Bind to a local port that is already in use

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

Quote:

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Originally Posted by Cooker

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

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

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.

Quote:

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Originally Posted by MikeAThon

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

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Quote:

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Originally Posted by 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.

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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:

Quote:

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Originally Posted by http://msdn2.microsoft.com/en-us/library/ms740618.aspx

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

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.

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.

Quote:

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Originally Posted by ludakot

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

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

1. UDP Receiver
   
   Code:

   ---

   #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
   
   Code:

   ---

   #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.

Quote:

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Originally Posted by 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.

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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.

Quote:

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Originally Posted by ludakot

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

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Linux version.
The source code of UDP sender

Code:

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#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) {}
}

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