question about sockets in c

2 Attachment(s)

Hello,

I am new to socket programming and have bought the book "TCP/IP Sockets in C".

I don't quite understand the relation between the client socket, the server socket and the socket returned by a call to accept().

I have posted two pictures.

In the second picture(client output) I have executed the client
by sending the message "Hello Server" to the server address 172.18.99.185-5001. After connection to the server I output the client address and the server address (using getsockname and getpeername()). So far I understand what's going on.

In the first picture I have executed the server. clntSock is the socket which is returned by a call to accept(). I dont understand
why the peer of clntSock has the address 172.18.99.177-2843.

I expected it to be 192.168.75.130-57994.

Thanks for your help!

Post your code.

The 192.168... address is class C private address, perhaps assigned by your router.
The 172.18... address is a class B private address, maybe assigned by your university?
What public IPv4 do you see when you go here: What Is My IP❓ - Shows your real public IP address - IPv4 - IPv6

Just for info: I run server and client in seperate virtual machines.

I have posted the code for client and server:

client:

Code:

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include "Practical.h" int main(int argc, char *argv[]) { if (argc < 3 || argc > 4) // Test for correct number of arguments DieWithUserMessage("Parameter(s)", "<Server Address> <Echo Word> [<Server Port>]"); char *servIP = argv[1]; // First arg: server IP address (dotted quad) char *echoString = argv[2]; // Second arg: string to echo // Third arg (optional): server port (numeric). 7 is well-known echo port in_port_t servPort = (argc == 4) ? atoi(argv[3]) : 7; // Create a reliable, stream socket using TCP int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (sock < 0) DieWithSystemMessage("socket() failed"); // Construct the server address structure struct sockaddr_in servAddr; // Server address memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure servAddr.sin_family = AF_INET; // IPv4 address family // Convert address int rtnVal = inet_pton(AF_INET, servIP, &servAddr.sin_addr.s_addr); if (rtnVal == 0) DieWithUserMessage("inet_pton() failed", "invalid address string"); else if (rtnVal < 0) DieWithSystemMessage("inet_pton() failed"); servAddr.sin_port = htons(servPort); // Server port // Establish the connection to the echo server(socket of echo server) if (connect(sock, (struct sockaddr *) &servAddr, sizeof(servAddr)) < 0) DieWithSystemMessage("connect() failed"); //print local address struct sockaddr_in clntAddrLcl; // client address memset(&clntAddrLcl, 0, sizeof(clntAddrLcl)); // Zero out structure clntAddrLcl.sin_family = AF_INET; // IPv4 address family int size = sizeof(clntAddrLcl); getsockname(sock, (struct sockaddr *) &clntAddrLcl, &size); fputs("client address after connect: ", stdout); PrintSocketAddress((struct sockaddr *) &clntAddrLcl, stdout); fputc('\n',stdout); //print foreign address struct sockaddr_in clntAddrFor; // foreign address memset(&clntAddrFor, 0, sizeof(clntAddrFor)); // Zero out structure clntAddrFor.sin_family = AF_INET; // IPv4 address family int size2 = sizeof(clntAddrFor); getpeername(sock, (struct sockaddr *) &clntAddrFor, &size2); fputs("address of server(peer of client) after connect: ", stdout); PrintSocketAddress((struct sockaddr *) &clntAddrFor, stdout); fputc('\n', stdout); size_t echoStringLen = strlen(echoString); // Determine input length //now client can send data to server // Send the string to the server over client sock unsigned int numBytes = send(sock, echoString, echoStringLen, 0); if (numBytes < 0) DieWithSystemMessage("send() failed"); else if (numBytes != echoStringLen) DieWithUserMessage("send()", "sent unexpected number of bytes"); // Receive the same string back from the server unsigned int totalBytesRcvd = 0; // Count of total bytes received // Count of total bytes received //fputs("\nReceived(clnt): \n", stdout); // Setup to print the echoed string while (totalBytesRcvd < echoStringLen) { char buffer[BUFSIZE]; // I/O buffer /* Receive up to the buffer size (minus 1 to leave space for a null terminator) bytes from the sender */ //receive message on clientsock numBytes = recv(sock, buffer, BUFSIZE - 1, 0); if (numBytes < 0) DieWithSystemMessage("recv() failed"); else if (numBytes == 0) DieWithUserMessage("recv()", "connection closed prematurely"); totalBytesRcvd += numBytes; // Keep tally of total bytes buffer[numBytes] = '\0'; // Terminate the string! //fputs(buffer, stdout); // Print the echo buffer } fputc('\n', stdout); // Print a final linefeed close(sock); exit(0); }

server:

Code:

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include "Practical.h" static const int MAXPENDING = 5; // Maximum outstanding connection requests int main(int argc, char *argv[]) { if (argc != 2) // Test for correct number of arguments DieWithUserMessage("Parameter(s)", "<Server Port>"); in_port_t servPort = atoi(argv[1]); // First arg: local port // Create socket for incoming connections int servSock; // Socket descriptor for server if ((servSock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) DieWithSystemMessage("socket() failed"); // Construct local address structure struct sockaddr_in servAddr; // Local address memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure servAddr.sin_family = AF_INET; // IPv4 address family servAddr.sin_addr.s_addr = htonl(INADDR_ANY); // Any incoming interface servAddr.sin_port = htons(servPort); // Local port // Bind to the local address if (bind(servSock, (struct sockaddr*) &servAddr, sizeof(servAddr)) < 0) DieWithSystemMessage("bind() failed"); // Mark the socket so it will listen for incoming connections if (listen(servSock, MAXPENDING) < 0) DieWithSystemMessage("listen() failed"); for (;:wink: { // Run forever struct sockaddr_in clntAddr; // Client address // Set length of client address structure (in-out parameter) socklen_t clntAddrLen = sizeof(clntAddr); // Wait for a client to connect int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen); if (clntSock < 0) DieWithSystemMessage("accept() failed"); //print address of clntsocket struct sockaddr_in clntAddrLcl; // client address memset(&clntAddrLcl, 0, sizeof(clntAddrLcl)); // Zero out structure clntAddrLcl.sin_family = AF_INET; // IPv4 address family int size = sizeof(clntAddrLcl); getsockname(clntSock, (struct sockaddr *) &clntAddrLcl, &size); fputs("address of clntSock after accept: ", stdout); PrintSocketAddress((struct sockaddr *) &clntAddrLcl, stdout); fputc('\n',stdout); //print address of peer of clntsocket struct sockaddr_in clntAddrFor; // client address memset(&clntAddrFor, 0, sizeof(clntAddrFor)); // Zero out structure clntAddrFor.sin_family = AF_INET; // IPv4 address family int size2 = sizeof(clntAddrFor); getpeername(clntSock, (struct sockaddr *) &clntAddrFor, &size2); fputs("address of peer of clntSock after accept: ", stdout); PrintSocketAddress((struct sockaddr *) &clntAddrFor, stdout); fputc('\n', stdout); // clntSock is connected to a client! char clntName[INET_ADDRSTRLEN]; // String to contain client address if (inet_ntop(AF_INET, &clntAddr.sin_addr.s_addr, clntName, sizeof(clntName)) != NULL) printf("Handling client %s/%d\n", clntName, ntohs(clntAddr.sin_port)); else puts("Unable to get client address"); HandleTCPClient(clntSock); } // NOT REACHED }

I have reposted the code for clien and server to make some variable names easier to understand.

client:

Code:

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include "Practical.h" int main(int argc, char *argv[]) { if (argc < 3 || argc > 4) // Test for correct number of arguments DieWithUserMessage("Parameter(s)", "<Server Address> <Echo Word> [<Server Port>]"); char *servIP = argv[1]; // First arg: server IP address (dotted quad) char *echoString = argv[2]; // Second arg: string to echo // Third arg (optional): server port (numeric). 7 is well-known echo port in_port_t servPort = (argc == 4) ? atoi(argv[3]) : 7; // Create a reliable, stream socket using TCP int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (sock < 0) DieWithSystemMessage("socket() failed"); // Construct the server address structure struct sockaddr_in servAddr; // Server address memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure servAddr.sin_family = AF_INET; // IPv4 address family // Convert address int rtnVal = inet_pton(AF_INET, servIP, &servAddr.sin_addr.s_addr); if (rtnVal == 0) DieWithUserMessage("inet_pton() failed", "invalid address string"); else if (rtnVal < 0) DieWithSystemMessage("inet_pton() failed"); servAddr.sin_port = htons(servPort); // Server port // Establish the connection to the echo server(socket of echo server) if (connect(sock, (struct sockaddr *) &servAddr, sizeof(servAddr)) < 0) DieWithSystemMessage("connect() failed"); //print local address struct sockaddr_in clientAddress; // client address memset(&clientAddress, 0, sizeof(clientAddress)); // Zero out structure clientAddress.sin_family = AF_INET; // IPv4 address family int size = sizeof(clientAddress); getsockname(sock, (struct sockaddr *) &clientAddress, &size); fputs("client address after connect: ", stdout); PrintSocketAddress((struct sockaddr *) &clientAddress, stdout); fputc('\n',stdout); //print foreign address struct sockaddr_in clientPeerAddress; // client peer address memset(&clientPeerAddress, 0, sizeof(clientPeerAddress)); // Zero out structure clientPeerAddress.sin_family = AF_INET; // IPv4 address family int size2 = sizeof(clientPeerAddress); getpeername(sock, (struct sockaddr *) &clientPeerAddress, &size2); fputs("address of server(peer of client) after connect: ", stdout); PrintSocketAddress((struct sockaddr *) &clientPeerAddress, stdout); fputc('\n', stdout); size_t echoStringLen = strlen(echoString); // Determine input length //now client can send data to server // Send the string to the server over client sock unsigned int numBytes = send(sock, echoString, echoStringLen, 0); if (numBytes < 0) DieWithSystemMessage("send() failed"); else if (numBytes != echoStringLen) DieWithUserMessage("send()", "sent unexpected number of bytes"); // Receive the same string back from the server unsigned int totalBytesRcvd = 0; // Count of total bytes received // Count of total bytes received //fputs("\nReceived(clnt): \n", stdout); // Setup to print the echoed string while (totalBytesRcvd < echoStringLen) { char buffer[BUFSIZE]; // I/O buffer /* Receive up to the buffer size (minus 1 to leave space for a null terminator) bytes from the sender */ //receive message on clientsock numBytes = recv(sock, buffer, BUFSIZE - 1, 0); if (numBytes < 0) DieWithSystemMessage("recv() failed"); else if (numBytes == 0) DieWithUserMessage("recv()", "connection closed prematurely"); totalBytesRcvd += numBytes; // Keep tally of total bytes buffer[numBytes] = '\0'; // Terminate the string! fputs(buffer, stdout); // Print the echo buffer } fputc('\n', stdout); // Print a final linefeed close(sock); exit(0); }

server:

Code:

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include "Practical.h" static const int MAXPENDING = 5; // Maximum outstanding connection requests int main(int argc, char *argv[]) { if (argc != 2) // Test for correct number of arguments DieWithUserMessage("Parameter(s)", "<Server Port>"); in_port_t servPort = atoi(argv[1]); // First arg: local port // Create socket for incoming connections int servSock; // Socket descriptor for server if ((servSock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) DieWithSystemMessage("socket() failed"); // Construct local address structure struct sockaddr_in servAddr; // Local address memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure servAddr.sin_family = AF_INET; // IPv4 address family servAddr.sin_addr.s_addr = htonl(INADDR_ANY); // Any incoming interface servAddr.sin_port = htons(servPort); // Local port // Bind to the local address if (bind(servSock, (struct sockaddr*) &servAddr, sizeof(servAddr)) < 0) DieWithSystemMessage("bind() failed"); // Mark the socket so it will listen for incoming connections if (listen(servSock, MAXPENDING) < 0) DieWithSystemMessage("listen() failed"); for (;;) { // Run forever struct sockaddr_in clntAddr; // Client address // Set length of client address structure (in-out parameter) socklen_t clntAddrLen = sizeof(clntAddr); // Wait for a client to connect int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen); if (clntSock < 0) DieWithSystemMessage("accept() failed"); //print address of clntsocket struct sockaddr_in clntsocketAddress; // client address memset(&clntsocketAddress, 0, sizeof(clntsocketAddress)); // Zero out structure clntsocketAddress.sin_family = AF_INET; // IPv4 address family int size = sizeof(clntsocketAddress); getsockname(clntSock, (struct sockaddr *) &clntsocketAddress, &size); fputs("address of clntSock after accept: ", stdout); PrintSocketAddress((struct sockaddr *) &clntsocketAddress, stdout); fputc('\n',stdout); //print address of peer of clntsocket struct sockaddr_in clntPeerSocketAddress; // peer of clntsocket address memset(&clntPeerSocketAddress, 0, sizeof(clntPeerSocketAddress)); // Zero out structure clntPeerSocketAddress.sin_family = AF_INET; // IPv4 address family int size2 = sizeof(clntPeerSocketAddress); getpeername(clntSock, (struct sockaddr *) &clntPeerSocketAddress, &size2); fputs("address of peer of clntSock after accept: ", stdout); PrintSocketAddress((struct sockaddr *) &clntPeerSocketAddress, stdout); fputc('\n', stdout); // clntSock is connected to a client! char clntName[INET_ADDRSTRLEN]; // String to contain client address if (inet_ntop(AF_INET, &clntAddr.sin_addr.s_addr, clntName, sizeof(clntName)) != NULL) printf("Handling client %s/%d\n", clntName, ntohs(clntAddr.sin_port)); else puts("Unable to get client address"); HandleTCPClient(clntSock); } // NOT REACHED }