ntohs() problems - c

Question

This program should catch a packet from offline dump file and decode it. Here I have problems with ntohs() function (it's near the end, headers description in the start). Why it does not work?
OS win7 x86, VS 2010 express.
#include "pcap.h"
#define SIZE_ETHERNET 14
#define ETHER_ADDR_LEN 6
/* 4 bytes IP address */
typedef struct ip_address{
u_char byte1;
u_char byte2;
u_char byte3;
u_char byte4;
}ip_address;
/* IPv4 header */
typedef struct ip_header{
u_char ver_ihl; // Version (4 bits) + Internet header length (4 bits)
u_char tos; // Type of service
u_short tlen; // Total length
u_short identification; // Identification
u_short flags_fo; // Flags (3 bits) + Fragment offset (13 bits)
u_char ttl; // Time to live
u_char proto; // Protocol
u_short crc; // Header checksum
ip_address saddr; // Source address
ip_address daddr; // Destination address
u_int op_pad; // Option + Padding
}ip_header;
/* UDP header*/
typedef struct udp_header{
u_short sport; // Source port
u_short dport; // Destination port
u_short len; // Datagram length
u_short crc; // Checksum
}udp_header;
typedef struct ethernet_address{
u_char byte1;
u_char byte2;
u_char byte3;
u_char byte4;
u_char byte5;
u_char byte6;
}ethernet_address;
/* Ethernet header */
typedef struct ethernet_header {
ethernet_address ether_dhost; /* Destination host address */
ethernet_address ether_shost; /* Source host address */
u_short ether_type; /* IP? ARP? RARP? etc */
};
/* TCP header */
typedef struct tcp_header {
u_short th_sport; /* source port */
u_short th_dport; /* destination port */
u_char th_offx2; /* data offset, rsvd */
#define TH_OFF(th) (((th)->th_offx2 & 0xf0) >> 4)
u_char th_flags;
#define TH_FIN 0x01
#define TH_SYN 0x02
#define TH_RST 0x04
#define TH_PUSH 0x08
#define TH_ACK 0x10
#define TH_URG 0x20
#define TH_ECE 0x40
#define TH_CWR 0x80
#define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
u_short th_win; /* window */
u_short th_sum; /* checksum */
u_short th_urp; /* urgent pointer */
}tcp_header;
/* prototype of the packet handler */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data);
/* Packet count */
int num = 1;
int main(int argc, char **argv)
{
pcap_t *adhandle;
char errbuf[PCAP_ERRBUF_SIZE];
char source[PCAP_BUF_SIZE];
u_int netmask;
char packet_filter[] = "";
struct bpf_program fcode;
if(argc != 2){
printf("usage: %s filename", argv[0]);
return -1;
}
/* Create the source string according to the new WinPcap syntax */
if ( pcap_createsrcstr( source, // variable that will keep the source string
PCAP_SRC_FILE, // we want to open a file
NULL, // remote host
NULL, // port on the remote host
argv[1], // name of the file we want to open
errbuf // error buffer
) != 0)
{
fprintf(stderr,"\nError creating a source string\n");
return -1;
}
/* Open the adapter */
if ( (adhandle= pcap_open(source, // name of the device
65536, // portion of the packet to capture.
// 65536 grants that the whole packet will be captured on all the MACs.
PCAP_OPENFLAG_PROMISCUOUS, // promiscuous mode
1000, // read timeout
NULL, // remote authentication
errbuf // error buffer
) ) == NULL)
{
fprintf(stderr,"\nUnable to open the adapter. %s is not supported by WinPcap\n");
return -1;
}
/* Check the link layer. We support only Ethernet for simplicity. */
if(pcap_datalink(adhandle) != DLT_EN10MB)
{
fprintf(stderr,"\nThis program works only on Ethernet networks.\n");
return -1;
}
/* if(d->addresses != NULL)
Retrieve the mask of the first address of the interface
netmask=((struct sockaddr_in *)(d->addresses->netmask))->sin_addr.S_un.S_addr;
else */
/* If the interface is without addresses we suppose to be in a C class network */
netmask=0xffffff;
//compile the filter
if (pcap_compile(adhandle, &fcode, packet_filter, 1, netmask) <0 )
{
fprintf(stderr,"\nUnable to compile the packet filter. Check the syntax.\n");
return -1;
}
//set the filter
if (pcap_setfilter(adhandle, &fcode)<0)
{
fprintf(stderr,"\nError setting the filter.\n");
return -1;
}
/* start the capture */
pcap_loop(adhandle, 0, packet_handler, NULL);
return 0;
}
/* Callback function invoked by libpcap for every incoming packet */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
struct tm ltime;
char timestr[16];
time_t local_tv_sec;
u_int ip_len;
u_short sport,dport;
ip_header *ih;
const struct ethernet_header *ethh; /* The ethernet header */
/*
* Unused variable
*/
(VOID)(param);
printf("Packet %d\n", num);
num++;
/* convert the timestamp to readable format */
local_tv_sec = header->ts.tv_sec;
localtime_s(&ltime, &local_tv_sec);
strftime( timestr, sizeof timestr, "%H:%M:%S", &ltime);
/* print timestamp and length of the packet */
// printf("%s.%.6d len:%d ", timestr, header->ts.tv_usec, header->len);
/* retireve the position of the ethernet header */
ethh = (struct ethernet_header*)(pkt_data);
/* retireve the position of the ip header */
ih = (ip_header *) (pkt_data + 14); //length of ethernet header
/* retireve the position of the udp header */
ip_len = (ih->ver_ihl & 0xf) * 4;
/* print ip addresses and ports */
printf("Eth Src: %x:%x:%x:%x:%x:%x >>> Dest: %x:%x:%x:%x:%x:%x\nIP Src: %d.%d.%d.%d >>> Dest: %d.%d.%d.%d\n",
ethh->ether_shost.byte1,
ethh->ether_shost.byte2,
ethh->ether_shost.byte3,
ethh->ether_shost.byte4,
ethh->ether_shost.byte5,
ethh->ether_shost.byte6,
ethh->ether_dhost.byte1,
ethh->ether_dhost.byte2,
ethh->ether_dhost.byte3,
ethh->ether_dhost.byte4,
ethh->ether_dhost.byte5,
ethh->ether_dhost.byte6,
ih->saddr.byte1,
ih->saddr.byte2,
ih->saddr.byte3,
ih->saddr.byte4,
ih->daddr.byte1,
ih->daddr.byte2,
ih->daddr.byte3,
ih->daddr.byte4
);
/* Panage protocols */
if(ih->proto == 0x11)
{
udp_header *uh;
uh = (udp_header *) ((u_char*)ih + ip_len);
printf("UDP Src: %d >>> Dest: %d\n", uh->sport,uh->dport);
}
else if(ih->proto == 0x06)
{
tcp_header *tcp = NULL;
/* convert from network byte order to host byte order */
tcp = (tcp_header *) ((u_char*)ih + ip_len);
/*HERE*/ sport = ntohs(tcp->th_sport);
/*HERE*/ dport = ntohs(tcp->th_dport);
printf("TCP Src: %d >>> Dest: %d\n", sport,dport);
}
}
error log:
1>------ Build started: Project: cw1, Configuration: Debug Win32 ------
1> cw1.c
1>cw1.obj : error LNK2019: unresolved external symbol __imp__ntohs#4 referenced in function _packet_handler
1>C:\Users\Medardas\Desktop\ComputerScience\C.SC251 - CW1\cw1\Debug\cw1.exe : fatal error LNK1120: 1 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

Try including <arpa/inet.h> or <netinet/in.h>. Failing that, check your linker settings.

Answer 2

That's a linker error. Consult the documentation of the function. At the bottom of that page (all such pages are laid out the same way) you will find the details of what header, lib and DLL are needed. In this case you need to link against Ws2_32.lib.