How to code Packet Sniffer in C with Sockets on Linux

Packet sniffer

Packet sniffers are programs that intercept the network traffic flowing in and out of a system through network interfaces.

So if you are browsing the internet then traffic is flowing and a packet sniffer would be able to catch it in the form of packets and display them for whatever reasons required.

Packet sniffers are used for various needs like analysing protocols, monitoring network, and assessing the security of a network.

Wireshark for example is the most popular packet sniffer out there and is available for all platforms. Its gui based and very easy to use.

In this post we are going to talk about how to code and make our own packet sniffer in C and on the linux platform. By Linux it means that the code sample shown here would work only on linux and not windows.

Packet sniffers can be coded by either using sockets api provided by the kernel, or by using some packet capture library like libpcap. In this tutorial we shall be covering the first method, that is by using sockets.

Basic Sniffer using sockets

To code a very simply sniffer in C the steps would be

1. Create a raw socket.
2. Put it in a recvfrom loop and receive data on it.

A raw socket when put in recvfrom loop receives all incoming packets. This is because it is not bound to a particular address or port.

sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_TCP);
while(1)
{
data_size = recvfrom(sock_raw , buffer , 65536 , 0 , &saddr , &saddr_size);
}

Thats all. The buffer will hold the data sniffed or picked up. The sniffing part is actually complete over here. The next task is to actually read the captured packet, analyse it and present it to the user in a readable format.

The following code shows an example of such a sniffer. Note that it sniffs only incoming packets.

Code

#include<stdio.h>	//For standard things
#include<stdlib.h>	//malloc
#include<string.h>	//memset
#include<netinet/ip_icmp.h>	//Provides declarations for icmp header
#include<netinet/udp.h>	//Provides declarations for udp header
#include<netinet/tcp.h>	//Provides declarations for tcp header
#include<netinet/ip.h>	//Provides declarations for ip header
#include<sys/socket.h>
#include<arpa/inet.h>

void ProcessPacket(unsigned char* , int);
void print_ip_header(unsigned char* , int);
void print_tcp_packet(unsigned char* , int);
void print_udp_packet(unsigned char * , int);
void print_icmp_packet(unsigned char* , int);
void PrintData (unsigned char* , int);

int sock_raw;
FILE *logfile;
int tcp=0,udp=0,icmp=0,others=0,igmp=0,total=0,i,j;
struct sockaddr_in source,dest;

int main()
{
	int saddr_size , data_size;
	struct sockaddr saddr;
	struct in_addr in;
	
	unsigned char *buffer = (unsigned char *)malloc(65536); //Its Big!
	
	logfile=fopen("log.txt","w");
	if(logfile==NULL) printf("Unable to create file.");
	printf("Starting...\n");
	//Create a raw socket that shall sniff
	sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_TCP);
	if(sock_raw < 0)
	{
		printf("Socket Error\n");
		return 1;
	}
	while(1)
	{
		saddr_size = sizeof saddr;
		//Receive a packet
		data_size = recvfrom(sock_raw , buffer , 65536 , 0 , &saddr , &saddr_size);
		if(data_size <0 )
		{
			printf("Recvfrom error , failed to get packets\n");
			return 1;
		}
		//Now process the packet
		ProcessPacket(buffer , data_size);
	}
	close(sock_raw);
	printf("Finished");
	return 0;
}

void ProcessPacket(unsigned char* buffer, int size)
{
	//Get the IP Header part of this packet
	struct iphdr *iph = (struct iphdr*)buffer;
	++total;
	switch (iph->protocol) //Check the Protocol and do accordingly...
	{
		case 1:  //ICMP Protocol
			++icmp;
			//PrintIcmpPacket(Buffer,Size);
			break;
		
		case 2:  //IGMP Protocol
			++igmp;
			break;
		
		case 6:  //TCP Protocol
			++tcp;
			print_tcp_packet(buffer , size);
			break;
		
		case 17: //UDP Protocol
			++udp;
			print_udp_packet(buffer , size);
			break;
		
		default: //Some Other Protocol like ARP etc.
			++others;
			break;
	}
	printf("TCP : %d   UDP : %d   ICMP : %d   IGMP : %d   Others : %d   Total : %d\r",tcp,udp,icmp,igmp,others,total);
}

void print_ip_header(unsigned char* Buffer, int Size)
{
	unsigned short iphdrlen;
		
	struct iphdr *iph = (struct iphdr *)Buffer;
	iphdrlen =iph->ihl*4;
	
	memset(&source, 0, sizeof(source));
	source.sin_addr.s_addr = iph->saddr;
	
	memset(&dest, 0, sizeof(dest));
	dest.sin_addr.s_addr = iph->daddr;
	
	fprintf(logfile,"\n");
	fprintf(logfile,"IP Header\n");
	fprintf(logfile,"   |-IP Version        : %d\n",(unsigned int)iph->version);
	fprintf(logfile,"   |-IP Header Length  : %d DWORDS or %d Bytes\n",(unsigned int)iph->ihl,((unsigned int)(iph->ihl))*4);
	fprintf(logfile,"   |-Type Of Service   : %d\n",(unsigned int)iph->tos);
	fprintf(logfile,"   |-IP Total Length   : %d  Bytes(Size of Packet)\n",ntohs(iph->tot_len));
	fprintf(logfile,"   |-Identification    : %d\n",ntohs(iph->id));
	//fprintf(logfile,"   |-Reserved ZERO Field   : %d\n",(unsigned int)iphdr->ip_reserved_zero);
	//fprintf(logfile,"   |-Dont Fragment Field   : %d\n",(unsigned int)iphdr->ip_dont_fragment);
	//fprintf(logfile,"   |-More Fragment Field   : %d\n",(unsigned int)iphdr->ip_more_fragment);
	fprintf(logfile,"   |-TTL      : %d\n",(unsigned int)iph->ttl);
	fprintf(logfile,"   |-Protocol : %d\n",(unsigned int)iph->protocol);
	fprintf(logfile,"   |-Checksum : %d\n",ntohs(iph->check));
	fprintf(logfile,"   |-Source IP        : %s\n",inet_ntoa(source.sin_addr));
	fprintf(logfile,"   |-Destination IP   : %s\n",inet_ntoa(dest.sin_addr));
}

void print_tcp_packet(unsigned char* Buffer, int Size)
{
	unsigned short iphdrlen;
	
	struct iphdr *iph = (struct iphdr *)Buffer;
	iphdrlen = iph->ihl*4;
	
	struct tcphdr *tcph=(struct tcphdr*)(Buffer + iphdrlen);
			
	fprintf(logfile,"\n\n***********************TCP Packet*************************\n");	
		
	print_ip_header(Buffer,Size);
		
	fprintf(logfile,"\n");
	fprintf(logfile,"TCP Header\n");
	fprintf(logfile,"   |-Source Port      : %u\n",ntohs(tcph->source));
	fprintf(logfile,"   |-Destination Port : %u\n",ntohs(tcph->dest));
	fprintf(logfile,"   |-Sequence Number    : %u\n",ntohl(tcph->seq));
	fprintf(logfile,"   |-Acknowledge Number : %u\n",ntohl(tcph->ack_seq));
	fprintf(logfile,"   |-Header Length      : %d DWORDS or %d BYTES\n" ,(unsigned int)tcph->doff,(unsigned int)tcph->doff*4);
	//fprintf(logfile,"   |-CWR Flag : %d\n",(unsigned int)tcph->cwr);
	//fprintf(logfile,"   |-ECN Flag : %d\n",(unsigned int)tcph->ece);
	fprintf(logfile,"   |-Urgent Flag          : %d\n",(unsigned int)tcph->urg);
	fprintf(logfile,"   |-Acknowledgement Flag : %d\n",(unsigned int)tcph->ack);
	fprintf(logfile,"   |-Push Flag            : %d\n",(unsigned int)tcph->psh);
	fprintf(logfile,"   |-Reset Flag           : %d\n",(unsigned int)tcph->rst);
	fprintf(logfile,"   |-Synchronise Flag     : %d\n",(unsigned int)tcph->syn);
	fprintf(logfile,"   |-Finish Flag          : %d\n",(unsigned int)tcph->fin);
	fprintf(logfile,"   |-Window         : %d\n",ntohs(tcph->window));
	fprintf(logfile,"   |-Checksum       : %d\n",ntohs(tcph->check));
	fprintf(logfile,"   |-Urgent Pointer : %d\n",tcph->urg_ptr);
	fprintf(logfile,"\n");
	fprintf(logfile,"                        DATA Dump                         ");
	fprintf(logfile,"\n");
		
	fprintf(logfile,"IP Header\n");
	PrintData(Buffer,iphdrlen);
		
	fprintf(logfile,"TCP Header\n");
	PrintData(Buffer+iphdrlen,tcph->doff*4);
		
	fprintf(logfile,"Data Payload\n");	
	PrintData(Buffer + iphdrlen + tcph->doff*4 , (Size - tcph->doff*4-iph->ihl*4) );
						
	fprintf(logfile,"\n###########################################################");
}

void print_udp_packet(unsigned char *Buffer , int Size)
{
	
	unsigned short iphdrlen;
	
	struct iphdr *iph = (struct iphdr *)Buffer;
	iphdrlen = iph->ihl*4;
	
	struct udphdr *udph = (struct udphdr*)(Buffer + iphdrlen);
	
	fprintf(logfile,"\n\n***********************UDP Packet*************************\n");
	
	print_ip_header(Buffer,Size);			
	
	fprintf(logfile,"\nUDP Header\n");
	fprintf(logfile,"   |-Source Port      : %d\n" , ntohs(udph->source));
	fprintf(logfile,"   |-Destination Port : %d\n" , ntohs(udph->dest));
	fprintf(logfile,"   |-UDP Length       : %d\n" , ntohs(udph->len));
	fprintf(logfile,"   |-UDP Checksum     : %d\n" , ntohs(udph->check));
	
	fprintf(logfile,"\n");
	fprintf(logfile,"IP Header\n");
	PrintData(Buffer , iphdrlen);
		
	fprintf(logfile,"UDP Header\n");
	PrintData(Buffer+iphdrlen , sizeof udph);
		
	fprintf(logfile,"Data Payload\n");	
	PrintData(Buffer + iphdrlen + sizeof udph ,( Size - sizeof udph - iph->ihl * 4 ));
	
	fprintf(logfile,"\n###########################################################");
}

void print_icmp_packet(unsigned char* Buffer , int Size)
{
	unsigned short iphdrlen;
	
	struct iphdr *iph = (struct iphdr *)Buffer;
	iphdrlen = iph->ihl*4;
	
	struct icmphdr *icmph = (struct icmphdr *)(Buffer + iphdrlen);
			
	fprintf(logfile,"\n\n***********************ICMP Packet*************************\n");	
	
	print_ip_header(Buffer , Size);
			
	fprintf(logfile,"\n");
		
	fprintf(logfile,"ICMP Header\n");
	fprintf(logfile,"   |-Type : %d",(unsigned int)(icmph->type));
			
	if((unsigned int)(icmph->type) == 11) 
		fprintf(logfile,"  (TTL Expired)\n");
	else if((unsigned int)(icmph->type) == ICMP_ECHOREPLY) 
		fprintf(logfile,"  (ICMP Echo Reply)\n");
	fprintf(logfile,"   |-Code : %d\n",(unsigned int)(icmph->code));
	fprintf(logfile,"   |-Checksum : %d\n",ntohs(icmph->checksum));
	//fprintf(logfile,"   |-ID       : %d\n",ntohs(icmph->id));
	//fprintf(logfile,"   |-Sequence : %d\n",ntohs(icmph->sequence));
	fprintf(logfile,"\n");

	fprintf(logfile,"IP Header\n");
	PrintData(Buffer,iphdrlen);
		
	fprintf(logfile,"UDP Header\n");
	PrintData(Buffer + iphdrlen , sizeof icmph);
		
	fprintf(logfile,"Data Payload\n");	
	PrintData(Buffer + iphdrlen + sizeof icmph , (Size - sizeof icmph - iph->ihl * 4));
	
	fprintf(logfile,"\n###########################################################");
}

void PrintData (unsigned char* data , int Size)
{
	
	for(i=0 ; i < Size ; i++)
	{
		if( i!=0 && i%16==0)   //if one line of hex printing is complete...
		{
			fprintf(logfile,"         ");
			for(j=i-16 ; j<i ; j++)
			{
				if(data[j]>=32 && data[j]<=128)
					fprintf(logfile,"%c",(unsigned char)data[j]); //if its a number or alphabet
				
				else fprintf(logfile,"."); //otherwise print a dot
			}
			fprintf(logfile,"\n");
		} 
		
		if(i%16==0) fprintf(logfile,"   ");
			fprintf(logfile," %02X",(unsigned int)data[i]);
				
		if( i==Size-1)  //print the last spaces
		{
			for(j=0;j<15-i%16;j++) fprintf(logfile,"   "); //extra spaces
			
			fprintf(logfile,"         ");
			
			for(j=i-i%16 ; j<=i ; j++)
			{
				if(data[j]>=32 && data[j]<=128) fprintf(logfile,"%c",(unsigned char)data[j]);
				else fprintf(logfile,".");
			}
			fprintf(logfile,"\n");
		}
	}
}

Compile and Run

$ gcc sniffer.c && sudo ./a.out

The program must be run as root user or superuser privileges. e.g. sudo ./a.out in ubuntu
The program creates raw sockets which require root access.

The output in the log file is something like this :

***********************TCP Packet*************************

IP Header
   |-IP Version        : 4
   |-IP Header Length  : 5 DWORDS or 20 Bytes
   |-Type Of Service   : 32
   |-IP Total Length   : 137  Bytes(Size of Packet)
   |-Identification    : 35640
   |-TTL      : 51
   |-Protocol : 6
   |-Checksum : 54397
   |-Source IP        : 174.143.119.91
   |-Destination IP   : 192.168.1.6

TCP Header
   |-Source Port      : 6667
   |-Destination Port : 38265
   |-Sequence Number    : 1237278529
   |-Acknowledge Number : 65363511
   |-Header Length      : 5 DWORDS or 20 BYTES
   |-Urgent Flag          : 0
   |-Acknowledgement Flag : 1
   |-Push Flag            : 1
   |-Reset Flag           : 0
   |-Synchronise Flag     : 0
   |-Finish Flag          : 0
   |-Window         : 9648
   |-Checksum       : 46727
   |-Urgent Pointer : 0

                        DATA Dump                         
IP Header
    45 20 00 89 8B 38 40 00 33 06 D4 7D AE 8F 77 5B         E [email protected]..}..w[
    C0 A8 01 06                                             ....
TCP Header
    1A 0B 95 79 49 BF 5F 41 03 E5 5E 37 50 18 25 B0         ...yI._A..^7P.%.
    B6 87 00 00                                             ....
Data Payload
    3A 6B 61 74 65 60 21 7E 6B 61 74 65 40 75 6E 61         :kate`!~kate@una
    66 66 69 6C 69 61 74 65 64 2F 6B 61 74 65 2F 78         ffiliated/kate/x
    2D 30 30 30 30 30 30 31 20 50 52 49 56 4D 53 47         -0000001 PRIVMSG
    20 23 23 63 20 3A 69 20 6E 65 65 64 20 65 78 61          ##c :i need exa
    63 74 6C 79 20 74 68 65 20 72 69 67 68 74 20 6E         ctly the right n
    75 6D 62 65 72 20 6F 66 20 73 6F 63 6B 73 21 0D         umber of socks!.
    0A                                                      .

###########################################################

***********************TCP Packet*************************

IP Header
   |-IP Version        : 4
   |-IP Header Length  : 5 DWORDS or 20 Bytes
   |-Type Of Service   : 32
   |-IP Total Length   : 186  Bytes(Size of Packet)
   |-Identification    : 56556
   |-TTL      : 48
   |-Protocol : 6
   |-Checksum : 22899
   |-Source IP        : 74.125.71.147
   |-Destination IP   : 192.168.1.6

TCP Header
   |-Source Port      : 80
   |-Destination Port : 49374
   |-Sequence Number    : 3136045905
   |-Acknowledge Number : 2488580377
   |-Header Length      : 5 DWORDS or 20 BYTES
   |-Urgent Flag          : 0
   |-Acknowledgement Flag : 1
   |-Push Flag            : 1
   |-Reset Flag           : 0
   |-Synchronise Flag     : 0
   |-Finish Flag          : 0
   |-Window         : 44765
   |-Checksum       : 15078
   |-Urgent Pointer : 0

                        DATA Dump                         
IP Header
    45 20 00 BA DC EC 00 00 30 06 59 73 4A 7D 47 93         E ......0.YsJ}G.
    C0 A8 01 06                                             ....
TCP Header
    00 50 C0 DE BA EC 43 51 94 54 B9 19 50 18 AE DD         .P....CQ.T..P...
    3A E6 00 00                                             :...
Data Payload
    48 54 54 50 2F 31 2E 31 20 33 30 34 20 4E 6F 74         HTTP/1.1 304 Not
    20 4D 6F 64 69 66 69 65 64 0D 0A 58 2D 43 6F 6E          Modified..X-Con
    74 65 6E 74 2D 54 79 70 65 2D 4F 70 74 69 6F 6E         tent-Type-Option
    73 3A 20 6E 6F 73 6E 69 66 66 0D 0A 44 61 74 65         s: nosniff..Date
    3A 20 54 68 75 2C 20 30 31 20 44 65 63 20 32 30         : Thu, 01 Dec 20
    31 31 20 31 33 3A 31 36 3A 34 30 20 47 4D 54 0D         11 13:16:40 GMT.
    0A 53 65 72 76 65 72 3A 20 73 66 66 65 0D 0A 58         .Server: sffe..X
    2D 58 53 53 2D 50 72 6F 74 65 63 74 69 6F 6E 3A         -XSS-Protection:
    20 31 3B 20 6D 6F 64 65 3D 62 6C 6F 63 6B 0D 0A          1; mode=block..
    0D 0A                                                   ..

###########################################################

Note

1. The above sniffer picks up only TCP packets, because of the declaration :

sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_TCP);

For UDP and ICMP the declaration has to be :

sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_UDP);
sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_ICMP);

You might be tempted to think of doing :

sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_IP);

but this will not work , since IPPROTO_IP is a dummy protocol not a real one.

2. This sniffer picks up only incoming packets.

3. It provides the application with IP frames, which means that ethernet headers are not available.

4. It is not very accurate since it misses out some packets even in the incoming ones.

5. libpcap can also be used to write sniffers. Code example here.

In Part 2 we shall see how the above mentioned drawbacks can be overcome.

Conclusion

The next part of this tutorial shows how to sniff Ethernet headers with a packet sniffer. Check this here:
How to code a Packet Sniffer in C with Linux Sockets - Part 2

To learn how to code a packet sniffer using Libpcap library check out this post:
How to code a Packet Sniffer in C with Libpcap on Linux