ip4数据包组装为ip6-libnet

Question

I have written a C-program that creates a TCP/IPv4-packet from the link-layer upwards. The goal of this program was to receive the single packet with an open socket, either raw or just a stream socket. The packet consists of an ethernet header, a tcp header and ipv4:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <libnet.h>
#include <time.h>

uint8_t *randmac();    //creates my random source-mac, shady I know but I got 
                       //tired of typing in the same input over and over
void gateway();        //gets my standart gateway i.e. the router (as far as 
                       //I understand the ethernet layer is for the router 
                       //and tcp and ip for the rest of the net.)



int main() {
uint32_t src, dst, seq, ack;
char payload[1024], destip[16];
uint16_t sp = 2001, dp = 2000;
libnet_ptag_t tcp;
libnet_ptag_t ip4;
libnet_ptag_t eth;
char errbuf[LIBNET_ERRBUF_SIZE];
int i, bytes;
uint8_t *dstmc;     //destination mac address (router)
uint8_t *smc;       //source mac address
int  len;
libnet_t * l;

l = libnet_init(LIBNET_LINK, NULL, errbuf);
if(l == NULL)

//getting the destination ip-address, works all right...

//payload is fine too

smc = malloc(6);
gatemac = malloc(6);

libnet_seed_prand(l);
src = libnet_get_prand(LIBNET_PRu32);
seq = 0;
ack = libnet_get_prand(LIBNET_PRu32);
dst = libnet_name2addr4(l, destip, LIBNET_DONT_RESOLVE);
smc = randmac();
gateway();

//getting the rest of the required addresses etc, works as well...




tcp = libnet_build_tcp(sp, dp, seq, ack, TH_SYN, 0, 0, 0, LIBNET_TCP_H + 
sizeof(payload),(uint8_t *) payload, sizeof(payload), l, 0);
if(tcp == -1)
printf("(1)unable because: %s\n", libnet_geterror(l));
perror("libnet_build_tcp()");

ip4 = libnet_build_ipv4(LIBNET_TCP_H + LIBNET_IPV4_H + sizeof(payload), 
IPPROTO_TCP, 1, 1024, 60, 0, 0, src, dst, NULL, 0, l, 0);   //or (uint8_t) 
if(ip4 == -1)
printf("(2)unable because: %s\n", libnet_geterror(l));
perror("libnet_build_ipv4()");

eth = libnet_build_ethernet(gatemac, smc, ETHERTYPE_IP, NULL, 0, l, 0);
if(eth == -1)
printf("(3) unable because: %s\n", libnet_geterror(l));
perror("libnet_build_ethernet()");


bytes = libnet_write(l);

//error handling

}

The program itself works all right, no errors, the packets are written and so on. But the funny thing is: I can't receive them with any kind of raw or ipv4 socket. So I turned on tcpdump, to see if the packets are even send at all and this came out:

 13:43:24.003324 IP 74.253.145.81 > 192.168.88.130: hopopt
 13:43:27.007860 IP 74.253.145.81 > 192.168.88.130: hopopt

 //Random source address, I also sent multiple packets to eliminate the 
 //chances of missing them.
 //There is also no doubt that these are truly my programmed packets due to 
 //their number and random addresses.

I did some researching and found out, that "hopopt" stands for hop-by-hop extension-header used by ipv6. Why does this happen when I clearly used the libnet_build_ipv4() function. And is there still a way to either properly receive these single packets or just get them to become normal ipv4?

Answer 1

It looks, like you do not use the correct order of parameters with libnet_build_ipv4 :

ip4 = libnet_build_ipv4(LIBNET_TCP_H + LIBNET_IPV4_H + sizeof(payload), 

IPPROTO_TCP, 1, 1024, 60, 0, 0, src, dst, NULL, 0, l, 0);

The correct order is:

libnet_ptag_t libnet_build_ipv4 (u_int16_t len,
u_int8_t tos, u_int16_t id, u_int16_t frag,
u_int8_t ttl, u_int8_t prot, u_int16_t sum,
u_int32_t src, u_int32_t dst, u_int8_t * payload,
u_int32_t payload_s, libnet_t * l, libnet_ptag_t ptag);

So, your sixth parameter (prot) is 0 , which exactly corresponds to the hop-by-hop-option (hopopt) you observe with tcpdump.

You probably want something like:

ip4 = libnet_build_ipv4(LIBNET_TCP_H + LIBNET_IPV4_H + sizeof(payload), //len
                        0,           // tos
                        1234,        // some id
                        0,           // No fragment
                        0x40,        // Standard TTL (64)
                        IPPROTO_TCP, // Next protocol goes here
                        0,           // Checksum, auto-filled?
                        src,         // IP source address
                        dst,         // IP destination address
                        NULL,        // payload
                        0,           // payload length
                        l,           // libnet handle
                        0            // ptag
);

If you fix the order of the parameters, it should work as expected! See also a libnet Tutorial here