How to send modified IPv6 packet through RAW socket?

Question

I'm trying to send a custom IPv6 header through a RAW socket in C Linux.
I already succeded in IPv4 using the IP_HDRINCL socket option, however, there's not an equivalent for IPv6.
I found a workaround here suggesting to use socket(AF_INET6, SOCK_RAW, IPPROTO_RAW) to have the same effect as enabling the IP_HDRINCL socket option.

The socket is created successfully and I'm not getting any error until I use the sendto function with my modified header.

I setup the socket like this:

static int socketFd = 0;
static struct sockaddr_in6 remote;

int main()
{
    socketFd = socket (PF_INET6, SOCK_RAW, IPPROTO_RAW);

    if (socketFd < 0)
    {
        printf ("An error ocurred while creating the socket.\n");
        exit (2);
    }

    remote.sin6_family = AF_INET6;
    remote.sin6_port = htons (25000);

    if (inet_pton (AF_INET6, "fd00:c0de::70d6:4ab9:115d:8cda", &(remote.sin6_addr)) != 1)
    {
        close (socketFd);
        printf ("Unable to parse IPv6 address.\n");
        exit (2);
    }

 /*More code */
  ...

  return 0;
}

And then, I have this callback function that should send my custom IPv6 packets but sendto fails returning EINVAL.

static void sendPacket ()
{
    char buffer[BUFSIZ];
    const size_t len = sizeof(struct ip6_hdr) + sizeof(struct UDP_hdr);
    struct ip6_hdr *ip6 = (struct ip6_hdr*) (buffer);
    struct UDP_hdr *udp = (struct UDP_hdr *) (buffer + sizeof(struct ip6_hdr));

    memset (buffer, 0, BUFSIZ);

    ip6->ip6_ctlun.ip6_un2_vfc = 0x60;
    ip6->ip6_dst = remote.sin6_addr;
    ip6->ip6_flow = 60;
    ip6->ip6_hops = 64;
    ip6->ip6_nxt = 17;
    ip6->ip6_plen = sizeof(struct UDP_hdr);

    if (inet_pton (AF_INET6, "fd00:c0de::62a4:4cff:1234:5678", &(ip6->ip6_src)) != 1)
    {
        printf ("Error while parsing spoofed IPv6 address.\n");
        return;
    }

    // Fabricate the UDP header. Source port number, redundant
    udp->uh_sport = htons (21000);
    // Destination port number
    udp->uh_dport = remote.sin6_port;
    udp->uh_ulen = htons (sizeof(struct UDP_hdr));

    if (sendto (socketFd, buffer, len, 0, (struct sockaddr *) &remote, sizeof(remote)) < 0)
    {
        perror ("sendto");
        printf ("Error while sending packet.\n");
    }
}

I've debugged the program and all the values in the ip6_hdr struct seem to be correct and also the ones in the struct UDP_hdr. Am I missing something?

score 4 · Answer 1 · answered Dec 12 '17 at 19:08

To whom it may be helpful, I got a similar code to work, but opening the socket instead with

socket(test->state.addr_family, SOCK_RAW, IPPROTO_UDP);

Setting the options with

int val = 1; ret = setsockopt(test->state.sockfd, IPPROTO_IPV6, IPV6_HDRINCL, &val, sizeof (val));

and setting the port to zero with remote.sin6_port = 0;, as explained in here to avoid the EINVAL error on the sendto call.

score 3 · Accepted Answer · answered Jul 16 '15 at 15:55

Ok, so I found a solution. To anyone struggling with this, the way I solved it was using the Pcap library (followed this example). However for those that must use AF_PACKETS, I found a very complete example which uses RAW sockets to accomplish this in this link.

How to send modified IPv6 packet through RAW socket?

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