[tinc] / src / net_packet.c

/*
    net_packet.c -- Handles in- and outgoing VPN packets
    Copyright (C) 1998-2005 Ivo Timmermans,
                  2000-2014 Guus Sliepen <guus@tinc-vpn.org>
                  2010      Timothy Redaelli <timothy@redaelli.eu>
                  2010      Brandon Black <blblack@gmail.com>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License along
    with this program; if not, write to the Free Software Foundation, Inc.,
    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/

#include "system.h"

#ifdef HAVE_ZLIB
#include <zlib.h>
#endif

#ifdef HAVE_LZO
#include LZO1X_H
#endif

#include "cipher.h"
#include "conf.h"
#include "connection.h"
#include "crypto.h"
#include "digest.h"
#include "device.h"
#include "ethernet.h"
#include "ipv4.h"
#include "ipv6.h"
#include "graph.h"
#include "logger.h"
#include "net.h"
#include "netutl.h"
#include "protocol.h"
#include "route.h"
#include "utils.h"
#include "xalloc.h"

#ifndef MAX
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#endif

int keylifetime = 0;
#ifdef HAVE_LZO
static char lzo_wrkmem[LZO1X_999_MEM_COMPRESS > LZO1X_1_MEM_COMPRESS ? LZO1X_999_MEM_COMPRESS : LZO1X_1_MEM_COMPRESS];
#endif

static void send_udppacket(node_t *, vpn_packet_t *);

unsigned replaywin = 16;
bool localdiscovery = true;
bool udp_discovery = true;
int udp_discovery_keepalive_interval = 9;
int udp_discovery_interval = 2;
int udp_discovery_timeout = 30;

#define MAX_SEQNO 1073741824

static void try_fix_mtu(node_t *n) {
        if(n->mtuprobes < 0)
                return;

        if(n->mtuprobes == 20 || n->minmtu >= n->maxmtu) {
                if(n->minmtu > n->maxmtu)
                        n->minmtu = n->maxmtu;
                else
                        n->maxmtu = n->minmtu;
                n->mtu = n->minmtu;
                logger(DEBUG_TRAFFIC, LOG_INFO, "Fixing MTU of %s (%s) to %d after %d probes", n->name, n->hostname, n->mtu, n->mtuprobes);
                n->mtuprobes = -1;
        }
}

static void udp_probe_timeout_handler(void *data) {
        node_t *n = data;
        if(!n->status.udp_confirmed)
                return;

        logger(DEBUG_TRAFFIC, LOG_INFO, "Too much time has elapsed since last UDP ping response from %s (%s), stopping UDP communication", n->name, n->hostname);
        n->status.udp_confirmed = false;
        n->mtuprobes = 0;
        n->minmtu = 0;
        n->maxmtu = MTU;
}

static void udp_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
        if(!DATA(packet)[0]) {
                logger(DEBUG_TRAFFIC, LOG_INFO, "Got UDP probe request %d from %s (%s)", packet->len, n->name, n->hostname);

                /* It's a probe request, send back a reply */

                /* Type 2 probe replies were introduced in protocol 17.3 */
                if ((n->options >> 24) >= 3) {
                        uint8_t *data = DATA(packet);
                        *data++ = 2;
                        uint16_t len16 = htons(len); memcpy(data, &len16, 2); data += 2;
                        struct timeval now;
                        gettimeofday(&now, NULL);
                        uint32_t sec = htonl(now.tv_sec); memcpy(data, &sec, 4); data += 4;
                        uint32_t usec = htonl(now.tv_usec); memcpy(data, &usec, 4); data += 4;
                        packet->len -= 10;
                } else {
                        /* Legacy protocol: n won't understand type 2 probe replies. */
                        DATA(packet)[0] = 1;
                }

                /* Temporarily set udp_confirmed, so that the reply is sent
                   back exactly the way it came in. */

                bool udp_confirmed = n->status.udp_confirmed;
                n->status.udp_confirmed = true;
                send_udppacket(n, packet);
                n->status.udp_confirmed = udp_confirmed;
        } else {
                length_t probelen = len;
                if (DATA(packet)[0] == 2) {
                        if (len < 3)
                                logger(DEBUG_TRAFFIC, LOG_WARNING, "Received invalid (too short) UDP probe reply from %s (%s)", n->name, n->hostname);
                        else {
                                uint16_t probelen16; memcpy(&probelen16, DATA(packet) + 1, 2); probelen = ntohs(probelen16);
                        }
                }
                logger(DEBUG_TRAFFIC, LOG_INFO, "Got type %d UDP probe reply %d from %s (%s)", DATA(packet)[0], probelen, n->name, n->hostname);

                /* It's a valid reply: now we know bidirectional communication
                   is possible using the address and socket that the reply
                   packet used. */
                n->status.udp_confirmed = true;

                if(udp_discovery) {
                        timeout_del(&n->udp_ping_timeout);
                        timeout_add(&n->udp_ping_timeout, &udp_probe_timeout_handler, n, &(struct timeval){udp_discovery_timeout, 0});
                }

                if(probelen >= n->maxmtu + 1) {
                        logger(DEBUG_TRAFFIC, LOG_INFO, "Increase in PMTU to %s (%s) detected, restarting PMTU discovery", n->name, n->hostname);
                        n->maxmtu = MTU;
                        /* Set mtuprobes to 1 so that try_mtu() doesn't reset maxmtu */
                        n->mtuprobes = 1;
                        return;
                }

                /* If applicable, raise the minimum supported MTU */

                if(probelen > n->maxmtu)
                        probelen = n->maxmtu;
                if(n->minmtu < probelen) {
                        n->minmtu = probelen;
                        try_fix_mtu(n);
                }

                /* Calculate RTT.
                   The RTT is the time between the MTU probe burst was sent and the first
                   reply is received.
                 */

                struct timeval now, diff;
                gettimeofday(&now, NULL);
                timersub(&now, &n->probe_time, &diff);

                struct timeval probe_timestamp = now;
                if (DATA(packet)[0] == 2 && packet->len >= 11) {
                        uint32_t sec; memcpy(&sec, DATA(packet) + 3, 4);
                        uint32_t usec; memcpy(&usec, DATA(packet) + 7, 4);
                        probe_timestamp.tv_sec = ntohl(sec);
                        probe_timestamp.tv_usec = ntohl(usec);
                }
                
                n->probe_counter++;

                if(n->probe_counter == 1) {
                        n->rtt = diff.tv_sec + diff.tv_usec * 1e-6;
                        n->probe_time = probe_timestamp;
                        logger(DEBUG_TRAFFIC, LOG_DEBUG, "%s (%s) RTT %.2f ms, rx packet loss %.2f %%", n->name, n->hostname, n->rtt * 1e3, n->packetloss * 1e2);
                }
        }
}

static length_t compress_packet(uint8_t *dest, const uint8_t *source, length_t len, int level) {
        if(level == 0) {
                memcpy(dest, source, len);
                return len;
        } else if(level == 10) {
#ifdef HAVE_LZO
                lzo_uint lzolen = MAXSIZE;
                lzo1x_1_compress(source, len, dest, &lzolen, lzo_wrkmem);
                return lzolen;
#else
                return -1;
#endif
        } else if(level < 10) {
#ifdef HAVE_ZLIB
                unsigned long destlen = MAXSIZE;
                if(compress2(dest, &destlen, source, len, level) == Z_OK)
                        return destlen;
                else
#endif
                        return -1;
        } else {
#ifdef HAVE_LZO
                lzo_uint lzolen = MAXSIZE;
                lzo1x_999_compress(source, len, dest, &lzolen, lzo_wrkmem);
                return lzolen;
#else
                return -1;
#endif
        }

        return -1;
}

static length_t uncompress_packet(uint8_t *dest, const uint8_t *source, length_t len, int level) {
        if(level == 0) {
                memcpy(dest, source, len);
                return len;
        } else if(level > 9) {
#ifdef HAVE_LZO
                lzo_uint lzolen = MAXSIZE;
                if(lzo1x_decompress_safe(source, len, dest, &lzolen, NULL) == LZO_E_OK)
                        return lzolen;
                else
#endif
                        return -1;
        }
#ifdef HAVE_ZLIB
        else {
                unsigned long destlen = MAXSIZE;
                if(uncompress(dest, &destlen, source, len) == Z_OK)
                        return destlen;
                else
                        return -1;
        }
#endif

        return -1;
}

/* VPN packet I/O */

static void receive_packet(node_t *n, vpn_packet_t *packet) {
        logger(DEBUG_TRAFFIC, LOG_DEBUG, "Received packet of %d bytes from %s (%s)",
                           packet->len, n->name, n->hostname);

        n->in_packets++;
        n->in_bytes += packet->len;

        route(n, packet);
}

static bool try_mac(node_t *n, const vpn_packet_t *inpkt) {
        if(n->status.sptps)
                return sptps_verify_datagram(&n->sptps, DATA(inpkt), inpkt->len);

#ifdef DISABLE_LEGACY
        return false;
#else
        if(!digest_active(n->indigest) || inpkt->len < sizeof(seqno_t) + digest_length(n->indigest))
                return false;

        return digest_verify(n->indigest, SEQNO(inpkt), inpkt->len - digest_length(n->indigest), DATA(inpkt) + inpkt->len - digest_length(n->indigest));
#endif
}

static bool receive_udppacket(node_t *n, vpn_packet_t *inpkt) {
        vpn_packet_t pkt1, pkt2;
        vpn_packet_t *pkt[] = { &pkt1, &pkt2, &pkt1, &pkt2 };
        int nextpkt = 0;
        size_t outlen;
        pkt1.offset = DEFAULT_PACKET_OFFSET;
        pkt2.offset = DEFAULT_PACKET_OFFSET;

        if(n->status.sptps) {
                if(!n->sptps.state) {
                        if(!n->status.waitingforkey) {
                                logger(DEBUG_TRAFFIC, LOG_DEBUG, "Got packet from %s (%s) but we haven't exchanged keys yet", n->name, n->hostname);
                                send_req_key(n);
                        } else {
                                logger(DEBUG_TRAFFIC, LOG_DEBUG, "Got packet from %s (%s) but he hasn't got our key yet", n->name, n->hostname);
                        }
                        return false;
                }
                inpkt->offset += 2 * sizeof(node_id_t);
                if(!sptps_receive_data(&n->sptps, DATA(inpkt), inpkt->len - 2 * sizeof(node_id_t))) {
                        logger(DEBUG_TRAFFIC, LOG_ERR, "Got bad packet from %s (%s)", n->name, n->hostname);
                        return false;
                }
                return true;
        }

#ifdef DISABLE_LEGACY
        return false;
#else
        if(!n->status.validkey_in) {
                logger(DEBUG_TRAFFIC, LOG_DEBUG, "Got packet from %s (%s) but he hasn't got our key yet", n->name, n->hostname);
                return false;
        }

        /* Check packet length */

        if(inpkt->len < sizeof(seqno_t) + digest_length(n->indigest)) {
                logger(DEBUG_TRAFFIC, LOG_DEBUG, "Got too short packet from %s (%s)",
                                        n->name, n->hostname);
                return false;
        }

        /* It's a legacy UDP packet, the data starts after the seqno */

        inpkt->offset += sizeof(seqno_t);

        /* Check the message authentication code */

        if(digest_active(n->indigest)) {
                inpkt->len -= digest_length(n->indigest);
                if(!digest_verify(n->indigest, SEQNO(inpkt), inpkt->len, SEQNO(inpkt) + inpkt->len)) {
                        logger(DEBUG_TRAFFIC, LOG_DEBUG, "Got unauthenticated packet from %s (%s)", n->name, n->hostname);
                        return false;
                }
        }
        /* Decrypt the packet */

        if(cipher_active(n->incipher)) {
                vpn_packet_t *outpkt = pkt[nextpkt++];
                outlen = MAXSIZE;

                if(!cipher_decrypt(n->incipher, SEQNO(inpkt), inpkt->len, SEQNO(outpkt), &outlen, true)) {
                        logger(DEBUG_TRAFFIC, LOG_DEBUG, "Error decrypting packet from %s (%s)", n->name, n->hostname);
                        return false;
                }

                outpkt->len = outlen;
                inpkt = outpkt;
        }

        /* Check the sequence number */

        seqno_t seqno;
        memcpy(&seqno, SEQNO(inpkt), sizeof seqno);
        seqno = ntohl(seqno);
        inpkt->len -= sizeof seqno;

        if(replaywin) {
                if(seqno != n->received_seqno + 1) {
                        if(seqno >= n->received_seqno + replaywin * 8) {
                                if(n->farfuture++ < replaywin >> 2) {
                                        logger(DEBUG_ALWAYS, LOG_WARNING, "Packet from %s (%s) is %d seqs in the future, dropped (%u)",
                                                n->name, n->hostname, seqno - n->received_seqno - 1, n->farfuture);
                                        return false;
                                }
                                logger(DEBUG_ALWAYS, LOG_WARNING, "Lost %d packets from %s (%s)",
                                                seqno - n->received_seqno - 1, n->name, n->hostname);
                                memset(n->late, 0, replaywin);
                        } else if (seqno <= n->received_seqno) {
                                if((n->received_seqno >= replaywin * 8 && seqno <= n->received_seqno - replaywin * 8) || !(n->late[(seqno / 8) % replaywin] & (1 << seqno % 8))) {
                                        logger(DEBUG_ALWAYS, LOG_WARNING, "Got late or replayed packet from %s (%s), seqno %d, last received %d",
                                                n->name, n->hostname, seqno, n->received_seqno);
                                        return false;
                                }
                        } else {
                                for(int i = n->received_seqno + 1; i < seqno; i++)
                                        n->late[(i / 8) % replaywin] |= 1 << i % 8;
                        }
                }

                n->farfuture = 0;
                n->late[(seqno / 8) % replaywin] &= ~(1 << seqno % 8);
        }

        if(seqno > n->received_seqno)
                n->received_seqno = seqno;

        n->received++;

        if(n->received_seqno > MAX_SEQNO)
                regenerate_key();

        /* Decompress the packet */

        length_t origlen = inpkt->len;

        if(n->incompression) {
                vpn_packet_t *outpkt = pkt[nextpkt++];

                if((outpkt->len = uncompress_packet(DATA(outpkt), DATA(inpkt), inpkt->len, n->incompression)) < 0) {
                        logger(DEBUG_TRAFFIC, LOG_ERR, "Error while uncompressing packet from %s (%s)",
                                                 n->name, n->hostname);
                        return false;
                }

                inpkt = outpkt;

                origlen -= MTU/64 + 20;
        }

        inpkt->priority = 0;

        if(!DATA(inpkt)[12] && !DATA(inpkt)[13])
                udp_probe_h(n, inpkt, origlen);
        else
                receive_packet(n, inpkt);
        return true;
#endif
}

void receive_tcppacket(connection_t *c, const char *buffer, int len) {
        vpn_packet_t outpkt;
        outpkt.offset = DEFAULT_PACKET_OFFSET;

        if(len > sizeof outpkt.data - outpkt.offset)
                return;

        outpkt.len = len;
        if(c->options & OPTION_TCPONLY)
                outpkt.priority = 0;
        else
                outpkt.priority = -1;
        memcpy(DATA(&outpkt), buffer, len);

        receive_packet(c->node, &outpkt);
}

static void send_sptps_packet(node_t *n, vpn_packet_t *origpkt) {
        if(!n->status.validkey && !n->connection)
                return;

        uint8_t type = 0;
        int offset = 0;

        if(!(DATA(origpkt)[12] | DATA(origpkt)[13])) {
                sptps_send_record(&n->sptps, PKT_PROBE, (char *)DATA(origpkt), origpkt->len);
                return;
        }

        if(routing_mode == RMODE_ROUTER)
                offset = 14;
        else
                type = PKT_MAC;

        if(origpkt->len < offset)
                return;

        vpn_packet_t outpkt;

        if(n->outcompression) {
                outpkt.offset = 0;
                int len = compress_packet(DATA(&outpkt) + offset, DATA(origpkt) + offset, origpkt->len - offset, n->outcompression);
                if(len < 0) {
                        logger(DEBUG_TRAFFIC, LOG_ERR, "Error while compressing packet to %s (%s)", n->name, n->hostname);
                } else if(len < origpkt->len - offset) {
                        outpkt.len = len + offset;
                        origpkt = &outpkt;
                        type |= PKT_COMPRESSED;
                }
        }

        /* If we have a direct metaconnection to n, and we can't use UDP, then
           don't bother with SPTPS and just use a "plaintext" PACKET message.
           We don't really care about end-to-end security since we're not
           sending the message through any intermediate nodes. */
        if(n->connection && origpkt->len > n->minmtu)
                send_tcppacket(n->connection, origpkt);
        else
                sptps_send_record(&n->sptps, type, DATA(origpkt) + offset, origpkt->len - offset);
        return;
}

static void adapt_socket(const sockaddr_t *sa, int *sock) {
        /* Make sure we have a suitable socket for the chosen address */
        if(listen_socket[*sock].sa.sa.sa_family != sa->sa.sa_family) {
                for(int i = 0; i < listen_sockets; i++) {
                        if(listen_socket[i].sa.sa.sa_family == sa->sa.sa_family) {
                                *sock = i;
                                break;
                        }
                }
        }
}

static void choose_udp_address(const node_t *n, const sockaddr_t **sa, int *sock) {
        /* Latest guess */
        *sa = &n->address;
        *sock = n->sock;

        /* If the UDP address is confirmed, use it. */
        if(n->status.udp_confirmed)
                return;

        /* Send every third packet to n->address; that could be set
           to the node's reflexive UDP address discovered during key
           exchange. */

        static int x = 0;
        if(++x >= 3) {
                x = 0;
                return;
        }

        /* Otherwise, address are found in edges to this node.
           So we pick a random edge and a random socket. */

        int i = 0;
        int j = rand() % n->edge_tree->count;
        edge_t *candidate = NULL;

        for splay_each(edge_t, e, n->edge_tree) {
                if(i++ == j) {
                        candidate = e->reverse;
                        break;
                }
        }

        if(candidate) {
                *sa = &candidate->address;
                *sock = rand() % listen_sockets;
        }

        adapt_socket(*sa, sock);
}

static void choose_local_address(const node_t *n, const sockaddr_t **sa, int *sock) {
        *sa = NULL;

        /* Pick one of the edges from this node at random, then use its local address. */

        int i = 0;
        int j = rand() % n->edge_tree->count;
        edge_t *candidate = NULL;

        for splay_each(edge_t, e, n->edge_tree) {
                if(i++ == j) {
                        candidate = e;
                        break;
                }
        }

        if (candidate && candidate->local_address.sa.sa_family) {
                *sa = &candidate->local_address;
                *sock = rand() % listen_sockets;
                adapt_socket(*sa, sock);
        }
}

static void send_udppacket(node_t *n, vpn_packet_t *origpkt) {
        vpn_packet_t pkt1, pkt2;
        vpn_packet_t *pkt[] = { &pkt1, &pkt2, &pkt1, &pkt2 };
        vpn_packet_t *inpkt = origpkt;
        int nextpkt = 0;
        vpn_packet_t *outpkt;
        int origlen = origpkt->len;
        size_t outlen;
#if defined(SOL_IP) && defined(IP_TOS)
        static int priority = 0;
        int origpriority = origpkt->priority;
#endif

        pkt1.offset = DEFAULT_PACKET_OFFSET;
        pkt2.offset = DEFAULT_PACKET_OFFSET;

        if(!n->status.reachable) {
                logger(DEBUG_TRAFFIC, LOG_INFO, "Trying to send UDP packet to unreachable node %s (%s)", n->name, n->hostname);
                return;
        }

        if(n->status.sptps)
                return send_sptps_packet(n, origpkt);

#ifdef DISABLE_LEGACY
        return;
#else
        /* Make sure we have a valid key */

        if(!n->status.validkey) {
                logger(DEBUG_TRAFFIC, LOG_INFO,
                                   "No valid key known yet for %s (%s), forwarding via TCP",
                                   n->name, n->hostname);
                send_tcppacket(n->nexthop->connection, origpkt);
                return;
        }

        if(n->options & OPTION_PMTU_DISCOVERY && inpkt->len > n->minmtu && (DATA(inpkt)[12] | DATA(inpkt)[13])) {
                logger(DEBUG_TRAFFIC, LOG_INFO,
                                "Packet for %s (%s) larger than minimum MTU, forwarding via %s",
                                n->name, n->hostname, n != n->nexthop ? n->nexthop->name : "TCP");

                if(n != n->nexthop)
                        send_packet(n->nexthop, origpkt);
                else
                        send_tcppacket(n->nexthop->connection, origpkt);

                return;
        }

        /* Compress the packet */

        if(n->outcompression) {
                outpkt = pkt[nextpkt++];

                if((outpkt->len = compress_packet(DATA(outpkt), DATA(inpkt), inpkt->len, n->outcompression)) < 0) {
                        logger(DEBUG_TRAFFIC, LOG_ERR, "Error while compressing packet to %s (%s)",
                                   n->name, n->hostname);
                        return;
                }

                inpkt = outpkt;
        }

        /* Add sequence number */

        seqno_t seqno = htonl(++(n->sent_seqno));
        memcpy(SEQNO(inpkt), &seqno, sizeof seqno);
        inpkt->len += sizeof seqno;

        /* Encrypt the packet */

        if(cipher_active(n->outcipher)) {
                outpkt = pkt[nextpkt++];
                outlen = MAXSIZE;

                if(!cipher_encrypt(n->outcipher, SEQNO(inpkt), inpkt->len, SEQNO(outpkt), &outlen, true)) {
                        logger(DEBUG_TRAFFIC, LOG_ERR, "Error while encrypting packet to %s (%s)", n->name, n->hostname);
                        goto end;
                }

                outpkt->len = outlen;
                inpkt = outpkt;
        }

        /* Add the message authentication code */

        if(digest_active(n->outdigest)) {
                if(!digest_create(n->outdigest, SEQNO(inpkt), inpkt->len, SEQNO(inpkt) + inpkt->len)) {
                        logger(DEBUG_TRAFFIC, LOG_ERR, "Error while encrypting packet to %s (%s)", n->name, n->hostname);
                        goto end;
                }

                inpkt->len += digest_length(n->outdigest);
        }

        /* Send the packet */

        const sockaddr_t *sa = NULL;
        int sock;

        if(n->status.send_locally)
                choose_local_address(n, &sa, &sock);
        if(!sa)
                choose_udp_address(n, &sa, &sock);

#if defined(SOL_IP) && defined(IP_TOS)
        if(priorityinheritance && origpriority != priority
           && listen_socket[n->sock].sa.sa.sa_family == AF_INET) {
                priority = origpriority;
                logger(DEBUG_TRAFFIC, LOG_DEBUG, "Setting outgoing packet priority to %d", priority);
                if(setsockopt(listen_socket[n->sock].udp.fd, SOL_IP, IP_TOS, &priority, sizeof(priority))) /* SO_PRIORITY doesn't seem to work */
                        logger(DEBUG_ALWAYS, LOG_ERR, "System call `%s' failed: %s", "setsockopt", sockstrerror(sockerrno));
        }
#endif

        if(sendto(listen_socket[sock].udp.fd, SEQNO(inpkt), inpkt->len, 0, &sa->sa, SALEN(sa->sa)) < 0 && !sockwouldblock(sockerrno)) {
                if(sockmsgsize(sockerrno)) {
                        if(n->maxmtu >= origlen)
                                n->maxmtu = origlen - 1;
                        if(n->mtu >= origlen)
                                n->mtu = origlen - 1;
                        try_fix_mtu(n);
                } else
                        logger(DEBUG_TRAFFIC, LOG_WARNING, "Error sending packet to %s (%s): %s", n->name, n->hostname, sockstrerror(sockerrno));
        }

end:
        origpkt->len = origlen;
#endif
}

static bool send_sptps_data_priv(node_t *to, node_t *from, int type, const void *data, size_t len) {
        node_t *relay = (to->via != myself && (type == PKT_PROBE || (len - SPTPS_DATAGRAM_OVERHEAD) <= to->via->minmtu)) ? to->via : to->nexthop;
        bool direct = from == myself && to == relay;
        bool relay_supported = (relay->options >> 24) >= 4;
        bool tcponly = (myself->options | relay->options) & OPTION_TCPONLY;

        /* Send it via TCP if it is a handshake packet, TCPOnly is in use, this is a relay packet that the other node cannot understand, or this packet is larger than the MTU.
           TODO: When relaying, the original sender does not know the end-to-end PMTU (it only knows the PMTU of the first hop).
                 This can lead to scenarios where large packets are sent over UDP to relay, but then relay has no choice but fall back to TCP. */

        if(type == SPTPS_HANDSHAKE || tcponly || (!direct && !relay_supported) || (type != PKT_PROBE && (len - SPTPS_DATAGRAM_OVERHEAD) > relay->minmtu)) {
                char buf[len * 4 / 3 + 5];
                b64encode(data, buf, len);
                /* If no valid key is known yet, send the packets using ANS_KEY requests,
                   to ensure we get to learn the reflexive UDP address. */
                if(from == myself && !to->status.validkey) {
                        to->incompression = myself->incompression;
                        return send_request(to->nexthop->connection, "%d %s %s %s -1 -1 -1 %d", ANS_KEY, from->name, to->name, buf, to->incompression);
                } else {
                        return send_request(to->nexthop->connection, "%d %s %s %d %s", REQ_KEY, from->name, to->name, REQ_SPTPS, buf);
                }
        }

        size_t overhead = 0;
        if(relay_supported) overhead += sizeof to->id + sizeof from->id;
        char buf[len + overhead]; char* buf_ptr = buf;
        if(relay_supported) {
                if(direct) {
                        /* Inform the recipient that this packet was sent directly. */
                        node_id_t nullid = {};
                        memcpy(buf_ptr, &nullid, sizeof nullid); buf_ptr += sizeof nullid;
                } else {
                        memcpy(buf_ptr, &to->id, sizeof to->id); buf_ptr += sizeof to->id;
                }
                memcpy(buf_ptr, &from->id, sizeof from->id); buf_ptr += sizeof from->id;

        }
        /* TODO: if this copy turns out to be a performance concern, change sptps_send_record() to add some "pre-padding" to the buffer and use that instead */
        memcpy(buf_ptr, data, len); buf_ptr += len;

        const sockaddr_t *sa = NULL;
        int sock;
        if(relay->status.send_locally)
                choose_local_address(relay, &sa, &sock);
        if(!sa)
                choose_udp_address(relay, &sa, &sock);
        logger(DEBUG_TRAFFIC, LOG_INFO, "Sending packet from %s (%s) to %s (%s) via %s (%s)", from->name, from->hostname, to->name, to->hostname, relay->name, relay->hostname);
        if(sendto(listen_socket[sock].udp.fd, buf, buf_ptr - buf, 0, &sa->sa, SALEN(sa->sa)) < 0 && !sockwouldblock(sockerrno)) {
                if(sockmsgsize(sockerrno)) {
                        // Compensate for SPTPS overhead
                        len -= SPTPS_DATAGRAM_OVERHEAD;
                        if(relay->maxmtu >= len)
                                relay->maxmtu = len - 1;
                        if(relay->mtu >= len)
                                relay->mtu = len - 1;
                        try_fix_mtu(relay);
                } else {
                        logger(DEBUG_TRAFFIC, LOG_WARNING, "Error sending UDP SPTPS packet to %s (%s): %s", relay->name, relay->hostname, sockstrerror(sockerrno));
                        return false;
                }
        }

        return true;
}

bool send_sptps_data(void *handle, uint8_t type, const void *data, size_t len) {
        return send_sptps_data_priv(handle, myself, type, data, len);
}

bool receive_sptps_record(void *handle, uint8_t type, const void *data, uint16_t len) {
        node_t *from = handle;

        if(type == SPTPS_HANDSHAKE) {
                if(!from->status.validkey) {
                        from->status.validkey = true;
                        from->status.waitingforkey = false;
                        logger(DEBUG_META, LOG_INFO, "SPTPS key exchange with %s (%s) succesful", from->name, from->hostname);
                }
                return true;
        }

        if(len > MTU) {
                logger(DEBUG_ALWAYS, LOG_ERR, "Packet from %s (%s) larger than maximum supported size (%d > %d)", from->name, from->hostname, len, MTU);
                return false;
        }

        vpn_packet_t inpkt;
        inpkt.offset = DEFAULT_PACKET_OFFSET;

        if(type == PKT_PROBE) {
                inpkt.len = len;
                memcpy(DATA(&inpkt), data, len);
                udp_probe_h(from, &inpkt, len);
                return true;
        }

        if(type & ~(PKT_COMPRESSED | PKT_MAC)) {
                logger(DEBUG_ALWAYS, LOG_ERR, "Unexpected SPTPS record type %d len %d from %s (%s)", type, len, from->name, from->hostname);
                return false;
        }

        /* Check if we have the headers we need */
        if(routing_mode != RMODE_ROUTER && !(type & PKT_MAC)) {
                logger(DEBUG_TRAFFIC, LOG_ERR, "Received packet from %s (%s) without MAC header (maybe Mode is not set correctly)", from->name, from->hostname);
                return false;
        } else if(routing_mode == RMODE_ROUTER && (type & PKT_MAC)) {
                logger(DEBUG_TRAFFIC, LOG_WARNING, "Received packet from %s (%s) with MAC header (maybe Mode is not set correctly)", from->name, from->hostname);
        }

        int offset = (type & PKT_MAC) ? 0 : 14;
        if(type & PKT_COMPRESSED) {
                length_t ulen = uncompress_packet(DATA(&inpkt) + offset, (const uint8_t *)data, len, from->incompression);
                if(ulen < 0) {
                        return false;
                } else {
                        inpkt.len = ulen + offset;
                }
                if(inpkt.len > MAXSIZE)
                        abort();
        } else {
                memcpy(DATA(&inpkt) + offset, data, len);
                inpkt.len = len + offset;
        }

        /* Generate the Ethernet packet type if necessary */
        if(offset) {
                switch(DATA(&inpkt)[14] >> 4) {
                        case 4:
                                DATA(&inpkt)[12] = 0x08;
                                DATA(&inpkt)[13] = 0x00;
                                break;
                        case 6:
                                DATA(&inpkt)[12] = 0x86;
                                DATA(&inpkt)[13] = 0xDD;
                                break;
                        default:
                                logger(DEBUG_TRAFFIC, LOG_ERR,
                                                   "Unknown IP version %d while reading packet from %s (%s)",
                                                   DATA(&inpkt)[14] >> 4, from->name, from->hostname);
                                return false;
                }
        }

        receive_packet(from, &inpkt);
        return true;
}

// This function tries to get SPTPS keys, if they aren't already known.
// This function makes no guarantees - it is up to the caller to check the node's state to figure out if the keys are available.
static void try_sptps(node_t *n) {
        if(n->status.validkey)
                return;

        logger(DEBUG_TRAFFIC, LOG_INFO, "No valid key known yet for %s (%s)", n->name, n->hostname);

        if(!n->status.waitingforkey)
                send_req_key(n);
        else if(n->last_req_key + 10 < now.tv_sec) {
                logger(DEBUG_ALWAYS, LOG_DEBUG, "No key from %s after 10 seconds, restarting SPTPS", n->name);
                sptps_stop(&n->sptps);
                n->status.waitingforkey = false;
                send_req_key(n);
        }

        return;
}

static void send_udp_probe_packet(node_t *n, int len) {
        vpn_packet_t packet;
        packet.offset = DEFAULT_PACKET_OFFSET;
        memset(DATA(&packet), 0, 14);
        randomize(DATA(&packet) + 14, len - 14);
        packet.len = len;
        packet.priority = 0;

        logger(DEBUG_TRAFFIC, LOG_INFO, "Sending UDP probe length %d to %s (%s)", len, n->name, n->hostname);

        send_udppacket(n, &packet);
}

// This function tries to establish a UDP tunnel to a node so that packets can be sent.
// If a tunnel is already established, it makes sure it stays up.
// This function makes no guarantees - it is up to the caller to check the node's state to figure out if UDP is usable.
static void try_udp(node_t* n) {
        if(!udp_discovery)
                return;

        struct timeval ping_tx_elapsed;
        timersub(&now, &n->udp_ping_sent, &ping_tx_elapsed);

        int interval = n->status.udp_confirmed ? udp_discovery_keepalive_interval : udp_discovery_interval;

        if(ping_tx_elapsed.tv_sec >= interval) {
                send_udp_probe_packet(n, MAX(n->minmtu, 16));
                n->udp_ping_sent = now;

                if(localdiscovery && !n->status.udp_confirmed && n->prevedge) {
                        n->status.send_locally = true;
                        send_udp_probe_packet(n, 16);
                        n->status.send_locally = false;
                }
        }
}

static length_t choose_initial_maxmtu(node_t *n) {
#ifdef IP_MTU

        int sock = -1;

        const sockaddr_t *sa = NULL;
        int sockindex;
        choose_udp_address(n, &sa, &sockindex);
        if(!sa)
                return MTU;

        sock = socket(sa->sa.sa_family, SOCK_DGRAM, IPPROTO_UDP);
        if(sock < 0) {
                logger(DEBUG_TRAFFIC, LOG_ERR, "Creating MTU assessment socket for %s (%s) failed: %s", n->name, n->hostname, sockstrerror(sockerrno));
                return MTU;
        }

        if(connect(sock, &sa->sa, SALEN(sa->sa))) {
                logger(DEBUG_TRAFFIC, LOG_ERR, "Connecting MTU assessment socket for %s (%s) failed: %s", n->name, n->hostname, sockstrerror(sockerrno));
                close(sock);
                return MTU;
        }

        int ip_mtu;
        socklen_t ip_mtu_len = sizeof ip_mtu;
        if(getsockopt(sock, IPPROTO_IP, IP_MTU, &ip_mtu, &ip_mtu_len)) {
                logger(DEBUG_TRAFFIC, LOG_ERR, "getsockopt(IP_MTU) on %s (%s) failed: %s", n->name, n->hostname, sockstrerror(sockerrno));
                close(sock);
                return MTU;
        }

        close(sock);

        /* getsockopt(IP_MTU) returns the MTU of the physical interface.
           We need to remove various overheads to get to the tinc MTU. */
        length_t mtu = ip_mtu;
        mtu -= (sa->sa.sa_family == AF_INET6) ? sizeof(struct ip6_hdr) : sizeof(struct ip);
        mtu -= 8; /* UDP */
        if(n->status.sptps) {
                mtu -= SPTPS_DATAGRAM_OVERHEAD;
                if((n->options >> 24) >= 4)
                        mtu -= sizeof(node_id_t) + sizeof(node_id_t);
        } else {
                mtu -= digest_length(n->outdigest);

                /* Now it's tricky. We use CBC mode, so the length of the
                   encrypted payload must be a multiple of the blocksize. The
                   sequence number is also part of the encrypted payload, so we
                   must account for it after correcting for the blocksize.
                   Furthermore, the padding in the last block must be at least
                   1 byte. */

                length_t blocksize = cipher_blocksize(n->outcipher);

                if(blocksize > 1) {
                        mtu /= blocksize;
                        mtu *= blocksize;
                        mtu--;
                }

                mtu -= 4; // seqno
        }

        if (mtu < 512) {
                logger(DEBUG_TRAFFIC, LOG_ERR, "getsockopt(IP_MTU) on %s (%s) returned absurdly small value: %d", n->name, n->hostname, ip_mtu);
                return MTU;
        }
        if (mtu > MTU)
                return MTU;

        logger(DEBUG_TRAFFIC, LOG_INFO, "Using system-provided maximum tinc MTU for %s (%s): %hd", n->name, n->hostname, mtu);
        return mtu;

#else

        return MTU;

#endif
}

/* This function tries to determines the MTU of a node.
   By calling this function repeatedly, n->minmtu will be progressively
   increased, and at some point, n->mtu will be fixed to n->minmtu.  If the MTU
   is already fixed, this function checks if it can be increased.
*/

static void try_mtu(node_t *n) {
        if(!(n->options & OPTION_PMTU_DISCOVERY))
                return;

        if(udp_discovery && !n->status.udp_confirmed) {
                n->mtuprobes = 0;
                n->minmtu = 0;
                n->maxmtu = MTU;
                return;
        }

        /* mtuprobes == 0..19: initial discovery, send bursts with 1 second interval, mtuprobes++
           mtuprobes ==    20: fix MTU, and go to -1
           mtuprobes ==    -1: send one >maxmtu probe every pingtimeout */

        struct timeval elapsed;
        timersub(&now, &n->probe_sent_time, &elapsed);
        if(n->mtuprobes >= 0) {
                if(n->mtuprobes != 0 && elapsed.tv_sec == 0 && elapsed.tv_usec < 333333)
                        return;
        } else {
                if(elapsed.tv_sec < pingtimeout)
                        return;
        }

        try_fix_mtu(n);

        if(n->mtuprobes < 0) {
                /* After the initial discovery, we only send one >maxmtu probe
                   to detect PMTU increases. */
                if(n->maxmtu + 1 < MTU)
                        send_udp_probe_packet(n, n->maxmtu + 1);
        } else {
                /* Before initial discovery begins, set maxmtu to the most likely value.
                   If it's underestimated, we will correct it after initial discovery. */
                if(n->mtuprobes == 0)
                        n->maxmtu = choose_initial_maxmtu(n);

                for (;;) {
                        /* Decreasing the number of probes per cycle might make the algorithm react faster to lost packets,
                           but it will typically increase convergence time in the no-loss case. */
                        const length_t probes_per_cycle = 8;

                        /* This magic value was determined using math simulations.
                           It will result in a 1329-byte first probe, followed (if there was a reply) by a 1407-byte probe.
                           Since 1407 is just below the range of tinc MTUs over typical networks,
                           this fine-tuning allows tinc to cover a lot of ground very quickly.
                           This fine-tuning is only valid for maxmtu = MTU; if maxmtu is smaller,
                           then it's better to use a multiplier of 1. Indeed, this leads to an interesting scenario
                           if choose_initial_maxmtu() returns the actual MTU value - it will get confirmed with one single probe. */
                        const float multiplier = (n->maxmtu == MTU) ? 0.97 : 1;

                        const float cycle_position = probes_per_cycle - (n->mtuprobes % probes_per_cycle) - 1;
                        const length_t minmtu = MAX(n->minmtu, 512);
                        const float interval = n->maxmtu - minmtu;

                        /* The core of the discovery algorithm is this exponential.
                           It produces very large probes early in the cycle, and then it very quickly decreases the probe size.
                           This reflects the fact that in the most difficult cases, we don't get any feedback for probes that
                           are too large, and therefore we need to concentrate on small offsets so that we can quickly converge
                           on the precise MTU as we are approaching it.
                           The last probe of the cycle is always 1 byte in size - this is to make sure we'll get at least one
                           reply per cycle so that we can make progress. */
                        const length_t offset = powf(interval, multiplier * cycle_position / (probes_per_cycle - 1));

                        length_t maxmtu = n->maxmtu;
                        send_udp_probe_packet(n, minmtu + offset);
                        /* If maxmtu changed, it means the probe was rejected by the system because it was too large.
                           In that case, we recalculate with the new maxmtu and try again. */
                        if(n->mtuprobes < 0 || maxmtu == n->maxmtu)
                                break;
                }

                if(n->mtuprobes >= 0)
                        n->mtuprobes++;
        }

        n->probe_counter = 0;
        n->probe_sent_time = now;
        n->probe_time = now;

        /* Calculate the packet loss of incoming traffic by comparing the rate of
           packets received to the rate with which the sequence number has increased.
           TODO: this is unrelated to PMTU discovery - it should be moved elsewhere.
         */

        if(n->received > n->prev_received)
                n->packetloss = 1.0 - (n->received - n->prev_received) / (float)(n->received_seqno - n->prev_received_seqno);
        else
                n->packetloss = n->received_seqno <= n->prev_received_seqno;

        n->prev_received_seqno = n->received_seqno;
        n->prev_received = n->received;
}

/* These functions try to establish a tunnel to a node (or its relay) so that
   packets can be sent (e.g. exchange keys).
   If a tunnel is already established, it tries to improve it (e.g. by trying
   to establish a UDP tunnel instead of TCP).  This function makes no
   guarantees - it is up to the caller to check the node's state to figure out
   if TCP and/or UDP is usable.  By calling this function repeatedly, the
   tunnel is gradually improved until we hit the wall imposed by the underlying
   network environment.  It is recommended to call this function every time a
   packet is sent (or intended to be sent) to a node, so that the tunnel keeps
   improving as packets flow, and then gracefully downgrades itself as it goes
   idle.
*/

static void try_tx_sptps(node_t *n) {
        /* If n is a TCP-only neighbor, we'll only use "cleartext" PACKET
           messages anyway, so there's no need for SPTPS at all. */

        if(n->connection && ((myself->options | n->options) & OPTION_TCPONLY))
                return;

        /* Otherwise, try to do SPTPS authentication with n if necessary. */

        try_sptps(n);

        /* Do we need to relay packets? */

        node_t *via = (n->via == myself) ? n->nexthop : n->via;

        /* If the relay doesn't support SPTPS, everything goes via TCP anyway. */

        if((via->options >> 24) < 4)
                return;

        /* If we do have a relay, try everything with that one instead. */

        if(via != n)
                return try_tx_sptps(via);

        try_udp(n);
        try_mtu(n);
}

static void try_tx_legacy(node_t *n) {
        /* Check if we already have a key, or request one. */

        if(!n->status.validkey) {
                if(n->last_req_key + 10 <= now.tv_sec) {
                        send_req_key(n);
                        n->last_req_key = now.tv_sec;
                }
                return;
        }

        try_udp(n);
        try_mtu(n);
}

void send_packet(node_t *n, vpn_packet_t *packet) {
        // If it's for myself, write it to the tun/tap device.

        if(n == myself) {
                if(overwrite_mac)
                         memcpy(DATA(packet), mymac.x, ETH_ALEN);
                n->out_packets++;
                n->out_bytes += packet->len;
                devops.write(packet);
                return;
        }

        logger(DEBUG_TRAFFIC, LOG_ERR, "Sending packet of %d bytes to %s (%s)", packet->len, n->name, n->hostname);

        // If the node is not reachable, drop it.

        if(!n->status.reachable) {
                logger(DEBUG_TRAFFIC, LOG_INFO, "Node %s (%s) is not reachable", n->name, n->hostname);
                return;
        }

        // Keep track of packet statistics.

        n->out_packets++;
        n->out_bytes += packet->len;

        // Check if it should be sent as an SPTPS packet.

        if(n->status.sptps) {
                send_sptps_packet(n, packet);
                try_tx_sptps(n);
                return;
        }

        // Determine which node to actually send it to.

        node_t *via = (packet->priority == -1 || n->via == myself) ? n->nexthop : n->via;

        if(via != n)
                logger(DEBUG_TRAFFIC, LOG_INFO, "Sending packet to %s via %s (%s)", n->name, via->name, n->via->hostname);

        // Try to send via UDP, unless TCP is forced.

        if(packet->priority == -1 || ((myself->options | via->options) & OPTION_TCPONLY)) {
                if(!send_tcppacket(via->connection, packet))
                        terminate_connection(via->connection, true);
                return;
        }

        send_udppacket(via, packet);
        try_tx_legacy(via);
}

void broadcast_packet(const node_t *from, vpn_packet_t *packet) {
        // Always give ourself a copy of the packet.
        if(from != myself)
                send_packet(myself, packet);

        // In TunnelServer mode, do not forward broadcast packets.
        // The MST might not be valid and create loops.
        if(tunnelserver || broadcast_mode == BMODE_NONE)
                return;

        logger(DEBUG_TRAFFIC, LOG_INFO, "Broadcasting packet of %d bytes from %s (%s)",
                           packet->len, from->name, from->hostname);

        switch(broadcast_mode) {
                // In MST mode, broadcast packets travel via the Minimum Spanning Tree.
                // This guarantees all nodes receive the broadcast packet, and
                // usually distributes the sending of broadcast packets over all nodes.
                case BMODE_MST:
                        for list_each(connection_t, c, connection_list)
                                if(c->edge && c->status.mst && c != from->nexthop->connection)
                                        send_packet(c->node, packet);
                        break;

                // In direct mode, we send copies to each node we know of.
                // However, this only reaches nodes that can be reached in a single hop.
                // We don't have enough information to forward broadcast packets in this case.
                case BMODE_DIRECT:
                        if(from != myself)
                                break;

                        for splay_each(node_t, n, node_tree)
                                if(n->status.reachable && n != myself && ((n->via == myself && n->nexthop == n) || n->via == n))
                                        send_packet(n, packet);
                        break;

                default:
                        break;
        }
}

static node_t *try_harder(const sockaddr_t *from, const vpn_packet_t *pkt) {
        node_t *n = NULL;
        bool hard = false;
        static time_t last_hard_try = 0;

        for splay_each(edge_t, e, edge_weight_tree) {
                if(!e->to->status.reachable || e->to == myself)
                        continue;

                if(sockaddrcmp_noport(from, &e->address)) {
                        if(last_hard_try == now.tv_sec)
                                continue;
                        hard = true;
                }

                if(!try_mac(e->to, pkt))
                        continue;

                n = e->to;
                break;
        }

        if(hard)
                last_hard_try = now.tv_sec;

        last_hard_try = now.tv_sec;
        return n;
}

void handle_incoming_vpn_data(void *data, int flags) {
        listen_socket_t *ls = data;
        vpn_packet_t pkt;
        char *hostname;
        node_id_t nullid = {};
        sockaddr_t addr = {};
        socklen_t addrlen = sizeof addr;
        node_t *from, *to;
        bool direct = false;

        pkt.offset = 0;
        int len = recvfrom(ls->udp.fd, DATA(&pkt), MAXSIZE, 0, &addr.sa, &addrlen);

        if(len <= 0 || len > MAXSIZE) {
                if(!sockwouldblock(sockerrno))
                        logger(DEBUG_ALWAYS, LOG_ERR, "Receiving packet failed: %s", sockstrerror(sockerrno));
                return;
        }

        pkt.len = len;

        sockaddrunmap(&addr); /* Some braindead IPv6 implementations do stupid things. */

        // Try to figure out who sent this packet.

        node_t *n = lookup_node_udp(&addr);

        if(!n) {
                // It might be from a 1.1 node, which might have a source ID in the packet.
                pkt.offset = 2 * sizeof(node_id_t);
                from = lookup_node_id(SRCID(&pkt));
                if(from && !memcmp(DSTID(&pkt), &nullid, sizeof nullid) && from->status.sptps) {
                        if(sptps_verify_datagram(&from->sptps, DATA(&pkt), pkt.len - 2 * sizeof(node_id_t)))
                                n = from;
                        else
                                goto skip_harder;
                }
        }

        if(!n) {
                pkt.offset = 0;
                n = try_harder(&addr, &pkt);
        }

skip_harder:
        if(!n) {
                if(debug_level >= DEBUG_PROTOCOL) {
                        hostname = sockaddr2hostname(&addr);
                        logger(DEBUG_PROTOCOL, LOG_WARNING, "Received UDP packet from unknown source %s", hostname);
                        free(hostname);
                }
                return;
        }

        if(n->status.sptps) {
                pkt.offset = 2 * sizeof(node_id_t);

                if(!memcmp(DSTID(&pkt), &nullid, sizeof nullid)) {
                        direct = true;
                        from = n;
                        to = myself;
                } else {
                        from = lookup_node_id(SRCID(&pkt));
                        to = lookup_node_id(DSTID(&pkt));
                }
                if(!from || !to) {
                        logger(DEBUG_PROTOCOL, LOG_WARNING, "Received UDP packet from %s (%s) with unknown source and/or destination ID", n->name, n->hostname);
                        return;
                }

                if(to != myself) {
                        send_sptps_data_priv(to, n, 0, DATA(&pkt), pkt.len - 2 * sizeof(node_id_t));
                        return;
                }
        } else {
                direct = true;
                from = n;
        }

        pkt.offset = 0;
        if(!receive_udppacket(from, &pkt))
                return;

        n->sock = ls - listen_socket;
        if(direct && sockaddrcmp(&addr, &n->address))
                update_node_udp(n, &addr);
}

void handle_device_data(void *data, int flags) {
        vpn_packet_t packet;
        packet.offset = DEFAULT_PACKET_OFFSET;
        packet.priority = 0;

        if(devops.read(&packet)) {
                myself->in_packets++;
                myself->in_bytes += packet.len;
                route(myself, &packet);
        }
}