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COAP-CLIENT(5) coap-client Manual COAP-CLIENT(5)


coap-client - 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] [-A type] [-B seconds] [-K interval] [-N] [-O num,text] [-P addr[:port]] [-T token] [-U] [[-k key] [-u user]] [[-c certfile] [-C cafile] [-R root_cafile]] URI


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 coap+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 or a literal IP address. Note that, for IPv6 address references, angle brackets are required (c.f. EXAMPLES).


-a addr

The local address of the interface that has to be used.

-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 datagams 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 listen on.


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 3, maximum is 9). Above 7, there is increased verbosity in GnuTLS and OpenSSL logging.

-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).

-K interval

Send a ping after interval seconds of inactivity. If not specified (or 0), keep-alive is disabled (default).


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 is converted to binary data.

-P addr[:port]

Address (and port) for proxy to use (automatically adds Proxy-Uri option to request).

-T token

Include the token to the request.


Never include Uri-Host or Uri-Port options.


(If supported by underlying (D)TLS library)

-k key

Pre-shared key for the specified user (-u option also required).

-u user

User identity for pre-shared key mode (-k option also required).


(If supported by underlying (D)TLS library)

-c certfile

Use the specified PEM file which contains the CERTIFICATE and PRIVATE KEY information.

-C cafile

PEM file containing the CA Certificate that was used to sign the certfile defined using -c certfile. This will trigger the validation of the server certificate. If certfile is self-signed (as defined by -c certfile), then you need to have on the command line the same filename for both the certfile and cafile (as in -c certfile -C certfile) to trigger validation.

-R root_cafile

PEM file containing the set of trusted root CAs that are to be used to validate the server certificate. The -C cafile does not have to be in this list and is "trusted" for the verification. Alternatively, this can point to a directory containing a set of CA PEM files.



coap-client coap://

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


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

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


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.


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.


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)


Please report bugs on the mailing list for libcoap:


The libcoap project <>

05/14/2022 coap-client 4.2.1