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ip/dhcpd: work around raspberry pi pxe firmware by providing dhcp option 66

The raspberry pi 4 PXE firmware insists on finding the tftp
server address by parsing dhcp option 66 as an ip address
string.

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

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

@@ -1136,8 +1136,7 @@

 	uchar *addrs[8];

 	uchar *op, *omax;

 	uchar x[nelem(addrs)*IPaddrlen], vopts[Maxoptlen];

-	char *p;

-	char *attr[100], **a;

+	char ip4str[16], *p, *attr[100], **a;

 	Ndbtuple *t;

 	Iplifc *lifc;

@@ -1278,6 +1277,21 @@

 			break;

 		case OBttl:

 			byteopt(rp, OBttl, 255);

+			break;

+

+		case ODtftpserver:

+			/* 

+			 * This option actually should contain a hostname, BUT:

+			 * It appears that Rpi4 PXE firmware ignores the siaddr

+			 * and instead insists on this (DHCP option 66) to contain

+			 * the IP address string of the TFTP server.

+			 */

+			snprint(ip4str, sizeof(ip4str), "%V", rp->bp->siaddr);

+			stringopt(rp, ODtftpserver, ip4str);

+			break;

+		case ODbootfile:

+			if(*rp->bp->file)

+				stringopt(rp, ODbootfile, rp->bp->file);

 			break;

 		}