DESCRIPTION

This man page focuses on the setting up of a CoAP client endpoint and hence creation of a CoAP session used to connect to a server. For a CoAP server endpoint, see coap_endpoint_server(3). There is no need to call coap_new_endpoint(3) for a client as well as one of the coap_new_client_server*() functions.

The CoAP stack’s global state is stored in a coap_context_t context object. Resources, Endpoints and Sessions are associated with this context object. There can be more than one coap_context_t object per application, it is up to the application to manage each one accordingly.

A CoAP session maintains the state of an ongoing connection between a Client and Server which is stored in a coap_session_t session object. A CoAP session is tracked by local port, CoAP protocol, remote IP address and remote port, or in the case of Unix Domain sockets, the local path and the remote path.

The session network traffic can be encrypted or un-encrypted if there is an underlying TLS library.

If (D)TLS is going to be used for encrypting the network traffic, then the (D)TLS information for Pre-Shared Keys (PSK) or Public Key Infrastructure (PKI) needs to be configured before any network traffic starts to flow. For Clients, this is done during the Client session set up.

For Clients, all the encryption information can be held at the (D)TLS context and CoAP context levels, or at the (D)TLS session and CoAP session levels. If defined at the context level, then when a session is created, it will inherit the context definitions, unless they have separately been defined for the session level, in which case the session version will get used. Typically the information will be configured at the session level for Clients.

In principle the set-up sequence for CoAP client endpoints looks like

coap_new_context()
coap_context_set_pki_root_cas() - if the root CAs need to be updated and using PKI
coap_new_client_session(), coap_new_client_session_pki() or coap_new_client_session_psk2()

Multiple client endpoints and hence sessions are supported per context.

Different CoAP protocols can be defined for proto - the current supported list is:

COAP_PROTO_UDP
COAP_PROTO_DTLS
COAP_PROTO_TCP
COAP_PROTO_TLS
COAP_PROTO_WS
COAP_PROTO_WSS

coap_tcp_is_supported(3), coap_dtls_is_supported(3), coap_tls_is_supported(3), coap_ws_is_supported(3) and coap_wss_is_supported(3) can be used for checking whether the underlying TCP, (D)TLS or WebSocket protocol support is available. See coap_tls_library(3) for further information on the types of (D)TLS sessions supported.

Libcoap supports 3 different socket types:

AF_INET  IPv4 IP addresses and ports
AF_INET6 IPv6 IP addresses and ports and can be dual IPv4/IPv6 stacked
AF_UNIX  Unix Domain using file path names

For AF_INET and AF_INET6, the client does not need to specify a local IP address and/or port as default values will get filled in. However for AF_UNIX, the local pathname must be provided and must be unique per client session. This unique local pathname will get deleted on the session being properly closed at application exit.

The client must specify IP and port when defining the coap_address_t (see coap_address_t(3)) for the remote end of the session if AF_INET or AF_INET6. If port is 0, then the default CoAP port is used instead. If AF_UNIX, the unix domain path to connect to must be specified.

FUNCTIONS

Function: coap_new_client_session()

The coap_new_client_session() function creates a client endpoint for a specific context and initiates a new client session to the specified server using the CoAP protocol proto as defined above. If the port is set to 0 in server (for AF_INET or AF_INET6), then the default CoAP port is used.

Normally local_if would be set to NULL, but by specifying local_if the source of the network session can be bound to a specific IP address or port. For AF_UNIX, local_if must be specified pointing to an appropriate coap_address_t. If local_if is defined, the address families for local_if and server must be identical. The session will initially have a reference count of 1.

To stop using a client session, the reference count must be decremented to 0 by calling coap_session_release(3). See coap_session(3). This will remove the client endpoint’s session and all its associated information.

Function: coap_new_client_session_pki()

The coap_new_client_session_pki() function, for a specific context, is used to configure the (D)TLS context using the setup_data variables as defined in the coap_dtls_pki_t structure in the newly created endpoint session - see coap_encryption(3). The connection is to the specified server using the CoAP protocol proto as defined above. If the port is set to 0 in server (for AF_INET or AF_INET6), then the default CoAP port is used.

Normally local_if would be set to NULL, but by specifying local_if the source of the network session can be bound to a specific IP address or port. For AF_UNIX, local_if must be specified pointing to an appropriate coap_address_t. If local_if is defined, the address families for local_if and server must be identical. The session will initially have a reference count of 1.

To stop using a client session, the reference count must be decremented to 0 by calling coap_session_release(3). See coap_session(3). This will remove the client endpoint’s session and all its associated information.

Function: coap_new_client_session_psk2()

The coap_new_client_session_psk2() function, for a specific context, is used to configure the (D)TLS context using the setup_data variables as defined in the coap_dtls_cpsk_t structure in the newly created endpoint session - see coap_encryption(3). The connection is to the specified server using the CoAP protocol proto as defined above. If the port is set to 0 in server (for AF_INET or AF_INET6), then the default CoAP port is used.

Normally local_if would be set to NULL, but by specifying local_if the source of the network session can be bound to a specific IP address or port. For AF_UNIX, local_if must be specified pointing to an appropriate coap_address_t. If local_if is defined, the address families for local_if and server must be identical. The session will initially have a reference count of 1.

To stop using a client session, the reference count must be decremented to 0 by calling coap_session_release(3). See coap_session(3). This will remove the client endpoint’s session and all its associated information.

Function: coap_session_set_mtu()

The coap_session_set_mtu() function is used to set the MTU size (the maximum message size) of the data in a packet, excluding any IP or TCP/UDP overhead to mtu for the client endpoint’s session. The default MTU is 1152.

Function: coap_session_max_pdu_size()

The coap_session_max_pdu_size() function is used to get the maximum MTU size of the data for the client endpoint’s session.

RETURN VALUES

coap_new_client_session(), coap_new_client_session_psk2(), coap_new_client_session_pki() return a newly created client. session or NULL if there is a creation failure.

coap_session_max_pdu_size() returns the MTU size.

EXAMPLES

CoAP Client Non-Encrypted Setup

#include <coap3/coap.h>

#include <netinet/in.h>

static coap_session_t *
setup_client_session (struct in_addr ip_address) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  /* See coap_address(3) */
  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5683);

  session = coap_new_client_session(context, NULL, &server, COAP_PROTO_UDP);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client Non-Encrypted Unix Domain Setup

#include <coap3/coap.h>

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

static coap_session_t *
setup_client_session (const char *server_ud) {
  coap_session_t *session;
  coap_address_t server;
  coap_address_t local;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  /* See coap_address(3) */
  coap_address_init(&server);
  server.addr.sa.sa_family = AF_UNIX;
  snprintf(server.addr.cun.sun_path, sizeof(server.addr.cun.sun_path),
           "%s", server_ud);

  /* Need to have a uniquely named local address */
  coap_address_init(&local);
  local.addr.sa.sa_family = AF_UNIX;
  snprintf(local.addr.cun.sun_path, sizeof(server.addr.cun.sun_path),
           "/tmp/client.%d", getpid());
  /* Only do this if you know it is safe to do so */
  unlink(local.addr.cun.sun_path);

  session = coap_new_client_session(context, &local, &server, COAP_PROTO_UDP);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client PKI Setup

#include <coap3/coap.h>

#include <netinet/in.h>

static int
verify_cn_callback(const char *cn,
                   const uint8_t *asn1_public_cert,
                   size_t asn1_length,
                   coap_session_t *c_session,
                   unsigned int depth,
                   int validated,
                   void *arg
) {
  /* Remove (void) definition if variable is used */
  (void)cn;
  (void)asn1_public_cert;
  (void)asn1_length;
  (void)c_session;
  (void)depth;
  (void)validated;
  (void)arg;

  /* Check that the CN is valid */

  /* ... */

  return 1;
}

static coap_session_t *
setup_client_session_pki (struct in_addr ip_address,
                          const char *public_cert_file,
                          const char *private_key_file,
                          const char *ca_file
) {
  coap_session_t *session;
  coap_address_t server;
  coap_dtls_pki_t dtls_pki;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  /* See coap_address(3) */
  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5684);

  memset (&dtls_pki, 0, sizeof (dtls_pki));

  /* See coap_encryption(3) */
  dtls_pki.version                 = COAP_DTLS_PKI_SETUP_VERSION;
  dtls_pki.verify_peer_cert        = 1;
  dtls_pki.check_common_ca         = 1;
  dtls_pki.allow_self_signed       = 1;
  dtls_pki.allow_expired_certs     = 1;
  dtls_pki.cert_chain_validation   = 1;
  dtls_pki.cert_chain_verify_depth = 1;
  dtls_pki.check_cert_revocation   = 1;
  dtls_pki.allow_no_crl            = 1;
  dtls_pki.allow_expired_crl       = 1;
  dtls_pki.allow_bad_md_hash       = 0;
  dtls_pki.allow_short_rsa_length  = 0;
  dtls_pki.is_rpk_not_cert         = 0; /* Set to 1 if RPK */
  dtls_pki.validate_cn_call_back   = verify_cn_callback;
  dtls_pki.cn_call_back_arg        = NULL;
  dtls_pki.validate_sni_call_back  = NULL;
  dtls_pki.sni_call_back_arg       = NULL;
  dtls_pki.additional_tls_setup_call_back = NULL;
  dtls_pki.client_sni              = NULL;
  dtls_pki.pki_key.key_type        = COAP_PKI_KEY_PEM;
  dtls_pki.pki_key.key.pem.ca_file = ca_file;
  dtls_pki.pki_key.key.pem.public_cert = public_cert_file;
  dtls_pki.pki_key.key.pem.private_key = private_key_file;

  session = coap_new_client_session_pki(context, NULL, &server,
                                        COAP_PROTO_DTLS, &dtls_pki);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client PSK Setup

#include <coap3/coap.h>

#include <stdio.h>
#include <netinet/in.h>

#ifndef min
#define min(a,b) ((a) < (b) ? (a) : (b))
#endif

static const coap_dtls_cpsk_info_t *
verify_ih_callback(coap_str_const_t *hint,
                   coap_session_t *c_session,
                   void *arg
) {
  coap_dtls_cpsk_info_t *psk_info = (coap_dtls_cpsk_info_t *)arg;
  /* Remove (void) definition if variable is used */
  (void)c_session;

  coap_log_info("Identity Hint '%.*s' provided\n", (int)hint->length, hint->s);

  /* Just use the defined information for now as passed in by arg */
  return psk_info;
}

static coap_dtls_cpsk_t dtls_psk;
static char client_sni[256];

static coap_session_t *
setup_client_session_psk (const char *uri,
                          struct in_addr ip_address,
                          const uint8_t *identity,
                          unsigned int identity_len,
                          const uint8_t *key,
                          unsigned int key_len
) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  /* See coap_address(3) */
  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5684);

  /* See coap_encryption(3) */
  memset (&dtls_psk, 0, sizeof(dtls_psk));
  dtls_psk.version = COAP_DTLS_CPSK_SETUP_VERSION;
  dtls_psk.validate_ih_call_back = verify_ih_callback;
  dtls_psk.ih_call_back_arg = &dtls_psk.psk_info;
  if (uri)
    memcpy(client_sni, uri, min(strlen(uri), sizeof(client_sni)-1));
  else
    memcpy(client_sni, "localhost", 9);
  dtls_psk.client_sni = client_sni;
  dtls_psk.psk_info.identity.s = identity;
  dtls_psk.psk_info.identity.length = identity_len;
  dtls_psk.psk_info.key.s = key;
  dtls_psk.psk_info.key.length = key_len;
  session = coap_new_client_session_psk2(context, NULL, &server,
                                        COAP_PROTO_DTLS, &dtls_psk);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

CoAP Client Anonymous PKI Setup

#include <coap3/coap.h>

#include <netinet/in.h>

static coap_session_t *
setup_client_session_dtls (struct in_addr ip_address) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  /* See coap_address(3) */
  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5683);

  session = coap_new_client_session(context, NULL, &server,
                                        COAP_PROTO_DTLS);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

SEE ALSO

coap_address(3), coap_block(3), coap_context(3), coap_encryption(3), coap_endpoint_server(3), coap_resource(3), coap_session(3) and coap_tls_library(3)

FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

"RFC8323: CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets"

for further information.

BUGS

Please report bugs on the mailing list for libcoap: libcoap-developers@lists.sourceforge.net or raise an issue on GitHub at https://github.com/obgm/libcoap/issues

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>