If the dhcp server delivers an interface-mtu option, parse it and store

the value in a new global intf_mtu for use by the application.

These changes were inspired by the patch provided by Robert Blayzor in
PR 187094, and will allow loader(8) to propagate the value to the kernel
along with the other nfs_diskless parms delivered via environment vars.

lib/libstand/bootp.c

@@ -39,6 +39,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/types.h>
+#include <sys/endian.h>
 #include <netinet/in.h>
 #include <netinet/in_systm.h>
 
@@ -393,6 +394,13 @@ vend_rfc1048(cp, len)
 				val = (const char *)cp;
 			strlcpy(hostname, val, sizeof(hostname));
 		}
+		if (tag == TAG_INTF_MTU) {
+			if ((val = getenv("dhcp.interface-mtu")) != NULL) {
+				intf_mtu = (u_int)strtoul(val, NULL, 0);
+			} else {
+				intf_mtu = be16dec(cp);
+			}
+		}
 #ifdef SUPPORT_DHCP
 		if (tag == TAG_DHCP_MSGTYPE) {
 			if(*cp != expected_dhcpmsgtype)

lib/libstand/bootp.h

@@ -91,6 +91,7 @@ struct bootp {
 #define TAG_DOMAINNAME		((unsigned char)  15)
 #define TAG_SWAPSERVER		((unsigned char)  16)
 #define TAG_ROOTPATH		((unsigned char)  17)
+#define TAG_INTF_MTU		((unsigned char)  26)
 
 #ifdef SUPPORT_DHCP
 #define TAG_REQ_ADDR		((unsigned char)  50)

lib/libstand/globals.c

@@ -32,5 +32,6 @@ struct	in_addr rootip;			/* root ip address */
 struct	in_addr swapip;			/* swap ip address */
 struct	in_addr gateip;			/* swap ip address */
 n_long	netmask = 0xffffff00;		/* subnet or net mask */
+u_int	intf_mtu;			/* interface mtu from bootp/dhcp */
 int	errno;				/* our old friend */
 

lib/libstand/net.h

@@ -83,6 +83,7 @@ extern	struct in_addr swapip;
 extern	struct in_addr gateip;
 extern	struct in_addr nameip;
 extern	n_long netmask;
+extern	u_int intf_mtu;
 
 extern	int debug;			/* defined in the machdep sources */