add code to dhcpd to allow the dhcp server to be multihomed
on the same interface and still properly match served ip addresses.

Reference: /n/atom/patch/applied2013/dhcpmultihome
Date: Thu Jun 20 01:01:25 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ip/dhcpd/ndb.c	Thu Jun 20 01:00:09 2013
+++ /sys/src/cmd/ip/dhcpd/ndb.c	Thu Jun 20 01:00:09 2013
@@ -20,8 +20,8 @@
 
 	for(ifc = ipifcs; ifc; ifc = ifc->next){
 		for(lifc = ifc->lifc; lifc != nil; lifc = lifc->next){
-			if(lifc->net[0] == 0)
-				continue;
+//			if(lifc->net[0] == 0)
+//				continue;
 			maskip(ip, lifc->mask, x);
 			if(memcmp(x, lifc->net, IPaddrlen) == 0)
 				return lifc;
@@ -60,6 +60,28 @@
 		parseipmask(mask, ip);
 }
 
+static int
+checkdb(void)
+{
+	long t;
+	static long ndbtime;
+
+	t = time(0);
+	if(db == nil){
+		db = ndbopen(ndbfile);
+		ndbtime = t;
+	}
+	else if(t - ndbtime > 30){
+		db = ndbreopendb(db);
+		ndbtime = t;
+	}
+	if(db == nil){
+		warning(1, "can't open db");
+		return -1;
+	}
+	return 0;
+}
+
 /*
  *  do an ipinfo with defaults
  */
@@ -70,12 +92,8 @@
 	Ndbtuple *t, *nt;
 	char *attrs[32], **p;
 
-	if(db == 0)
-		db = ndbopen(ndbfile);
-	if(db == 0){
-		warning(1, "can't open db");
+	if(checkdb() == -1)
 		return -1;
-	}
 
 	p = attrs;
 	*p++ = "ip";
@@ -170,6 +188,45 @@
 	return 0;
 }
 
+static int
+mksamenet(Info *ii, Info *gii)
+{
+	uchar x[IPaddrlen];
+	Ipifc *ifc;
+	Iplifc *lifc, *nexus;
+
+	maskip(ii->ipaddr, gii->ipmask, x);
+	if(ipcmp(x, gii->ipnet) == 0)
+		return 0;
+
+	/* find ii's interface */
+	for(ifc = ipifcs;; ifc = ifc->next){
+		if(ifc == nil)
+			return -1;
+		for(lifc = ifc->lifc; lifc != nil; lifc = lifc->next){
+			maskip(ii->ipaddr, lifc->mask, x);
+			if(ipcmp(x, lifc->net) == 0)
+				goto found;
+		}
+	}
+found:
+	nexus = lifc;
+	/* check to see that gii appears on same interface */
+	for(lifc = ifc->lifc;; lifc = lifc->next){
+		if(lifc == nil)
+			return -1;
+		if(ipcmp(lifc->net, gii->ipnet) == 0)
+			break;
+	}
+
+	/* fixup incorrect gii information */
+	if(lookupip(nexus->ip, gii, 1) < 0)
+		return -1;
+	syslog(0, blog, "nexus %I %I is %s:%I",
+		ii->ipaddr, gii->ipaddr, ifc->dev, nexus->net);
+	return 0;
+}
+
 static uchar zeroes[6];
 
 /*
@@ -185,12 +242,8 @@
 	char *hwval, hwbuf[33];
 	uchar ciaddr[IPaddrlen];
 
-	if(db == 0)
-		db = ndbopen(ndbfile);
-	if(db == 0){
-		warning(1, "can't open db");
+	if(checkdb() == -1)
 		return -1;
-	}
 
 	memset(iip, 0, sizeof(*iip));
 
@@ -245,7 +298,7 @@
 			parseip(ciaddr, nt->val);
 			if(lookupip(ciaddr, iip, 0) < 0)
 				continue;
-			if(samenet(riip->ipaddr, iip)){
+			if(mksamenet(iip, riip) == 0){
 				ndbfree(t);
 				return 0;
 			}