Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 89537)
+++ channels/chan_sip.c	(working copy)
@@ -1712,6 +1712,8 @@
 static int get_destination(struct sip_pvt *p, struct sip_request *oreq);
 static int get_msg_text(char *buf, int len, struct sip_request *req);
 static int transmit_state_notify(struct sip_pvt *p, int state, int full, int timeout);
+static int get_domain(const char *str, char *domain, int len); 
+static int get_realm(const struct sip_request *req, char *realm, int len);
 
 /*--- Constructing requests and responses */
 static void initialize_initreq(struct sip_pvt *p, struct sip_request *req);
@@ -6738,14 +6740,18 @@
 	struct sip_request resp;
 	char tmp[512];
 	int seqno = 0;
+	char realm[MAXHOSTNAMELEN];
 
 	if (reliable && (sscanf(get_header(req, "CSeq"), "%d ", &seqno) != 1)) {
 		ast_log(LOG_WARNING, "Unable to determine sequence number from '%s'\n", get_header(req, "CSeq"));
 		return -1;
 	}
+	/* Choose Realm */
+	get_realm(req, realm, sizeof(realm));
+
 	/* Stale means that they sent us correct authentication, but 
 	   based it on an old challenge (nonce) */
-	snprintf(tmp, sizeof(tmp), "Digest algorithm=MD5, realm=\"%s\", nonce=\"%s\"%s", global_realm, randdata, stale ? ", stale=true" : "");
+	snprintf(tmp, sizeof(tmp), "Digest algorithm=MD5, realm=\"%s\", nonce=\"%s\"%s", realm, randdata, stale ? ", stale=true" : "");
 	respprep(&resp, p, msg, req);
 	add_header(&resp, header, tmp);
 	add_header_contentLength(&resp, 0);
@@ -6753,6 +6759,68 @@
 	return send_response(p, &resp, reliable, seqno);
 }
 
+/*! 
+ \brief Extract domain from SIP To/From header
+ \return -1 on error, 1 if domain string is empty, 0 if domain was properly extracted
+ \note TODO: Such code is all over SIP channel, there is a sense to organize 
+      this patern in one function
+*/
+static int get_domain(const char *str, char *domain, int len) 
+{
+	char tmpf[256];
+	char *a, *from;
+
+	*domain = '\0';
+	ast_copy_string(tmpf, str, sizeof(tmpf));
+	from = get_in_brackets(tmpf);
+	if (!ast_strlen_zero(from)) {
+		if (strncasecmp(from, "sip:", 4)) {
+			ast_log(LOG_WARNING, "Huh?  Not a SIP header (%s)?\n", from);
+			return -1;
+		}
+		from += 4;
+	} else
+		from = NULL;
+
+	if (from) {
+		/* Strip any params or options from user */
+		if ((a = strchr(from, ';')))
+			*a = '\0';
+		/* Strip port from domain if present */
+		if ((a = strchr(from, ':')))
+			*a = '\0';
+		if ((a = strchr(from, '@'))) {
+			*a = '\0';
+			ast_copy_string(domain, a + 1, len);
+		} else
+			ast_copy_string(domain, from, len);
+	}
+
+	return ast_strlen_zero(domain);
+}
+
+/*! 
+  \brief Choose realm based on From header or use globaly configured realm. 
+  Realm from From header should be listed among served domains in config file: domain=... 
+*/
+static int get_realm(const struct sip_request *req, char *realm, int len)
+{
+	char fromdomain[MAXHOSTNAMELEN];
+	char *r = NULL;
+
+	if (!AST_LIST_EMPTY(&domain_list) && 
+	    !get_domain(get_header(req, "From"), fromdomain, sizeof(fromdomain)))
+	{
+		if (check_sip_domain(fromdomain, NULL, 0))
+			r = fromdomain;
+	}
+	if (!r)
+		r = global_realm;
+	ast_copy_string(realm, r, len);
+
+	return 0;
+}
+
 /*! \brief Add text body to SIP message */
 static int add_text(struct sip_request *req, const char *text)
 {
@@ -9123,7 +9191,10 @@
 		ast_copy_string(a1_hash, md5secret, sizeof(a1_hash));
 	else {
 		char a1[256];
-		snprintf(a1, sizeof(a1), "%s:%s:%s", username, global_realm, secret);
+		char realm[MAXHOSTNAMELEN];
+		/* Choose Realm */
+		get_realm(req, realm, sizeof(realm));
+		snprintf(a1, sizeof(a1), "%s:%s:%s", username, realm, secret);
 		ast_md5_hash(a1_hash, a1);
 	}
 
Index: configs/sip.conf.sample
===================================================================
--- configs/sip.conf.sample	(revision 89537)
+++ configs/sip.conf.sample	(working copy)
@@ -38,6 +38,12 @@
 				; asterisk.conf, it defaults to that system name
 				; Realms MUST be globally unique according to RFC 3261
 				; Set this to your host name or domain name
+				; You can serve multiple Realms specifying several
+				; 'domain=...' directives (see below). 
+				; In this case Realm will be based on request 'From' header
+				; and should match one of domain names.
+				; Otherwise default 'realm=...' will be used.
+
 bindport=5060			; UDP Port to bind to (SIP standard port is 5060)
 				; bindport is the local UDP port that Asterisk will listen on
 bindaddr=0.0.0.0		; IP address to bind to (0.0.0.0 binds to all)