Fix assorted issues in client host name lookup.

The code for matching clients to pg_hba.conf lines that specify host names
(instead of IP address ranges) failed to complain if reverse DNS lookup
failed; instead it silently didn't match, so that you might end up getting
a surprising "no pg_hba.conf entry for ..." error, as seen in bug #9518
from Mike Blackwell.  Since we don't want to make this a fatal error in
situations where pg_hba.conf contains a mixture of host names and IP
addresses (clients matching one of the numeric entries should not have to
have rDNS data), remember the lookup failure and mention it as DETAIL if
we get to "no pg_hba.conf entry".  Apply the same approach to forward-DNS
lookup failures, too, rather than treating them as immediate hard errors.

Along the way, fix a couple of bugs that prevented us from detecting an
rDNS lookup error reliably, and make sure that we make only one rDNS lookup
attempt; formerly, if the lookup attempt failed, the code would try again
for each host name entry in pg_hba.conf.  Since more or less the whole
point of this design is to ensure there's only one lookup attempt not one
per entry, the latter point represents a performance bug that seems
sufficient justification for back-patching.

Also, adjust src/port/getaddrinfo.c so that it plays as well as it can
with this code.  Which is not all that well, since it does not have actual
support for rDNS lookup, but at least it should return the expected (and
required by spec) error codes so that the main code correctly perceives the
lack of functionality as a lookup failure.  It's unlikely that PG is still
being used in production on any machines that require our getaddrinfo.c,
so I'm not excited about working harder than this.

To keep the code in the various branches similar, this includes
back-patching commits c424d0d1 and
1997f34d into 9.2 and earlier.

Back-patch to 9.1 where the facility for hostnames in pg_hba.conf was
introduced.

src/backend/libpq/auth.c

@@ -425,15 +425,25 @@ ClientAuthentication(Port *port)
 								   NI_NUMERICHOST);
 
 #define HOSTNAME_LOOKUP_DETAIL(port) \
-				(port->remote_hostname				  \
-				 ? (port->remote_hostname_resolv == +1					\
-					? errdetail_log("Client IP address resolved to \"%s\", forward lookup matches.", port->remote_hostname) \
-					: (port->remote_hostname_resolv == 0				\
-					   ? errdetail_log("Client IP address resolved to \"%s\", forward lookup not checked.", port->remote_hostname) \
-					   : (port->remote_hostname_resolv == -1			\
-						  ? errdetail_log("Client IP address resolved to \"%s\", forward lookup does not match.", port->remote_hostname) \
-						  : 0)))										\
-				 : 0)
+				(port->remote_hostname ? \
+				 (port->remote_hostname_resolv == +1 ? \
+				  errdetail_log("Client IP address resolved to \"%s\", forward lookup matches.", \
+								port->remote_hostname) : \
+				  port->remote_hostname_resolv == 0 ? \
+				  errdetail_log("Client IP address resolved to \"%s\", forward lookup not checked.", \
+								port->remote_hostname) : \
+				  port->remote_hostname_resolv == -1 ? \
+				  errdetail_log("Client IP address resolved to \"%s\", forward lookup does not match.", \
+								port->remote_hostname) : \
+				  port->remote_hostname_resolv == -2 ? \
+				  errdetail_log("Could not translate client host name \"%s\" to IP address: %s.", \
+								port->remote_hostname, \
+								gai_strerror(port->remote_hostname_errcode)) : \
+				  0) \
+				 : (port->remote_hostname_resolv == -2 ? \
+					errdetail_log("Could not resolve client IP address to a host name: %s.", \
+								  gai_strerror(port->remote_hostname_errcode)) : \
+					0))
 
 				if (am_walsender)
 				{

src/backend/libpq/hba.c

@@ -592,35 +592,47 @@ check_hostname(hbaPort *port, const char *hostname)
 	int			ret;
 	bool		found;
 
+	/* Quick out if remote host name already known bad */
+	if (port->remote_hostname_resolv < 0)
+		return false;
+
 	/* Lookup remote host name if not already done */
 	if (!port->remote_hostname)
 	{
 		char		remote_hostname[NI_MAXHOST];
 
-		if (pg_getnameinfo_all(&port->raddr.addr, port->raddr.salen,
-							   remote_hostname, sizeof(remote_hostname),
-							   NULL, 0,
-							   0) != 0)
+		ret = pg_getnameinfo_all(&port->raddr.addr, port->raddr.salen,
+								 remote_hostname, sizeof(remote_hostname),
+								 NULL, 0,
+								 NI_NAMEREQD);
+		if (ret != 0)
+		{
+			/* remember failure; don't complain in the postmaster log yet */
+			port->remote_hostname_resolv = -2;
+			port->remote_hostname_errcode = ret;
 			return false;
+		}
 
 		port->remote_hostname = pstrdup(remote_hostname);
 	}
 
+	/* Now see if remote host name matches this pg_hba line */
 	if (!hostname_match(hostname, port->remote_hostname))
 		return false;
 
-	/* Lookup IP from host name and check against original IP */
-
+	/* If we already verified the forward lookup, we're done */
 	if (port->remote_hostname_resolv == +1)
 		return true;
-	if (port->remote_hostname_resolv == -1)
-		return false;
 
+	/* Lookup IP from host name and check against original IP */
 	ret = getaddrinfo(port->remote_hostname, NULL, NULL, &gai_result);
 	if (ret != 0)
-		ereport(ERROR,
-				(errmsg("could not translate host name \"%s\" to address: %s",
-						port->remote_hostname, gai_strerror(ret))));
+	{
+		/* remember failure; don't complain in the postmaster log yet */
+		port->remote_hostname_resolv = -2;
+		port->remote_hostname_errcode = ret;
+		return false;
+	}
 
 	found = false;
 	for (gai = gai_result; gai; gai = gai->ai_next)

src/backend/postmaster/postmaster.c

@@ -3952,8 +3952,23 @@ BackendInitialize(Port *port)
 	 */
 	port->remote_host = strdup(remote_host);
 	port->remote_port = strdup(remote_port);
-	if (log_hostname)
-		port->remote_hostname = port->remote_host;
+
+	/*
+	 * If we did a reverse lookup to name, we might as well save the results
+	 * rather than possibly repeating the lookup during authentication.
+	 *
+	 * Note that we don't want to specify NI_NAMEREQD above, because then we'd
+	 * get nothing useful for a client without an rDNS entry.  Therefore, we
+	 * must check whether we got a numeric IPv4 or IPv6 address, and not save
+	 * it into remote_hostname if so.  (This test is conservative and might
+	 * sometimes classify a hostname as numeric, but an error in that
+	 * direction is safe; it only results in a possible extra lookup.)
+	 */
+	if (log_hostname &&
+		ret == 0 &&
+		strspn(remote_host, "0123456789.") < strlen(remote_host) &&
+		strspn(remote_host, "0123456789ABCDEFabcdef:") < strlen(remote_host))
+		port->remote_hostname = strdup(remote_host);
 
 	/*
 	 * Ready to begin client interaction.  We will give up and exit(1) after a

src/include/getaddrinfo.h

@@ -82,6 +82,9 @@
 #ifndef NI_NUMERICSERV
 #define NI_NUMERICSERV	2
 #endif
+#ifndef NI_NAMEREQD
+#define NI_NAMEREQD		4
+#endif
 
 #ifndef NI_MAXHOST
 #define NI_MAXHOST	1025

src/include/libpq/libpq-be.h

@@ -104,6 +104,20 @@ extern int	ssl_renegotiation_limit;
  * still available when a backend is running (see MyProcPort).	The data
  * it points to must also be malloc'd, or else palloc'd in TopMemoryContext,
  * so that it survives into PostgresMain execution!
+ *
+ * remote_hostname is set if we did a successful reverse lookup of the
+ * client's IP address during connection setup.
+ * remote_hostname_resolv tracks the state of hostname verification:
+ *	+1 = remote_hostname is known to resolve to client's IP address
+ *	-1 = remote_hostname is known NOT to resolve to client's IP address
+ *	 0 = we have not done the forward DNS lookup yet
+ *	-2 = there was an error in name resolution
+ * If reverse lookup of the client IP address fails, remote_hostname will be
+ * left NULL while remote_hostname_resolv is set to -2.  If reverse lookup
+ * succeeds but forward lookup fails, remote_hostname_resolv is also set to -2
+ * (the case is distinguishable because remote_hostname isn't NULL).  In
+ * either of the -2 cases, remote_hostname_errcode saves the lookup return
+ * code for possible later use with gai_strerror.
  */
 
 typedef struct Port
@@ -116,12 +130,8 @@ typedef struct Port
 	char	   *remote_host;	/* name (or ip addr) of remote host */
 	char	   *remote_hostname;/* name (not ip addr) of remote host, if
 								 * available */
-	int			remote_hostname_resolv; /* +1 = remote_hostname is known to
-										 * resolve to client's IP address; -1
-										 * = remote_hostname is known NOT to
-										 * resolve to client's IP address; 0 =
-										 * we have not done the forward DNS
-										 * lookup yet */
+	int			remote_hostname_resolv; /* see above */
+	int			remote_hostname_errcode;		/* see above */
 	char	   *remote_port;	/* text rep of remote port */
 	CAC_state	canAcceptConnections;	/* postmaster connection status */
 

src/port/getaddrinfo.c

@@ -181,7 +181,7 @@ getaddrinfo(const char *node, const char *service,
 		else if (hints.ai_flags & AI_NUMERICHOST)
 		{
 			if (!inet_aton(node, &sin.sin_addr))
-				return EAI_FAIL;
+				return EAI_NONAME;
 		}
 		else
 		{
@@ -347,8 +347,8 @@ gai_strerror(int errcode)
 /*
  * Convert an ipv4 address to a hostname.
  *
- * Bugs:	- Only supports NI_NUMERICHOST and NI_NUMERICSERV
- *		  It will never resolv a hostname.
+ * Bugs:	- Only supports NI_NUMERICHOST and NI_NUMERICSERV behavior.
+ *		  It will never resolve a hostname.
  *		- No IPv6 support.
  */
 int
@@ -375,6 +375,10 @@ getnameinfo(const struct sockaddr * sa, int salen,
 		return EAI_FAMILY;
 #endif
 
+	/* Unsupported flags. */
+	if (flags & NI_NAMEREQD)
+		return EAI_AGAIN;
+
 	if (node)
 	{
 		if (sa->sa_family == AF_INET)
@@ -397,7 +401,7 @@ getnameinfo(const struct sockaddr * sa, int salen,
 			ret = snprintf(service, servicelen, "%d",
 						   ntohs(((struct sockaddr_in *) sa)->sin_port));
 		}
-		if (ret == -1 || ret > servicelen)
+		if (ret == -1 || ret >= servicelen)
 			return EAI_MEMORY;
 	}