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 (file)
--- 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;
 }

diff --git a/networking/udhcp/common.h b/networking/udhcp/common.h
index 38ede01..b1d629b 100644 (file)
--- a/networking/udhcp/common.h
+++ b/networking/udhcp/common.h
@@ -14,6 +14,9 @@
 
 #define DEFAULT_SCRIPT  "/usr/share/udhcpc/default.script"
 
+#define SERVER_PORT  67
+#define CLIENT_PORT  68
+
 extern const uint8_t MAC_BCAST_ADDR[6]; /* six all-ones */
 
 /*** packet.h ***/
@@ -21,7 +24,7 @@ extern const uint8_t MAC_BCAST_ADDR[6]; /* six all-ones */
 #include <netinet/udp.h>
 #include <netinet/ip.h>
 
-#define DHCP_OPTIONS_BUFSIZE 308
+#define DHCP_OPTIONS_BUFSIZE  308
 
 struct dhcpMessage {
        uint8_t op;
@@ -93,7 +96,7 @@ int listen_socket(/*uint32_t ip,*/ int port, const char *inf);
 int arpping(uint32_t test_ip, uint32_t from_ip, uint8_t *from_mac, const char *interface);
 
 #if ENABLE_FEATURE_UDHCP_DEBUG
-# define DEBUG(str, args...) bb_info_msg(str, ## args)
+# define DEBUG(str, args...) bb_info_msg("### " str, ## args)
 #else
 # define DEBUG(str, args...) do {;} while (0)
 #endif

diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index e9b728a..f54bc08 100644 (file)
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -452,7 +452,8 @@ int udhcpc_main(int argc, char **argv)
 
                        if (listen_mode == LISTEN_KERNEL)
                                len = udhcp_recv_packet(&packet, sockfd);
-                       else len = get_raw_packet(&packet, sockfd);
+                       else
+                               len = get_raw_packet(&packet, sockfd);
 
                        if (len == -1) { /* error is severe, reopen socket */
                                DEBUG("error on read, %s, reopening socket", strerror(errno));

diff --git a/networking/udhcp/dhcprelay.c b/networking/udhcp/dhcprelay.c
index a6483fc..c243cc1 100644 (file)
--- a/networking/udhcp/dhcprelay.c
+++ b/networking/udhcp/dhcprelay.c
@@ -155,12 +155,12 @@ static int init_sockets(char **client, int num_clients,
        int i, n;
 
        /* talk to real server on bootps */
-       fds[0] = listen_socket(/*INADDR_ANY,*/ 67, server);
+       fds[0] = listen_socket(/*INADDR_ANY,*/ SERVER_PORT, server);
        n = fds[0];
 
        for (i = 1; i < num_clients; i++) {
                /* listen for clients on bootps */
-               fds[i] = listen_socket(/*INADDR_ANY,*/ 67, client[i-1]);
+               fds[i] = listen_socket(/*INADDR_ANY,*/ SERVER_PORT, client[i-1]);
                if (fds[i] > n)
                        n = fds[i];
        }
@@ -289,7 +289,7 @@ int dhcprelay_main(int argc, char **argv)
        struct sockaddr_in server_addr;
 
        server_addr.sin_family = AF_INET;
-       server_addr.sin_port = htons(67);
+       server_addr.sin_port = htons(SERVER_PORT);
        if (argc == 4) {
                if (!inet_aton(argv[3], &server_addr.sin_addr))
                        bb_perror_msg_and_die("didn't grok server");

diff --git a/networking/udhcp/options.h b/networking/udhcp/options.h
index c98aec4..e9eeefb 100644 (file)
--- a/networking/udhcp/options.h
+++ b/networking/udhcp/options.h
@@ -28,9 +28,6 @@ enum {
 /*****************************************************************/
 
 /* DHCP protocol -- see RFC 2131 */
-#define SERVER_PORT            67
-#define CLIENT_PORT            68
-
 #define DHCP_MAGIC             0x63825363