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ip/dhcp6d: work arround non-ethernet based client duid

in ndb, we use the ethernet mac to identify the client.
in dhcpv6, there is just a uniqueue device id that
might even be generated randomly. to find the ethernet
address of a client, check the duid type and only use
it when the dudid is of type 1 (link layer) or 3 (link
layer address + time) and the link layer address type
is 1 (ethernet). otherwise, assume the source ip is
a link local address and extract it from that.

this hack works for thinkpad t495, which uses random
uuid based client duid.

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

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

@@ -31,7 +31,8 @@

 	Udphdr		*udp;

 	Ipifc		*ifc;

-	uchar		mac[Eaddrlen];

+	int		ncid;

+	uchar		cid[256];

 	uchar		ips[IPaddrlen*8];

 	int		nips;

@@ -236,6 +237,31 @@

 	return r;

 }

+static uchar*

+clientea(Req *r)

+{

+	static uchar ea[Eaddrlen];

+	u32int type;

+ 	uchar *ip;

+

+	if(r->ncid >= 4+Eaddrlen){

+		type = r->cid[0]<<24 | r->cid[1]<<16 | r->cid[2]<<8 | r->cid[3];

+		switch(type){

+		case 0x00010001:

+		case 0x00030001:

+			return r->cid + r->ncid - Eaddrlen;

+		}

+	}

+	ip = r->udp->raddr;

+	ea[0] = ip[8] ^ 2;

+	ea[1] = ip[9];

+	ea[2] = ip[10];

+	ea[3] = ip[13];

+	ea[4] = ip[14];

+	ea[5] = ip[15];

+	return ea;

+}

+

 static void

 clearotab(void)

 {

@@ -387,7 +413,7 @@

 		if((r->db = opendb()) == nil)

 			continue;

-		r->nips = lookupips(r->ips, sizeof(r->ips), r->db, r->mac)/IPaddrlen;

+		r->nips = lookupips(r->ips, sizeof(r->ips), r->db, clientea(r))/IPaddrlen;

 		if(debug){

 			for(i=0; i<r->nips; i++)

 				fprint(2, "ip=%I\n", r->ips+i*IPaddrlen);

@@ -411,10 +437,11 @@

 	if((p = gettlv(1, &len, r->req.p, r->req.e)) == nil)

 		return -1;

-	if(len < 4+4+Eaddrlen || n < len)

+	if(len > sizeof(r->cid) || n < len)

 		return -1;

-	memmove(r->mac, p+len-Eaddrlen, Eaddrlen);

 	memmove(w, p, len);

+	memmove(r->cid, p, len);

+	r->ncid = len;

 	return len;

 }