libcoap 4.3.2


coap-client, coap-client-gnutls, coap-client-mbedtls, coap-client-openssl, coap-client-notls — CoAP Client based on libcoap


coap-client [-a addr] [-b [num,]size] [-e text] [-f file] [-l loss] [-m method] [-o file] [-p port] [-r] [-s duration] [-t type] [-v num] [-w] [-A type] [-B seconds] [-E oscore_conf_file[,seq_file]] [-G count] [-H hoplimit] [-K interval] [-L value] [-N] [-O num,text] [-P scheme://addr[:port]] [-T token] [-U] [-V num] [-X size] [[-h match_hint_file] [-k key] [-u user]] [[-c certfile] [-j keyfile] [-n] [-C cafile] [-J pkcs11_pin] [-M rpk_file] [-R trust_casfile]] URI

For coap-client versions that use libcoap compiled for different (D)TLS libraries, coap-client-notls, coap-client-gnutls, coap-client-openssl, coap-client-mbedtls or coap-client-tinydtls may be available. Otherwise, coap-client uses the default libcoap (D)TLS support.


coap-client is a CoAP client to communicate with 6LoWPAN devices via the protocol CoAP (RFC 7252) using the URI given as argument on the command line. The URI must have the scheme coap, coap+tcp, coaps or coaps+tcp.

coaps and coaps+tcp are only supported when coap-client is built with support for secure (D)TLS communication.

If coaps or coaps+tcp is being used, provided the CoAP server supports PKI and is configured with a certificate and private key, the coap-client does not need to have a Pre-Shared Key (-k) or certificate (-c) configured.

The URI’s host part may be a DNS name, a literal IP address or a Unix domain name. For Unix domain names, %2F is used as the / separator to differentiate between the host and patch definitions. For IPv6 address references, angle brackets are required (c.f. EXAMPLES) to delimit the host portion of the URI.

OPTIONS - General

-a addr
The local address of the interface that has to be used. Note: Do not use this option if the interface is likely to be transient - i.e. it is a tunnel interface that may come and go, as this is likely to cause "No such device" errors on transmission.
-b [num,]size
The block size to be used in GET/PUT/POST requests (value must be a multiple of 16 not larger than 1024 as libcoap uses a fixed maximum PDU size of 1400 bytes). If num is present, the request chain will start at block num. When the server includes a Block2 option in its response to a GET request, coap-client will automatically retrieve the subsequent block from the server until there are no more outstanding blocks for the requested content.
-e text
Include text as payload (use percent-encoding for non-ASCII characters).
-f file
File to send with PUT/POST (use - for STDIN).
-l list
Fail to send some datagrams specified by a comma separated list of numbers or number ranges (debugging only).
-l loss%
Randomly failed to send datagrams with the specified probability - 100% all datagrams, 0% no datagrams (debugging only).
-m method
The request method for action (get|put|post|delete), default is get. (Note that the string passed to -m is compared case-insensitive.)
-o file
A filename to store data retrieved with GET.
-p port
The port to send from.
Use reliable protocol (TCP or TLS).
-s duration
Subscribe to / observe the resource specified by URI for the given duration in seconds.
-t type

Content format for given resource for PUT/POST. type must be either a numeric value reflecting a valid CoAP content format or a string describing a registered format. The following registered content format descriptors are supported, with alternative shortcuts given in parentheses:

text/plain (plain)
application/link-format (link, link-format)
application/xml (xml)
application/octet-stream (binary, octet-stream)
application/exi (exi)
application/json (json)
application/cbor (cbor)
-v num
The verbosity level to use (default 4, maximum is 8) for general CoAP logging.
Append a newline to received data.
-A type
Accepted media type. type must be either a numeric value reflecting a valid CoAP content format or a string that specifies a registered format as described for option -t.
-B seconds
Break operation after waiting given seconds (default is 90).
-E oscore_conf_file[,seq_file]
oscore_conf_file contains OSCORE configuration. See coap-oscore-conf(5) for definitions. Optional seq_file (which is created if needed) is used to save the current transmit sequence number, so on client restart sequence numbers continue to increase and are not reset to prevent anti-replay mechanisms being triggered.
-G count
Repeat the Request count times with a second delay between each one. Must have a value between 1 and 255 inclusive. Default is 1.
-H hoplimit
Set the Hop Limit count to hoplimit for proxies. Must have a value between 1 and 255 inclusive. Default is 16.
-K interval
Send a ping after interval seconds of inactivity. If not specified (or 0), keep-alive is disabled (default).
-L value

Sum of one or more COAP_BLOCK_* flag values for different block handling methods. Default is 1 (COAP_BLOCK_USE_LIBCOAP).

Send NON-confirmable message. If option -N is not specified, a confirmable message will be sent.
-O num,text
Add option num with contents of text to the request. If the text begins with 0x, then the hex text (two [0-9a-f] per byte) is converted to binary data.
-P scheme://addr[:port]
Scheme, address and optional port to define how to connect to a CoAP proxy (automatically adds Proxy-Uri option to request) to forward the request to. Scheme is one of coap, coaps, coap+tcp and coaps+tcp.
-T token
Define the initial starting token for the request (up to 24 characters).
Never include Uri-Host or Uri-Port options.
-V num
The verbosity level to use (default 3, maximum is 7) for (D)TLS library logging.
-X size
Maximum message size to use for TCP based connections (default is 8388864). Maximum value of 2^32 -1.


(If supported by underlying (D)TLS library)

-h match_hint_file
This is a file that contains one or more lines of received Identity Hints to match to use different user identity and associated pre-shared key (PSK) (comma separated) instead of the -k key and -u user options. E.g., per line hint_to_match,use_user,with_key A line that starts with # is treated as a comment. Note: -k key and -u user still need to be defined for the default case in case there is no match.
-k key
Pre-shared key for the specified user identity (-u option also required).
-u user
User identity to send for pre-shared key mode (-k option also required).


(If supported by underlying (D)TLS library)

Note: If any one of certfile, keyfile or cafile is in PKCS11 URI naming format (pkcs11: prefix), then any remaining non PKCS11 URI file definitions have to be in DER, not PEM, format. Otherwise all of certfile, keyfile or cafile are in PEM format.

-c certfile
PEM file or PKCS11 URI for the certificate. The private key can also be in the PEM file, or has the same PKCS11 URI. If not, the private key is defined by -j keyfile.
-j keyfile
PEM file or PKCS11 URI for the private key for the certificate in -c certfile if the parameter is different from certfile in -c certfile.
Disable remote peer certificate checking.
-C cafile
PEM file or PKCS11 URI for the CA certificate that was used to sign the server certfile. Ideally the client certificate should be signed by the same CA so that mutual authentication can take place. The contents of cafile are added to the trusted store of root CAs. Using the -C or -R options will trigger the validation of the server certificate unless overridden by the -n option.
-J pkcs11_pin
The user pin to unlock access to the PKCS11 token.
-M rpk_file
Raw Public Key (RPK) PEM file or PKCS11 URI that contains both PUBLIC KEY and PRIVATE KEY or just EC PRIVATE KEY. (GnuTLS and TinyDTLS(PEM) support only). -C cafile or -R trust_casfile are not required.
-R trust_casfile
PEM file containing the set of trusted root CAs that are to be used to validate the server certificate. Alternatively, this can point to a directory containing a set of CA PEM files. The -C cafile CA does not have to be in this list and is trusted for the validation. Using -R trust_casfile disables common CA mutual authentication which can only be done by using -C cafile. Using the -C or -R options will will trigger the validation of the server certificate unless overridden by the -n option.


  • Example
coap-client coap://

Query the resource / from server (using the GET method).

  • Example
coap-client -m get coap://[::1]/

Query the resource / on localhost using the GET method to get back the summary information.

  • Example
coap-client -m get coap://%2Fsome%2Funix%2Fdomain%2Fpath/time

Query the resource /time on server listening on datagram Unix domain /some/unix/domain/path using the GET method to get back the current time. The %2F is the hex encoding for / and indicates which is the host definition separator and the simple / is for the path definition separator.

  • Example
coap-client -m get coap://[::1]/.well-known/core

Query on the resource .well-known/core on localhost to get back a list of the known resources along with their attribute definitions.

  • Example
echo -n "mode=on" | coap-client -m put \
coap://[2001:db8:c001:f00d:221:2eff:ff00:2704]:5683/actuators/leds?color=r -f-

Send text mode=on to resource actuators/leds?color=r on the endpoint with address 2001:db8:c001:f00d:221:2eff:ff00:2704 and port 5683. Note that the port 5683 is the default port and isn’t actually required in this instance.

  • Example
coap-client -m put coap://[fec0::3]/ck -T 3a -t binary -f to_upload

Put the contents of file to_upload with content type binary (i.e. application/octet-stream) into resource ck on fec0::3 using a token of 3a via the PUT method.


There are no configuration files.


Failure (syntax or usage error; configuration error; document processing failure; unexpected error)


coap-server(5) and coap-oscore-conf(5)


Please report bugs on the mailing list for libcoap: or raise an issue on GitHub at


The libcoap project <>