Question

I am writing a Java interposer (using LD_PRELOAD method) that modifies the recipient information in network communication system calls (connect/sendto).

Whenever Java tries to connect to another socket, I modify the intended recipient IP and port. Java uses IPv4-mapped-IPv6 addresses. So, I need to extract the IPv4 part of it. I achieve this using the method prescribed by Nicolas Bachschmidt at link.

The problem I am facing is that for every IPv4-mapped-IPv6 address, the result string (IPv4 part) I obtain is always 0.0.0.1. Instead it should be 10.0.0.1 (for ::ffff:10.0.0.1). I have tried this with different IP addresses. The result is always the same.

Two things I would like to mention that I think may be related:

1. When I tested the same program a month ago on my local network (that has 192.168.1.XXX IP addresses), the program worked correctly. Point being (I don’t think) there is any problem with code. To verify this, I asked a question on stackoverflow to convert IPv4-mapped-IPv6 addresses to IPv4, the link of which is mentioned earlier).

2. I am trying to test this program now on my university network (that has 10.XXX.XXX.XXX IP addresses) and VirtualBox (NAT mode that also gives 10.XXX.XXX.XXX addresses). However, I have tried to connect to 10.0.0.1 and 12.0.0.1 in these cases. Both give 0.0.0.1.

What am I doing wrong?

UPDATE: In Java, socket connection is done by the usual method:

Socket conn = new Socket("10.0.0.1", 50021);

The code to interpose this connect() system call is as follows:

int connect(int fd, const struct sockaddr *sk, socklen_t sl)
{
    struct sockaddr_in      *lsk_in  = (struct sockaddr_in *)  sk;
    struct sockaddr_in6     *lsk_in6 = (struct sockaddr_in6 *) sk;

    struct sockaddr_in      addr4;

    unsigned int            len;
    int                     nbytes, oport, tport, ret, i;
    char                    ip_address[30];
    char                    buffer[1024];   
    char                    tempBuffer[1024];   

    if((lsk_in->sin_family == AF_INET) || (lsk_in->sin_family == AF_INET6))
    {
        if(lsk_in->sin_family == AF_INET)
        {
            oport = ntohs(lsk_in->sin_port);
            memcpy(&addr4.sin_addr.s_addr, &lsk_in->sin_addr.s_addr, sizeof(addr4.sin_addr.s_addr));
        }
        else if(lsk_in->sin_family == AF_INET6)
        {
            oport = ntohs(lsk_in6->sin6_port);

            //This is where the problem is. I always get 0.0.0.1
            memcpy(&addr4.sin_addr.s_addr, lsk_in6->sin6_addr.s6_addr+12, sizeof(addr4.sin_addr.s_addr)); 
        }

        memset(buffer, '\0', sizeof(buffer));
        sprintf(buffer, "%s%c%s%c%i", NAT_VM_CONNECT_RULE, NAT_VM_DELIMITER, (char *)inet_ntoa(addr4.sin_addr), NAT_VM_DELIMITER, oport);

        nbytes = send(sock, buffer, strlen(buffer), 0);
        if(DEBUG_MODE)
            fprintf(stdout, "[LD_INTERPOSER] Sent[%s]\n", buffer);

        memset(buffer, '\0', sizeof(buffer));
        nbytes = recv(sock, buffer, sizeof(buffer), 0);

        fprintf(stderr, "[LD_INTERPOSER] Received CONNECT [%s]\n", buffer);

        memset(ip_address, '\0', sizeof(ip_address));
        int pos = strrchr(buffer, NAT_VM_DELIMITER) - buffer;

        strncpy(ip_address, buffer, pos);
        ip_address[pos] = '\0';
        tport = atoi(buffer + pos + 1);

        if(lsk_in->sin_family == AF_INET)
        {
            lsk_in->sin_addr.s_addr = inet_addr(ip_address + 7);
            lsk_in->sin_port = htons(tport);
        }
        else if(lsk_in->sin_family == AF_INET6)
        {
            inet_pton(AF_INET6, ip_address, &(lsk_in6->sin6_addr));
            lsk_in6->sin6_port = htons(tport);
        }

        fprintf(stderr, "[LD_INTERPOSER] IP[%s], Port[%d] for VM[%s]\n", ip_address, tport, vm_ip);
    }

    return real_connect(fd, sk, sl);
}

Answer 1

Thanks to @ugoren hex dump technique (in comments), I was able to figure out that the IPv6 structure itself contained a 0.0.0.1 address. I realized that the problem may be due to different JDKs. The Java project was built using OpenJDK 7 while the PC I was using had OpenJDK 6. When I updated the JDK to version 7, the error disappeared. However, it has landed me to another error which is documented at a new stackoverflow question which I am still unable to resolve.