PCAP functions in tcp raw program. - c

Question

I am having some trouble with some functions that won't compile properly. They are pretty much borrowed from an example program to get a better understanding of how network programming works and to see if I could put together a reliable TCP connection (3 way handshake) using RAW sockets. These are the functions:
void recieve(u_char *args, const struct pcap_pkthdr *pkthdr, const u_char *buffer)
{
const int one = 1;
u_char *ptr;
int LEN = args; /* FIRST WARNING POINTS HERE */
struct ipheader *ip;
struct tcpheader *tcp;
ip = (struct ipheader *)(buffer + LEN);
tcp = (struct tcpheader *)(buffer + LEN + sizeof (struct ipheader));
printf("%d\n", LEN);
printf("Packet recieved. ACK number: %d\n", ntohl (tcp->tcph_seqnum));
printf("Packet recieved. SEQ number: %d\n", nthol (tcp->tcph_acknum));
s_seq = nthol (tcp->tcph_seqnum);
send_syn_ack(s_seq, dip, sip, dport, sport);
sleep(100);
}
and
void capture()
{
pcap_t *pd;
bpf_u_int32 netmask;
bpf_u_int32 localnet;
char *filter = ("ip dest host %s", dstip); /* SECOND WARNING POINTS HERE */
char *dev = NULL;
char errbuf[PCAP_ERRBUF_SIZE];
struct bpf_program filterprog;
int dl = 0, dl_len = 0;
if ((pd = pcap_open_live(dev, 1514, 1, 500, errbuf)) == NULL)
{
fprintf(stderr, "cannot open device %s: %s\n", dev, errbuf);
exit(1);
}
pcap_lookupnet(dev, &localnet, &netmask, errbuf);
pcap_compile(pd, &filterprog, filter, 0, localnet);
if (pcap_setfilter(pd, &filterprog) == - 1)
{
fprintf(stderr, "cannot set pcap filter %s: %s\n", filter, errbuf);
exit(1);
}
pcap_freecode(&filterprog);
dl = pcap_datalink(pd);
switch(dl) {
case 1:
dl_len = 14;
break;
default:
dl_len = 14;
break;
}
if (pcap_loop(pd, -1, recieve, (u_char *)dl_len) < 0) { /* LAST WARNING HERE */
fprintf(stderr, "cannot get raw packet: %s\n", pcap_geterr(pd));
exit(1);
}
}
and these are the error messages:
In function ‘recieve’:
warning: initialization makes integer from pointer without a cast
In function ‘capture’:
warning: initialization makes pointer from integer without a cast
Warning: cast to pointer from integer of different size
I get the first and last warning even in the example program. Are these two functions any good, and is there a simpler and cleaner way to achieve what they are meant to? if not, what should I do to get rid of these warnings? Thanks :)

Answer 1

u_char is a not a standard type. Equivalent is a uint8_t which is a part of C99 standard. More or less the first error please try,
int LEN = *args
uint8_t is defined in the library stdint.h
In the second error place, do the following,
`char *s = "ip dest host ";
char *filter = (char *)malloc(strlen(s) + strlen(dstip) + 1);
strcpy(filter,s);
strcat(filter,dstip);'
Please include for string functions strlen, strcpy and strcat.

Answer 2

Well, the warnings are correct - in recieve(), the first argument is u_char *args, and in the line you flag, you are attempting to assign args to int LEN; args isn't an int, and you're not explicitly asking for it to be converted to an int, so the compiler warns about it.
For the second one, I'm not sure what type dstip is, since the declaration/definition is not included in the code you posted, but if it's not a char *, that would explain the second message.
For the third one, I'd have to know what pcap_loop() is expecting in place of the u_char * you are passing it, but that's likely also a mismatch.