of "A" DNS records only, i.e.
those representing IPv4 addresses. This can be useful when a host lack
end-to-end dual-stack connectivity. This overrides the global
"dns-accept-family" directive and is equivalent to value "ipv4".

doc/management.txt

@@ -192,6 +192,11 @@ list of options is :
 
   -Ws : master-worker mode with support of `notify` type of systemd service.
 
+  -4 : force DNS resolvers to query and accept IPv4 addresses only ("A"
+       records). This can be used when facing difficulties in certain
+       environments lacking end-to-end dual-stack connectivity. It overrides
+       the global "dns-accept-family" directive and forces it to "ipv4".
+
   -c : only performs a check of the configuration files and exits before trying
     to bind. The exit status is zero if everything is OK, or non-zero if an
     error is encountered. Presence of warnings will be reported if any.

include/haproxy/resolvers-t.h

@@ -97,6 +97,7 @@ enum {
 	RSLV_ACCEPT_IPV4   = 0x01,
 	RSLV_ACCEPT_IPV6   = 0x02,
 	RSLV_ACCEPT_MASK   = RSLV_ACCEPT_IPV4 | RSLV_ACCEPT_IPV6,
+	RSLV_FORCED_FAMILY = 0x04,
 };
 
 /* NOTE: big endian structure */

src/haproxy.c

@@ -110,6 +110,7 @@
 #include <haproxy/proto_tcp.h>
 #include <haproxy/proxy.h>
 #include <haproxy/regex.h>
+#include <haproxy/resolvers.h>
 #include <haproxy/sample.h>
 #include <haproxy/server.h>
 #include <haproxy/session.h>
@@ -662,6 +663,7 @@ static void usage(char *name)
 		"        -q quiet mode : don't display messages\n"
 		"        -c check mode : only check config files and exit\n"
 		"        -cc check condition : evaluate a condition and exit\n"
+		"        -4 force resolvers to consider IPv4 responses only\n"
 		"        -n sets the maximum total # of connections (uses ulimit -n)\n"
 		"        -m limits the usable amount of memory (in MB)\n"
 		"        -N sets the default, per-proxy maximum # of connections (%d)\n"
@@ -1591,6 +1593,8 @@ static void init_args(int argc, char **argv)
 				argc--;
 				check_condition = *argv;
 			}
+			else if (*flag == '4')
+				resolv_accept_families = RSLV_ACCEPT_IPV4 | RSLV_FORCED_FAMILY;
 			else if (*flag == 'c')
 				arg_mode |= MODE_CHECK;
 			else if (*flag == 'D')

src/resolvers.c

@@ -3962,7 +3962,11 @@ static int cfg_parse_dns_accept_family(char **args, int section_type, struct pro
 			goto usage;
 	}
 
-	resolv_accept_families = accept_families;
+	/* we ignore the settings if it was forced on the cmdline, but we still
+	 * parse it for config validity checks.
+	 */
+	if (!(resolv_accept_families & RSLV_FORCED_FAMILY))
+		resolv_accept_families = accept_families;
 	return 0;
  usage:
 	memprintf(err, "'%s' expects a comma-delimited list of 'ipv4' and 'ipv6' but got '%s'.", args[0], args[1]);