Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 140020)
+++ channels/chan_sip.c	(working copy)
@@ -574,6 +574,10 @@
 /*! \brief Codecs that we support by default: */
 static int global_capability = AST_FORMAT_ULAW | AST_FORMAT_ALAW | AST_FORMAT_GSM | AST_FORMAT_H263;
 
+/*! \brief Global list of addresses dynamic peers are not allowed to use */
+static struct ast_ha *global_contact_ha = NULL;
+static int global_dynamic_exclude_static = 0;
+
 /* Object counters */
 static int suserobjs = 0;                /*!< Static users */
 static int ruserobjs = 0;                /*!< Realtime users */
@@ -1130,6 +1134,7 @@
 	
 	struct sockaddr_in defaddr;	/*!<  Default IP address, used until registration */
 	struct ast_ha *ha;		/*!<  Access control list */
+	struct ast_ha *contactha;       /*!<  Restrict what IPs are allowed in the Contact header (for registration) */
 	struct ast_variable *chanvars;	/*!<  Variables to set for channel created by user */
 	struct sip_pvt *mwipvt;		/*!<  Subscription for MWI */
 	int lastmsg;
@@ -8159,7 +8164,7 @@
 	const char *useragent;
 	struct hostent *hp;
 	struct ast_hostent ahp;
-	struct sockaddr_in oldsin;
+	struct sockaddr_in oldsin, testsin;
 
 	ast_copy_string(contact, get_header(req, "Contact"), sizeof(contact));
 
@@ -8240,13 +8245,26 @@
 	} else
 		port = STANDARD_SIP_PORT;
 	oldsin = peer->addr;
+
+	/* Check that they're allowed to register at this IP */
+	/* XXX This could block for a long time XXX */
+	hp = ast_gethostbyname(n, &ahp);
+	if (!hp)  {
+		ast_log(LOG_WARNING, "Invalid host '%s'\n", n);
+		*peer->fullcontact = '\0';
+		ast_string_field_set(pvt, our_contact, "");
+		return PARSE_REGISTER_FAILED;
+	}
+	memcpy(&testsin.sin_addr, hp->h_addr, sizeof(testsin.sin_addr));
+	if (	ast_apply_ha(global_contact_ha, &testsin) != AST_SENSE_ALLOW ||
+			ast_apply_ha(peer->contactha, &testsin) != AST_SENSE_ALLOW) {
+		ast_log(LOG_WARNING, "Host '%s' disallowed by rule\n", n);
+		*peer->fullcontact = '\0';
+		ast_string_field_set(pvt, our_contact, "");
+		return PARSE_REGISTER_FAILED;
+	}
+
 	if (!ast_test_flag(&peer->flags[0], SIP_NAT_ROUTE)) {
-		/* XXX This could block for a long time XXX */
-		hp = ast_gethostbyname(n, &ahp);
-		if (!hp)  {
-			ast_log(LOG_WARNING, "Invalid host '%s'\n", n);
-			return PARSE_REGISTER_FAILED;
-		}
 		peer->addr.sin_family = AF_INET;
 		memcpy(&peer->addr.sin_addr, hp->h_addr, sizeof(peer->addr.sin_addr));
 		peer->addr.sin_port = htons(port);
@@ -17118,6 +17136,9 @@
 					if (!peer->addr.sin_port)
 						peer->addr.sin_port = htons(STANDARD_SIP_PORT);
 				}
+				if (global_dynamic_exclude_static) {
+					global_contact_ha = ast_append_ha("deny", (char *)ast_inet_ntoa(peer->addr.sin_addr), global_contact_ha);
+				}
 			}
 		} else if (!strcasecmp(v->name, "defaultip")) {
 			if (ast_get_ip(&peer->defaddr, v->value)) {
@@ -17126,6 +17147,8 @@
 			}
 		} else if (!strcasecmp(v->name, "permit") || !strcasecmp(v->name, "deny")) {
 			peer->ha = ast_append_ha(v->name, v->value, peer->ha);
+		} else if (!strcasecmp(v->name, "contactpermit") || !strcasecmp(v->name, "contactdeny")) {
+			peer->contactha = ast_append_ha(v->name + 7, v->value, peer->contactha);
 		} else if (!strcasecmp(v->name, "port")) {
 			if (!realtime && ast_test_flag(&peer->flags[1], SIP_PAGE2_DYNAMIC))
 				peer->defaddr.sin_port = htons(atoi(v->value));
@@ -17293,6 +17316,9 @@
 	clear_sip_domains();
 	authl = NULL;
 
+	ast_free_ha(global_contact_ha);
+	global_contact_ha = NULL;
+
 	/* First, destroy all outstanding registry calls */
 	/* This is needed, since otherwise active registry entries will not be destroyed */
 	ASTOBJ_CONTAINER_TRAVERSE(&regl, 1, do {
@@ -17443,6 +17469,10 @@
 			ast_set2_flag(&global_flags[1], ast_true(v->value), SIP_PAGE2_IGNOREREGEXPIRE);	
 		} else if (!strcasecmp(v->name, "t1min")) {
 			global_t1min = atoi(v->value);
+		} else if (!strcasecmp(v->name, "dynamic_exclude_static") || !strcasecmp(v->name, "dynamic_excludes_static")) {
+			global_dynamic_exclude_static = ast_true(v->value);
+		} else if (!strcasecmp(v->name, "contactpermit") || !strcasecmp(v->name, "contactdeny")) {
+			global_contact_ha = ast_append_ha(v->name + 7, v->value, global_contact_ha);
 		} else if (!strcasecmp(v->name, "rtautoclear")) {
 			int i = atoi(v->value);
 			if (i > 0)
Index: configs/sip.conf.sample
===================================================================
--- configs/sip.conf.sample	(revision 140020)
+++ configs/sip.conf.sample	(working copy)
@@ -136,6 +136,16 @@
                                 ; your localnet setting. Unless you have some sort of strange network
                                 ; setup you will not need to enable this.
 
+;dynamic_exclude_static = yes   ; Disallow all dynamic hosts from registering
+                                ; as any IP address used for staticly defined
+                                ; hosts.  This helps avoid the configuration
+                                ; error of allowing your users to register at
+                                ; the same address as a SIP provider.
+
+;contactdeny=0.0.0.0/0.0.0.0           ; Use contactpermit and contactdeny to
+;contactpermit=172.16.0.0/255.255.0.0  ; restrict at what IPs your users may
+                                       ; register their phones.
+
 ;
 ; If regcontext is specified, Asterisk will dynamically create and destroy a
 ; NoOp priority 1 extension for a given peer who registers or unregisters with
@@ -501,6 +511,10 @@
 ;                             outboundproxy
 ;                             rfc2833compensate
 ;                             t38pt_usertpsource
+;                             contactpermit         ; Limit what a host may register as (a neat trick
+;                             contactdeny           ; is to register at the same IP as a SIP provider,
+;                                                   ; then call oneself, and get redirected to that
+;                                                   ; same location).
 
 ;[sip_proxy]
 ; For incoming calls only. Example: FWD (Free World Dialup)