Tcp socket programming in linux

How to Code Raw Sockets in C on Linux

Raw sockets can be used to construct a packet manually inside an application. In normal sockets when any data is send over the network, the kernel of the operating system adds some headers to it like IP header and TCP header. So an application only needs to take care of what data it is sending and what reply it is expecting.

But there are other cases when an application needs to set its own headers. Raw sockets are used in security related applications like nmap , packets sniffer etc.

In this article we are going to program raw sockets on linux using native sockets.

Windows for example does not support raw socket programming directly. To program raw sockets on windows a packet crafting library like winpcap has to be used.

In this article we are going to do some raw socket programming by constructing a raw TCP packet and sending it over the network. Before programming raw sockets, it is recommended that you learn about the basics of socket programming in c.

Raw TCP packets

A TCP packet is constructed like this

Packet = IP Header + TCP Header + Data

The plus means to attach the binary data side by side. So when making a raw tcp packet we need to know how to construct the headers properly. The structures of all headers are established standards which are described in RFCs.

IP Header Structure

The structure of IP Header as given by RFC 791 :

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The «Source Address» field stores the ip address of the system sending the packet and the «Destination Address» contains the ip address of the destination system. Ip addresses are stored in long number format. The «Protocol» field stores a number that indicates the protocol, which is TCP in this case.

Structure of TCP header

The structure of a TCP header as given by RFC 793

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Port | Destination Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Acknowledgment Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data | |U|A|P|R|S|F| | | Offset| Reserved |R|C|S|S|Y|I| Window | | | |G|K|H|T|N|N| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | Urgent Pointer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

So we need to construct the headers according to the formats specified above.

Raw tcp sockets

Create a raw socket like this

int s = socket (AF_INET, SOCK_RAW, IPPROTO_TCP);

The above function call creates a raw socket of protocol TCP. This means that we have to provide the TCP header along with the data. The kernel or the network stack of Linux shall provide the IP header.

If we want to provide the IP header as well then there are 2 ways of doing this

1. Use protocol IPPROTO_RAW — This will allow to specify the IP header and everything that is contained in the packet.

int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);

2. Set the IP_HDRINCL socket option to 1 — This is same as the above. Just another way of doing.

int s = socket (AF_INET, SOCK_RAW, IPPROTO_TCP); int one = 1; const int *val = &one; if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one))

When using the IP_HDRINCL the protocol used in the socket function is effectively of no use.

In this example we are creating raw sockets where we specify the Ip header and TCP header. The packet that moves out of the machine actually has 1 more header attached to it called the Ethernet header. So the actual packet structure is somewhat like this. Packet = Ethernet header + Ip header + TCP header + Data Take a look at the packets sniffed by wireshark to understand this better. It is important to note here that the Ethernet header is provided by the OS kernel and we do not have to construct it. However it is possible to make such raw packets where we can even specify the ethernet header, but we shall look into those in a separate article.

Below is an example code which constructs a raw TCP packet with some data

Final Code

/* Raw TCP packets */ #include //for printf #include //memset #include //for socket ofcourse #include //for exit(0); #include //For errno - the error number #include //Provides declarations for tcp header #include //Provides declarations for ip header #include // inet_addr #include // sleep() /* 96 bit (12 bytes) pseudo header needed for tcp header checksum calculation */ struct pseudo_header < u_int32_t source_address; u_int32_t dest_address; u_int8_t placeholder; u_int8_t protocol; u_int16_t tcp_length; >; /* Generic checksum calculation function */ unsigned short csum(unsigned short *ptr,int nbytes) < register long sum; unsigned short oddbyte; register short answer; sum=0; while(nbytes>1) < sum+=*ptr++; nbytes-=2; >if(nbytes==1) < oddbyte=0; *((u_char*)&oddbyte)=*(u_char*)ptr; sum+=oddbyte; >sum = (sum>>16)+(sum & 0xffff); sum = sum + (sum>>16); answer=(short)~sum; return(answer); > int main (void) < //Create a raw socket int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP); if(s == -1) < //socket creation failed, may be because of non-root privileges perror("Failed to create socket"); exit(1); >//Datagram to represent the packet char datagram[4096] , source_ip[32] , *data , *pseudogram; //zero out the packet buffer memset (datagram, 0, 4096); //IP header struct iphdr *iph = (struct iphdr *) datagram; //TCP header struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip)); struct sockaddr_in sin; struct pseudo_header psh; //Data part data = datagram + sizeof(struct iphdr) + sizeof(struct tcphdr); strcpy(data , "ABCDEFGHIJKLMNOPQRSTUVWXYZ"); //some address resolution strcpy(source_ip , "192.168.1.2"); sin.sin_family = AF_INET; sin.sin_port = htons(80); sin.sin_addr.s_addr = inet_addr ("1.2.3.4"); //Fill in the IP Header iph->ihl = 5; iph->version = 4; iph->tos = 0; iph->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr) + strlen(data); iph->id = htonl (54321); //Id of this packet iph->frag_off = 0; iph->ttl = 255; iph->protocol = IPPROTO_TCP; iph->check = 0; //Set to 0 before calculating checksum iph->saddr = inet_addr ( source_ip ); //Spoof the source ip address iph->daddr = sin.sin_addr.s_addr; //Ip checksum iph->check = csum ((unsigned short *) datagram, iph->tot_len); //TCP Header tcph->source = htons (1234); tcph->dest = htons (80); tcph->seq = 0; tcph->ack_seq = 0; tcph->doff = 5; //tcp header size tcph->fin=0; tcph->syn=1; tcph->rst=0; tcph->psh=0; tcph->ack=0; tcph->urg=0; tcph->window = htons (5840); /* maximum allowed window size */ tcph->check = 0; //leave checksum 0 now, filled later by pseudo header tcph->urg_ptr = 0; //Now the TCP checksum psh.source_address = inet_addr( source_ip ); psh.dest_address = sin.sin_addr.s_addr; psh.placeholder = 0; psh.protocol = IPPROTO_TCP; psh.tcp_length = htons(sizeof(struct tcphdr) + strlen(data) ); int psize = sizeof(struct pseudo_header) + sizeof(struct tcphdr) + strlen(data); pseudogram = malloc(psize); memcpy(pseudogram , (char*) &psh , sizeof (struct pseudo_header)); memcpy(pseudogram + sizeof(struct pseudo_header) , tcph , sizeof(struct tcphdr) + strlen(data)); tcph->check = csum( (unsigned short*) pseudogram , psize); //IP_HDRINCL to tell the kernel that headers are included in the packet int one = 1; const int *val = &one; if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0) < perror("Error setting IP_HDRINCL"); exit(0); >//loop if you want to flood :) while (1) < //Send the packet if (sendto (s, datagram, iph->tot_len , 0, (struct sockaddr *) &sin, sizeof (sin)) < 0) < perror("sendto failed"); >//Data send successfully else < printf ("Packet Send. Length : %d \n" , iph->tot_len); > // sleep for 1 seconds sleep(1); > return 0; > //Complete

Compile and Run

Compile by program by doing a gcc raw_socket.c at the terminal. Remember to run the program with root privileges. Raw sockets require root privileges.

$ gcc raw_socket.c -o raw_socket $ sudo ./raw_socket

Note the while loop in the above program. It has been put for testing purpose and should be removed if you dont intend to flood the target.

Use a packet sniffer like wireshark to check the output and verify that the packets have actually been generated and send over the network. Also note that if some kind of firewall like firestarter is running then it might block raw packets.

Resources

A Tech Enthusiast, Blogger, Linux Fan and a Software Developer. Writes about Computer hardware, Linux and Open Source software and coding in Python, Php and Javascript. He can be reached at [email protected] .

39 Comments

1. Vladyslav February 28, 2023 at 4:04 pm Hi, thank you for the great example!
   One question. You use direct mapping of the structures into memory. How do you know that there will be no problems with alignment? Did you just check it with “sizeof” beforehand?

1. Silver Moon Post author July 26, 2020 at 8:08 pm if you need to receive the data then it is not recommended to use raw sockets.
   Just use normal tcp or udp sockets to send and receive data.

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