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copensslsocks

SSL connection via SOCKS5 proxy

I am trying to write a C code to open an SSL connection with a remote server via a SOCKS5 proxy server.

I have successfully opened a tcp connection with the SOCKS5 proxy server. However, I do not know how to make an SSL connection with the remote server (different from proxy server).

// p->ai_addr has the SOCKS5 proxy address
m_sock_fd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
connect(m_sock_fd, p->ai_addr, p->ai_addrlen);

char socks5_request[256] = {0}, response[256] = {0};
sprintf(socks5_request, "\5\1\0");
send(m_sock_fd, socks5_request, 4, 0);
recv(m_sock_fd, response, 256, 0);

assert(response[0] == '\5' && response[1] == '\0'); // OK

m_ssl_ctx = SSL_CTX_new(SSLv23_client_method());
m_ssl = SSL_new(m_ssl_ctx);
SSL_set_fd(m_ssl, m_sock_fd);

SSL_connect(m_ssl); // returns -1

SSL_connect failed and gave error:1408F10B:SSL routines:ssl3_get_record:wrong version number:ssl/record/ssl3_record.c:332:

I have two questions.

  1. Why am I getting this error?
  2. where do I specify the remote IP address when making the SSL connection?

Update 4/2/2019

Code below is for sending connect request to proxy server. The last call for CheckConnectivity() found the connection closed.

struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr("*.*.*.*");
addr.sin_port = htons(443);

SOCKSRequest connect_request;

// 5: protocol number, 1: connect, 0: reserved, 1: ipv4
connect_request.m_version = '\5';
connect_request.m_type = '\1';
connect_request.m_reserved = '\0';
connect_request.m_addr_type = '\1';

connect_request.m_ip = addr.sin_addr.s_addr;
connect_request.m_port = addr.sin_port;

CheckConnectivitiy();

if (!send(m_sock_fd, &connect_request, 4, 0))
    return 0;

CheckConnectivitiy();

if (!send(m_sock_fd, &connect_request.m_ip, sizeof(in_addr), 0))
    return 0;

CheckConnectivitiy();

if (!send(m_sock_fd, &connect_request.m_port, 2, 0))
    return 0;

CheckConnectivitiy();  // connection was closed by peer here!

Implementation of CheckConnectivity.

int CheckConnectivitiy() const
{
    int error = 0;
    socklen_t len = sizeof (error);
    int retval = getsockopt (m_sock_fd, SOL_SOCKET, SO_ERROR, &error, &len);

    if (retval != 0) 
    {
        /* there was a problem getting the error code */
        fprintf(stderr, "error getting socket error code: %s\n", strerror(retval));
        return 0;
    }

    if (error != 0) 
    {
        /* socket has a non zero error status */
        fprintf(stderr, "socket error: %s\n", strerror(error));
        return 0;
    }

    return 1;
}
Solution

  • You need only 1 connection, you to the proxy. You tell the proxy where it should connect to next (you are not doing that!). Only if the proxy successfully connects will you have a valid connection to the target server. Only then can you send the SSL/TLS handshake, as if you had connected to the target server directly.

    I suggest you read RFC 1928 for the SOCKS v5 protocol. You are sending an Authentication request and reading its response, but you are not sending a Connect request and reading is response, before you attempt to use SSL/TLS. That is why the handshake is failing. You are not connected to the target server yet.