TPROXY - Transparent proxy - UDP program - RHEL6

udp_tproxy.c

/*
 * # iptables -t mangle -N DIVERT
 * # iptables -t mangle -A PREROUTING -p udp -m socket -j DIVERT
 * # iptables -t mangle -A DIVERT -j MARK --set-mark 1
 * # iptables -t mangle -A DIVERT -j ACCEPT
 * # ip rule add fwmark 1 lookup 100
 * # ip route add local 0.0.0.0/0 dev lo table 100
 * # iptables -t mangle -A PREROUTING -p udp --dport 9201 -j TPROXY --tproxy-mark 0x1/0x1 --on-port 19201
 *
 * Tested on RHEL 6
 */

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <string.h>
#include <errno.h>
#include <sys/ioctl.h>

#define MAX_RECV_BUF    (1000)

int handle_msg (struct msghdr *msg);
int send_transparently (struct msghdr *msg, struct sockaddr_in *dstaddr);

int main (int argc, char **argv)
{
        int                     s;
        short int               port;
        struct sockaddr_in      servaddr;
        struct sockaddr_in      clntaddr;
        int                     n;
        int                     ret;
        struct msghdr           msg;
        char                    cntrlbuf[64];
        struct iovec            iov[1];
        char                    buffer[MAX_RECV_BUF];
        char                    *endptr;

        if (argc < 2)
        {
                printf ("usage: %s <port>\n", argv[0]);
                return -1;
        }

        port = strtol (argv[1], &endptr, 0);
        if (*endptr || port <= 0)
        {
                fprintf (stderr, "invalid port number %s.\n", argv[1]);
                return -2;
        }

        if ((s = socket (AF_INET, SOCK_DGRAM, 0)) < 0)
        {
                fprintf (stderr, "error creating listening socket.\n");
                return -3;
        }

        n=1;
        setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &n, sizeof(n));
        setsockopt(s, SOL_SOCKET, SO_BROADCAST, &n, sizeof(n));

        n=1;
        ret = setsockopt (s, SOL_IP, IP_TRANSPARENT, &n, sizeof(int));
        if (ret != 0)
        {
                fprintf (stderr, "error setting transparency for listening socket. err (#%d %s)\n", errno, strerror(errno));
                close (s);
                return -4;
        }
        n=1;
        ret = setsockopt (s, IPPROTO_IP, IP_RECVORIGDSTADDR, &n, sizeof(int));
        if (ret != 0)
        {
                fprintf (stderr, "error setting the listening socket to IP_TRANSPARENT. err (#%d %s)\n", errno, strerror(errno));
                close (s);
                return -5;
        }

        memset (&servaddr, 0, sizeof (servaddr));
        servaddr.sin_family = AF_INET;
        servaddr.sin_addr.s_addr = htonl (INADDR_ANY);
        servaddr.sin_port = htons (port);


        if (bind (s, (struct sockaddr *) &servaddr, sizeof (servaddr)) < 0)
        {
                fprintf (stderr, "error calling bind()\n");
                return -6;
        }

        while (1)
        {
                msg.msg_name = &clntaddr;
                msg.msg_namelen = sizeof(clntaddr);
                msg.msg_control = cntrlbuf;
                msg.msg_controllen = sizeof(cntrlbuf);
                iov[0].iov_base = buffer;
                iov[0].iov_len = sizeof (buffer);
                msg.msg_iov = iov;
                msg.msg_iovlen = 1;
                ret = recvmsg (s, &msg, 0);
                if (ret <= 0)
                {
                        fprintf (stderr, "error calling recvmsg(). err (#%d %s)\n", errno, strerror(errno));
                        break;
                }

                msg.msg_iov[0].iov_len = ret;
                handle_msg (&msg);
        }

        close (s);

        return 0;
}

int handle_msg (struct msghdr *msg)
{
        struct sockaddr_in      *clntaddr;
        struct sockaddr_in      dstaddr={0,};
        struct cmsghdr          *cmsg;
        int                     ret;
        int                     found=0;

        clntaddr =  msg->msg_name;
        printf ("recvd msg from %X:%d\n", clntaddr->sin_addr.s_addr, clntaddr->sin_port);

        /* get original destination address */
        for (cmsg = CMSG_FIRSTHDR(msg); cmsg; cmsg = CMSG_NXTHDR(msg, cmsg))
        {
                if (cmsg->cmsg_level == SOL_IP && cmsg->cmsg_type == IP_RECVORIGDSTADDR)
                {
                        memcpy (&dstaddr, CMSG_DATA(cmsg), sizeof (struct sockaddr_in));
                        dstaddr.sin_family = AF_INET;
                        printf ("original dst address %X:%d\n", dstaddr.sin_addr.s_addr, dstaddr.sin_port);
                        found = 1;
                }
        }

        if (! found)
        {
                return -1;
        }

        ret = send_transparently (msg, &dstaddr);
        if (ret <= 0)
        {
                return -2;
        }

        return 0;
}

int send_transparently (struct msghdr *msg, struct sockaddr_in *dstaddr)
{
        int     d;
        int     n;
        int     ret;

        if (msg == NULL || dstaddr == NULL)
        {
                return -1;
        }

        d = socket (AF_INET, SOCK_DGRAM, 0);
        if (d == -1)
        {
                fprintf (stderr, "error creating socket (#%d %s)\n", errno, strerror(errno));
                return -2;
        }

        n=1;
        ret = setsockopt (d, SOL_IP, IP_TRANSPARENT, &n, sizeof(int));
        if (ret != 0)
        {
                fprintf (stderr, "error setting transparency towards destination. err (#%d %s)\n", errno, strerror(errno));
                close (d);
                return -3;
        }

        ret = bind (d, (struct sockaddr *)msg->msg_name, sizeof (struct sockaddr_in));
        if (ret != 0)
        {
                fprintf (stderr, "error binding to client . err (#%d %s)\n", errno, strerror(errno));
                close (d);
                return -4;
        }

        ret = sendto (d, msg->msg_iov[0].iov_base, msg->msg_iov[0].iov_len, 0, (struct sockaddr *)dstaddr, sizeof (*dstaddr));
        if (ret <= 0)
        {
                fprintf (stderr, "error sending to detination. err (#%d %s)\n", errno, strerror(errno));
                close (d);
                return -5;
        }

        close (d);

        return 0;
}