Raw Socket Programming in Python on Linux – Code Examples
Raw sockets allow a program or application to provide custom headers for the specific protocol(tcp ip) which are otherwise provided by the kernel/os network stack.
In more simple terms its for adding custom headers instead of headers provided by the underlying operating system.
Raw socket support is available natively in the socket api in linux. This is different from windows where it is absent (it became available in windows 2000/xp/xp sp1 but was removed later).
Although raw sockets don’t find much use in common networking applications, they are used widely in applications related to network security.
TCP IP Headers
In this article we are going to create raw tcp/ip packets. For this we need to know how to make proper ip header and tcp headers. A packet = Ip header + Tcp header + data.
So lets have a look at the structures.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Every single number is 1 bit. So for example the Version field is 4 bit. The header must be constructed exactly like shown.
Next comes the TCP header. According to 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Create a raw socket
Raw socket can be created in python like this
#create a raw socket try: s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW) except socket.error , msg: print 'Socket could not be created. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit()
To create raw socket, the program must have root privileges on the system. For example on ubuntu run the program with sudo.
The above example creates a raw socket of type IPPROTO_RAW which is a raw IP packet. Means that we provide everything including the ip header.
Once the socket is created, next thing is to create and construct the packet that is to be send out. C like structures are not available in python, therefore the functions called pack and unpack have to be used to create the packet in the structure specified above.
IP Header
So first, lets make the IP Header:
source_ip = '192.168.1.101' dest_ip = '192.168.1.1' # or socket.gethostbyname('www.google.com') # ip header fields ip_ihl = 5 ip_ver = 4 ip_tos = 0 ip_tot_len = 0 # kernel will fill the correct total length ip_id = 54321 #Id of this packet ip_frag_off = 0 ip_ttl = 255 ip_proto = socket.IPPROTO_TCP ip_check = 0 # kernel will fill the correct checksum ip_saddr = socket.inet_aton ( source_ip ) #Spoof the source ip address if you want to ip_daddr = socket.inet_aton ( dest_ip ) ip_ihl_ver = (version
Now ip_header has the data for the ip header. Now the usage of pack function, it packs some values has bytes, some as 16bit fields and some as 32 bit fields.
Tcp Header
Next comes the tcp header:
# tcp header fields tcp_source = 1234 # source port tcp_dest = 80 # destination port tcp_seq = 454 tcp_ack_seq = 0 tcp_doff = 5 #4 bit field, size of tcp header, 5 * 4 = 20 bytes #tcp flags tcp_fin = 0 tcp_syn = 1 tcp_rst = 0 tcp_psh = 0 tcp_ack = 0 tcp_urg = 0 tcp_window = socket.htons (5840) # maximum allowed window size tcp_check = 0 tcp_urg_ptr = 0 tcp_offset_res = (tcp_doff
The construction of the tcp header is similar to the ip header. The tcp header has a field called checksum which needs to be filled in correctly. A pseudo header is constructed to compute the checksum.
The checksum is calculated over the tcp header along with the data. Checksum is necessary to detect errors in the transmission on the receiver side.
Code
Here is the full code to send a raw packet
''' Raw sockets on Linux ''' # some imports import socket, sys, time 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]) + (ord(msg[i+1]) >16) + (s & 0xffff); s = s + (s >> 16); #complement and mask to 4 byte short s = ~s & 0xffff return s # the main function def main(): #create a raw socket try: s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW) except socket.error , msg: print 'Socket could not be created. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() # tell kernel not to put in headers, since we are providing it, when using IPPROTO_RAW this is not necessary # s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1) # now start constructing the packet packet = ''; source_ip = '1.2.3.4' dest_ip = '192.168.1.1' # or socket.gethostbyname('www.google.com') # ip header fields ip_ihl = 5 ip_ver = 4 ip_tos = 0 ip_tot_len = 0 # kernel will fill the correct total length ip_id = 54321 #Id of this packet ip_frag_off = 0 ip_ttl = 255 ip_proto = socket.IPPROTO_TCP ip_check = 0 # kernel will fill the correct checksum ip_saddr = socket.inet_aton ( source_ip ) #Spoof the source ip address if you want to ip_daddr = socket.inet_aton ( dest_ip ) ip_ihl_ver = (ip_ver
Output
Run the above program from the command line using python. Make sure to run with root privileges using sudo, since raw sockets need root privileges.
$ sudo python raw_socket.py sending packet. send sending packet. send sending packet. send all packets send $
Run the above program from the terminal and check the network traffic using a packet sniffer like wireshark. It should show the packet.
You could also use tcpdump in a separate terminal to check that the packets were indeed sent. We need to provide the filter expression to tcpdump to check for the exact packet that we generated from our python program.
Here is a quick example of how it would look:
$ sudo tcpdump -n -i enp1s0 'tcp and src 1.2.3.4' tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on enp1s0, link-type EN10MB (Ethernet), capture size 262144 bytes 16:50:10.228944 IP 1.2.3.4.1234 > 192.168.1.1.80: Flags [S], seq 454:472, win 53270, length 18: HTTP 16:50:11.230101 IP 1.2.3.4.1234 > 192.168.1.1.80: Flags [S], seq 454:472, win 53270, length 18: HTTP 16:50:12.231100 IP 1.2.3.4.1234 > 192.168.1.1.80: Flags [S], seq 454:472, win 53270, length 18: HTTP
In the above tcpdump output note that the source ip is 1.2.3.4 which we used in our program. To learn more about tcpdump check this post:
Tcpdump Tutorial - How to Sniff and Analyse Packets from Commandline
Conclusion
Raw sockets find application in the field of network security. The above example can be used to code a tcp syn flood program.
Syn flood programs are used in Dos attacks. Raw sockets are also used to code packet sniffers, port scanners etc.
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] .
27 Comments
- me September 19, 2019 at 2:02 am On reading the comments, it is clear that you don’t understand your code. You copied bits from different parts. And the result is incorrect code that confuses everybody.
- Visitor May 4, 2019 at 1:01 pm The reason for everyone’s difficulty is due to the fact that the socket being used is operating at the wrong TCP/IP layer. TCP/IP. Application (Web/HTTP, FTP, SSH, Multiplayer games, etc…)
v
Transport (TCP, UDP, …)
v
Network (IP, IPX, …)
v
Link (Ethernet, 802.11 partially) The hundreds of network protocols which make up our interconnected world can to all some degree be placed with in this stack, some of the most important are shown above. It is called a stack because each layer provides services to the one above and uses services provided by those below it. The code on this page attempts to send data from network layer up (hence the need for creating an IP packet). However, the socket is configured as AF_INET which already provides the necessary functions for creating and sending IP packets. The result is that the code puts an IP packet with in another IP packet. Wireshark will not recognize the inner IP packet. The solution here is to either just send the TCP data or to change the socket’s config. On Linux, you should use… s = socket(AF_PACKET, SOCK_RAW, IPPROTO_RAW) Apart from this, the code is very helpful, thanks!!
- me September 18, 2019 at 7:07 pm Thank you very much for your knowledgeable input.
What we want is to spoof the IP, so, we do need to craft the IP packet. Therefore, I think the only option left is, as you say, to use AF_PACKET instead of INET A thing that I would have liked to see in action is useful DATA sent instead of writing “Hello how are you” to a web page. I suppose http get code would have been much more interesting. Anyone can do that and see what reply we get?
> def checksum(msg):
> vals=map(ord,msg)
> if ((len(vals)>>1) < s=sum([vals[i]+(vals[i+1]< while s >0xffff: s=(s>>16)+(s & 0xffff)
> return s ^ 0xffff # ones complement