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ip/dhcp6d: ignore short and from non-local source packets

--- a/sys/src/cmd/ip/dhcp6d.c

+++ b/sys/src/cmd/ip/dhcp6d.c

@@ -113,6 +113,21 @@

 	return nil;

 }

+static Iplifc*

+localonifc(Ipifc *ifc, uchar ip[IPaddrlen])

+{

+	Iplifc *lifc;

+	uchar net[IPaddrlen];

+

+	for(lifc = ifc->lifc; lifc != nil; lifc = lifc->next){

+		maskip(ip, lifc->mask, net);

+		if(ipcmp(net, lifc->net) == 0)

+			return lifc;

+	}

+

+	return nil;

+}

+

 static int

 openlisten(char *net)

 {

@@ -310,20 +325,26 @@

 		break;

 	}

-	while((n = read(fd, ibuf, sizeof(ibuf))) > Udphdrsize+4){

-		r->req.p = ibuf+Udphdrsize;

-		r->req.e = ibuf+n;

+	while((n = read(fd, ibuf, sizeof(ibuf))) > 0){

+		if(n < Udphdrsize+4)

+			continue;

+		r->udp = (Udphdr*)ibuf;

+		if(isv4(r->udp->raddr))

+			continue;

+		if((r->ifc = findifc(netmtpt, r->udp->ifcaddr)) == nil)

+			continue;

+		if(localonifc(r->ifc, r->udp->raddr) == nil)

+			continue;

+

 		memmove(obuf, ibuf, Udphdrsize);

-		r->udp = (Udphdr*)obuf;

+		r->req.p = ibuf+Udphdrsize;

+		r->req.e = ibuf+n;

 		r->resp.p = obuf+Udphdrsize;

 		r->resp.e = &obuf[sizeof(obuf)];

 		r->tra = r->req.p[1]<<16 | r->req.p[2]<<8 | r->req.p[3];

 		r->req.t = r->req.p[0];

-

-		if((r->ifc = findifc(netmtpt, r->udp->ifcaddr)) == nil)

-			continue;

 		if(debug)

 		fprint(2, "%I->%I(%s) typ=%d tra=%x\n",