Make dhclient use bootpc (68) as the source port for unicast DHCPREQUEST

packets instead of allowing the protocol stack to pick a random source port.

This fixes the behaviour where dhclient would never transition from RENEWING
to BOUND without going through REBINDING in networks which are paranoid about
DHCP spoofing, such as most mainstream cable-broadband ISP networks.

Reviewed by:	brooks
Obtained from:	OpenBSD (partly - I'm not convinced their solution can work)
MFC after:	1 week (pending re approval)

3 files changed, 41 insertions, 18 deletions

diff --git a/sbin/dhclient/bpf.c b/sbin/dhclient/bpf.c
index 8a669e1..9f8e45f 100644
--- a/sbin/dhclient/bpf.c
+++ b/sbin/dhclient/bpf.c
@@ -90,11 +90,23 @@ if_register_bpf(struct interface_info *info)
 void
 if_register_send(struct interface_info *info)
 {
+	int sock, on = 1;
+
 	/*
 	 * If we're using the bpf API for sending and receiving, we
 	 * don't need to register this interface twice.
 	 */
 	info->wfdesc = info->rfdesc;
+
+	/*
+	 * Use raw socket for unicast send.
+	 */
+	if ((sock = socket(AF_INET, SOCK_RAW, IPPROTO_UDP)) == -1)
+		error("socket(SOCK_RAW): %m");
+	if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on,
+	    sizeof(on)) == -1)
+		error("setsockopt(IP_HDRINCL): %m");
+	info->ufdesc = sock;
 }
 
 /*
@@ -244,35 +256,32 @@ send_packet(struct interface_info *interface, struct dhcp_packet *raw,
 {
 	unsigned char buf[256];
 	struct iovec iov[2];
+	struct msghdr msg;
 	int result, bufp = 0;
-	int sock;
-
-	if (to->sin_addr.s_addr != INADDR_BROADCAST) {
-		note("SENDING DIRECT");
-		/* We know who the server is, send the packet via
-		   normal socket interface */
-
-		if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) >= 0) {
-			result = sendto (sock, (char *)raw, len, 0,
-					 (struct sockaddr *)to, sizeof *to);
-			close(sock);
-			if (result > 0)
-				return result;
-			}
-		}
 
 	/* Assemble the headers... */
-	assemble_hw_header(interface, buf, &bufp, hto);
+	if (to->sin_addr.s_addr == INADDR_BROADCAST)
+		assemble_hw_header(interface, buf, &bufp, hto);
 	assemble_udp_ip_header(buf, &bufp, from.s_addr,
 	    to->sin_addr.s_addr, to->sin_port, (unsigned char *)raw, len);
 
-	/* Fire it off */
 	iov[0].iov_base = (char *)buf;
 	iov[0].iov_len = bufp;
 	iov[1].iov_base = (char *)raw;
 	iov[1].iov_len = len;
 
-	result = writev(interface->wfdesc, iov, 2);
+	/* Fire it off */
+	if (to->sin_addr.s_addr == INADDR_BROADCAST)
+		result = writev(interface->wfdesc, iov, 2);
+	else {
+		memset(&msg, 0, sizeof(msg));
+		msg.msg_name = (struct sockaddr *)to;
+		msg.msg_namelen = sizeof(*to);
+		msg.msg_iov = iov;
+		msg.msg_iovlen = 2;
+		result = sendmsg(interface->ufdesc, &msg, 0);
+	}
+
 	if (result < 0)
 		warning("send_packet: %m");
 	return (result);
diff --git a/sbin/dhclient/dhcpd.h b/sbin/dhclient/dhcpd.h
index 8097f14..bd4c9c0 100644
--- a/sbin/dhclient/dhcpd.h
+++ b/sbin/dhclient/dhcpd.h
@@ -37,6 +37,8 @@
  * Enterprises.  To learn more about the Internet Software Consortium,
  * see ``http://www.vix.com/isc''.  To learn more about Vixie
  * Enterprises, see ``http://www.vix.com''.
+ *
+ * $FreeBSD$
  */
 
 #include <sys/types.h>
@@ -194,6 +196,7 @@ struct interface_info {
 	char			 name[IFNAMSIZ];
 	int			 rfdesc;
 	int			 wfdesc;
+	int			 ufdesc;
 	unsigned char		*rbuf;
 	size_t			 rbuf_max;
 	size_t			 rbuf_offset;
diff --git a/sbin/dhclient/packet.c b/sbin/dhclient/packet.c
index 484953c..2e90cc8 100644
--- a/sbin/dhclient/packet.c
+++ b/sbin/dhclient/packet.c
@@ -135,6 +135,17 @@ assemble_udp_ip_header(unsigned char *buf, int *bufix, u_int32_t from,
 	ip.ip_dst.s_addr = to;
 
 	ip.ip_sum = wrapsum(checksum((unsigned char *)&ip, sizeof(ip), 0));
+
+	/*
+	 * While the BPF -- used for broadcasts -- expects a "true" IP header
+	 * with all the bytes in network byte order, the raw socket interface
+	 * which is used for unicasts expects the ip_len field to be in host
+	 * byte order.  In both cases, the checksum has to be correct, so this
+	 * is as good a place as any to turn the bytes around again.
+	 */
+	if (to != INADDR_BROADCAST)
+		ip.ip_len = ntohs(ip.ip_len);
+
 	memcpy(&buf[*bufix], &ip, sizeof(ip));
 	*bufix += sizeof(ip);