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Linux Raw Socket Programming - What Lies Beneath a Socket?

Linux Raw Socket Programming - What Lies Beneath a Socket?

Calculate IP Checksum
Now let's analyze the IP header values. Check out Figure 2 for IP Header structure. The first entry in the header is the IP version, which is 4 (IPV4). The next entry is the header length, which is 5 for most packets. The next entry is "Type of Service", which is 0. Note that TOS occupies 8 bits of header. So lets calculate our first 16 bit chunk. We know 4 is 100 and 5 is 101 in binary. So our first chunk is:
0100 0101 0000 0000, which is 4500. Next, 16 bits occupy the total length of the packet. Lets assume the length as 40 bytes (This includes the TCP Header too. The TCP Header's minimum size is 20 bytes). So the second chunk is :
0000 0000 0010 1000, which is 0028. If our Identification field value is 1, then the third chunk becomes:
0000 0000 0000 0001, which is 0001
Fragmentation Offset and flags : 0
which gives the fourth chunk as 0000. Our 8-bit Time to Live value is 255, which is FF. The protocol number is 6 in our case (TCP). So the fifth chunk is FF06. Assume header checksum as 0 so the sixth chunk is 0000. Next field is the source address. So with a source address of, we get the seventh and eighth chunk as 221A and 110A. Next is the destination address, which is say We get 0102 and 1304 chunks. By adding up all the chunks, we get 17B59. Now remove the first digit and add it with the rest. i.e 1+7B59 = 7B5A. Subtract the number from FFFF, which gives 84A5, which is our IP Checksum value.

Session Hijacking
Session Hijacking can be done effortlessly with a few lines of code using raw socket. Before proceeding to hijack an open TCP Session, we need to understand the TCP connection termination process. Unlike TCP connection initialization, which is a three-way process, connection termination takes place with the exchange of four-way packets. The client who needs to terminate the connection sends a FIN segment to the server (TCP Packet with the FIN flag set) indicating that it has finished sending the data. The server, upon receiving the FIN segment, does not terminate the connection but enters into a "passive close" (CLOSE_WAIT) state and sends an ACK for the FIN back to the client with the sequence number incremented by one. Now the server enters into LAST_ACK state. When the client gets the last ACK from the server, it enters into a TIME_WAIT state, and sends an ACK back to the server with the sequence number incremented by one. When the server gets the ACK from the client, it closes the connection.

As you can see, the connection termination process of TCP is complex, since data integrity is ensured with every packet transferred. Before trying to hijack a TCP connection, we need to understand the TIME_WAIT state. Why should any client be made to wait even after receiving connection termination confirmation from the server? Consider this instance (termed as "incarnation") with two systems, A and B, communicating. After terminating the connection, if these two clients want to communicate again, they should not be allowed to establish a connection before a certain period. This is because stray packets (if there are any) transferred during the initial session should not confuse the second session initialization. So TCP has set the TIME_WAIT period to be twice the MSL (Maximum Segment Lifetime) for the packet. We can spoof our TCP packets and can try to reset an established TCP connection with the following steps:

  1. Sniff a TCP Connection. In Linux we need to set our Network Interface to Promiscuous mode. This can be done by specifying the Socket Type in SocketOpt structure as 'PACKET_MR_PROMISC' e-g sockopt.mr_type= PACKET_MR_PROMISC
  2. Check if the packet has ACK flag set. If set, the Acknowledgment number is recorded (which will be our next packet sequence number) along with the source IP.
  3. Establish a raw socket with spoofed IP and send out the FIN packet to the client with the recorded sequence number. Make sure that you have also set your ACK flag.

Session Hijacking can also be done with the RST flag.

An Example
I have built two tools for Linux, one called choke, which SYN floods a target system with randomly spoofed IP address. The other one is a TCP connection terminator, which sets your Ethernet card in Promiscuous mode and sniffs on the local subnet, analyze the sniffed packet, retrieve the source IP, destination IP and ports if the protocol being used is TCP and reset the connection by interchanging the sequence and acknowledgment number. With old kernels writing code to read directly from the Datalink layer was shunned mostly because of the inherent complexity involved.

If you are familiar with an Unix environment, you can use the BSD packet Filter or Data Link provider Interface (DLPI). With the new releases of Linux kernel, you can use an elegant interface called SOCK_PACKET. Explaining BSD Packet Filter and DLPI is beyond the scope of this article so lets deal with Linux specific SOCK_PACKET Interface. Similar to the creation of raw socket, creating SOCK_PACKET socket requires administrative privilege too.
You can create a socket like this:

sd=socket(AF_INET, SOCK_PACKET, htons(ETH_P_ALL));

which will receive all the frames from the datalink layer. Remember you are sniffing a subnet and with high traffic network, the socket will act really strange. You can instead sniff only IP packets using ETH_P_IP frame option. Other options include ETH_P_ARP and ETH_P_IPV6. (Check out linux/if_ether.h). If your kernel and network device supports Promiscuous mode, you can do an ioctl of SIOCGIFLAGS by fetching the flag and setting IFF_PROMISC flag and again storing the flags using SIOCGIFLAGS. Check this from termin.c (Available on site) code:

fd = socket(AF_INET, SOCK_PACKET, htons(0x3)))
ifinfo.ifr_ifru.ifru_flags |= IFF_PROMISC;

If you don't know what ioctl does, check out the man page. It lets you manipulate the device parameters of files. And for instructing your network adapter to pass on all the packets it gets to your application, you should set your socket option to SOL_PACKET and bind it to the socket using PACKET_ADD_MEMBERSHIP.

sock = socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP));
(void *)&sockopt, sizeof(sockopt));

The major disadvantage of using SOCK_PACKET is, it does not support kernel filtering and buffering; so multiple frames cannot be passed to our code with a single read method. And also ETH_P_IP returns packets even from a loopback device, so the real challenge lies in discerning and discarding useless frames.

One problem I faced during connection termination was that most of the packets I sniffed came from my loopback device. So every time I try to hijack a connection I had to check if the source and destination IP are same. Then I wait for an ACK flag set packet. This is to ensure that we mess up with only established connections. Once I find an ACK packet I extract the acknowledgment number and use it as the sequence number in my own packet with IP addresses swapped and will send a FIN packet to the server with the acknowledgment number as the summation of original packet sequence number and the data length.


Since the server gets the correct sequence number, which it is expecting, it will disconnect the connection. Boom!

Broadcast Amplifiers
IP Spoofing has always helped hackers in devising new DDOS attacks. One such technique is to send a large amount of ICMP echo request (Ping) traffic to all known IP broadcast addresses with the spoofed source address of the victim. The router delivering traffic to the broadcast address enables in broadcasting the request, which results in most of the hosts on the network taking the ICMP echo request and replying to it. First it kills the target and next it increases the network traffic. Unfortunately C lets you code one such spoofer for broadcast amplification with ease. Many hacker tools also perform similar function like Smurf.
After creating a raw socket we need to set our socket option to enable us to send broadcast datagrams with spoofed IP in the network. This can be done effortlessly as follows:

setsockopt(sd, SOL_SOCKET, SO_BROADCAST, (char *) &bcast, sizeof(bcast));

Remember SO_BROADCAST applies only for Datagram packets. Creating the header is just the same as we have done in the previous example. Let us analyze the ICMP related headers in our packet.

struct icmp *icmp;
icmp = (struct icmp *) (packet + sizeof(struct ip));
icmp->icmp_type = 8;
icmp->icmp_code = 0;
icmp->icmp_cksum = htons(~(ICMP_ECHO << 8));

Note that we are setting our ICMP packet type as 8, which represents Echo request. (Check out RFC 1700 for other types). The ICMP code stands for the nature of error in the communication. Let us make it 0, which stands for 'Net Unreachable' (Who cares for error messages). After construction the echo reply, just send it to any valid broadcast address, or your subnet broadcast address (Say, and check out the burst in network traffic.

I am not a hacker. Nor am I a C programming expert. I have just pointed out few flaws in our networking and communication architecture, which the hackers have exploited all these years. Few years back Yahoo was down for 3 hours-a victim of Spoofing with broadcast amplifiers. With ARP Spoofing already getting popular and hackers exploring possibilities of exploits for undetectable DDOS attack, we can expect more network chaos in the future. If you still swear on TCP/IP as the best communication protocol, tell me, why?

More Stories By Frank Jennings

Frank Jennings works in the Communication Designs Group of Pramati Technologies

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Most Recent Comments
falgun 02/08/05 03:47:41 AM EST

nice tutorial keep up the good work!

srinivas 12/10/04 02:39:33 AM EST

I am happy with this tutorial, this is one of the most valuable tutorials never found anywhere

keep it up

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