DNS: Add basic support for asynchronous DNS resolving

This patch is a simple implementation for the proposal discussed in
https://mail.openvswitch.org/pipermail/ovs-dev/2017-August/337038.html and
https://mail.openvswitch.org/pipermail/ovs-dev/2017-October/340013.html.

It enables ovs-vswitchd and other utilities to use DNS names when specifying
OpenFlow and OVSDB remotes.

Below are some of the features and limitations of this patch:
    - Resolving is asynchornous in daemon context, avoiding blocking main loop;
    - Resolving is synchronous in general utility context;
    - Both IPv4 and IPv6 are supported;
    - The resolving API is thread-safe;
    - Depends on the unbound library;
    - When multiple ip addresses are returned, only the first one is used;
    - /etc/nsswitch.conf isn't respected as unbound library doesn't look at it;
    - For async-resolving, caller need to retry later; there is no callback.

Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/socket-util.c

@@ -48,6 +48,7 @@
 #include "netlink-protocol.h"
 #include "netlink-socket.h"
 #endif
+#include "dns-resolve.h"
 
 VLOG_DEFINE_THIS_MODULE(socket_util);
 
@@ -56,6 +57,12 @@ static int getsockopt_int(int fd, int level, int option, const char *optname,
 static struct sockaddr_in *sin_cast(const struct sockaddr *);
 static struct sockaddr_in6 *sin6_cast(const struct sockaddr *);
 static const struct sockaddr *sa_cast(const struct sockaddr_storage *);
+static bool parse_sockaddr_components(struct sockaddr_storage *ss,
+                                      char *host_s,
+                                      const char *port_s,
+                                      uint16_t default_port,
+                                      const char *s,
+                                      bool resolve_host);
 
 /* Sets 'fd' to non-blocking mode.  Returns 0 if successful, otherwise a
  * positive errno value. */
@@ -419,11 +426,31 @@ inet_parse_port_host_tokens(char *s, char **portp, char **hostp)
     inet_parse_tokens__(s, 1, hostp, portp);
 }
 
+static bool
+parse_sockaddr_components_dns(struct sockaddr_storage *ss OVS_UNUSED,
+                              char *host_s,
+                              const char *port_s OVS_UNUSED,
+                              uint16_t default_port OVS_UNUSED,
+                              const char *s OVS_UNUSED)
+{
+    char *tmp_host_s;
+
+    dns_resolve(host_s, &tmp_host_s);
+    if (tmp_host_s != NULL) {
+        parse_sockaddr_components(ss, tmp_host_s, port_s,
+                                  default_port, s, false);
+        free(tmp_host_s);
+        return true;
+    }
+    return false;
+}
+
 static bool
 parse_sockaddr_components(struct sockaddr_storage *ss,
                           char *host_s,
                           const char *port_s, uint16_t default_port,
-                          const char *s)
+                          const char *s,
+                          bool resolve_host)
 {
     struct sockaddr_in *sin = sin_cast(sa_cast(ss));
     int port;
@@ -445,7 +472,6 @@ parse_sockaddr_components(struct sockaddr_storage *ss,
         sin6->sin6_family = AF_INET6;
         sin6->sin6_port = htons(port);
         if (!addr || !*addr || !ipv6_parse(addr, &sin6->sin6_addr)) {
-            VLOG_ERR("%s: bad IPv6 address \"%s\"", s, addr ? addr : "");
             goto exit;
         }
 
@@ -468,13 +494,19 @@ parse_sockaddr_components(struct sockaddr_storage *ss,
         sin->sin_family = AF_INET;
         sin->sin_port = htons(port);
         if (host_s && !ip_parse(host_s, &sin->sin_addr.s_addr)) {
-            VLOG_ERR("%s: bad IPv4 address \"%s\"", s, host_s);
-            goto exit;
+            goto resolve;
         }
     }
 
     return true;
 
+resolve:
+    if (resolve_host && parse_sockaddr_components_dns(ss, host_s, port_s,
+                                                      default_port, s)) {
+        return true;
+    } else if (!resolve_host) {
+        VLOG_ERR("%s: bad IP address \"%s\"", s, host_s);
+    }
 exit:
     memset(ss, 0, sizeof *ss);
     return false;
@@ -505,7 +537,8 @@ inet_parse_active(const char *target_, int default_port,
         VLOG_ERR("%s: port must be specified", target_);
         ok = false;
     } else {
-        ok = parse_sockaddr_components(ss, host, port, default_port, target_);
+        ok = parse_sockaddr_components(ss, host, port, default_port,
+                                       target_, true);
     }
     if (!ok) {
         memset(ss, 0, sizeof *ss);
@@ -625,7 +658,8 @@ inet_parse_passive(const char *target_, int default_port,
         VLOG_ERR("%s: port must be specified", target_);
         ok = false;
     } else {
-        ok = parse_sockaddr_components(ss, host, port, default_port, target_);
+        ok = parse_sockaddr_components(ss, host, port, default_port,
+                                       target_, true);
     }
     if (!ok) {
         memset(ss, 0, sizeof *ss);
@@ -747,7 +781,7 @@ inet_parse_address(const char *target_, struct sockaddr_storage *ss)
 {
     char *target = xstrdup(target_);
     char *host = unbracket(target);
-    bool ok = parse_sockaddr_components(ss, host, NULL, 0, target_);
+    bool ok = parse_sockaddr_components(ss, host, NULL, 0, target_, false);
     if (!ok) {
         memset(ss, 0, sizeof *ss);
     }