cast multiple structs to a char string - c

Question

this is my first time posting on stack overflow so be gentle. I am writing a networking program in c to run on linux machines. The goal of my program is to be able to capture packets sent to it, change the source ip and hw address, rebuild the packet with the new info and send it back out onto the wire. My question relates to the rebuilding process. I have some structs that I am using to hold information about various headers in my programs. Detailed here
struct my_ip
{
u_int8_t ip_vhl; /* header length, version */
#define IP_V(ip) (((ip)->ip_vhl & 0xf0) >> 4)
#define IP_HL(ip) ((ip)->ip_vhl & 0x0f)
u_int8_t ip_tos; /* type of service */
u_int16_t ip_len; /* total length */
u_int16_t ip_id; /* identification */
u_int16_t ip_off; /* fragment offset field */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_int8_t ip_ttl; /* time to live */
u_int8_t ip_p; /* protocol */
u_int16_t ip_sum; /* checksum */
struct in_addr ip_src,ip_dst; /* source and dest address */
};
/* UDP header */
struct sniff_udp
{
u_short uh_sport; /* source port */
u_short uh_dport; /* destination port */
u_short uh_ulen; /* udp length */
u_short uh_sum; /* udp checksum */
};
#define SIZE_UDP 8 /* length of UDP header */
#define SIZE_ETHERNET 14
As well as a few other structs from the pcap library(like ether_header). I cast the u_char* to these structs like so
struct my_ip* ip = (struct my_ip*)(packet + sizeof(struct ether_header));
struct ether_header* eptr = (struct ether_header *) packet;
Where packet is a u_char holding the entirety of the packet
My question is, once I have modified data within these structures how do I cast all of my stucts back into a single u_char string? I am trying to cast each struct to fill a different segment of string in the same way a packet is structured
This is the code I have so far.
void buildPacket(sniff_udp *udp, ether_header *ethh, my_ip *ip, u_char *payload, u_char *buffer)
{
memset(buffer,0, (sizeof(udp)+sizeof(ethh)+sizeof(ip)+sizeof(payload)));
buffer=(u_char *)(ethh); // adds layer 2 header
(buffer+SIZE_ETHERNET)= (u_char *)ip; // adds layer 3 header
(buffer+SIZE_ETHERNET+sizeof(ip))=(u_char *) udp; // adds protocol header
(buffer+SIZE_ETHERNET+sizeof(ip)+SIZE_UDP)=(u_char *)payload; // adds payload
}
This isn't the correct way to do it from what I've gathered. How can I cast multiple structs to the same string?

Answer 1

Something like
(buffer+SIZE_ETHERNET)= (u_char *)ip; // adds layer 3 header
isn't valid because the lefthand operator of = won't be a (modifable) lvalue.
You can use memcpy() to copy contents of memory. The correct code should be like this:
void buildPacket(sniff_udp *udp, ether_header *ethh, my_ip *ip, u_char *payload, u_char *buffer)
{
memset(buffer,0, (sizeof(udp)+sizeof(ethh)+sizeof(ip)+sizeof(payload)));
memcpy(buffer, ethh, SIZE_ETHERNET); // adds layer 2 header
memcpy(buffer+SIZE_ETHERNET, ip, sizeof(ip)); // adds layer 3 header
memcpy(buffer+SIZE_ETHERNET+sizeof(ip), udp, SIZE_UDP); // adds protocol header
memcpy(buffer+SIZE_ETHERNET+sizeof(ip)+SIZE_UDP, payload, sizeof(payload)); // adds payload
}
This code doesn't seem correct because sizeof(udp), sizeof(ethh), sizeof(ip) and sizeof(payload) will return the size of pointers, not what is pointed, and I don't think it is what you want. Use correct size instead of them.