bug links

[oweals/gnunet.git] / src / transport / gnunet-communicator-udp.c

/*
     This file is part of GNUnet
     Copyright (C) 2010-2014, 2018, 2019 GNUnet e.V.

     GNUnet is free software: you can redistribute it and/or modify it
     under the terms of the GNU Affero General Public License as published
     by the Free Software Foundation, either version 3 of the License,
     or (at your option) any later version.

     GNUnet 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
     Affero General Public License for more details.

     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.

     SPDX-License-Identifier: AGPL3.0-or-later
*/

/**
 * @file transport/gnunet-communicator-udp.c
 * @brief Transport plugin using UDP.
 * @author Christian Grothoff
 *
 * TODO:
 * - add and use util/ check for IPv6 availability (#5553)
 * - consider imposing transmission limits in the absence
 *   of ACKs; or: maybe this should be done at TNG service level?
 *   (at least the receiver might want to enforce limits on
 *    KX/DH operations per sender in here) (#5552)
 * - overall, we should look more into flow control support
 *   (either in backchannel, or general solution in TNG service)
 * - handle addresses discovered from broadcasts (#5551)
 *   (think: what was the story again on address validation?
 *    where is the API for that!?!)
 * - support DNS names in BINDTO option (#5528)
 * - support NAT connection reversal method (#5529)
 * - support other UDP-specific NAT traversal methods (#) 
 */
#include "platform.h"
#include "gnunet_util_lib.h"
#include "gnunet_protocols.h"
#include "gnunet_signatures.h"
#include "gnunet_constants.h"
#include "gnunet_nt_lib.h"
#include "gnunet_nat_service.h"
#include "gnunet_statistics_service.h"
#include "gnunet_transport_communication_service.h"

/**
 * How often do we rekey based on time (at least)
 */ 
#define REKEY_TIME_INTERVAL GNUNET_TIME_UNIT_DAYS

/**
 * How long do we wait until we must have received the initial KX?
 */ 
#define PROTO_QUEUE_TIMEOUT GNUNET_TIME_UNIT_MINUTES

/**
 * How often do we broadcast our presence on the LAN?
 */ 
#define BROADCAST_FREQUENCY GNUNET_TIME_UNIT_MINUTES

/**
 * How often do we scan for changes to our network interfaces?
 */ 
#define INTERFACE_SCAN_FREQUENCY GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 5)

/**
 * AES key size.
 */
#define AES_KEY_SIZE (256/8)

/**
 * AES (GCM) IV size.
 */
#define AES_IV_SIZE (96/8)

/**
 * Size of the GCM tag.
 */
#define GCM_TAG_SIZE (128/8)

/**
 * If we fall below this number of available KCNs,
 * we generate additional ACKs until we reach
 * #KCN_TARGET.
 * Should be large enough that we don't generate ACKs all
 * the time and still have enough time for the ACK to
 * arrive before the sender runs out. So really this 
 * should ideally be based on the RTT.
 */
#define KCN_THRESHOLD 92

/**
 * How many KCNs do we keep around *after* we hit
 * the #KCN_THRESHOLD? Should be larger than
 * #KCN_THRESHOLD so we do not generate just one
 * ACK at the time.
 */
#define KCN_TARGET 128

/**
 * What is the maximum delta between KCN sequence numbers
 * that we allow. Used to expire 'ancient' KCNs that likely
 * were dropped by the network.  Must be larger than
 * KCN_TARGET (otherwise we generate new KCNs all the time),
 * but not too large (otherwise packet loss may cause
 * sender to fall back to KX needlessly when sender runs
 * out of ACK'ed KCNs due to losses).
 */
#define MAX_SQN_DELTA 160

/**
 * How many shared master secrets do we keep around 
 * at most per sender?  Should be large enough so 
 * that we generally have a chance of sending an ACK
 * before the sender already rotated out the master
 * secret.  Generally values around #KCN_TARGET make
 * sense. Might make sense to adapt to RTT if we had
 * a good measurement...
 */
#define MAX_SECRETS 128

/**
 * How often do we rekey based on number of bytes transmitted?
 * (additionally randomized).
 */ 
#define REKEY_MAX_BYTES (1024LLU * 1024 * 1024 * 4LLU)

/**
 * Address prefix used by the communicator.
 */

#define COMMUNICATOR_ADDRESS_PREFIX "udp"

/**
 * Configuration section used by the communicator.
 */
#define COMMUNICATOR_CONFIG_SECTION "communicator-udp"

GNUNET_NETWORK_STRUCT_BEGIN


/**
 * Signature we use to verify that the ephemeral key was really chosen by
 * the specified sender.  If possible, the receiver should respond with
 * a `struct UDPAck` (possibly via backchannel).
 */
struct UdpHandshakeSignature
{
  /**
   * Purpose must be #GNUNET_SIGNATURE_COMMUNICATOR_UDP_HANDSHAKE
   */
  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;

  /**
   * Identity of the inititor of the UDP connection (UDP client).
   */ 
  struct GNUNET_PeerIdentity sender;

  /**
   * Presumed identity of the target of the UDP connection (UDP server)
   */ 
  struct GNUNET_PeerIdentity receiver;

  /**
   * Ephemeral key used by the @e sender.
   */ 
  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral;

  /**
   * Monotonic time of @e sender, to possibly help detect replay attacks
   * (if receiver persists times by sender).
   */ 
  struct GNUNET_TIME_AbsoluteNBO monotonic_time;
};


/**
 * "Plaintext" header at beginning of KX message. Followed
 * by encrypted `struct UDPConfirmation`.
 */
struct InitialKX
{

  /**
   * Ephemeral key for KX.
   */ 
  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral;

  /**
   * HMAC for the following encrypted message, using GCM.  HMAC uses
   * key derived from the handshake with sequence number zero.
   */ 
  char gcm_tag[GCM_TAG_SIZE];

};


/**
 * Encrypted continuation of UDP initial handshake, followed
 * by message header with payload.
 */
struct UDPConfirmation
{
  /**
   * Sender's identity
   */
  struct GNUNET_PeerIdentity sender;

  /**
   * Sender's signature of type #GNUNET_SIGNATURE_COMMUNICATOR_UDP_HANDSHAKE
   */
  struct GNUNET_CRYPTO_EddsaSignature sender_sig;

  /**
   * Monotonic time of @e sender, to possibly help detect replay attacks
   * (if receiver persists times by sender).
   */ 
  struct GNUNET_TIME_AbsoluteNBO monotonic_time;

  /* followed by messages */

  /* padding may follow actual messages */
};


/**
 * UDP key acknowledgement.  May be sent via backchannel. Allows the
 * sender to use `struct UDPBox` with the acknowledge key henceforth.
 */ 
struct UDPAck
{

  /**
   * Type is #GNUNET_MESSAGE_TYPE_COMMUNICATOR_UDP_ACK.
   */ 
  struct GNUNET_MessageHeader header;

  /**
   * Sequence acknowledgement limit. Specifies current maximum sequence
   * number supported by receiver.
   */ 
  uint32_t sequence_max GNUNET_PACKED;
  
  /**
   * CMAC of the base key being acknowledged.
   */ 
  struct GNUNET_HashCode cmac;

};


/**
 * Signature we use to verify that the broadcast was really made by
 * the peer that claims to have made it.  Basically, affirms that the
 * peer is really using this IP address (albeit possibly not in _our_
 * LAN).  Makes it difficult for peers in the LAN to claim to 
 * be just any global peer -- an attacker must have at least
 * shared a LAN with the peer they're pretending to be here.
 */
struct UdpBroadcastSignature
{
  /**
   * Purpose must be #GNUNET_SIGNATURE_COMMUNICATOR_UDP_BROADCAST
   */
  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;

  /**
   * Identity of the inititor of the UDP broadcast.
   */ 
  struct GNUNET_PeerIdentity sender;

  /**
   * Hash of the sender's UDP address.
   */ 
  struct GNUNET_HashCode h_address;
};


/**
 * Broadcast by peer in LAN announcing its presence.  Unusual in that
 * we don't pad these to full MTU, as we cannot prevent being
 * recognized in LAN as GNUnet peers if this feature is enabled
 * anyway.  Also, the entire message is in cleartext.
 */
struct UDPBroadcast
{

  /**
   * Sender's peer identity.
   */
  struct GNUNET_PeerIdentity sender;

  /**
   * Sender's signature of type
   * #GNUNET_SIGNATURE_COMMUNICATOR_UDP_BROADCAST
   */
  struct GNUNET_CRYPTO_EddsaSignature sender_sig;  
  
};


/**
 * UDP message box.  Always sent encrypted, only allowed after
 * the receiver sent a `struct UDPAck` for the base key!
 */ 
struct UDPBox
{

  /**
   * Key and IV identification code. KDF applied to an acknowledged
   * base key and a sequence number.  Sequence numbers must be used
   * monotonically increasing up to the maximum specified in 
   * `struct UDPAck`. Without further `struct UDPAck`s, the sender
   * must fall back to sending handshakes!
   */
  struct GNUNET_ShortHashCode kid;

  /**
   * 128-bit authentication tag for the following encrypted message,
   * from GCM.  MAC starts at the @e body_start that follows and
   * extends until the end of the UDP payload.  If the @e hmac is
   * wrong, the receiver should check if the message might be a
   * `struct UdpHandshakeSignature`.
   */ 
  char gcm_tag[GCM_TAG_SIZE];
  
};


GNUNET_NETWORK_STRUCT_END

/**
 * Shared secret we generated for a particular sender or receiver.
 */
struct SharedSecret;


/**
 * Pre-generated "kid" code (key and IV identification code) to
 * quickly derive master key for a `struct UDPBox`.
 */
struct KeyCacheEntry
{

  /**
   * Kept in a DLL.
   */
  struct KeyCacheEntry *next;
  
  /**
   * Kept in a DLL.
   */
  struct KeyCacheEntry *prev;

  /**
   * Key and IV identification code. KDF applied to an acknowledged
   * base key and a sequence number.  Sequence numbers must be used
   * monotonically increasing up to the maximum specified in 
   * `struct UDPAck`. Without further `struct UDPAck`s, the sender
   * must fall back to sending handshakes!
   */
  struct GNUNET_ShortHashCode kid;

  /**
   * Corresponding shared secret.
   */
  struct SharedSecret *ss;

  /**
   * Sequence number used to derive this entry from master key.
   */ 
  uint32_t sequence_number;
};


/**
 * Information we track per sender address we have recently been
 * in contact with (decryption from sender).
 */
struct SenderAddress;

/**
 * Information we track per receiving address we have recently been
 * in contact with (encryption to receiver).
 */
struct ReceiverAddress;

/**
 * Shared secret we generated for a particular sender or receiver.
 */
struct SharedSecret
{
  /**
   * Kept in a DLL.
   */
  struct SharedSecret *next;

  /**
   * Kept in a DLL.
   */
  struct SharedSecret *prev;

  /**
   * Kept in a DLL, sorted by sequence number. Only if we are decrypting.
   */
  struct KeyCacheEntry *kce_head;
  
  /**
   * Kept in a DLL, sorted by sequence number. Only if we are decrypting.
   */
  struct KeyCacheEntry *kce_tail;

  /**
   * Sender we use this shared secret with, or NULL.
   */
  struct SenderAddress *sender;

  /**
   * Receiver we use this shared secret with, or NULL.
   */
  struct ReceiverAddress *receiver;
  
  /**
   * Master shared secret.
   */ 
  struct GNUNET_HashCode master;

  /**
   * CMAC is used to identify @e master in ACKs.
   */ 
  struct GNUNET_HashCode cmac;

  /**
   * Up to which sequence number did we use this @e master already?
   * (for encrypting only)
   */ 
  uint32_t sequence_used;

  /**
   * Up to which sequence number did the other peer allow us to use
   * this key, or up to which number did we allow the other peer to
   * use this key?
   */
  uint32_t sequence_allowed;

  /**
   * Number of active KCN entries.
   */ 
  unsigned int active_kce_count;
};


/**
 * Information we track per sender address we have recently been
 * in contact with (we decrypt messages from the sender).
 */
struct SenderAddress
{

  /**
   * To whom are we talking to.
   */
  struct GNUNET_PeerIdentity target;  

  /**
   * Entry in sender expiration heap.
   */ 
  struct GNUNET_CONTAINER_HeapNode *hn;

  /**
   * Shared secrets we used with @e target, first used is head.
   */
  struct SharedSecret *ss_head;

  /**
   * Shared secrets we used with @e target, last used is tail.
   */
  struct SharedSecret *ss_tail;
  
  /**
   * Address of the other peer.
   */
  struct sockaddr *address;
  
  /**
   * Length of the address.
   */
  socklen_t address_len;

  /**
   * Timeout for this sender.
   */
  struct GNUNET_TIME_Absolute timeout;

  /**
   * Length of the DLL at @a ss_head.
   */ 
  unsigned int num_secrets;
  
  /**
   * Which network type does this queue use?
   */
  enum GNUNET_NetworkType nt;
  
};


/**
 * Information we track per receiving address we have recently been
 * in contact with (encryption to receiver).
 */
struct ReceiverAddress
{

  /**
   * To whom are we talking to.
   */
  struct GNUNET_PeerIdentity target;  
  
  /**
   * Shared secrets we received from @e target, first used is head.
   */
  struct SharedSecret *ss_head;

  /**
   * Shared secrets we received with @e target, last used is tail.
   */
  struct SharedSecret *ss_tail;

  /**
   * Address of the receiver in the human-readable format
   * with the #COMMUNICATOR_ADDRESS_PREFIX.
   */ 
  char *foreign_addr;

  /**
   * Address of the other peer.
   */
  struct sockaddr *address;
  
  /**
   * Length of the address.
   */
  socklen_t address_len;

  /**
   * Entry in sender expiration heap.
   */ 
  struct GNUNET_CONTAINER_HeapNode *hn;

  /**
   * Message queue we are providing for the #ch.
   */
  struct GNUNET_MQ_Handle *mq;

  /**
   * handle for this queue with the #ch.
   */
  struct GNUNET_TRANSPORT_QueueHandle *qh;

  /**
   * Timeout for this receiver address.
   */
  struct GNUNET_TIME_Absolute timeout;

  /**
   * MTU we allowed transport for this receiver right now.
   */
  size_t mtu;
  
  /**
   * Length of the DLL at @a ss_head.
   */ 
  unsigned int num_secrets;

  /**
   * Number of BOX keys from ACKs we have currently 
   * available for this receiver.
   */ 
  unsigned int acks_available;
  
  /**
   * Which network type does this queue use?
   */
  enum GNUNET_NetworkType nt;
  
};


/**
 * Interface we broadcast our presence on.
 */
struct BroadcastInterface
{

  /**
   * Kept in a DLL.
   */
  struct BroadcastInterface *next;

  /**
   * Kept in a DLL.
   */
  struct BroadcastInterface *prev;

  /**
   * Task for this broadcast interface.
   */
  struct GNUNET_SCHEDULER_Task *broadcast_task;
  
  /**
   * Sender's address of the interface.
   */
  struct sockaddr *sa;
  
  /**
   * Broadcast address to use on the interface.
   */
  struct sockaddr *ba;

  /**
   * Message we broadcast on this interface.
   */ 
  struct UDPBroadcast bcm;
  
  /**
   * If this is an IPv6 interface, this is the request
   * we use to join/leave the group.
   */
  struct ipv6_mreq mcreq;
  
  /**
   * Number of bytes in @e sa.
   */ 
  socklen_t salen;

  /**
   * Was this interface found in the last #iface_proc() scan?
   */
  int found;
};


/**
 * Cache of pre-generated key IDs.
 */
static struct GNUNET_CONTAINER_MultiShortmap *key_cache;

/**
 * ID of read task
 */
static struct GNUNET_SCHEDULER_Task *read_task;

/**
 * ID of timeout task
 */
static struct GNUNET_SCHEDULER_Task *timeout_task;

/**
 * ID of master broadcast task
 */
static struct GNUNET_SCHEDULER_Task *broadcast_task;

/**
 * For logging statistics.
 */
static struct GNUNET_STATISTICS_Handle *stats;

/**
 * Our environment.
 */
static struct GNUNET_TRANSPORT_CommunicatorHandle *ch;

/**
 * Receivers (map from peer identity to `struct ReceiverAddress`)
 */
static struct GNUNET_CONTAINER_MultiPeerMap *receivers;

/**
 * Senders (map from peer identity to `struct SenderAddress`)
 */
static struct GNUNET_CONTAINER_MultiPeerMap *senders;

/**
 * Expiration heap for senders (contains `struct SenderAddress`)
 */
static struct GNUNET_CONTAINER_Heap *senders_heap;

/**
 * Expiration heap for receivers (contains `struct ReceiverAddress`)
 */
static struct GNUNET_CONTAINER_Heap *receivers_heap;

/**
 * Broadcast interface tasks. Kept in a DLL.
 */
static struct BroadcastInterface *bi_head;

/**
 * Broadcast interface tasks. Kept in a DLL.
 */
static struct BroadcastInterface *bi_tail;

/**
 * Our socket.
 */
static struct GNUNET_NETWORK_Handle *udp_sock;

/** 
 * #GNUNET_YES if #udp_sock supports IPv6.
 */ 
static int have_v6_socket;

/**
 * Our public key.
 */
static struct GNUNET_PeerIdentity my_identity;

/**
 * Our private key.
 */
static struct GNUNET_CRYPTO_EddsaPrivateKey *my_private_key;

/**
 * Our configuration.
 */
static const struct GNUNET_CONFIGURATION_Handle *cfg;

/**
 * Network scanner to determine network types.
 */
static struct GNUNET_NT_InterfaceScanner *is;

/**
 * Connection to NAT service.
 */
static struct GNUNET_NAT_Handle *nat;

/**
 * Port number to which we are actually bound.
 */ 
static uint16_t my_port;


/**
 * An interface went away, stop broadcasting on it.
 *
 * @param bi entity to close down
 */
static void
bi_destroy (struct BroadcastInterface *bi)
{
  if (AF_INET6 == bi->sa->sa_family)
  {
    /* Leave the multicast group */
    if (GNUNET_OK !=
        GNUNET_NETWORK_socket_setsockopt
        (udp_sock,
         IPPROTO_IPV6,
         IPV6_LEAVE_GROUP,
         &bi->mcreq,
         sizeof (bi->mcreq)))
    {
      GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                           "setsockopt");
    }
  }
  GNUNET_CONTAINER_DLL_remove (bi_head,
                               bi_tail,
                               bi);
  GNUNET_SCHEDULER_cancel (bi->broadcast_task);
  GNUNET_free (bi->sa);
  GNUNET_free_non_null (bi->ba);
  GNUNET_free (bi);
}


/**
 * Destroys a receiving state due to timeout or shutdown.
 *
 * @param receiver entity to close down
 */
static void
receiver_destroy (struct ReceiverAddress *receiver)
{
  struct GNUNET_MQ_Handle *mq;
  
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Disconnecting receiver for peer `%s'\n",
              GNUNET_i2s (&receiver->target));
  if (NULL != (mq = receiver->mq))
  {
    receiver->mq = NULL;
    GNUNET_MQ_destroy (mq);
  }
  if (NULL != receiver->qh)
  {
    GNUNET_TRANSPORT_communicator_mq_del (receiver->qh);
    receiver->qh = NULL;
  }
  GNUNET_assert (GNUNET_YES ==
                 GNUNET_CONTAINER_multipeermap_remove (receivers,
                                                       &receiver->target,
                                                       receiver));
  GNUNET_assert (receiver ==
                 GNUNET_CONTAINER_heap_remove_node (receiver->hn));
  GNUNET_STATISTICS_set (stats,
                         "# receivers active",
                         GNUNET_CONTAINER_multipeermap_size (receivers),
                         GNUNET_NO);
  GNUNET_free (receiver->address);
  GNUNET_free (receiver->foreign_addr);
  GNUNET_free (receiver);
}


/**
 * Free memory used by key cache entry.
 *
 * @param kce the key cache entry
 */ 
static void
kce_destroy (struct KeyCacheEntry *kce)
{
  struct SharedSecret *ss = kce->ss;

  ss->active_kce_count--;
  GNUNET_CONTAINER_DLL_remove (ss->kce_head,
                               ss->kce_tail,
                               kce);
  GNUNET_assert (GNUNET_YES ==
                 GNUNET_CONTAINER_multishortmap_remove (key_cache,
                                                        &kce->kid,
                                                        kce));
  GNUNET_free (kce);
}


/**
 * Compute @a kid.
 *
 * @param msec master secret for HMAC calculation
 * @param serial number for the @a smac calculation
 * @param kid[out] where to write the key ID
 */
static void
get_kid (const struct GNUNET_HashCode *msec,
         uint32_t serial,
         struct GNUNET_ShortHashCode *kid)
{
  uint32_t sid = htonl (serial);

  GNUNET_CRYPTO_hkdf (kid,
                      sizeof (*kid),
                      GCRY_MD_SHA512,
                      GCRY_MD_SHA256,
                      &sid,
                      sizeof (sid),
                      msec,
                      sizeof (*msec),
                      "UDP-KID",
                      strlen ("UDP-KID"),
                      NULL, 0);
}


/**
 * Setup key cache entry for sequence number @a seq and shared secret @a ss.
 *
 * @param ss shared secret
 * @param seq sequence number for the key cache entry
 */
static void
kce_generate (struct SharedSecret *ss,
              uint32_t seq)
{
  struct KeyCacheEntry *kce;

  GNUNET_assert (0 < seq);
  kce = GNUNET_new (struct KeyCacheEntry);
  kce->ss = ss;
  kce->sequence_number = seq;
  get_kid (&ss->master,
           seq,
           &kce->kid);
  GNUNET_CONTAINER_DLL_insert (ss->kce_head,
                               ss->kce_tail,
                               kce);
  ss->active_kce_count++;
  (void) GNUNET_CONTAINER_multishortmap_put (key_cache,
                                             &kce->kid,
                                             kce,
                                             GNUNET_CONTAINER_MULTIHASHMAPOPTION_MULTIPLE);
  GNUNET_STATISTICS_set (stats,
                         "# KIDs active",
                         GNUNET_CONTAINER_multishortmap_size (key_cache),
                         GNUNET_NO);
}


/**
 * Destroy @a ss and associated key cache entries.
 *
 * @param ss shared secret to destroy
 */
static void
secret_destroy (struct SharedSecret *ss)
{
  struct SenderAddress *sender;
  struct ReceiverAddress *receiver;
  struct KeyCacheEntry *kce;

  if (NULL != (sender = ss->sender))
  {
    GNUNET_CONTAINER_DLL_remove (sender->ss_head,
                                 sender->ss_tail,
                                 ss);
    sender->num_secrets--;
  }
  if (NULL != (receiver = ss->receiver))
  {
    GNUNET_CONTAINER_DLL_remove (receiver->ss_head,
                                 receiver->ss_tail,
                                 ss);
    receiver->num_secrets--;
    receiver->acks_available
      -= (ss->sequence_allowed - ss->sequence_used);
  }
  while (NULL != (kce = ss->kce_head))
    kce_destroy (kce);
  GNUNET_STATISTICS_update (stats,
                            "# Secrets active",
                            -1,
                            GNUNET_NO);
  GNUNET_STATISTICS_set (stats,
                         "# KIDs active",
                         GNUNET_CONTAINER_multishortmap_size (key_cache),
                         GNUNET_NO);
  GNUNET_free (ss);
}


/**
 * Functions with this signature are called whenever we need
 * to close a sender's state due to timeout.
 *
 * @param sender entity to close down
 */
static void
sender_destroy (struct SenderAddress *sender)
{
  GNUNET_assert (GNUNET_YES ==
                 GNUNET_CONTAINER_multipeermap_remove (senders,
                                                       &sender->target,
                                                       sender));
  GNUNET_assert (sender ==
                 GNUNET_CONTAINER_heap_remove_node (sender->hn));
  GNUNET_STATISTICS_set (stats,
                         "# senders active",
                         GNUNET_CONTAINER_multipeermap_size (senders),
                         GNUNET_NO);
  GNUNET_free (sender->address);
  GNUNET_free (sender);
}


/**
 * Compute @a key and @a iv.
 *
 * @param msec master secret for calculation
 * @param serial number for the @a smac calculation
 * @param key[out] where to write the decrption key
 * @param iv[out] where to write the IV
 */
static void
get_iv_key (const struct GNUNET_HashCode *msec,
            uint32_t serial,
            char key[AES_KEY_SIZE],
            char iv[AES_IV_SIZE])
{
  uint32_t sid = htonl (serial);
  char res[AES_KEY_SIZE + AES_IV_SIZE];

  GNUNET_CRYPTO_hkdf (res,
                      sizeof (res),
                      GCRY_MD_SHA512,
                      GCRY_MD_SHA256,
                      &sid,
                      sizeof (sid),
                      msec,
                      sizeof (*msec),
                      "UDP-IV-KEY",
                      strlen ("UDP-IV-KEY"),
                      NULL, 0);
  memcpy (key,
          res,
          AES_KEY_SIZE);
  memcpy (iv,
          &res[AES_KEY_SIZE],
          AES_IV_SIZE);
}


/**
 * Increment sender timeout due to activity.
 *
 * @param sender address for which the timeout should be rescheduled
 */
static void
reschedule_sender_timeout (struct SenderAddress *sender)
{
  sender->timeout
    = GNUNET_TIME_relative_to_absolute (GNUNET_CONSTANTS_IDLE_CONNECTION_TIMEOUT);
  GNUNET_CONTAINER_heap_update_cost (sender->hn,
                                     sender->timeout.abs_value_us);
}


/**
 * Increment receiver timeout due to activity.
 *
 * @param receiver address for which the timeout should be rescheduled
 */
static void
reschedule_receiver_timeout (struct ReceiverAddress *receiver)
{
  receiver->timeout
    = GNUNET_TIME_relative_to_absolute (GNUNET_CONSTANTS_IDLE_CONNECTION_TIMEOUT);
  GNUNET_CONTAINER_heap_update_cost (receiver->hn,
                                     receiver->timeout.abs_value_us);
}


/**
 * Task run to check #receiver_heap and #sender_heap for timeouts.
 *
 * @param cls unused, NULL
 */
static void
check_timeouts (void *cls)
{
  struct GNUNET_TIME_Relative st;
  struct GNUNET_TIME_Relative rt;
  struct GNUNET_TIME_Relative delay;
  struct ReceiverAddress *receiver;
  struct SenderAddress *sender;
  
  (void) cls;
  timeout_task = NULL;
  rt = GNUNET_TIME_UNIT_FOREVER_REL;
  while (NULL != (receiver = GNUNET_CONTAINER_heap_peek (receivers_heap)))
  {
    rt = GNUNET_TIME_absolute_get_remaining (receiver->timeout);
    if (0 != rt.rel_value_us)
      break;
    receiver_destroy (receiver);
  }
  st = GNUNET_TIME_UNIT_FOREVER_REL;
  while (NULL != (sender = GNUNET_CONTAINER_heap_peek (senders_heap)))
  {
    st = GNUNET_TIME_absolute_get_remaining (receiver->timeout);
    if (0 != st.rel_value_us)
      break;
    sender_destroy (sender);
  }
  delay = GNUNET_TIME_relative_min (rt,
                                    st);
  if (delay.rel_value_us < GNUNET_TIME_UNIT_FOREVER_REL.rel_value_us)
    timeout_task = GNUNET_SCHEDULER_add_delayed (delay,
                                                 &check_timeouts,
                                                 NULL);  
}
                          

/**
 * Calcualte cmac from master in @a ss.
 *
 * @param ss[in,out] data structure to complete
 */
static void
calculate_cmac (struct SharedSecret *ss)
{
  GNUNET_CRYPTO_hkdf (&ss->cmac,
                      sizeof (ss->cmac),
                      GCRY_MD_SHA512,
                      GCRY_MD_SHA256,
                      "CMAC",
                      strlen ("CMAC"),
                      &ss->master,
                      sizeof (ss->master),
                      "UDP-CMAC",
                      strlen ("UDP-CMAC"),
                      NULL, 0);
}


/**
 * We received @a plaintext_len bytes of @a plaintext from a @a sender.
 * Pass it on to CORE.  
 *
 * @param queue the queue that received the plaintext
 * @param plaintext the plaintext that was received
 * @param plaintext_len number of bytes of plaintext received
 */ 
static void
pass_plaintext_to_core (struct SenderAddress *sender,
                        const void *plaintext,
                        size_t plaintext_len)
{
  const struct GNUNET_MessageHeader *hdr = plaintext;

  while (ntohs (hdr->size) < plaintext_len)
  {
    GNUNET_STATISTICS_update (stats,
                              "# bytes given to core",
                              ntohs (hdr->size),
                              GNUNET_NO);
    (void) GNUNET_TRANSPORT_communicator_receive (ch,
                                                  &sender->target,
                                                  hdr,
                                                  NULL /* no flow control possible */,
                                                  NULL);
    /* move on to next message, if any */
    plaintext_len -= ntohs (hdr->size);
    if (plaintext_len < sizeof (*hdr))
      break;
    hdr = plaintext + ntohs (hdr->size);
  }
  GNUNET_STATISTICS_update (stats,
                            "# bytes padding discarded",
                            plaintext_len,
                            GNUNET_NO);
}


/**
 * Setup @a cipher based on shared secret @a msec and
 * serial number @a serial.
 *
 * @param msec master shared secret
 * @param serial serial number of cipher to set up
 * @param cipher[out] cipher to initialize
 */
static void
setup_cipher (const struct GNUNET_HashCode *msec,
              uint32_t serial,
              gcry_cipher_hd_t *cipher)
{
  char key[AES_KEY_SIZE];
  char iv[AES_IV_SIZE];

  gcry_cipher_open (cipher,
                    GCRY_CIPHER_AES256 /* low level: go for speed */,
                    GCRY_CIPHER_MODE_GCM,
                    0 /* flags */);
  get_iv_key (msec,
              serial,
              key,
              iv);
  gcry_cipher_setkey (*cipher,
                      key,
                      sizeof (key));
  gcry_cipher_setiv (*cipher,
                     iv,
                     sizeof (iv));
}


/**
 * Try to decrypt @a buf using shared secret @a ss and key/iv 
 * derived using @a serial.
 *
 * @param ss shared secret
 * @param tag GCM authentication tag
 * @param serial serial number to use
 * @param in_buf input buffer to decrypt
 * @param in_buf_size number of bytes in @a in_buf and available in @a out_buf
 * @param out_buf where to write the result
 * @return #GNUNET_OK on success
 */
static int
try_decrypt (const struct SharedSecret *ss,
             const char tag[GCM_TAG_SIZE],
             uint32_t serial,
             const char *in_buf,
             size_t in_buf_size,
             char *out_buf)
{
  gcry_cipher_hd_t cipher;

  setup_cipher (&ss->master,
                serial,
                &cipher);
  GNUNET_assert (0 ==
                 gcry_cipher_decrypt (cipher,
                                      out_buf,
                                      in_buf_size,
                                      in_buf,
                                      in_buf_size));
  if (0 !=
      gcry_cipher_checktag (cipher,
                            tag,
                            GCM_TAG_SIZE))
  {
    gcry_cipher_close (cipher);
    GNUNET_STATISTICS_update (stats,
                              "# AEAD authentication failures",
                              1,
                              GNUNET_NO);
    return GNUNET_SYSERR;
  }
  gcry_cipher_close (cipher);
  return GNUNET_OK;
}


/**
 * Setup shared secret for decryption.
 *
 * @param ephemeral ephemeral key we received from the other peer
 * @return new shared secret
 */
static struct SharedSecret *
setup_shared_secret_dec (const struct GNUNET_CRYPTO_EcdhePublicKey *ephemeral)
{
  struct SharedSecret *ss;

  ss = GNUNET_new (struct SharedSecret);
  GNUNET_CRYPTO_eddsa_ecdh (my_private_key,
                            ephemeral,
                            &ss->master);
  return ss;
}


/**
 * Setup shared secret for encryption.
 *
 * @param ephemeral ephemeral key we are sending to the other peer
 * @param receiver[in,out] queue to initialize encryption key for
 * @return new shared secret
 */
static struct SharedSecret *
setup_shared_secret_enc (const struct GNUNET_CRYPTO_EcdhePrivateKey *ephemeral,
                         struct ReceiverAddress *receiver)
{
  struct SharedSecret *ss;

  ss = GNUNET_new (struct SharedSecret);
  GNUNET_CRYPTO_ecdh_eddsa (ephemeral,
                            &receiver->target.public_key,
                            &ss->master);
  calculate_cmac (ss);
  ss->receiver = receiver;
  GNUNET_CONTAINER_DLL_insert (receiver->ss_head,
                               receiver->ss_tail,
                               ss);
  receiver->num_secrets++;
  GNUNET_STATISTICS_update (stats,
                            "# Secrets active",
                            1,
                            GNUNET_NO);
  return ss;
}


/**
 * Setup the MQ for the @a receiver.  If a queue exists,
 * the existing one is destroyed.  Then the MTU is
 * recalculated and a fresh queue is initialized.
 *
 * @param receiver receiver to setup MQ for
 */ 
static void
setup_receiver_mq (struct ReceiverAddress *receiver);


/**
 * We received an ACK for @a pid. Check if it is for
 * the receiver in @a value and if so, handle it and
 * return #GNUNET_NO. Otherwise, return #GNUNET_YES.
 *
 * @param cls a `const struct UDPAck`
 * @param pid peer the ACK is from
 * @param value a `struct ReceiverAddress`
 * @return #GNUNET_YES to continue to iterate
 */
static int
handle_ack (void *cls,
            const struct GNUNET_PeerIdentity *pid,
            void *value)
{
  const struct UDPAck *ack = cls;
  struct ReceiverAddress *receiver = value;

  (void) pid;
  for (struct SharedSecret *ss = receiver->ss_head;
       NULL != ss;
       ss = ss->next)
  {
    if (0 == memcmp (&ack->cmac,
                     &ss->cmac,
                     sizeof (struct GNUNET_HashCode)))
    {
      uint32_t allowed;
      
      allowed = ntohl (ack->sequence_max);
                            
      if (allowed > ss->sequence_allowed)
      {
        receiver->acks_available += (allowed - ss->sequence_allowed);
        if ((allowed - ss->sequence_allowed)
            == receiver->acks_available)
        {
          /* we just incremented from zero => MTU change! */
          setup_receiver_mq (receiver);
        }
        ss->sequence_allowed = allowed;
        /* move ss to head to avoid discarding it anytime soon! */
        GNUNET_CONTAINER_DLL_remove (receiver->ss_head,
                                     receiver->ss_tail,
                                     ss);
        GNUNET_CONTAINER_DLL_insert (receiver->ss_head,
                                     receiver->ss_tail,
                                     ss);
      }
      return GNUNET_NO;
    }
  }
  return GNUNET_YES;
}


/**
 * Test if we have received a valid message in plaintext.
 * If so, handle it.
 *
 * @param sender peer to process inbound plaintext for
 * @param buf buffer we received
 * @param buf_size number of bytes in @a buf
 */ 
static void
try_handle_plaintext (struct SenderAddress *sender,
                      const void *buf,
                      size_t buf_size)
{
  const struct GNUNET_MessageHeader *hdr
    = (const struct GNUNET_MessageHeader *) buf;
  const struct UDPAck *ack
    = (const struct UDPAck *) buf;
  uint16_t type;

  if (sizeof (*hdr) > buf_size)
    return; /* not even a header */
  if (ntohs (hdr->size) > buf_size)
    return; /* not even a header */
  type = ntohs (hdr->type);
  switch (type)
  {
  case GNUNET_MESSAGE_TYPE_COMMUNICATOR_UDP_ACK:
    /* lookup master secret by 'cmac', then update sequence_max */
    GNUNET_CONTAINER_multipeermap_get_multiple (receivers,
                                                &sender->target,
                                                &handle_ack,
                                                (void *) ack);
    /* There could be more messages after the ACK, handle those as well */
    buf += ntohs (hdr->size);
    buf_size -= ntohs (hdr->size);
    pass_plaintext_to_core (sender,
                            buf,
                            buf_size);
    break;
  case GNUNET_MESSAGE_TYPE_COMMUNICATOR_UDP_PAD:
    /* skip padding */
    break;
  default:
    pass_plaintext_to_core (sender,
                            buf,
                            buf_size);
  }
}


/**
 * We established a shared secret with a sender. We should try to send
 * the sender an `struct UDPAck` at the next opportunity to allow the
 * sender to use @a ss longer (assuming we did not yet already
 * recently).
 *
 * @param ss shared secret to generate ACKs for
 */
static void
consider_ss_ack (struct SharedSecret *ss)
{
  GNUNET_assert (NULL != ss->sender);
  /* drop ancient KeyCacheEntries */
  while ( (NULL != ss->kce_head) &&
          (MAX_SQN_DELTA < ss->kce_head->sequence_number - ss->kce_tail->sequence_number) )
    kce_destroy (ss->kce_tail);
  if (ss->active_kce_count < KCN_THRESHOLD)
  {
    struct UDPAck ack;

    while (ss->active_kce_count < KCN_TARGET)
      kce_generate (ss,
                    ++ss->sequence_allowed);
    ack.header.type = htons (GNUNET_MESSAGE_TYPE_COMMUNICATOR_UDP_ACK);
    ack.header.size = htons (sizeof (ack));
    ack.sequence_max = htonl (ss->sequence_allowed);
    ack.cmac = ss->cmac;
    GNUNET_TRANSPORT_communicator_notify (ch,
                                          &ss->sender->target,
                                          COMMUNICATOR_ADDRESS_PREFIX,
                                          &ack.header);
  }
}


/**
 * We received a @a box with matching @a kce.  Decrypt and process it.
 *
 * @param box the data we received
 * @param box_len number of bytes in @a box
 * @param kce key index to decrypt @a box
 */ 
static void
decrypt_box (const struct UDPBox *box,
             size_t box_len,
             struct KeyCacheEntry *kce)
{
  struct SharedSecret *ss = kce->ss;
  char out_buf[box_len - sizeof (*box)];

  GNUNET_assert (NULL != ss->sender);
  if (GNUNET_OK !=
      try_decrypt (ss,
                   box->gcm_tag,
                   kce->sequence_number,
                   (const char *) &box[1],
                   sizeof (out_buf),
                   out_buf))
  {
    GNUNET_STATISTICS_update (stats,
                              "# Decryption failures with valid KCE",
                              1,
                              GNUNET_NO);
    kce_destroy (kce);
    return;
  }
  kce_destroy (kce);
  GNUNET_STATISTICS_update (stats,
                            "# bytes decrypted with BOX",
                            sizeof (out_buf),
                            GNUNET_NO);
  try_handle_plaintext (ss->sender,
                        out_buf,
                        sizeof (out_buf));
  consider_ss_ack (ss);
}


/**
 * Closure for #find_sender_by_address()
 */
struct SearchContext
{
  /**
   * Address we are looking for.
   */
  const struct sockaddr *address;

  /**
   * Number of bytes in @e address.
   */
  socklen_t address_len;

  /**
   * Return value to set if we found a match.
   */
  struct SenderAddress *sender;
};


/**
 * Find existing `struct SenderAddress` by matching addresses.
 *
 * @param cls a `struct SearchContext`
 * @param key ignored, must match already
 * @param value a `struct SenderAddress`
 * @return #GNUNET_YES if not found (continue to search), #GNUNET_NO if found
 */
static int
find_sender_by_address (void *cls,
                        const struct GNUNET_PeerIdentity *key,
                        void *value)
{
  struct SearchContext *sc = cls;
  struct SenderAddress *sender = value;

  if ( (sender->address_len == sc->address_len) &&
       (0 == memcmp (sender->address,
                     sc->address,
                     sender->address_len)) )
  {
    sc->sender = sender;
    return GNUNET_NO; /* stop iterating! */
  }
  return GNUNET_YES;
}


/**
 * Create sender address for @a target.  Note that we
 * might already have one, so a fresh one is only allocated
 * if one does not yet exist for @a address.
 *
 * @param target peer to generate address for
 * @param address target address 
 * @param address_len number of bytes in @a address
 * @return data structure to keep track of key material for
 *         decrypting data from @a target
 */
static struct SenderAddress *
setup_sender (const struct GNUNET_PeerIdentity *target,
              const struct sockaddr *address,
              socklen_t address_len)
{
  struct SenderAddress *sender;
  struct SearchContext sc = {
    .address = address,
    .address_len = address_len,
    .sender = NULL
  };

  GNUNET_CONTAINER_multipeermap_get_multiple (senders,
                                              target,
                                              &find_sender_by_address,
                                              &sc);
  if (NULL != sc.sender)
  {
    reschedule_sender_timeout (sc.sender);
    return sc.sender;
  }
  sender = GNUNET_new (struct SenderAddress);
  sender->target = *target;
  sender->address = GNUNET_memdup (address,
                                   address_len);
  sender->address_len = address_len;
  (void) GNUNET_CONTAINER_multipeermap_put (senders,
                                            &sender->target,
                                            sender,
                                            GNUNET_CONTAINER_MULTIHASHMAPOPTION_MULTIPLE);
  GNUNET_STATISTICS_set (stats,
                         "# senders active",
                         GNUNET_CONTAINER_multipeermap_size (receivers),
                         GNUNET_NO);
  sender->timeout
    = GNUNET_TIME_relative_to_absolute (GNUNET_CONSTANTS_IDLE_CONNECTION_TIMEOUT);
  sender->hn = GNUNET_CONTAINER_heap_insert (senders_heap,
                                             sender,
                                             sender->timeout.abs_value_us);
  sender->nt = GNUNET_NT_scanner_get_type (is,
                                           address,
                                           address_len);
  if (NULL == timeout_task)
    timeout_task = GNUNET_SCHEDULER_add_now (&check_timeouts,
                                             NULL);
  return sender;
}


/**
 * Check signature from @a uc against @a ephemeral.
 *
 * @param ephermal key that is signed
 * @param uc signature of claimant
 * @return #GNUNET_OK if signature is valid
 */
static int
verify_confirmation (const struct GNUNET_CRYPTO_EcdhePublicKey *ephemeral,
                     const struct UDPConfirmation *uc)
{
  struct UdpHandshakeSignature uhs;
                        
  uhs.purpose.purpose = htonl (GNUNET_SIGNATURE_COMMUNICATOR_UDP_HANDSHAKE);
  uhs.purpose.size = htonl (sizeof (uhs));
  uhs.sender = uc->sender;
  uhs.receiver = my_identity;
  uhs.ephemeral = *ephemeral;
  uhs.monotonic_time = uc->monotonic_time;
  return GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_COMMUNICATOR_UDP_HANDSHAKE,
                                     &uhs.purpose,
                                     &uc->sender_sig,
                                     &uc->sender.public_key);
}


/**
 * Socket read task. 
 *
 * @param cls NULL
 */
static void
sock_read (void *cls)
{
  struct sockaddr_storage sa;
  socklen_t salen = sizeof (sa);
  char buf[UINT16_MAX];
  ssize_t rcvd;

  (void) cls;
  read_task
      = GNUNET_SCHEDULER_add_read_net (GNUNET_TIME_UNIT_FOREVER_REL,
                                       udp_sock,
                                       &sock_read,
                                       NULL);
  rcvd = GNUNET_NETWORK_socket_recvfrom (udp_sock,
                                         buf,
                                         sizeof (buf),
                                         (struct sockaddr *) &sa,
                                         &salen);
  if (-1 == rcvd)
  {
    GNUNET_log_strerror (GNUNET_ERROR_TYPE_DEBUG,
                         "recv");
    return;
  }

  /* first, see if it is a UDPBox */
  if (rcvd > sizeof (struct UDPBox))
  {
    const struct UDPBox *box;
    struct KeyCacheEntry *kce;

    box = (const struct UDPBox *) buf;
    kce = GNUNET_CONTAINER_multishortmap_get (key_cache,
                                              &box->kid);
    if (NULL != kce)
    {
      decrypt_box (box,
                   (size_t) rcvd,
                   kce);
      return;
    }
  }

  /* next, check if it is a broadcast */
  if (sizeof (struct UDPBroadcast) == rcvd)
  {
    const struct UDPBroadcast *ub;
    struct UdpBroadcastSignature uhs;

    ub = (const struct UDPBroadcast *) buf;
    uhs.purpose.purpose = htonl (GNUNET_SIGNATURE_COMMUNICATOR_UDP_BROADCAST);
    uhs.purpose.size = htonl (sizeof (uhs));
    uhs.sender = ub->sender;
    GNUNET_CRYPTO_hash (&sa,
                        salen,
                        &uhs.h_address);
    if (GNUNET_OK ==
        GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_COMMUNICATOR_UDP_BROADCAST,
                                    &uhs.purpose,
                                    &ub->sender_sig,
                                    &ub->sender.public_key))
    {
      GNUNET_STATISTICS_update (stats,
                                "# broadcasts received",
                                1,
                                GNUNET_NO);
      // FIXME #5551: we effectively just got a HELLO!
      // trigger verification NOW!
      return;
    }
    /* continue with KX, mostly for statistics... */
  }
  

  /* finally, test if it is a KX */
  if (rcvd < sizeof (struct UDPConfirmation) + sizeof (struct InitialKX))
  {
    GNUNET_STATISTICS_update (stats,
                              "# messages dropped (no kid, too small for KX)",
                              1,
                              GNUNET_NO);
    return;
  }

  {
    const struct InitialKX *kx;
    struct SharedSecret *ss;
    char pbuf[rcvd - sizeof (struct InitialKX)];
    const struct UDPConfirmation *uc;
    struct SenderAddress *sender;

    kx = (const struct InitialKX *) buf;
    ss = setup_shared_secret_dec (&kx->ephemeral);
    if (GNUNET_OK !=
        try_decrypt (ss,
                     kx->gcm_tag,
                     0,
                     &buf[sizeof (*kx)],
                     sizeof (pbuf),
                     pbuf))
    {
      GNUNET_free (ss);
      GNUNET_STATISTICS_update (stats,
                                "# messages dropped (no kid, AEAD decryption failed)",
                                1,
                                GNUNET_NO);
      return;
    }
    uc = (const struct UDPConfirmation *) pbuf;
    if (GNUNET_OK !=
        verify_confirmation (&kx->ephemeral,
                             uc))
    {
      GNUNET_break_op (0);
      GNUNET_free (ss);
      GNUNET_STATISTICS_update (stats,
                                "# messages dropped (sender signature invalid)",
                                1,
                                GNUNET_NO);
      return;
    }
    calculate_cmac (ss);
    sender = setup_sender (&uc->sender,
                           (const struct sockaddr *) &sa,
                           salen);
    ss->sender = sender;
    GNUNET_CONTAINER_DLL_insert (sender->ss_head,
                                 sender->ss_tail,
                                 ss);
    sender->num_secrets++;
    GNUNET_STATISTICS_update (stats,
                              "# Secrets active",
                              1,
                              GNUNET_NO);
    GNUNET_STATISTICS_update (stats,
                              "# messages decrypted without BOX",
                              1,
                              GNUNET_NO);
    try_handle_plaintext (sender,
                          &uc[1],
                          sizeof (pbuf) - sizeof (*uc));
    consider_ss_ack (ss);
    if (sender->num_secrets > MAX_SECRETS)
      secret_destroy (sender->ss_tail);
  }
}


/**
 * Convert UDP bind specification to a `struct sockaddr *`
 *
 * @param bindto bind specification to convert
 * @param[out] sock_len set to the length of the address
 * @return converted bindto specification
 */
static struct sockaddr *
udp_address_to_sockaddr (const char *bindto,
                         socklen_t *sock_len)
{
  struct sockaddr *in;
  unsigned int port;
  char dummy[2];
  char *colon;
  char *cp;
  
  if (1 == SSCANF (bindto,
                   "%u%1s",
                   &port,
                   dummy))
  {
    /* interpreting value as just a PORT number */
    if (port > UINT16_MAX)
    {
      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                  "BINDTO specification `%s' invalid: value too large for port\n",
                  bindto);
      return NULL;
    }
    /* FIXME #V6: add test to util/ for IPv6 availability,
       and depending on the result, go directly for v4-only */
    if (GNUNET_YES ==
        GNUNET_CONFIGURATION_get_value_yesno (cfg,
                                              COMMUNICATOR_CONFIG_SECTION,
                                              "DISABLE_V6"))
    {
      struct sockaddr_in *i4;
      
      i4 = GNUNET_malloc (sizeof (struct sockaddr_in));
      i4->sin_family = AF_INET;
      i4->sin_port = htons ((uint16_t) port);
      *sock_len = sizeof (struct sockaddr_in);
      in = (struct sockaddr *) i4;
    }
    else
    {
      struct sockaddr_in6 *i6;
      
      i6 = GNUNET_malloc (sizeof (struct sockaddr_in6));
      i6->sin6_family = AF_INET6;
      i6->sin6_port = htons ((uint16_t) port);
      *sock_len = sizeof (struct sockaddr_in6);
      in = (struct sockaddr *) i6;
    }
    return in;
  }
  cp = GNUNET_strdup (bindto);
  colon = strrchr (cp, ':');
  if (NULL != colon)
  {
    /* interpet value after colon as port */
    *colon = '\0';
    colon++;
    if (1 == SSCANF (colon,
                     "%u%1s",
                     &port,
                     dummy))
    {
      /* interpreting value as just a PORT number */
      if (port > UINT16_MAX)
      {
        GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                    "BINDTO specification `%s' invalid: value too large for port\n",
                    bindto);
        GNUNET_free (cp);
        return NULL;
      }
    }
    else
    {
      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                  "BINDTO specification `%s' invalid: last ':' not followed by number\n",
                  bindto);
      GNUNET_free (cp);
      return NULL;
    }
  }
  else
  {
    /* interpret missing port as 0, aka pick any free one */
    port = 0;
  }
  {
    /* try IPv4 */
    struct sockaddr_in v4;

    if (1 == inet_pton (AF_INET,
                        cp,
                        &v4))
    {
      v4.sin_port = htons ((uint16_t) port);
      in = GNUNET_memdup (&v4,
                          sizeof (v4));
      *sock_len = sizeof (v4);
      GNUNET_free (cp);
      return in;
    }
  }
  {
    /* try IPv6 */
    struct sockaddr_in6 v6;
    const char *start;

    start = cp;
    if ( ('[' == *cp) &&
         (']' == cp[strlen (cp)-1]) )
    {
      start++; /* skip over '[' */
      cp[strlen (cp) -1] = '\0'; /* eat ']' */
    }
    if (1 == inet_pton (AF_INET6,
                        start,
                        &v6))
    {
      v6.sin6_port = htons ((uint16_t) port);
      in = GNUNET_memdup (&v6,
                          sizeof (v6));
      *sock_len = sizeof (v6);
      GNUNET_free (cp);
      return in;
    }
  }
  /* #5528 FIXME (feature!): maybe also try getnameinfo()? */
  GNUNET_free (cp);
  return NULL;
}


/**
 * Pad @a dgram by @a pad_size using @a out_cipher.
 *
 * @param out_cipher cipher to use
 * @param dgram datagram to pad
 * @param pad_size number of bytes of padding to append
 */
static void
do_pad (gcry_cipher_hd_t out_cipher,
        char *dgram,
        size_t pad_size)
{
  char pad[pad_size];

  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_WEAK,
                              pad,
                              sizeof (pad));
  if (sizeof (pad) > sizeof (struct GNUNET_MessageHeader))
  {
    struct GNUNET_MessageHeader hdr = {
      .size = htons (sizeof (pad)),
      .type = htons (GNUNET_MESSAGE_TYPE_COMMUNICATOR_UDP_PAD)
    };
    
    memcpy (pad,
            &hdr,
            sizeof (hdr));
  }
  GNUNET_assert (0 ==
                 gcry_cipher_encrypt (out_cipher,
                                      dgram,
                                      sizeof (pad),
                                      pad,
                                      sizeof (pad)));
}


/**
 * Signature of functions implementing the sending functionality of a
 * message queue.
 *
 * @param mq the message queue
 * @param msg the message to send
 * @param impl_state our `struct ReceiverAddress`
 */
static void
mq_send (struct GNUNET_MQ_Handle *mq,
         const struct GNUNET_MessageHeader *msg,
         void *impl_state)
{
  struct ReceiverAddress *receiver = impl_state;
  uint16_t msize = ntohs (msg->size);

  GNUNET_assert (mq == receiver->mq);
  if (msize > receiver->mtu)
  {
    GNUNET_break (0);
    receiver_destroy (receiver);
    return;
  }
  reschedule_receiver_timeout (receiver);
  
  if (0 == receiver->acks_available)
  {
    /* use KX encryption method */
    struct UdpHandshakeSignature uhs;
    struct UDPConfirmation uc;
    struct InitialKX kx;
    struct GNUNET_CRYPTO_EcdhePrivateKey epriv;
    char dgram[receiver->mtu +
               sizeof (uc) +
               sizeof (kx)];
    size_t dpos;
    gcry_cipher_hd_t out_cipher;
    struct SharedSecret *ss;

    /* setup key material */
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CRYPTO_ecdhe_key_create2 (&epriv));

    ss = setup_shared_secret_enc (&epriv,
                                  receiver);
    setup_cipher (&ss->master,
                  0,
                  &out_cipher);
    /* compute 'uc' */
    uc.sender = my_identity;
    uc.monotonic_time = GNUNET_TIME_absolute_hton (GNUNET_TIME_absolute_get_monotonic (cfg));
    uhs.purpose.purpose = htonl (GNUNET_SIGNATURE_COMMUNICATOR_UDP_HANDSHAKE);
    uhs.purpose.size = htonl (sizeof (uhs));
    uhs.sender = my_identity;
    uhs.receiver = receiver->target;
    GNUNET_CRYPTO_ecdhe_key_get_public (&epriv,
                                        &uhs.ephemeral);
    uhs.monotonic_time = uc.monotonic_time;
    GNUNET_assert (GNUNET_OK ==
                   GNUNET_CRYPTO_eddsa_sign (my_private_key,
                                             &uhs.purpose,
                                             &uc.sender_sig));
    /* Leave space for kx */
    dpos = sizeof (struct GNUNET_CRYPTO_EcdhePublicKey);
    /* Append encrypted uc to dgram */
    GNUNET_assert (0 ==
                   gcry_cipher_encrypt (out_cipher,
                                        &dgram[dpos],
                                        sizeof (uc),
                                        &uc,
                                        sizeof (uc)));
    dpos += sizeof (uc);
    /* Append encrypted payload to dgram */
    GNUNET_assert (0 ==
                   gcry_cipher_encrypt (out_cipher,
                                        &dgram[dpos],
                                        msize,
                                        msg,
                                        msize));
    dpos += msize;
    do_pad (out_cipher,
            &dgram[dpos],
            sizeof (dgram) - dpos);
    /* Datagram starts with kx */
    kx.ephemeral = uhs.ephemeral;
    GNUNET_assert (0 ==
                   gcry_cipher_gettag (out_cipher,
                                       kx.gcm_tag,
                                       sizeof (kx.gcm_tag)));
    gcry_cipher_close (out_cipher);
    memcpy (dgram,
            &kx,
            sizeof (kx));
    if (-1 ==
        GNUNET_NETWORK_socket_sendto (udp_sock,
                                      dgram,
                                      sizeof (dgram),
                                      receiver->address,
                                      receiver->address_len))
      GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                           "send");
    GNUNET_MQ_impl_send_continue (mq);
    return;
  } /* End of KX encryption method */

  /* begin "BOX" encryption method, scan for ACKs from tail! */
  for (struct SharedSecret *ss = receiver->ss_tail;
       NULL != ss;
       ss = ss->prev)
  {
    if (ss->sequence_used < ss->sequence_allowed)
    {
      char dgram[sizeof (struct UDPBox) + receiver->mtu];
      struct UDPBox *box;
      gcry_cipher_hd_t out_cipher;
      size_t dpos;

      box = (struct UDPBox *) dgram;
      ss->sequence_used++;
      get_kid (&ss->master,
               ss->sequence_used,
               &box->kid);
      setup_cipher (&ss->master,
                    ss->sequence_used,
                    &out_cipher);
      /* Append encrypted payload to dgram */
      dpos = sizeof (struct UDPBox);
      GNUNET_assert (0 ==
                     gcry_cipher_encrypt (out_cipher,
                                          &dgram[dpos],
                                          msize,
                                          msg,
                                          msize));
      dpos += msize;
      do_pad (out_cipher,
              &dgram[dpos],
              sizeof (dgram) - dpos);
      GNUNET_assert (0 ==
                     gcry_cipher_gettag (out_cipher,
                                         box->gcm_tag,
                                         sizeof (box->gcm_tag)));
      gcry_cipher_close (out_cipher);
      if (-1 ==
          GNUNET_NETWORK_socket_sendto (udp_sock,
                                        dgram,
                                        sizeof (dgram),
                                        receiver->address,
                                        receiver->address_len))
        GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                             "send");
      GNUNET_MQ_impl_send_continue (mq);
      receiver->acks_available--;
      if (0 == receiver->acks_available)
      {
        /* We have no more ACKs => MTU change! */
        setup_receiver_mq (receiver);
      }
      return;
    }
  }
  GNUNET_assert (0);
}


/**
 * Signature of functions implementing the destruction of a message
 * queue.  Implementations must not free @a mq, but should take care
 * of @a impl_state.
 *
 * @param mq the message queue to destroy
 * @param impl_state our `struct ReceiverAddress`
 */
static void
mq_destroy (struct GNUNET_MQ_Handle *mq,
            void *impl_state)
{
  struct ReceiverAddress *receiver = impl_state;

  if (mq == receiver->mq)
  {
    receiver->mq = NULL;
    receiver_destroy (receiver);
  }
}


/**
 * Implementation function that cancels the currently sent message.
 *
 * @param mq message queue
 * @param impl_state our `struct RecvierAddress`
 */
static void
mq_cancel (struct GNUNET_MQ_Handle *mq,
           void *impl_state)
{
  /* Cancellation is impossible with UDP; bail */
  GNUNET_assert (0);
}


/**
 * Generic error handler, called with the appropriate
 * error code and the same closure specified at the creation of
 * the message queue.
 * Not every message queue implementation supports an error handler.
 *
 * @param cls our `struct ReceiverAddress`
 * @param error error code
 */
static void
mq_error (void *cls,
          enum GNUNET_MQ_Error error)
{
  struct ReceiverAddress *receiver = cls;

  GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
              "MQ error in queue to %s: %d\n",
              GNUNET_i2s (&receiver->target),
              (int) error);
  receiver_destroy (receiver);
}


/**
 * Setup the MQ for the @a receiver.  If a queue exists,
 * the existing one is destroyed.  Then the MTU is
 * recalculated and a fresh queue is initialized.
 *
 * @param receiver receiver to setup MQ for
 */ 
static void
setup_receiver_mq (struct ReceiverAddress *receiver)
{
  size_t base_mtu;
  
  if (NULL != receiver->qh)
  {
    GNUNET_TRANSPORT_communicator_mq_del (receiver->qh);
    receiver->qh = NULL;
  }
  GNUNET_assert (NULL == receiver->mq);
  switch (receiver->address->sa_family)
  {
  case AF_INET:
    base_mtu
      = 1480 /* Ethernet MTU, 1500 - Ethernet header - VLAN tag */
      - sizeof (struct GNUNET_TUN_IPv4Header) /* 20 */
      - sizeof (struct GNUNET_TUN_UdpHeader) /* 8 */;
    break;
  case AF_INET6:
    base_mtu
      = 1280 /* Minimum MTU required by IPv6 */
      - sizeof (struct GNUNET_TUN_IPv6Header) /* 40 */
      - sizeof (struct GNUNET_TUN_UdpHeader) /* 8 */;
    break;
  default:
    GNUNET_assert (0);
    break;
  }
  if (0 == receiver->acks_available)
  {
    /* MTU based on full KX messages */
    receiver->mtu
      = base_mtu
      - sizeof (struct InitialKX) /* 48 */
      - sizeof (struct UDPConfirmation); /* 104 */
  }
  else
  {
    /* MTU based on BOXed messages */
    receiver->mtu
      = base_mtu - sizeof (struct UDPBox);
  }
  /* => Effective MTU for CORE will range from 1080 (IPv6 + KX) to
     1404 (IPv4 + Box) bytes, depending on circumstances... */
  receiver->mq
    = GNUNET_MQ_queue_for_callbacks (&mq_send,
                                     &mq_destroy,
                                     &mq_cancel,
                                     receiver,
                                     NULL,
                                     &mq_error,
                                     receiver);
  receiver->qh
    = GNUNET_TRANSPORT_communicator_mq_add (ch,
                                            &receiver->target,
                                            receiver->foreign_addr,
                                            receiver->mtu,
                                            receiver->nt,
                                            GNUNET_TRANSPORT_CS_OUTBOUND,
                                            receiver->mq);
}


/**
 * Setup a receiver for transmission.  Setup the MQ processing and
 * inform transport that the queue is ready. 
 *
 * @param 
 */ 
static struct ReceiverAddress *
receiver_setup (const struct GNUNET_PeerIdentity *target,
                const struct sockaddr *address,
                socklen_t address_len)
{
  struct ReceiverAddress *receiver;

  receiver = GNUNET_new (struct ReceiverAddress);
  receiver->address = GNUNET_memdup (address,
                                     address_len);
  receiver->address_len = address_len;
  receiver->target = *target;
  receiver->nt = GNUNET_NT_scanner_get_type (is,
                                             address,
                                             address_len);
  (void) GNUNET_CONTAINER_multipeermap_put (receivers,
                                            &receiver->target,
                                            receiver,
                                            GNUNET_CONTAINER_MULTIHASHMAPOPTION_MULTIPLE);
  receiver->timeout
    = GNUNET_TIME_relative_to_absolute (GNUNET_CONSTANTS_IDLE_CONNECTION_TIMEOUT);
  receiver->hn = GNUNET_CONTAINER_heap_insert (receivers_heap,
                                               receiver,
                                               receiver->timeout.abs_value_us);
  GNUNET_STATISTICS_set (stats,
                         "# receivers active",
                         GNUNET_CONTAINER_multipeermap_size (receivers),
                         GNUNET_NO);
  switch (address->sa_family)
  {
  case AF_INET:
    GNUNET_asprintf (&receiver->foreign_addr,
                     "%s-%s",
                     COMMUNICATOR_ADDRESS_PREFIX,
                     GNUNET_a2s (receiver->address,
                                 receiver->address_len));
    break;
  case AF_INET6:
    GNUNET_asprintf (&receiver->foreign_addr,
                     "%s-%s",
                     COMMUNICATOR_ADDRESS_PREFIX,
                     GNUNET_a2s (receiver->address,
                                 receiver->address_len));
    break;
  default:
    GNUNET_assert (0);
  }

  setup_receiver_mq (receiver);

  if (NULL == timeout_task)
    timeout_task = GNUNET_SCHEDULER_add_now (&check_timeouts,
                                             NULL);
  return receiver;
}


/**
 * Function called by the transport service to initialize a
 * message queue given address information about another peer.
 * If and when the communication channel is established, the
 * communicator must call #GNUNET_TRANSPORT_communicator_mq_add()
 * to notify the service that the channel is now up.  It is
 * the responsibility of the communicator to manage sane
 * retries and timeouts for any @a peer/@a address combination
 * provided by the transport service.  Timeouts and retries
 * do not need to be signalled to the transport service.
 *
 * @param cls closure
 * @param peer identity of the other peer
 * @param address where to send the message, human-readable
 *        communicator-specific format, 0-terminated, UTF-8
 * @return #GNUNET_OK on success, #GNUNET_SYSERR if the provided address is invalid
 */
static int
mq_init (void *cls,
         const struct GNUNET_PeerIdentity *peer,
         const char *address)
{
  struct ReceiverAddress *receiver;
  const char *path;
  struct sockaddr *in;
  socklen_t in_len;
  
  if (0 != strncmp (address,
                    COMMUNICATOR_ADDRESS_PREFIX "-",
                    strlen (COMMUNICATOR_ADDRESS_PREFIX "-")))
  {
    GNUNET_break_op (0);
    return GNUNET_SYSERR;
  }
  path = &address[strlen (COMMUNICATOR_ADDRESS_PREFIX "-")];
  in = udp_address_to_sockaddr (path,
                                &in_len);  
  receiver = receiver_setup (peer,
                             in,
                             in_len);
  (void) receiver;
  return GNUNET_OK;  
}


/**
 * Iterator over all receivers to clean up.
 *
 * @param cls NULL
 * @param target unused
 * @param value the queue to destroy
 * @return #GNUNET_OK to continue to iterate
 */
static int
get_receiver_delete_it (void *cls,
                        const struct GNUNET_PeerIdentity *target,
                        void *value)
{
  struct ReceiverAddress *receiver = value;

  (void) cls;
  (void) target;
  receiver_destroy (receiver);
  return GNUNET_OK;
}


/**
 * Iterator over all senders to clean up.
 *
 * @param cls NULL
 * @param target unused
 * @param value the queue to destroy
 * @return #GNUNET_OK to continue to iterate
 */
static int
get_sender_delete_it (void *cls,
                      const struct GNUNET_PeerIdentity *target,
                      void *value)
{
  struct SenderAddress *sender = value;

  (void) cls;
  (void) target;
  sender_destroy (sender);
  return GNUNET_OK;
}


/**
 * Shutdown the UNIX communicator.
 *
 * @param cls NULL (always)
 */
static void
do_shutdown (void *cls)
{
  if (NULL != nat)
  {
     GNUNET_NAT_unregister (nat);
     nat = NULL;
  }
  while (NULL != bi_head)
    bi_destroy (bi_head);
  if (NULL != broadcast_task)
  {
    GNUNET_SCHEDULER_cancel (broadcast_task);
    broadcast_task = NULL;
  }
  if (NULL != read_task)
  {
    GNUNET_SCHEDULER_cancel (read_task);
    read_task = NULL;
  }
  if (NULL != udp_sock)
  {
    GNUNET_break (GNUNET_OK ==
                  GNUNET_NETWORK_socket_close (udp_sock));
    udp_sock = NULL;
  }
  GNUNET_CONTAINER_multipeermap_iterate (receivers,
                                         &get_receiver_delete_it,
                                         NULL);
  GNUNET_CONTAINER_multipeermap_destroy (receivers);
  GNUNET_CONTAINER_multipeermap_iterate (senders,
                                         &get_sender_delete_it,
                                         NULL);
  GNUNET_CONTAINER_multipeermap_destroy (senders);
  GNUNET_CONTAINER_multishortmap_destroy (key_cache);
  GNUNET_CONTAINER_heap_destroy (senders_heap);
  GNUNET_CONTAINER_heap_destroy (receivers_heap);
  if (NULL != ch)
  {
    GNUNET_TRANSPORT_communicator_disconnect (ch);
    ch = NULL;
  }
  if (NULL != stats)
  {
    GNUNET_STATISTICS_destroy (stats,
                               GNUNET_NO);
    stats = NULL;
  }
  if (NULL != my_private_key)
  {
    GNUNET_free (my_private_key);
    my_private_key = NULL;
  }
  if (NULL != is)
  {
     GNUNET_NT_scanner_done (is);
     is = NULL;
  }
}


/**
 * Function called when the transport service has received a
 * backchannel message for this communicator (!) via a different return
 * path. Should be an acknowledgement.
 *
 * @param cls closure, NULL
 * @param sender which peer sent the notification
 * @param msg payload
 */
static void
enc_notify_cb (void *cls,
               const struct GNUNET_PeerIdentity *sender,
               const struct GNUNET_MessageHeader *msg)
{
  const struct UDPAck *ack;
  
  (void) cls;
  if ( (ntohs (msg->type) != GNUNET_MESSAGE_TYPE_COMMUNICATOR_UDP_ACK) ||
       (ntohs (msg->size) != sizeof (struct UDPAck)) )
  {
    GNUNET_break_op (0);
    return;
  }
  ack = (const struct UDPAck *) msg;
  GNUNET_CONTAINER_multipeermap_get_multiple (receivers,
                                              sender,
                                              &handle_ack,
                                              (void *) ack);
}


/**
 * Signature of the callback passed to #GNUNET_NAT_register() for
 * a function to call whenever our set of 'valid' addresses changes.
 *
 * @param cls closure
 * @param app_ctx[in,out] location where the app can store stuff
 *                  on add and retrieve it on remove
 * @param add_remove #GNUNET_YES to add a new public IP address, 
 *                   #GNUNET_NO to remove a previous (now invalid) one
 * @param ac address class the address belongs to
 * @param addr either the previous or the new public IP address
 * @param addrlen actual length of the @a addr
 */
static void
nat_address_cb (void *cls,
                void **app_ctx,
                int add_remove,
                enum GNUNET_NAT_AddressClass ac,
                const struct sockaddr *addr,
                socklen_t addrlen)
{
  char *my_addr;
  struct GNUNET_TRANSPORT_AddressIdentifier *ai;

  if (GNUNET_YES == add_remove)
  {
    enum GNUNET_NetworkType nt;

    GNUNET_asprintf (&my_addr,
                     "%s-%s",
                     COMMUNICATOR_ADDRESS_PREFIX,
                     GNUNET_a2s (addr,
                                 addrlen));
    nt = GNUNET_NT_scanner_get_type (is,
                                     addr,
                                     addrlen); 
    ai = GNUNET_TRANSPORT_communicator_address_add (ch,
                                                    my_addr,
                                                    nt,
                                                    GNUNET_TIME_UNIT_FOREVER_REL);
    GNUNET_free (my_addr);
    *app_ctx = ai;
  }
  else
  {
    ai = *app_ctx;
    GNUNET_TRANSPORT_communicator_address_remove (ai);
    *app_ctx = NULL;
  }
}


/**
 * Broadcast our presence on one of our interfaces.
 *
 * @param cls a `struct BroadcastInterface`
 */
static void
ifc_broadcast (void *cls)
{
  struct BroadcastInterface *bi = cls;
  struct GNUNET_TIME_Relative delay;

  delay = BROADCAST_FREQUENCY;
  delay.rel_value_us = GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK,
                                                 delay.rel_value_us);
  bi->broadcast_task
    = GNUNET_SCHEDULER_add_delayed (INTERFACE_SCAN_FREQUENCY,
                                    &ifc_broadcast,
                                    bi);
  
  switch (bi->sa->sa_family) {
  case AF_INET:
    {
      static int yes = 1;
      static int no = 0;
      ssize_t sent;
    
      if (GNUNET_OK !=
          GNUNET_NETWORK_socket_setsockopt (udp_sock,
                                            SOL_SOCKET,
                                            SO_BROADCAST,
                                            &yes,
                                            sizeof (int)))
        GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                             "setsockopt");
      sent = GNUNET_NETWORK_socket_sendto (udp_sock,
                                           &bi->bcm,
                                           sizeof (bi->bcm),
                                           bi->ba,
                                           bi->salen);
      if (-1 == sent)
        GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                             "sendto");
      if (GNUNET_OK !=
          GNUNET_NETWORK_socket_setsockopt (udp_sock,
                                            SOL_SOCKET,
                                            SO_BROADCAST,
                                            &no,
                                            sizeof (int)))
        GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                             "setsockopt");
      break;
    }
  case AF_INET6:
    {
      ssize_t sent;
      struct sockaddr_in6 dst;

      dst.sin6_family = AF_INET6;
      dst.sin6_port = htons (my_port);
      dst.sin6_addr = bi->mcreq.ipv6mr_multiaddr;
      dst.sin6_scope_id = ((struct sockaddr_in6*) bi->ba)->sin6_scope_id;

      sent = GNUNET_NETWORK_socket_sendto (udp_sock,
                                           &bi->bcm,
                                           sizeof (bi->bcm),
                                           (const struct sockaddr *)
                                           &dst,
                                           sizeof (dst));
      if (-1 == sent)
        GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                             "sendto");
      break;
    }
  default:
    GNUNET_break (0);
    break;
  }
}


/**
 * Callback function invoked for each interface found.
 * Activates/deactivates broadcast interfaces.
 *
 * @param cls NULL
 * @param name name of the interface (can be NULL for unknown)
 * @param isDefault is this presumably the default interface
 * @param addr address of this interface (can be NULL for unknown or unassigned)
 * @param broadcast_addr the broadcast address (can be NULL for unknown or unassigned)
 * @param netmask the network mask (can be NULL for unknown or unassigned)
 * @param addrlen length of the address
 * @return #GNUNET_OK to continue iteration, #GNUNET_SYSERR to abort
 */
static int
iface_proc (void *cls,
            const char *name,
            int isDefault,
            const struct sockaddr *addr,
            const struct sockaddr *broadcast_addr,
            const struct sockaddr *netmask, socklen_t addrlen)
{
  struct BroadcastInterface *bi;
  enum GNUNET_NetworkType network;
  struct UdpBroadcastSignature ubs;

  (void) cls;
  (void) netmask;
  network = GNUNET_NT_scanner_get_type (is,
                                        addr,
                                        addrlen);
  if (GNUNET_NT_LOOPBACK == network)
  {
    /* Broadcasting on loopback does not make sense */
    return GNUNET_YES;
  }
  if (NULL == addr)
    return GNUNET_YES; /* need to know our address! */
  for (bi = bi_head; NULL != bi; bi = bi->next)
  {
    if ( (bi->salen == addrlen) &&
         (0 == memcmp (addr,
                       bi->sa,
                       addrlen)) )
    {
      bi->found = GNUNET_YES;
      return GNUNET_OK;
    }
  }

  if ( (AF_INET6 == addr->sa_family) &&
       (NULL == broadcast_addr) )
    return GNUNET_OK; /* broadcast_addr is required for IPv6! */
  if ( (AF_INET6 == addr->sa_family) &&
       (GNUNET_YES != have_v6_socket) )
    return GNUNET_OK; /* not using IPv6 */
  
  bi = GNUNET_new (struct BroadcastInterface);
  bi->sa = GNUNET_memdup (addr,
                          addrlen);
  if (NULL != broadcast_addr)
    bi->ba = GNUNET_memdup (broadcast_addr,
                            addrlen);
  bi->salen = addrlen;
  bi->found = GNUNET_YES;
  bi->bcm.sender = my_identity;
  ubs.purpose.purpose = htonl (GNUNET_SIGNATURE_COMMUNICATOR_UDP_BROADCAST);
  ubs.purpose.size = htonl (sizeof (ubs));
  ubs.sender = my_identity;
  GNUNET_CRYPTO_hash (addr,
                      addrlen,
                      &ubs.h_address);
  GNUNET_assert (GNUNET_OK ==
                 GNUNET_CRYPTO_eddsa_sign (my_private_key,
                                           &ubs.purpose,
                                           &bi->bcm.sender_sig));
  bi->broadcast_task = GNUNET_SCHEDULER_add_now (&ifc_broadcast,
                                                 bi);
  GNUNET_CONTAINER_DLL_insert (bi_head,
                               bi_tail,
                               bi);
  if ( (AF_INET6 == addr->sa_family) &&
       (NULL != broadcast_addr) )
  {
    /* Create IPv6 multicast request */
    const struct sockaddr_in6 *s6
      = (const struct sockaddr_in6 *) broadcast_addr;

    GNUNET_assert (1 ==
                   inet_pton (AF_INET6,
                              "FF05::13B",
                              &bi->mcreq.ipv6mr_multiaddr));
    
    /* http://tools.ietf.org/html/rfc2553#section-5.2:
     *
     * IPV6_JOIN_GROUP
     *
     * Join a multicast group on a specified local interface.  If the
     * interface index is specified as 0, the kernel chooses the local
     * interface.  For example, some kernels look up the multicast
     * group in the normal IPv6 routing table and using the resulting
     * interface; we do this for each interface, so no need to use
     * zero (anymore...).
     */
    bi->mcreq.ipv6mr_interface = s6->sin6_scope_id;

    /* Join the multicast group */
    if (GNUNET_OK !=
        GNUNET_NETWORK_socket_setsockopt
        (udp_sock,
         IPPROTO_IPV6,
         IPV6_JOIN_GROUP,
         &bi->mcreq,
         sizeof (bi->mcreq)))
    {
      GNUNET_log_strerror (GNUNET_ERROR_TYPE_WARNING,
                           "setsockopt");
    }
  }
  return GNUNET_OK;
}


/**
 * Scan interfaces to broadcast our presence on the LAN.
 *
 * @param cls NULL, unused
 */
static void
do_broadcast (void *cls)
{
  struct BroadcastInterface *bin;
  
  (void) cls;
  for (struct BroadcastInterface *bi = bi_head;
       NULL != bi;
       bi = bi->next)
    bi->found = GNUNET_NO;
  GNUNET_OS_network_interfaces_list (&iface_proc,
                                     NULL);
  for (struct BroadcastInterface *bi = bi_head;
       NULL != bi;
       bi = bin)
  {
    bin = bi->next;
    if (GNUNET_NO == bi->found)
      bi_destroy (bi);
  }
  broadcast_task
    = GNUNET_SCHEDULER_add_delayed (INTERFACE_SCAN_FREQUENCY,
                                    &do_broadcast,
                                    NULL);
}


/**
 * Setup communicator and launch network interactions.
 *
 * @param cls NULL (always)
 * @param args remaining command-line arguments
 * @param cfgfile name of the configuration file used (for saving, can be NULL!)
 * @param c configuration
 */
static void
run (void *cls,
     char *const *args,
     const char *cfgfile,
     const struct GNUNET_CONFIGURATION_Handle *c)
{
  char *bindto;
  struct sockaddr *in;
  socklen_t in_len;
  struct sockaddr_storage in_sto;
  socklen_t sto_len;
  
  (void) cls;
  cfg = c;
  if (GNUNET_OK !=
      GNUNET_CONFIGURATION_get_value_filename (cfg,
                                               COMMUNICATOR_CONFIG_SECTION,
                                               "BINDTO",
                                               &bindto))
  {
    GNUNET_log_config_missing (GNUNET_ERROR_TYPE_ERROR,
                               COMMUNICATOR_CONFIG_SECTION,
                               "BINDTO");
    return;
  }

  in = udp_address_to_sockaddr (bindto,
                                &in_len);
  if (NULL == in)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Failed to setup UDP socket address with path `%s'\n",
                bindto);
    GNUNET_free (bindto);
    return;
  }
  udp_sock = GNUNET_NETWORK_socket_create (in->sa_family,
                                           SOCK_DGRAM,
                                           IPPROTO_UDP);
  if (NULL == udp_sock)
  {
    GNUNET_log_strerror (GNUNET_ERROR_TYPE_ERROR,
                         "socket");
    GNUNET_free (in);
    GNUNET_free (bindto);
    return;
  }
  if (AF_INET6 == in->sa_family)
    have_v6_socket = GNUNET_YES;
  if (GNUNET_OK !=
      GNUNET_NETWORK_socket_bind (udp_sock,
                                  in,
                                  in_len))
  {
    GNUNET_log_strerror_file (GNUNET_ERROR_TYPE_ERROR,
                              "bind",
                              bindto);
    GNUNET_NETWORK_socket_close (udp_sock);
    udp_sock = NULL;
    GNUNET_free (in);
    GNUNET_free (bindto);
    return;
  }
  /* We might have bound to port 0, allowing the OS to figure it out;
     thus, get the real IN-address from the socket */
  sto_len = sizeof (in_sto);
  if (0 != getsockname (GNUNET_NETWORK_get_fd (udp_sock),
                        (struct sockaddr *) &in_sto,
                        &sto_len))
  {
    memcpy (&in_sto,
            in,
            in_len);
    sto_len = in_len;
  }
  GNUNET_free (in);
  GNUNET_free (bindto);
  in = (struct sockaddr *) &in_sto;
  in_len = sto_len;
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "Bound to `%s'\n",
              GNUNET_a2s ((const struct sockaddr *) &in_sto,
                          sto_len));
  switch (in->sa_family)
  {
  case AF_INET:
    my_port = ntohs (((struct sockaddr_in *) in)->sin_port);
    break;
  case AF_INET6:
    my_port = ntohs (((struct sockaddr_in6 *) in)->sin6_port);
    break;
  default:
    GNUNET_break (0);
    my_port = 0;
  }
  stats = GNUNET_STATISTICS_create ("C-UDP",
                                    cfg);
  senders = GNUNET_CONTAINER_multipeermap_create (32,
                                                  GNUNET_YES);
  receivers = GNUNET_CONTAINER_multipeermap_create (32,
                                                    GNUNET_YES);
  senders_heap = GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
  receivers_heap = GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
  key_cache = GNUNET_CONTAINER_multishortmap_create (1024,
                                                     GNUNET_YES);
  GNUNET_SCHEDULER_add_shutdown (&do_shutdown,
                                 NULL);
  is = GNUNET_NT_scanner_init ();
  my_private_key = GNUNET_CRYPTO_eddsa_key_create_from_configuration (cfg);
  if (NULL == my_private_key)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                _("Transport service is lacking key configuration settings. Exiting.\n"));
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  GNUNET_CRYPTO_eddsa_key_get_public (my_private_key,
                                      &my_identity.public_key);
  /* start reading */
  read_task = GNUNET_SCHEDULER_add_read_net (GNUNET_TIME_UNIT_FOREVER_REL,
                                             udp_sock,
                                             &sock_read,
                                             NULL);
  ch = GNUNET_TRANSPORT_communicator_connect (cfg,
                                              COMMUNICATOR_CONFIG_SECTION,
                                              COMMUNICATOR_ADDRESS_PREFIX,
                                              GNUNET_TRANSPORT_CC_UNRELIABLE,
                                              &mq_init,
                                              NULL,
                                              &enc_notify_cb,
                                              NULL);
  if (NULL == ch)
  {
    GNUNET_break (0);
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  /* start broadcasting */
  if (GNUNET_YES !=
      GNUNET_CONFIGURATION_get_value_yesno (cfg,
                                            COMMUNICATOR_CONFIG_SECTION,
                                            "DISABLE_BROADCAST"))
  {
    broadcast_task = GNUNET_SCHEDULER_add_now (&do_broadcast,
                                               NULL);
  }  
  nat = GNUNET_NAT_register (cfg,
                             COMMUNICATOR_CONFIG_SECTION,
                             IPPROTO_UDP,
                             1 /* one address */,
                             (const struct sockaddr **) &in,
                             &in_len,
                             &nat_address_cb,
                             NULL /* FIXME: support reversal: #5529 */,
                             NULL /* closure */);
}


/**
 * The main function for the UNIX communicator.
 *
 * @param argc number of arguments from the command line
 * @param argv command line arguments
 * @return 0 ok, 1 on error
 */
int
main (int argc,
      char *const *argv)
{
  static const struct GNUNET_GETOPT_CommandLineOption options[] = {
    GNUNET_GETOPT_OPTION_END
  };
  int ret;

  if (GNUNET_OK !=
      GNUNET_STRINGS_get_utf8_args (argc, argv,
                                    &argc, &argv))
    return 2;

  ret =
      (GNUNET_OK ==
       GNUNET_PROGRAM_run (argc, argv,
                           "gnunet-communicator-udp",
                           _("GNUnet UDP communicator"),
                           options,
                           &run,
                           NULL)) ? 0 : 1;
  GNUNET_free ((void*) argv);
  return ret;
}


/* end of gnunet-communicator-udp.c */