udhcpc: filter unwanted packets in kernel
(Cristian Ionescu-Idbohrn <cristian.ionescu-idbohrn@axis.com>)
diff --git a/networking/udhcp/clientsocket.c b/networking/udhcp/clientsocket.c
index 541f883..954db53 100644
--- a/networking/udhcp/clientsocket.c
+++ b/networking/udhcp/clientsocket.c
@@ -30,22 +30,75 @@
#include <linux/if_packet.h>
#include <linux/if_ether.h>
#endif
+#include <linux/filter.h>
#include "common.h"
+#define SERVER_AND_CLIENT_PORTS ((SERVER_PORT << 16) + CLIENT_PORT)
int raw_socket(int ifindex)
{
int fd;
struct sockaddr_ll sock;
- DEBUG("Opening raw socket on ifindex %d", ifindex);
+ /*
+ * Comment:
+ *
+ * I've selected not to see LL header, so BPF doesn't see it, too.
+ * The filter may also pass non-IP and non-ARP packets, but we do
+ * a more complete check when receiving the message in userspace.
+ *
+ * and filter shamelessly stolen from:
+ *
+ * http://www.flamewarmaster.de/software/dhcpclient/
+ *
+ * There are a few other interesting ideas on that page (look under
+ * "Motivation"). Use of netlink events is most interesting. Think
+ * of various network servers listening for events and reconfiguring.
+ * That would obsolete sending HUP signals and/or make use of restarts.
+ *
+ * Copyright: 2006, 2007 Stefan Rompf <sux@loplof.de>.
+ * License: GPL v2.
+ *
+ * TODO: make conditional?
+ */
+ static const struct sock_filter filter_instr[] = {
+ /* check for udp */
+ BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 9),
+ BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, IPPROTO_UDP, 2, 0), /* L5, L1, is UDP? */
+ /* ugly check for arp on ethernet-like and IPv4 */
+ BPF_STMT(BPF_LD|BPF_W|BPF_ABS, 2), /* L1: */
+ BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, 0x08000604, 3, 4), /* L3, L4 */
+ /* skip IP header */
+ BPF_STMT(BPF_LDX|BPF_B|BPF_MSH, 0), /* L5: */
+ /* check udp source and destination ports */
+ BPF_STMT(BPF_LD|BPF_W|BPF_IND, 0),
+ BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, SERVER_AND_CLIENT_PORTS, 0, 1), /* L3, L4 */
+ /* returns */
+ BPF_STMT(BPF_RET|BPF_K, ~0UL), /* L3: pass */
+ BPF_STMT(BPF_RET|BPF_K, 0), /* L4: reject */
+ };
+ static const struct sock_fprog filter_prog = {
+ .len = sizeof(filter_instr) / sizeof(filter_instr[0]),
+ /* casting const away: */
+ .filter = (struct sock_filter *) filter_instr,
+ };
+
+ DEBUG("opening raw socket on ifindex %d", ifindex);
+
fd = xsocket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP));
+ DEBUG("got raw socket fd %d", fd);
+
+ /* Ignoring error (kernel may lack support for this) */
+ if (setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER, &filter_prog,
+ sizeof(filter_prog)) >= 0)
+ DEBUG("attached filter to raw socket fd %d", fd);
sock.sll_family = AF_PACKET;
sock.sll_protocol = htons(ETH_P_IP);
sock.sll_ifindex = ifindex;
xbind(fd, (struct sockaddr *) &sock, sizeof(sock));
+ DEBUG("bound to raw socket fd %d", fd);
return fd;
}