Amit Kucheria wrote:
> > > I have set some IP options for my sockets (IP_HDRINCL & IP_ROUTER_ALERT)
> > > using setsockopt(), but when I do a getsockopt(), nothing is reflected.
> > > I know I am forgetting something very elementary here, but cant put my
> > > finger on it. Any pointers are welcome.
> >
> > IP_HDRINCL and IP_ROUTER_ALERT aren't "options", in the sense of
> > IP_OPTIONS.
>
> So what you mean is that they are not _IP_ options; they are
> more of _raw_ socket options?
Yes.
> > Calling getsockopt() on any of the above options should return
> > whatever value was set by a preceding setsockopt() call. However, each
> > of these options is independent; setting one of them won't change any
> > of the others.
> >
> > Also, if you set both IP_HDRINCL and IP_OPTIONS, IP_OPTIONS is
> > ignored.
>
> Then I guess the right question to ask here is:
>
> How can I create an IP packet using raw sockets with the IP_ROUTER_ALERT
> option set?
>
> Once I allocate a buffer and fill out the IP and TCP headers, HOW then
> should i add the IP_ROUTER_ALERT option and change the length of the IP
> header (ip->ihl and ip->tot_len) corresponding to this option.
Just append the option (code byte, length byte, padding) to the IP
header, updating the length fields accordingly.
> ------ sample code of what i am doing currently -----------
+ u8 *ra;
> iph = (struct iphdr *)buffer;
+ ra = (u8 *) buffer + sizeof(struct iphdr);
> tcph = (struct tcphdr *) buffer + sizeof(struct iphdr);
[Note: This is incorrect; tcph would be offset by 20 TCP headers (400
bytes), not 20 bytes. You would need to enclose the addition in
parentheses (and "buffer" needs to be "char *" or similar).]
tcph = (struct tcphdr *) (ra + 4);
> printf("Set pointer to IP and TCP headers!\n");
>
> memset(buffer, 0, PACKET_SIZE); /* zero out the buffer */
>
> /* fill in the ip header */
- iph->ihl = 5;
+ iph->ihl = 6;
> iph->version = 4;
> iph->tos = 0;
- iph->tot_len = sizeof(struct iphdr)
- + sizeof(struct tcphdr); /* no payload */
+ iph->tot_len = sizeof(struct iphdr)
+ + 4
+ + sizeof(struct tcphdr); /* no payload */
> iph->id = htonl(23456); /* some value..not imp */
> iph->frag_off = 0;
> iph->ttl = 255;
> iph->protocol = 48; /* protocol: 6 = TCP, 48 = new */
> iph->check = 0; /* compute checksum later */
> iph->saddr = inet_addr("1.2.3.4"); /* some spoofed src address */
> iph->daddr = dest.sin_addr.s_addr;
> printf("IP header filled\n");
+ ra[0] = IPOPT_RA; /* option code */
+ ra[1] = 4; /* option length */
+ ra[2] = 0; /* padding */
+ ra[3] = 0; /* padding */
> /* fill in the tcp header */
[snip]
--
Glynn Clements <glynn.clements@virgin.net>
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