CURLOPT_DNS_SERVERS: set name servers if possible

docs/libcurl/curl_easy_setopt.3

@@ -2045,6 +2045,20 @@ resolves, by including a string in the linked list that uses the format
 and port number must exactly match what was already added previously.
 
 (Added in 7.21.3)
+.IP CURLOPT_DNS_SERVERS
+Set the list of DNS servers to be used instead of the system default.
+The format of the dns servers option is:
+
+host[:port][,host[:port]]...
+
+For example:
+
+192.168.1.100,192.168.1.101,3.4.5.6
+
+This option requires that libcurl was built with a resolver backend that
+supports this operation. The c-ares backend is the only such one.
+
+(Added in 7.24.0)
 .SH SSL and SECURITY OPTIONS
 .IP CURLOPT_SSLCERT
 Pass a pointer to a zero terminated string as parameter. The string should be

include/curl/curl.h

@@ -1486,6 +1486,9 @@ typedef enum {
   /* allow GSSAPI credential delegation */
   CINIT(GSSAPI_DELEGATION, LONG, 210),
 
+  /* Set the name servers to use for DNS resolution */
+  CINIT(DNS_SERVERS, OBJECTPOINT, 211),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;
 

lib/asyn-ares.c

@@ -600,4 +600,30 @@ Curl_addrinfo *Curl_resolver_getaddrinfo(struct connectdata *conn,
   }
   return NULL; /* no struct yet */
 }
+
+CURLcode Curl_set_dns_servers(struct SessionHandle *data,
+                              char *servers)
+{
+  CURLcode result = CURLE_NOT_BUILT_IN;
+#if (ARES_VERSION >= 0x010704)
+  int ares_result = ares_set_servers_csv(data->state.resolver, servers);
+  switch(ares_result) {
+  case ARES_SUCCESS:
+    break;
+  case ARES_ENOMEM:
+    result = CURLE_OUT_OF_MEMORY;
+    break;
+  case ARES_ENOTINITIALIZED:
+  case ARES_ENODATA:
+  case ARES_EBADSTR:
+  default:
+    result = CURLE_BAD_FUNCTION_ARGUMENT;
+    break;
+  }
+#else /* too old c-ares version! */
+  (void)data;
+  (void)servers;
+#endif
+  return result;
+}
 #endif /* CURLRES_ARES */

lib/asyn-thread.c

@@ -696,4 +696,13 @@ Curl_addrinfo *Curl_resolver_getaddrinfo(struct connectdata *conn,
 
 #endif /* !HAVE_GETADDRINFO */
 
+CURLcode Curl_set_dns_servers(struct SessionHandle *data,
+                              char *servers)
+{
+  (void)data;
+  (void)servers;
+  return CURLE_NOT_BUILT_IN;
+
+}
+
 #endif /* CURLRES_THREADED */

lib/hostip.h

@@ -195,4 +195,9 @@ Curl_cache_addr(struct SessionHandle *data, Curl_addrinfo *addr,
 extern sigjmp_buf curl_jmpenv;
 #endif
 
+/*
+ * Function provided by the resolver backend to set DNS servers to use.
+ */
+CURLcode Curl_set_dns_servers(struct SessionHandle *data, char *servers);
+
 #endif /* HEADER_CURL_HOSTIP_H */

lib/hostsyn.c

@@ -66,5 +66,16 @@
  **********************************************************************/
 #ifdef CURLRES_SYNCH
 
+/*
+ * Function provided by the resolver backend to set DNS servers to use.
+ */
+CURLcode Curl_set_dns_servers(struct SessionHandle *data,
+                              char *servers)
+{
+  (void)data;
+  (void)servers;
+  return CURLE_NOT_BUILT_IN;
+
+}
 
 #endif /* truly sync */

lib/url.c

@@ -2531,6 +2531,10 @@ CURLcode Curl_setopt(struct SessionHandle *data, CURLoption option,
       data->set.ssl.authtype = CURL_TLSAUTH_NONE;
     break;
 #endif
+  case CURLOPT_DNS_SERVERS:
+    result = Curl_set_dns_servers(data, va_arg(param, char *));
+    break;
+
   default:
     /* unknown tag and its companion, just ignore: */
     result = CURLE_UNKNOWN_OPTION;