Implement second fallback mode for DNS requests.

There is some more shardware/software out there which has problems
if two DNS requests are sent using the same tuple

  (source addr, source port, dest addr, dest port)

This can range from firewalls to load balancers.  Some of the vendors
already fixed it in response to this problem.  Still, we need a way
to make glibc work with broken environments.  The single-request-reopen
flag can be used or we fall back automatically to this mode.

1 files changed, 49 insertions, 19 deletions

diff --git a/resolv/res_send.c b/resolv/res_send.c
index 39c69da2bc..971a4afb6f 100644
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -908,24 +908,11 @@ send_vc(res_state statp,
 }
 
 static int
-send_dg(res_state statp,
-	const u_char *buf, int buflen, const u_char *buf2, int buflen2,
-	u_char **ansp, int *anssizp,
-	int *terrno, int ns, int *v_circuit, int *gotsomewhere, u_char **anscp,
-	u_char **ansp2, int *anssizp2, int *resplen2)
+reopen (res_state statp, int *terrno, int ns)
 {
-	const HEADER *hp = (HEADER *) buf;
-	const HEADER *hp2 = (HEADER *) buf2;
-	u_char *ans = *ansp;
-	int orig_anssizp = *anssizp;
-	struct sockaddr_in6 *nsap = EXT(statp).nsaddrs[ns];
-	struct timespec now, timeout, finish;
-	struct pollfd pfd[1];
-        int ptimeout;
-	struct sockaddr_in6 from;
-	int resplen, n;
-
 	if (EXT(statp).nssocks[ns] == -1) {
+		struct sockaddr_in6 *nsap = EXT(statp).nsaddrs[ns];
+
 		/* only try IPv6 if IPv6 NS and if not failed before */
 		if ((EXT(statp).nscount6 > 0) && !statp->ipv6_unavail) {
 			if (__builtin_expect (__have_o_nonblock >= 0, 1)) {
@@ -1000,6 +987,26 @@ send_dg(res_state statp,
 		}
 	}
 
+	return 1;
+}
+
+static int
+send_dg(res_state statp,
+	const u_char *buf, int buflen, const u_char *buf2, int buflen2,
+	u_char **ansp, int *anssizp,
+	int *terrno, int ns, int *v_circuit, int *gotsomewhere, u_char **anscp,
+	u_char **ansp2, int *anssizp2, int *resplen2)
+{
+	const HEADER *hp = (HEADER *) buf;
+	const HEADER *hp2 = (HEADER *) buf2;
+	u_char *ans = *ansp;
+	int orig_anssizp = *anssizp;
+	struct timespec now, timeout, finish;
+	struct pollfd pfd[1];
+        int ptimeout;
+	struct sockaddr_in6 from;
+	int resplen, n;
+
 	/*
 	 * Compute time for the total operation.
 	 */
@@ -1008,8 +1015,15 @@ send_dg(res_state statp,
 		seconds /= statp->nscount;
 	if (seconds <= 0)
 		seconds = 1;
-	bool single_request = (statp->options & RES_SNGLKUP) != 0;// XXX
+	bool single_request = (statp->options & RES_SNGLKUP) != 0;
+	bool single_request_reopen = (statp->options & RES_SNGLKUPREOP) != 0;
 	int save_gotsomewhere = *gotsomewhere;
+
+	int retval;
+ retry_reopen:
+	retval = reopen (statp, terrno, ns);
+	if (retval <= 0)
+		return retval;
  retry:
 	evNowTime(&now);
 	evConsTime(&timeout, seconds, 0);
@@ -1064,6 +1078,14 @@ send_dg(res_state statp,
 			*gotsomewhere = save_gotsomewhere;
 			goto retry;
 		      }
+		    else if (!single_request_reopen)
+		      {
+			statp->options |= RES_SNGLKUPREOP;
+			single_request_reopen = true;
+			*gotsomewhere = save_gotsomewhere;
+			__res_iclose (statp, false);
+			goto retry_reopen;
+		      }
 
 		    *resplen2 = 1;
 		    return resplen;
@@ -1092,7 +1114,8 @@ send_dg(res_state statp,
 			Perror(statp, stderr, "send", errno);
 			goto err_out;
 		}
-		if (nwritten != 0 || buf2 == NULL || single_request)
+		if (nwritten != 0 || buf2 == NULL
+		    || single_request || single_request_reopen)
 		  pfd[0].events = POLLIN;
 		else
 		  pfd[0].events = POLLIN | POLLOUT;
@@ -1306,8 +1329,15 @@ send_dg(res_state statp,
 			recvresp2 = 1;
 		/* Repeat waiting if we have a second answer to arrive.  */
 		if ((recvresp1 & recvresp2) == 0) {
-			if (single_request)
+			if (single_request || single_request_reopen) {
 				pfd[0].events = POLLOUT;
+				if (single_request_reopen) {
+					__res_iclose (statp, false);
+					retval = reopen (statp, terrno, ns);
+					if (retval <= 0)
+						return retval;
+				}
+			}
 			goto wait;
 		}
 		/*