Bind to a local port that is already in use

[Cooker]

I also make the first experiment on Ubuntu Linux. The result is the same as on Windows Vista. I just share this information.

[ludakot]

I also make the first experiment on Ubuntu Linux. The result is the same as on Windows Vista. I just share this information.

Are you willing to share the source code so I can try it myself on gentoo?

[j0nas]

Well, it's only security issue if you do TCP bind to ANY address. If you bind to a specific address, no other TCP server can "steal" that specific address+port.

Yes, it is a security issue, but it's not unique to Windows. The issue arises in all BSD socket implementations that support the SO_REUSEADDR option. AFAIK, all implementations do so, inclusive of Linux, Sun etc.

Windows introduced the SO_EXCLUSIVEADDRUSE option to address this issue. See "Using SO_EXCLUSIVEADDRUSE" at http://msdn2.microsoft.com/en-us/library/ms740618.aspx . Here's a quote:

I am not certain if other socket implementations (i.e., other than that on Windows) also have the SO_EXCLUSIVEADDRUSE option.

Mike

[MikeAThon]

Well, it's only security issue if you do TCP bind to ANY address. If you bind to a specific address, no other TCP server can "steal" that specific address+port.

I don't think I agree. The entire purpose of the SO_EXCLUSIVEADDR option is to prevent a malicious program from "stealing" the addr+port. Without SO_EXCLUSIVEADDR, it's my understanding that a malicious program can do exactly that:

... such reuse can be executed by malicious applications to disrupt the application

.

[ludakot]

Still waiting on that source code. I'm either doing something wrong or my OS has some security mechanism such as SO_EXCLUSIVEADDR at kernel level or something.

[Cooker]

Are you willing to share the source code so I can try it myself on gentoo?

At first, You can run the following source code on Windows OS.

1. UDP Receiver
   

   #include <stdio.h>
   #include <stdlib.h>
   
   #include <winsock2.h>              // for Winsock
   #pragma comment(lib,"Ws2_32.lib")  // for Winsock 
   
   void ErrExit(SOCKET fd, const char *format, ...);
   
   
   int main(int argc, char *argv[])
   {
   
   	WORD wVersionRequested = MAKEWORD(2, 2);
   	WSADATA wsaData;
   	int err = 0;
   	SOCKET sockfd = 0;
   	struct sockaddr_in local, remote;
   	int localport = 0;
   	int nrecvbytes = 0, fromlen = 0;
   	char recvbuf[128];
   	BOOL bOptVal = TRUE;
   	int bOptLen = sizeof(BOOL);
   
   	if (argc != 3) {
   		printf("Usage:\n");
   		printf("      program local_addr local_port\n");
   		printf("Description:\n");
   		printf("      Receive UDP packets from local_addr:local_port\n");
   		return -1;
   	} 
   	
   	localport = strtol(argv[2], NULL, 10);
   	
   	// Initiate use of the Winsock DLL
   	err = WSAStartup(wVersionRequested, &wsaData);
   	if (err != 0) {
   		// Could not find a usable WinSock DLL
   		printf("Couldn't find a usable WinSock DLL.\n");
   		return -1;
   	}
   
   	// Confirm that the WinSock DLL supports 2.2.              
   	if (LOBYTE( wsaData.wVersion ) != 2 || HIBYTE( wsaData.wVersion ) != 2) 
   		ErrExit(0, "Bad winsock version.\n");
   
   	// Create a socket that is bound to a specific transport service provider
   	sockfd = socket(AF_INET, SOCK_DGRAM, 0);
   	if (sockfd == INVALID_SOCKET) 
   		ErrExit(0, "Can't open a socket with error code %d\n", WSAGetLastError());
   
   	// Contain information about local IP address and port
   	local.sin_family = AF_INET;         
       	local.sin_port = htons(localport);     
       	local.sin_addr.s_addr = inet_addr(argv[1]);
       	memset(local.sin_zero, '\0', sizeof local.sin_zero);
   
   	if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (char*)&bOptVal, bOptLen) == SOCKET_ERROR) {
   		printf("Can't set SO_REUSEADDR option.\n");
   	}
   
   	// Associate a local address with a socket
       	err = bind(sockfd, (struct sockaddr *)&local, sizeof local);
   	if (err == SOCKET_ERROR)
   		ErrExit(sockfd, "Can't bind to a local address and port with error code %d\n", WSAGetLastError());
   
   	memset(&remote, 0, sizeof remote);
   	memset(recvbuf, 0, sizeof recvbuf);
   
   	for (;;) 
   	{
   		fromlen = sizeof remote;
   		nrecvbytes = recvfrom(sockfd, recvbuf, sizeof recvbuf, 0, (struct sockaddr *)&remote, &fromlen);
   		
   		if (nrecvbytes == SOCKET_ERROR)
   			ErrExit(sockfd, "Can't receive messages with error code %d\n", WSAGetLastError());
   		
   		recvbuf[nrecvbytes] = '\0';
   		printf("Message [%s] from %s:%d\n", recvbuf, inet_ntoa(remote.sin_addr), ntohs(remote.sin_port));		
   	}
   
   	return 0;
   }
   
   void ErrExit(SOCKET fd, const char *format, ...)
   {
   	va_list args;
   
   	va_start(args, format);
   	vfprintf(stderr, format, args);
   	va_end(args);
   
   	if (fd != NULL)
   		closesocket(fd);
   
   	WSACleanup();
   	exit(1);
   }

2. UDP sender
   

   #include <stdio.h>
   #include <stdlib.h>
   #include <conio.h>
   #include <time.h>
   
   #include <winsock2.h>              // for Winsock
   #pragma comment(lib,"Ws2_32.lib")  // for Winsock 
   
   void ErrExit(SOCKET fd, const char *format, ...);
   void wait(int seconds);
   
   int main(int argc, char *argv[])
   {
   
   	WORD wVersionRequested = MAKEWORD(2, 2);
   	WSADATA wsaData;
   	int err = 0;
   	SOCKET sockfd = 0;
   	struct sockaddr_in local, remote;
   	int localport = 0, remoteport = 0, cycle = 0;
   	int nsentbytes = 0;
   	BOOL bOptVal = TRUE;
   	int bOptLen = sizeof(BOOL);
   	
   
   	if (argc != 7) {
   		printf("Usage:\n");
   		printf("      program local_addr local_port cycle message remote_addr remote_port\n");
   		printf("Description:\n");
   		printf("      Send a UDP packet to remote_addr:remote_port from local_addr:local_port every cycle seconds\n");
   		return -1;
   	} 
   	
   	localport = strtol(argv[2], NULL, 10);
   	remoteport = strtol(argv[6], NULL, 10);
   	cycle = strtol(argv[3], NULL, 10);	
   	
   	// Initiate use of the Winsock DLL
   	err = WSAStartup(wVersionRequested, &wsaData);
   	if (err != 0) {
   		// Could not find a usable WinSock DLL
   		printf("Couldn't find a usable WinSock DLL.\n");
   		return -1;
   	}
   
   	// Confirm that the WinSock DLL supports 2.2.              
   	if (LOBYTE( wsaData.wVersion ) != 2 || HIBYTE( wsaData.wVersion ) != 2) 
   		ErrExit(0, "Bad winsock version.\n");
   
   	// Create a socket that is bound to a specific transport service provider
   	sockfd = socket(AF_INET, SOCK_DGRAM, 0);
   	if (sockfd == INVALID_SOCKET) 
   		ErrExit(0, "Can't open a socket with error code %d\n", WSAGetLastError());
   
   	// Contain information about local IP address and port
   	local.sin_family = AF_INET;         
       	local.sin_port = htons(localport);     
       	local.sin_addr.s_addr = inet_addr(argv[1]);
       	memset(local.sin_zero, '\0', sizeof local.sin_zero);
   
   	if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (char*)&bOptVal, bOptLen) == SOCKET_ERROR) {
   		printf("Can't set SO_REUSEADDR option.\n");
   	}
   
   	// Associate a local address with a socket
       	err = bind(sockfd, (struct sockaddr *)&local, sizeof local);
   	if (err == SOCKET_ERROR)
   		ErrExit(sockfd, "Can't bind to a local address and port with error code %d\n", WSAGetLastError());
   
   	// Contain information about remote IP address and port
   	remote.sin_family = AF_INET;         
       	remote.sin_port = htons(remoteport);     
       	remote.sin_addr.s_addr = inet_addr(argv[5]);
       	memset(remote.sin_zero, '\0', sizeof remote.sin_zero);
   
   	printf("==> Press any key to quit the program. <==\n");
   	for (;;) 
   	{
   		if (kbhit())
   			break;
   		nsentbytes = sendto(sockfd, argv[4], strlen(argv[4]), 0, (struct sockaddr  *)&remote, sizeof remote);
   		if (nsentbytes == SOCKET_ERROR) 
   			ErrExit(sockfd, "Can't send meesages to remote address with error code%d\n", WSAGetLastError());
   		wait(cycle);
   	}
   
   	system("pause");
   	return 0;
   }
   
   void ErrExit(SOCKET fd, const char *format, ...)
   {
   	va_list args;
   
   	va_start(args, format);
   	vfprintf(stderr, format, args);
   	va_end(args);
   
   	if (fd != NULL)
   		closesocket(fd);
   
   	WSACleanup();
   	system("pause");
   	exit(1);
   }
   
   void wait(int seconds)
   {
   	clock_t endwait;
   	endwait = clock () + seconds * CLK_TCK ;
   	while (clock() < endwait) {}
   }

The source code which runs on Ubuntu Linux isn't in my current PC. I will post later.

[Cooker]

Still waiting on that source code. I'm either doing something wrong or my OS has some security mechanism such as SO_EXCLUSIVEADDR at kernel level or something.

This SO_EXCLUSIVEADDR option is available on Microsoft Windows. On a Linux, all the socket information is in the man page for socket in section 7. You can type "man 7 socket" to get all these goodies and find if there exists SO_EXCLUSIVEADDRUSE option. That's my way.

[Cooker]

Are you willing to share the source code so I can try it myself on gentoo?

Linux version.
The source code of UDP sender

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>

#include <errno.h>
#include <stdarg.h> 
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define INVALID_SOCKET -1
#define SOCKET_ERROR   -1
#define CLK_TCK        CLOCKS_PER_SEC

void ErrExit(int fd, const char *format, ...);
void wait(int seconds);

int main(int argc, char *argv[])
{

	int err = 0;
	int sockfd = 0;
	struct sockaddr_in local, remote;
	int localport = 0, remoteport = 0, cycle = 0;
	int nsentbytes = 0;
	int optval;
	
	if (argc != 7) {
		printf("Usage:\n");
		printf("      program local_addr local_port cycle message remote_addr remote_port\n");
		printf("Description:\n");
		printf("      Send a UDP packet to remote_addr:remote_port from local_addr:local_port every cycle seconds\n");
		return -1;
	} 
	
	localport = strtol(argv[2], NULL, 10);
	remoteport = strtol(argv[6], NULL, 10);
	cycle = strtol(argv[3], NULL, 10);	
	
	// Create a socket that is bound to a specific transport service provider
	sockfd = socket(AF_INET, SOCK_DGRAM, 0);
	if (sockfd == INVALID_SOCKET) 
		ErrExit(0, "Can't open a socket with error %s\n", strerror(errno));

	// Contain information about local IP address and port
	local.sin_family = AF_INET;         
    	local.sin_port = htons(localport);     
    	local.sin_addr.s_addr = inet_addr(argv[1]);
    	memset(local.sin_zero, '\0', sizeof local.sin_zero);

	optval = 1;
	if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (char*)&optval, sizeof optval) == SOCKET_ERROR) {
		printf("Can't set SO_REUSEADDR option.\n");
	}

	// Associate a local address with a socket
    	err = bind(sockfd, (struct sockaddr *)&local, sizeof local);
	if (err == SOCKET_ERROR)
		ErrExit(sockfd, "Can't bind to a local address and port with error %s\n", strerror(errno));

	// Contain information about remote IP address and port
	remote.sin_family = AF_INET;         
    	remote.sin_port = htons(remoteport);     
    	remote.sin_addr.s_addr = inet_addr(argv[5]);
    	memset(remote.sin_zero, '\0', sizeof remote.sin_zero);

	for (;;) 
	{
		nsentbytes = sendto(sockfd, argv[4], strlen(argv[4]), 0, (struct sockaddr  *)&remote, sizeof remote);
		if (nsentbytes == SOCKET_ERROR) 
			ErrExit(sockfd, "Can't send meesages to remote address with error %s\n", strerror(errno));
		wait(cycle);
	}

	return 0;
}

void ErrExit(int fd, const char *format, ...)
{
	va_list args;

	va_start(args, format);
	vfprintf(stderr, format, args);
	va_end(args);

	if (fd != NULL)
		close(fd);
	exit(1);
}

void wait(int seconds)
{
	clock_t endwait;
	endwait = clock () + seconds * CLK_TCK ;
	while (clock() < endwait) {}
}

I use 'tcpdump' to check UDP message at remote PC side.