error in ip address received - c

Question

I am implementing raw sockets in C. I have two programs (say server and client). Client is sending info to server and then server is sending ACK. Client has sent info and server has successfully received it. Now server is sending back ACK. Now, Client has parsed ethernet header successfully. I have problem in receiving the IP address.
1. What should be the type of arguments of the function CreateIPHeader()?
2. How to print IP addresses in ParseIPHeader()?
3. What should be the data type of value returned by ParseIPHeader, if I want just the IP addresses?
struct iphdr *CreateIPHeader(char *src_ip,char *dst_ip)
{
struct iphdr *ip_header;
ip_header=malloc(sizeof(struct iphdr));
//OTHER FIELDS OF IP HEADER
ip_header->saddr = inet_ntoa(*((struct in_addr *)(src_ip)));
ip_header->daddr = inet_ntoa(*((struct in_addr *)(dst_ip)));
ip_header->check=ComputeChecksum((unsigned char *)ip_header,ip_header->ihl*4);
printf("\nip to be sent = %s",ip_header->saddr); //printing correct IP
printf("\nip to be rcvd = %s",ip_header->daddr);
return(ip_header);
}
char* ParseIPHeader(unsigned char *packet,int len)
{
struct iphdr *ip_header,*ret_ip;
unsigned char *out;
struct ethhdr *ethernet_header;
out=malloc(2048);
memset(out, 0, 2048);
ethernet_header=(struct ethhdr *) out;
ret_ip=(struct iphdr *) (out + sizeof(struct ethhdr));
if(ntohs(ethernet_header->h_proto)==ETH_P_IP)
{
if(len>=(sizeof(struct ethhdr)+sizeof(struct iphdr)))
{
ip_header=(struct iphdr*)(packet+sizeof(struct ethhdr));
ret_ip->saddr = ip_header->daddr;
ret_ip->daddr = ip_header->saddr;
printf("daddr SENT = %s",ret_ip->daddr); //how to print them?
printf("saddr SENT = %s",ret_ip->saddr);
}
else
printf("IP packet does not have full header\n");
}
else
{
//not an IP packet
}
return out;
}
int main()
{
unsigned char in[2048];
int len;
char *rcv_ip;
Struct iphdr *ip_header;
memset(in,0,2048);
len=recvfrom(raw,in,2048,0,(struct sockaddr *)&packet_info,&packet_info_size);
rcv_ip=ParseIPHeader(in,len); /*I want this function to return me the ip addresses which I would use in the next line.*/
ip_header =CreateIPHeader(rcv_ip+5,rcv_ip);
memset(in,0,2048);
memcpy(in+sizeof(struct ethhdr),ip_header,ip_header->ihl*4);
sendrawpacket(raw,in,pkt_len);
free(ip_header);
return 0;
}
Any help would be appreciated.
Thanks :)

Answer 1

Instead of:
printf("\nDest Addr %s \n",inet_ntoa(*((struct in_addr *)&((ip_header->daddr)))));
printf("\nSource Addr %s \n",inet_ntoa(*((struct in_addr *)&(ip_header->saddr))));
Perhaps:
printf("\nDest Addr %s \n",inet_ntoa(ip_header->daddr));
printf("\nSource Addr %s \n",inet_ntoa(ip_header->saddr));

Answer 2

If indeed the printf is causing you to segfault and not something else, then perhaps:
struct in_addr dest;
dest.s_addr = ip_header->daddr;
printf("\nDest Addr %s \n", inet_ntoa(dest));
This is because (assuming you are using struct iphdr *ip_header) ip_header->daddr has a type of __u32 and inet_ntoa takes a struct in_addr.

Answer 3

There are a couple things wrong.
CreateIPHeader
You are confusing inet_ntoa() with inet_aton(). When creating the IP header, you want to set ip_header->saddr using something like
inet_aton(src_ip, (struct in_addr *) &ip_header->saddr)
Remember, the s_addr and d_addr in struct iphdr are of type __u32, not char arrays. Thus, when you make this change, the printf statements in CreateIPHeader will break.
main
The reason why your printf's are working is due to Problem #1. On receive, you call CreateIPHeader to create an incorrect struct iphdr (see above), which now incorrectly has C-strings assigned to saddr and daddr. Later, abc points to this incorrect header; your printf's magically work because abc->saddr and abc->daddr actually are C strings.
You are only overwriting the iphdr portion of packet_buffer (your memcpy statement). You must also change the h_dest and h_src values in struct ethhdr.
You really don't need to convert the IP addresses into a C string and then convert them back into IP addresses.
Also, you aren't freeing ip_header.
In general, you can do something like this:
int main()
{
unsigned char in[2048];
unsigned char out[2048];
int len;
memset(in, 0, 2048);
memset(out, 0, 2048);
len = recvfrom(raw, in, 2048, 0,
(struct sockaddr *) &packet_info, &packet_info_size);
struct ethhdr *in_eth = (struct ethhdr *) in;
if (ntohs(ethernet_header->h_proto) == ETH_P_IP &&
len >= sizeof(struct ethhdr) + sizeof(struct iphdr) {
struct iphdr *in_ip = (struct iphdr*) (in + sizeof(struct ethhdr));
/* create outbound packet, starting with eth header */
struct ethhdr *out_eth = (struct ethhdr *) out;
/* ... set h_dest and h_src */
struct iphdr *out_ip = (struct iphdr *) (out + sizeof(struct ethhdr));
out_ip->saddr = in_ip->daddr;
out_ip->daddr = in_ip->saddr;
/* calculate the IPv4 checksum, packet len etc */
sendrawpacket(raw, out, pkt_len);
}
return 0;
};
No guarantee that's bug-free. Just wrote it in the browser.