congestvpn/UdpVpn.cpp

#include "UdpVpn.hpp"

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#include <poll.h>
#include <errno.h>

#include "ip_header.hpp"

static const size_t VPN_MTU = 1460; // TODO determine this -- issue #3

UdpVpn::UdpVpn()
    : _stopped(false), _vpn_mtu(VPN_MTU), _tun_dev("cvpn%d"), _peer(nullptr)
{
    _tun_dev.set_mtu(VpnPacket::get_tunnelled_mtu(_vpn_mtu));
    _socket = socket(AF_INET6, SOCK_DGRAM, 0);
    if(_socket < 0)
        throw UdpVpn::InitializationError("Cannot create socket", errno, true);
}

UdpVpn::~UdpVpn() {
    close(_socket);
}

void UdpVpn::run() {
    int rc;
    int start_at_fd = 0;  // read from polled fds in round-robin fashion
    int cur_fd;
    int nfds = 2;
    struct pollfd poll_fds[2];

    // poll_fds[0]: tun device
    poll_fds[0].fd = _tun_dev.get_fd();
    poll_fds[0].events = POLLIN;

    // poll_fds[1]: UDP socket device
    poll_fds[1].fd = _socket;
    poll_fds[1].events = POLLIN;

    while(!_stopped) {
        rc = poll(poll_fds, nfds, -1);

        if(rc < 0) {
            if(errno == EINTR)  // Interrupt.
                continue;
            throw UdpVpn::NetError(
                    "Error polling from interface", errno, true);
        }
        else if(rc == 0)  // Nothing to read
            continue;

        cur_fd = start_at_fd;
        do {
            if(poll_fds[cur_fd].revents & POLLIN) {
                if(cur_fd == 0)
                    receive_from_tun();
                else if(cur_fd == 1)
                    receive_from_udp();
                break;
            }

            cur_fd = (cur_fd + 1) % nfds;
        } while(cur_fd != start_at_fd);

        start_at_fd = (start_at_fd + 1) % nfds;
    }
}

size_t UdpVpn::read_from_tun(char* buffer, size_t len) {
    // We know that there is data available -- use `read()`
    return _tun_dev.read(buffer, len);
}

size_t UdpVpn::read_from_tun(TunnelledPacket& packet) {
    size_t payload_space = packet.get_payload_space();
    size_t nread = read_from_tun(packet.get_payload(), payload_space);
    packet.set_payload_size(nread);
    if(!packet.parse_as_ipv6()) {
        debugf("Ignoring packet with invalid header\n");
        return 0;
    }
    if(nread != packet.get_ipv6_header().packet_length()) {
        debugf("Ignoring packet with bad size (expected %d, got %d, buffer %d)\n",
               packet.get_ipv6_header().packet_length(), nread, payload_space);
        return 0;
    }
    return nread;
}

size_t UdpVpn::read_from_udp(char* buffer, size_t len,
        sockaddr_in6& peer_addr)
{
    ssize_t nread;
    socklen_t peer_addr_len = sizeof(peer_addr);
    nread = recvfrom(_socket, buffer, len, 0,
            (struct sockaddr*) &peer_addr, &peer_addr_len);

    if(nread < 0)
        throw UdpVpn::NetError("Cannot receive datagram", errno, true);
    if(nread == 0)
        return 0;

    if(peer_addr.sin6_family != AF_INET6) {
        debugf("WARNING: Received non-ipv6 family datagram %d. Ignoring.\n",
                peer_addr.sin6_family);
        return 0;
    }
    if(peer_addr_len != sizeof(peer_addr)) {
        debugf("WARNING: received unexpected source address length %u."
                "Ignoring.\n",
                peer_addr_len);
        return 0;
    }

    return nread;
}

size_t UdpVpn::read_from_udp(VpnPacket& packet, sockaddr_in6& peer_addr) {
    packet.upon_reception(); // The packet is not read yet, but it has arrived
    size_t nread =
        read_from_udp(packet.get_data(), packet.get_data_space(), peer_addr);
    packet.set_data_size(nread);
    return nread;
}

size_t UdpVpn::transmit_to_peer(VpnPacket& packet) {
    if(!_peer) {
        debugf("Dropping packet: no peer yet.\n");
        return 0;
    }
    return _peer->write(packet);
}

void UdpVpn::receive_from_tun() {
    VpnPacket packet(_vpn_mtu);
    TunnelledPacket tunnelled = TunnelledPacket::create(packet);
    size_t nread = read_from_tun(tunnelled);
    if(nread == 0)
        return;

    if(!_peer) {
        debugf("Dropping packet: no peer yet.\n");
        return;
    }
    packet.set_peer(_peer.get());

    kdebugf("Transmitting %s -> %s, size %d\n",
            format_address(tunnelled.get_ipv6_header().source.s6_addr),
            format_address(tunnelled.get_ipv6_header().dest.s6_addr),
            nread);

    packet.prepare_for_sending();
    transmit_to_peer(packet);
}

void UdpVpn::receive_from_udp() {
    VpnPacket packet(_vpn_mtu);
    sockaddr_in6 peer_ext_addr;
    size_t nread = read_from_udp(packet, peer_ext_addr);
    if(nread == 0)
        return;

    // If we don't have a peer yet -- we're just setting the peer to nullptr.
    packet.set_peer(_peer.get());

    for(VpnPacketTLV tlv=packet.first_tlv();
            !tlv.past_the_end();
            tlv.seek_next_tlv())
    {
        switch(tlv.get_type()) {
            case VpnPacket::PAYLOAD_TYPE_TUNNELLED:
                {
                    TunnelledPacket tunnelled(tlv);
                    acquire_peer(tunnelled, peer_ext_addr);
                    receive_tunnelled_tlv(tunnelled);
                }
                break;

            case VpnPacket::PAYLOAD_TYPE_UNDEF:
            default:
                debugf("#%d+%lu: ignoring TLV with bad type %d.\n",
                        packet.get_seqno(), tlv.get_offset(),
                        tlv.get_type());
                break;
        }
    }
}

void UdpVpn::receive_tunnelled_tlv(TunnelledPacket& packet) {
    // Reinject into tun
    kdebugf("Reinjecting tunnelled packet of size %d\n",
            packet.get_payload_size());
    _tun_dev.write(packet.get_payload(), packet.get_payload_size());
}