/*

 * Socket Utilities

 */

#include <assert.h>

#include <c-stdaux.h>

#include <errno.h>

#include <net/if.h>

#include <stdlib.h>

#include <string.h>

#include <sys/ioctl.h>

#include <sys/socket.h>

#include "socket.h"

/**

 * socket_SIOCGIFNAME() - resolve an ifindex to an ifname

 * @socket:                     socket to operate on

 * @ifindex:                    index of network interface to resolve

 * @ifname:                     buffer to store resolved name

 *

 * This uses the SIOCGIFNAME ioctl to resolve an ifindex to an ifname. The

 * buffer provided in @ifnamep must be at least IFNAMSIZ bytes in size. The

 * maximum ifname length is IFNAMSIZ-1, and this function always

 * zero-terminates the result.

 *

 * This function is similar to if_indextoname(3) provided by glibc, but it

 * allows to specify the target socket explicitly. This allows the caller to

 * control the target network-namespace, rather than relying on the network

 * namespace of the running process.

 *

 * Return: 0 on success, negative kernel error code on failure.

 */

int socket_SIOCGIFNAME(int socket, int ifindex, char (*ifnamep)[IFNAMSIZ]) {

        struct ifreq req = { .ifr_ifindex = ifindex };

        int r;

        r = ioctl(socket, SIOCGIFNAME, &req;;

        if (r < 0)

                return -errno;

        /*

         * The linux kernel guarantees that an interface name is always

         * zero-terminated, and it always fully fits into IFNAMSIZ bytes,

         * including the zero-terminator.

         */

        memcpy(ifnamep, req.ifr_name, IFNAMSIZ);

        return 0;

}

/**

 * socket_bind_if() - bind socket to a network interface

 * @socket:                     socket to operate on

 * @ifindex:                    index of network interface to bind to, or 0

 *

 * This binds the socket given via @socket to the network interface specified

 * via @ifindex. It uses the underlying SO_BINDTODEVICE ioctl of the linux

 * kernel. However, if available, if prefers the newer SO_BINDTOIFINDEX ioctl,

 * which avoids resolving the interface name temporarily, and thus does not

 * suffer from a race-condition.

 *

 * Return: 0 on success, negative error code on failure.

 */

int socket_bind_if(int socket, int ifindex) {

        char ifname[IFNAMSIZ] = {};

        int r;

        c_assert(ifindex >= 0);

        /*

         * We first try the newer SO_BINDTOIFINDEX. If it is not available on

         * the running kernel, we fall back to SO_BINDTODEVICE. This, however,

         * requires us to first resolve the ifindex to an ifname. Note that

         * this is racy, since the device name might theoretically change

         * asynchronously.

         *

         * Using 0 as ifindex will remove the device-binding. For

         * SO_BINDTOIFINDEX we simply pass-through the 0 to the kernel, which

         * recognizes this correctly. For SO_BINDTODEVICE we pass the empty

         * string, which the kernel recognizes as a request to remove the

         * binding.

         *

         * The commit introducing SO_BINDTOIFINDEX first appeared in linux-5.1:

         *

         *     commit f5dd3d0c9638a9d9a02b5964c4ad636f06cf7e2c

         *     Author: David Herrmann <dh.herrmann@gmail.com>

         *     Date:   Tue Jan 15 14:42:14 2019 +0100

         *

         *         net: introduce SO_BINDTOIFINDEX sockopt

         *

         * In older kernels, setsockopt(2) is guaranteed to return ENOPROTOOPT

         * for this ioctl.

         */

#ifdef SO_BINDTOIFINDEX

        r = setsockopt(socket,

                       SOL_SOCKET,

                       SO_BINDTOIFINDEX,

                       &ifindex,

                       sizeof(ifindex));

        if (r >= 0)

                return 0;

        else if (errno != ENOPROTOOPT)

                return -errno;

#endif /* SO_BINDTOIFINDEX */

        if (ifindex > 0) {

                r = socket_SIOCGIFNAME(socket, ifindex, &ifname);

                if (r)

                        return r;

        }

        r = setsockopt(socket,

                       SOL_SOCKET,

                       SO_BINDTODEVICE,

                       ifname,

                       strlen(ifname));

        if (r < 0)

                return -errno;

        return 0;

}