Don't set PAM_RHOST for Unix sockets.

Since commit 2f1d2b7a we have set PAM_RHOST to "[local]" for Unix
sockets.  This caused Linux PAM's libaudit integration to make DNS
requests for that name.  It's not exactly clear what value PAM_RHOST
should have in that case, but it seems clear that we shouldn't set it
to an unresolvable name, so don't do that.

Back-patch to 9.6.  Bug #15520.

Author: Thomas Munro
Reviewed-by: Peter Eisentraut
Reported-by: Albert Schabhuetl
Discussion: https://postgr.es/m/15520-4c266f986998e1c5%40postgresql.org

src/backend/libpq/auth.c

@@ -2162,18 +2162,6 @@ CheckPAMAuth(Port *port, const char *user, const char *password)
 {
 	int			retval;
 	pam_handle_t *pamh = NULL;
-	char		hostinfo[NI_MAXHOST];
-
-	retval = pg_getnameinfo_all(&port->raddr.addr, port->raddr.salen,
-								hostinfo, sizeof(hostinfo), NULL, 0,
-								port->hba->pam_use_hostname ? 0 : NI_NUMERICHOST | NI_NUMERICSERV);
-	if (retval != 0)
-	{
-		ereport(WARNING,
-				(errmsg_internal("pg_getnameinfo_all() failed: %s",
-								 gai_strerror(retval))));
-		return STATUS_ERROR;
-	}
 
 	/*
 	 * We can't entirely rely on PAM to pass through appdata --- it appears
@@ -2219,6 +2207,27 @@ CheckPAMAuth(Port *port, const char *user, const char *password)
 		return STATUS_ERROR;
 	}
 
+	if (port->hba->conntype != ctLocal)
+	{
+		char		hostinfo[NI_MAXHOST];
+		int			flags;
+
+		if (port->hba->pam_use_hostname)
+			flags = 0;
+		else
+			flags = NI_NUMERICHOST | NI_NUMERICSERV;
+
+		retval = pg_getnameinfo_all(&port->raddr.addr, port->raddr.salen,
+									hostinfo, sizeof(hostinfo), NULL, 0,
+									flags);
+		if (retval != 0)
+		{
+			ereport(WARNING,
+					(errmsg_internal("pg_getnameinfo_all() failed: %s",
+									 gai_strerror(retval))));
+			return STATUS_ERROR;
+		}
+
 		retval = pam_set_item(pamh, PAM_RHOST, hostinfo);
 
 		if (retval != PAM_SUCCESS)
@@ -2229,6 +2238,7 @@ CheckPAMAuth(Port *port, const char *user, const char *password)
 			pam_passwd = NULL;
 			return STATUS_ERROR;
 		}
+	}
 
 	retval = pam_set_item(pamh, PAM_CONV, &pam_passw_conv);