Why is my router dropping packets from my raw sockets application? - c

Question

I have an application that is sending hand crafted SOCK_RAW packets from a PF_PACKET socket. The packets are being created and sent as the screenshot from Wireshark shows. The packets being sent are TCP SYN packets with an expected TCP SYN/ACK response. However, no response is being received, again as the screenshot shows. I assume that this is because the router is dropping the packets for some reason. Any ideas what the reason could be? Or is there some other reason why I am not receiving any responses.
The full code is quite long because it takes a lot of code to get the IP address and the MAC address of the router to build the ethernet header with. So I have only included the most relevant code. If that is not enough please leave a comment and I will post the full code.
fd_socket[z] = socket(PF_PACKET, SOCK_RAW|SOCK_NONBLOCK, htons(ETH_P_ALL));
if(fd_socket[z] == -1)
{
perror("socket");
return EXIT_FAILURE;
}
/* clear structure */
memset(&my_addr[z], 0, sizeof(struct sockaddr_ll));
my_addr[z].sll_family = PF_PACKET;
my_addr[z].sll_protocol = htons(ETH_P_ALL);
str_devname = ifname;
//strcpy (str_devname, ifname);
/* initialize interface struct */
strncpy (s_ifr.ifr_name, str_devname, sizeof(s_ifr.ifr_name));
/* Get the broad cast address */
ec = ioctl(fd_socket[z], SIOCGIFINDEX, &s_ifr);
if(ec == -1)
{
perror("iotcl");
return EXIT_FAILURE;
}
/* update with interface index */
i_ifindex = s_ifr.ifr_ifindex;
s_ifr.ifr_mtu = 7200;
/* update the mtu through ioctl */
ec = ioctl(fd_socket[z], SIOCSIFMTU, &s_ifr);
if(ec == -1)
{
perror("iotcl");
return EXIT_FAILURE;
}
/* set sockaddr info */
memset(&my_addr[z], 0, sizeof(struct sockaddr_ll));
my_addr[z].sll_family = AF_PACKET;
my_addr[z].sll_protocol = ETH_P_ALL;
my_addr[z].sll_ifindex = i_ifindex;
/* bind port */
if (bind(fd_socket[z], (struct sockaddr *)&my_addr[z], sizeof(struct sockaddr_ll)) == -1)
{
perror("bind");
return EXIT_FAILURE;
}
/* prepare Tx ring request */
s_packet_req.tp_block_size = c_buffer_sz;
s_packet_req.tp_frame_size = c_buffer_sz;
s_packet_req.tp_block_nr = c_buffer_nb;
s_packet_req.tp_frame_nr = c_buffer_nb;
/* calculate memory to mmap in the kernel */
size = s_packet_req.tp_block_size * s_packet_req.tp_block_nr;
/* set packet loss option */
tmp = mode_loss;
if (setsockopt(fd_socket[z], SOL_PACKET, PACKET_LOSS, (char *)&tmp, sizeof(tmp))<0)
{
perror("setsockopt: PACKET_LOSS");
return EXIT_FAILURE;
}
/* send TX ring request */
if (setsockopt(fd_socket[z], SOL_PACKET, PACKET_TX_RING, (char *)&s_packet_req, sizeof(s_packet_req))<0)
{
perror("setsockopt: PACKET_TX_RING");
return EXIT_FAILURE;
}
/* change send buffer size */
if(c_sndbuf_sz) {
printf("send buff size = %d\n", c_sndbuf_sz);
if (setsockopt(fd_socket[z], SOL_SOCKET, SO_SNDBUF, &c_sndbuf_sz, sizeof(c_sndbuf_sz))< 0)
{
perror("getsockopt: SO_SNDBUF");
return EXIT_FAILURE;
}
}
/* get data offset */
data_offset = TPACKET_HDRLEN - sizeof(struct sockaddr_ll);
/* mmap Tx ring buffers memory */
ps_header_start = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd_socket[z], 0);
if (ps_header_start == (void*)-1)
{
perror("mmap");
return EXIT_FAILURE;
}
int i,j;
int i_index = 0;
char * data;
int first_loop = 1;
struct tpacket_hdr * ps_header;
int ec_send = 0;
for(i=1; i <= c_packet_nb; i++)
{
int i_index_start = i_index;
int loop = 1;
/* get free buffer */
do {
ps_header = ((struct tpacket_hdr *)((void *)ps_header_start + (c_buffer_sz*i_index)));
data = ((void*) ps_header) + data_offset;
switch((volatile uint32_t)ps_header->tp_status)
{
case TP_STATUS_AVAILABLE:
/* fill data in buffer */
if(first_loop) {
//Datagram to represent the packet
char datagram[4096] , source_ip[32] , *data2, *pseudogram;
//zero out the packet buffer
memset (datagram, 0, 4096);
//Ethernet header
struct ether_header *eh = (struct ether_header *) datagram;
//IP header
struct iphdr *iph = (struct iphdr *) (datagram + sizeof (struct ether_header));
//TCP header
struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ether_header) + sizeof (struct ip));
struct sockaddr_in sin;
struct pseudo_header psh;
//Data part
data2 = datagram + sizeof(struct ether_header) + sizeof(struct iphdr) + sizeof(struct tcphdr);
strcpy(data2 , "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
//some address resolution
strcpy(source_ip , inet_ntoa(ipaddr->sin_addr));
sin.sin_family = AF_INET;
sin.sin_port = htons(80);
if (fscanf(fp, "%253s", server) == 1)
sin.sin_addr.s_addr = inet_addr (server);
else
{
done = 1;
break;
}
//Fill in the Ethernet Header
eh->ether_dhost[0] = arp_resp->sender_mac[0];
eh->ether_dhost[1] = arp_resp->sender_mac[1];
eh->ether_dhost[2] = arp_resp->sender_mac[2];
eh->ether_dhost[3] = arp_resp->sender_mac[3];
eh->ether_dhost[4] = arp_resp->sender_mac[4];
eh->ether_dhost[5] = arp_resp->sender_mac[5];
memcpy(eh->ether_shost, ifr.ifr_hwaddr.sa_data, HWADDR_len);
eh->ether_type = 0x0008;
//Fill in the IP Header
iph->ihl = 5;
iph->version = 4;
iph->tos = 0;
iph->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr) + strlen(data);
iph->id = htonl (54321); //Id of this packet
iph->frag_off = 0;
iph->ttl = 255;
iph->protocol = IPPROTO_TCP;
iph->check = 0; //Set to 0 before calculating checksum
iph->saddr = inet_addr ( source_ip ); //Spoof the source ip address
iph->daddr = sin.sin_addr.s_addr;
//Ip checksum
iph->check = csum ((unsigned short *) datagram, iph->tot_len);
//TCP Header
tcph->source = htons (1234);
tcph->dest = htons (80);
tcph->seq = 0;
tcph->ack_seq = 0;
tcph->doff = 5; //tcp header size
tcph->fin=0;
tcph->syn=1;
tcph->rst=0;
tcph->psh=0;
tcph->ack=0;
tcph->urg=0;
tcph->window = htons (5840); // maximum allowed window size
tcph->check = 0; //leave checksum 0 now, filled later by pseudo header
tcph->urg_ptr = 0;
//Now the TCP checksum
psh.source_address = inet_addr( source_ip );
psh.dest_address = sin.sin_addr.s_addr;
psh.placeholder = 0;
psh.protocol = IPPROTO_TCP;
psh.tcp_length = htons(sizeof(struct tcphdr) + strlen(data) );
int psize = sizeof(struct pseudo_header) + sizeof(struct tcphdr) + strlen(data);
pseudogram = malloc(psize);
memcpy(pseudogram , (char*) &psh , sizeof (struct pseudo_header));
memcpy(pseudogram + sizeof(struct pseudo_header) , tcph , sizeof(struct tcphdr) + strlen(data));
tcph->check = csum( (unsigned short*) pseudogram , psize);
memcpy(data, datagram, 4096);
free(pseudogram);
// for(j=0;j<c_packet_sz;j++)
// data[j] = j;
}
loop = 0;
break;
case TP_STATUS_WRONG_FORMAT:
printf("An error has occured during transfer\n");
exit(EXIT_FAILURE);
break;
default:
/* nothing to do => schedule : useful if no SMP */
usleep(0);
break;
}
}
while(loop == 1);
i_index ++;
if(i_index >= c_buffer_nb)
{
i_index = 0;
first_loop = 0;
}
/* update packet len */
ps_header->tp_len = c_packet_sz;
/* set header flag to USER (trigs xmit)*/
ps_header->tp_status = TP_STATUS_SEND_REQUEST;
/* if smp mode selected */
if(!mode_thread)
{
/* send all packets */
if( ((i&c_send_mask)==0) || (ec_send < 0) || (i == c_packet_nb) )
{
/* send all buffers with TP_STATUS_SEND_REQUEST */
/* Don't wait end of transfer */
//ec_send = (int) task_send((void*)0);
}
}
else if(c_error) {
if(i == (c_packet_nb/2))
{
int ec_close;
if(c_error == 1) {
ec_close = close(fd_socket[z]);
}
if(c_error == 2) {
if (setsockopt(fd_socket[z], SOL_PACKET, PACKET_TX_RING, (char *)&s_packet_req, sizeof(s_packet_req))<0)
{
perror("setsockopt: PACKET_TX_RING");
//return EXIT_FAILURE;
}
}
break;
}
}
}
//int ec_send;
static int total=0;
int blocking = 1;
/* send all buffers with TP_STATUS_SEND_REQUEST */
/* Wait end of transfer */
ec_send = sendto(fd_socket[z],NULL,0,(blocking? 0 : MSG_DONTWAIT),(struct sockaddr *) ps_sockaddr,sizeof(struct sockaddr_ll));
if(ec_send < 0) {
perror("sendto");
}
else if ( ec_send == 0 ) {
/* nothing to do => schedule : useful if no SMP */
usleep(0);
}
else {
total += ec_send/(c_packet_sz);
printf("send %d packets (+%d bytes)\n",total, ec_send);
fflush(0);
}
//ps_header_start = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd_socket[z], 0);
if (munmap(ps_header_start, size) == -1)
{
perror("munmap");
exit(EXIT_FAILURE);
}
close(fd_socket[z]);
EDIT
A similar program that uses SOCK_RAW on a PF_INET socket also sends hand crafted packets doing a TCP SYN and does in fact receive the expected TCP SYN/ACK response. The only difference in the way the packets are constructed is that the PF_PACKET version has to have the ethernet headers added as well. The below screenshot shows a packet sent and a packet received which shows that the ethernet headers are the same for both the PF_INET version and the PF_PACKET version. So it is a mystery what the actual difference is that leads to this behaviour.
The PF_INET program code is as follows
/*
Raw TCP packets
*/
#include <stdio.h> //for printf
#include <string.h> //memset
#include <sys/socket.h> //for socket ofcourse
#include <stdlib.h> //for exit(0);
#include <errno.h> //For errno - the error number
#include <netinet/tcp.h> //Provides declarations for tcp header
#include <netinet/ip.h> //Provides declarations for ip header
#include <arpa/inet.h> // inet_addr
#include <unistd.h> // sleep()
/*
96 bit (12 bytes) pseudo header needed for tcp header checksum calculation
*/
struct pseudo_header
{
u_int32_t source_address;
u_int32_t dest_address;
u_int8_t placeholder;
u_int8_t protocol;
u_int16_t tcp_length;
};
/*
Generic checksum calculation function
*/
unsigned short csum(unsigned short *ptr,int nbytes)
{
register long sum;
unsigned short oddbyte;
register short answer;
sum=0;
while(nbytes>1) {
sum+=*ptr++;
nbytes-=2;
}
if(nbytes==1) {
oddbyte=0;
*((u_char*)&oddbyte)=*(u_char*)ptr;
sum+=oddbyte;
}
sum = (sum>>16)+(sum & 0xffff);
sum = sum + (sum>>16);
answer=(short)~sum;
return(answer);
}
int main (void)
{
//Create a raw socket
int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);
if(s == -1)
{
//socket creation failed, may be because of non-root privileges
perror("Failed to create socket");
exit(1);
}
//Datagram to represent the packet
char datagram[4096] , source_ip[32] , *data , *pseudogram;
//zero out the packet buffer
memset (datagram, 0, 4096);
//IP header
struct iphdr *iph = (struct iphdr *) datagram;
//TCP header
struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip));
struct sockaddr_in sin;
struct pseudo_header psh;
//Data part
data = datagram + sizeof(struct iphdr) + sizeof(struct tcphdr);
strcpy(data , "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
//some address resolution
strcpy(source_ip , "192.168.1.170");
sin.sin_family = AF_INET;
sin.sin_port = htons(80);
sin.sin_addr.s_addr = inet_addr ("51.89.233.84");
//Fill in the IP Header
iph->ihl = 5;
iph->version = 4;
iph->tos = 0;
iph->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr) + strlen(data);
iph->id = htonl (54321); //Id of this packet
iph->frag_off = 0;
iph->ttl = 255;
iph->protocol = IPPROTO_TCP;
iph->check = 0; //Set to 0 before calculating checksum
iph->saddr = inet_addr ( source_ip ); //Spoof the source ip address
iph->daddr = sin.sin_addr.s_addr;
//Ip checksum
iph->check = csum ((unsigned short *) datagram, iph->tot_len);
//TCP Header
tcph->source = htons (1234);
tcph->dest = htons (80);
tcph->seq = 0;
tcph->ack_seq = 0;
tcph->doff = 5; //tcp header size
tcph->fin=0;
tcph->syn=1;
tcph->rst=0;
tcph->psh=0;
tcph->ack=0;
tcph->urg=0;
tcph->window = htons (5840); /* maximum allowed window size */
tcph->check = 0; //leave checksum 0 now, filled later by pseudo header
tcph->urg_ptr = 0;
//Now the TCP checksum
psh.source_address = inet_addr( source_ip );
psh.dest_address = sin.sin_addr.s_addr;
psh.placeholder = 0;
psh.protocol = IPPROTO_TCP;
psh.tcp_length = htons(sizeof(struct tcphdr) + strlen(data) );
int psize = sizeof(struct pseudo_header) + sizeof(struct tcphdr) + strlen(data);
pseudogram = malloc(psize);
memcpy(pseudogram , (char*) &psh , sizeof (struct pseudo_header));
memcpy(pseudogram + sizeof(struct pseudo_header) , tcph , sizeof(struct tcphdr) + strlen(data));
tcph->check = csum( (unsigned short*) pseudogram , psize);
//IP_HDRINCL to tell the kernel that headers are included in the packet
int one = 1;
const int *val = &one;
if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
{
perror("Error setting IP_HDRINCL");
exit(0);
}
//loop if you want to flood :)
while (1)
{
//Send the packet
if (sendto (s, datagram, iph->tot_len , 0, (struct sockaddr *) &sin, sizeof (sin)) < 0)
{
perror("sendto failed");
}
//Data send successfully
else
{
printf ("Packet Send. Length : %d \n" , iph->tot_len);
}
// sleep for 1 seconds
sleep(1);
}
return 0;
}
EDIT
On closer inspection of the packets being sent out on the wire I noticed that the IP header checksum is wrong for the PF_PACKET version. Also the byte order needs to be reversed for values added that are more than a single byte in length. Why doesn't the checksum function work for the PF_PACKET version. Here is the csum function:
/*
Generic checksum calculation function
*/
unsigned short csum(unsigned short *ptr,int nbytes)
{
register long sum;
unsigned short oddbyte;
register short answer;
sum=0;
while(nbytes>1) {
sum+=*ptr++;
nbytes-=2;
}
if(nbytes==1) {
oddbyte=0;
*((u_char*)&oddbyte)=*(u_char*)ptr;
sum+=oddbyte;
}
sum = (sum>>16)+(sum & 0xffff);
sum = sum + (sum>>16);
answer=(short)~sum;
return(answer);
}

Answer 1

The ip4 checksum is only calculated over the ip header, if I get it correctly. So if you pass the total lenght of the whole packet to the checlsum calculation function, I would not be surprised, if you get a wrong checksum. I wonder though why it happend to work in the second program.