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9bootpxe: continues the war against random DHCPv6 DUIDs

Some UEFI implementations use random UUID based DUID instead of
ethernet address, but use ethernet derived link-local addresses.
So extract the MAC from our IPv6 address.

--- a/sys/src/boot/efi/pxe.c

+++ b/sys/src/boot/efi/pxe.c

@@ -367,6 +367,7 @@

 	char *x;

 	int opt;

 	int len;

+	uint type;

 	memset(mymac, 0, sizeof(mymac));

 	memset(serverip, 0, sizeof(serverip));

@@ -389,7 +390,15 @@

 			break;

 		switch(opt){

 		case 1:	/* Client DUID */

-			memmove(mymac, p+len-6, 6);

+			if(len < 4+6)

+				break;

+			type = p[0]<<24 | p[1]<<16 | p[2]<<8 | p[3];

+			switch(type){

+			case 0x00010001:

+			case 0x00030001:

+				memmove(mymac, p+len-6, 6);

+				break;

+			}

 			break;

 		case 59: /* Boot File URL */

 			for(x = (char*)p; x < (char*)p+len; x++){

@@ -401,6 +410,21 @@

 			break;

 		}

 		p += len;

+	}

+

+	/*

+	 * some UEFI implementations use random UUID based DUID instead of

+	 * ethernet address, but use ethernet derived link-local addresses.

+	 * so extract the MAC from our IPv6 address.

+	 */

+	if((mymac[0]|mymac[1]|mymac[2]|mymac[3]|mymac[4]|mymac[5]) == 0){

+		p = pxe->Mode->StationIp;

+		mymac[0] = p[8] ^ 2;

+		mymac[1] = p[9];

+		mymac[2] = p[10];

+		mymac[3] = p[13];

+		mymac[4] = p[14];

+		mymac[5] = p[15];

 	}

 }