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udhcpc: filter unwanted packets in kernel

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(Cristian Ionescu-Idbohrn <cristian.ionescu-idbohrn@axis.com>)
This commit is contained in:
Denis Vlasenko
2008-01-25 22:46:34 +00:00
parent 893988182a
commit b76b9a4328
5 changed files with 64 additions and 10 deletions
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55
networking/udhcp/clientsocket.c
View File

@@ -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;
}