--- /usr/src/sys/sys/socket.h.orig	Thu May  4 12:56:39 2006
+++ /usr/src/sys/sys/socket.h	Fri May  5 00:14:41 2006
@@ -118,6 +118,7 @@
 #define	SO_ACCEPTFILTER	0x1000		/* there is an accept filter */
 #define	SO_BINTIME	0x2000		/* timestamp received dgram traffic */
 #endif
+#define	SO_DONTFRAGMENT	0x4000		/* don't fragment packet */
 
 /*
  * Additional options, not kept in so_options.
--- /usr/src/sys/kern/uipc_socket.c.orig	Thu May  4 13:00:50 2006
+++ /usr/src/sys/kern/uipc_socket.c	Fri May  5 00:11:39 2006
@@ -1553,6 +1553,7 @@
 #ifdef MAC
 	struct mac extmac;
 #endif
+	struct	inpcb *inp = sotoinpcb(so);
 
 	error = 0;
 	if (sopt->sopt_level != SOL_SOCKET) {
@@ -1594,6 +1595,7 @@
 		case SO_TIMESTAMP:
 		case SO_BINTIME:
 		case SO_NOSIGPIPE:
+		case SO_DONTFRAGMENT:
 			error = sooptcopyin(sopt, &optval, sizeof optval,
 					    sizeof optval);
 			if (error)
@@ -1603,6 +1605,23 @@
 				so->so_options |= sopt->sopt_name;
 			else
 				so->so_options &= ~sopt->sopt_name;
+
+			switch (sopt->sopt_name) {
+
+#define OPTSET(bit) do {						\
+	INP_LOCK(inp);							\
+	if (optval)							\
+		inp->inp_flags |= bit;					\
+	else								\
+		inp->inp_flags &= ~bit;					\
+	INP_UNLOCK(inp);						\
+} while (0)
+
+			case SO_DONTFRAGMENT:
+				OPTSET(INP_DONTFRAG);
+				break;
+			}
+
 			SOCK_UNLOCK(so);
 			break;