I Wrote A Simple TCP Server. I Get Weird Output - Why? C/C++ Socket Programming

Posted on 2008-10-31
Last Modified: 2013-11-13
Simple server that listens on port 10030. Once a client connects to it, the server sends back a message to the client says "Hello To Socket Programming World!" then closes the socket. Use telnet as the client. It works fine, but the server output is not as expected.

Why does the client's ip address appear as follows:

Sent 35 bytes to client : ::aff:c0fd:7fbf
Sent 35 bytes to client : ::ffff:
Sent 35 bytes to client : ::ffff:
Sent 35 bytes to client : ::ffff:

1. The fist IP address appears as ::aff:c0fd:7fbf
the second appears as ::ffff:
I think i only should get, i'm not using any IPv6. Do you know why?

2. Why IP at first time appears differently than the rest?

Look at this function - void *get_in_addr(struct sockaddr *clientAddress) as its the one that fetches client's IP address...

Please help!








using namespace std;


#define PORT "10030" // the port users will be connecting to - the port we (server) will be listening on

#define BACKLOG 10   // how many pending connections queue will hold


//describe the program

void progDesc()


	cout<<"This server listens for a connection from client, andreplies\n";

	cout<<"back to the client with a message, then closes the socket"<<endl;



//get sockaddr, IPv4 or IPv6:

void *get_in_addr(struct sockaddr *clientAddress)


	void *addr;


	//check the version of client's IP

	if (clientAddress->sa_family == AF_INET) {

		struct sockaddr_in *ipv4 = (struct sockaddr_in *)clientAddress;

		addr = &(ipv4->sin_addr);

		return addr;


		struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)clientAddress;

		addr = &(ipv6->sin6_addr);

		return addr;





int main()


	int serverSockD;           //the server's socket that listen incoming connections

	int clientSockD;           //the client's socket that works as a medium between client and server to send(), recv(), etc

	struct addrinfo hints;     //get inputs about the server information


	struct addrinfo *servInfo; 


	struct sockaddr clientInfo;			     //obtains clients information once accept succeed

	socklen_t clientInfo_size; 			     //holds the size of cleintInfo

	char msg[] = "Hello To Socket Programming World!\n"; //message that is sent back to client


	//set size of hints to zero


	memset(&hints, 0, sizeof hints);




	//fill some of the server info (your current host info)


	hints.ai_family = AF_UNSPEC;


	hints.ai_socktype = SOCK_STREAM;

        hints.ai_flags = AI_PASSIVE;     //your (the server's) local ip address will be specified 

                                         //automatically. The server can have more than one ip address.


	//get a more specific server's information, such as IP Addresses, and pass the output to servInfo

	int status = getaddrinfo(NULL, PORT, &hints, &servInfo);


	//make sure getaddrinfo succeeded

	if(status != 0)




		fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));


		return 1;




        //create a socket - this socket will be used to listen on PORT for incoming connections

	serverSockD = socket(servInfo->ai_family, servInfo->ai_socktype, servInfo->ai_protocol);


	if(serverSockD == -1 )




		cout<<"Error: failed to create a socket.\n";


		return 2;





	//now bind the server socket to the ip address & port specified

	if(bind(serverSockD, servInfo->ai_addr, servInfo->ai_addrlen) == -1) 


		perror("server: bind");

	        return 3;



	//listen to the incoming connection, take up to 10 connections

	if(listen(serverSockD, BACKLOG) == -1) 



	    return 4;



	//get the size of sockaddr struct of clientAddr

	clientInfo_size = sizeof clientInfo;


	//holds number of bytes sent

	int bytes_sent;


	//holds an IP address of the client's

	char printableClientIP[INET6_ADDRSTRLEN];


	//holds the exact IP address for the client() you accepted his/her connection

	void *clientIP;


	//listen forever, and accept all connections that come in

	//The only way we get out of the loop is in case of a crash 

	//or a user-initiated shutdown of the server



		//accept pending connection. Information of incoming connection will go to clientInfo

		clientSockD = accept(serverSockD, &clientInfo, &clientInfo_size);


		if(clientSockD == -1)



	   		return 5;



		//send msg to the client & number of bytes sent will be returned

		bytes_sent = send(clientSockD, msg, strlen(msg), 0);


		//get_in_addr gives you the current client IP Address

		clientIP = get_in_addr(&clientInfo);


		//convert clientIP from numeric binary form to standard text form

		inet_ntop(clientInfo.sa_family, clientIP, printableClientIP, sizeof printableClientIP);


		//print to screen the IP Address of the client & how many bytes are sent

		printf("Sent %d bytes to client : %s\n", bytes_sent, printableClientIP);


		//close the current connection & repeat the loop to take the next connection




	//free linkedlist of struct addrinfo *servInfo




	return 0;


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Question by:F-J-K
  • 2
  • 2
LVL 39

Expert Comment

ID: 22856031
>>>> I think i only should get, i'm not using any IPv6.
Indeed, you don't as the higher numbers default to ffff. But the utility producing the output you posted was able to handle both IPv4 abd IPv6 hence you got it converted to the more universal one.

>>>> Why IP at first time appears differently than the rest?
The first IP is the one your provider had returned for the router after getting a request. The second IP is your local IP which is only valid at your own computer. It would be used by your network device for local use only. In case you would operate more than one computer using the connection, you would need a router providing a local C-Net at your LAN, normally with xxx often is 0 or 1 and yyy goes from 1 to 255, where all computers needing a connection would get  one of the (only locally usable) IP address of the C-Net.

Author Comment

ID: 22858253
>>Indeed, you don't as the higher numbers default to ffff. But the utility producing the output you posted was able to handle both IPv4 abd IPv6 hence you got it converted to the more universal one.

Remember, i'm returning "addr", it either returns a total IPv4 or a total IPv6 ....
clientAddress->sa_family == AF_INET
if my cleintAddress is IPv4, then we return addr with IPv4. There won't be a chance to go to "else". Thats what makes me confuse.

>>The first IP is the one your provider had returned for the router after getting a request.
I'm using a telnet to connect to the simple tcp server,  the telnet is running on same machine "". No third parties involved, such as routers, etc. Can you explain what do you exactly mean by "your provider" & what the router got to do with this as i'm not connecting from outside the network.

Thanks for your answer...I hope to hear from you!
LVL 53

Assisted Solution

Infinity08 earned 200 total points
ID: 22861038
Your network interface supports IPv6 addresses, so that's what it uses. The IPv6 address ::ffff: is the exact same address as the IPv4 address

>> if my cleintAddress is IPv4

Your client has an address, which might be represented as either IPv4 or IPv6. In this case, your protocol stack chose to represent it as IPv6.
LVL 39

Accepted Solution

itsmeandnobodyelse earned 300 total points
ID: 22861264
>>>> the telnet is running on same machine
Yes, but the telnet could run on each other computer in the wide world. Hence it uses an internal IP address which is already IPv6 and whcih was mapped to the you have configured as only (default) OP address. That a mapping occurs you can see at the second line.

>>>> you exactly mean by "your provider"
Sorry I assumed you were connected to the internet. But even for a stand-alone computer, you have a TCP/IP configuration which internally uses IP addresses that were different to

Author Closing Comment

ID: 31512169
Thanks! Your answers were clear and perfect! .......... Can't thank you enough!

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