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crosscoap is a CoAP (Constrained Application Protocol) UDP server which translates incoming CoAP requests to corresponding HTTP requests which are sent to a backend HTTP server. The HTTP responses from the backend are translated back to CoAP and sent over to the CoAP client.

crosscoap can be used in front of any HTTP server to quickly allow CoAP clients to consume content from that HTTP application, without adding specific CoAP functionality to the application itself.


Go must be installed and $GOPATH properly set. To install crosscoap, run:

go get

This should build the crosscoap executable in $GOPATH/bin.

crosscoap relies on the go-coap library to handle the CoAP packets; it is installed as part of the go get.

Command usage

To proxy incoming CoAP packets (UDP port 5683) to a local HTTP server listening on port 8000:

crosscoap -listen -backend

Command-line switches:

  • -listen LISTEN_ADDR_PORT: The address and UDP port on which to listen for incoming CoAP UDP requests (example:
  • -backend BACKEND_URL: The URL of the HTTP backend server (example:
  • -errorlog FILENAME: Log errors to file (default is logging errors to stderr) (example: /tmp/crosscoap-error.log)
  • -accesslog: Log every request to file (example: /tmp/crosscoap-access.log)

Example: fetching Mars weather data over CoAP

The following command will start a CoAP server on UDP port 5683; incoming requests will be translated to HTTP and proxied to the Mars Weather MAAS API service:

crosscoap -listen -backend -accesslog /tmp/coap-mars-access.log

Once this is running, you can get the latest Mars weather information with a CoAP client. Here is an example with coap-cli (line breaks were added to the response for clarity):

coap get coap://localhost/latest

(2.05) {"report":
        {"terrestrial_date": "2015-10-12",
         "sol": 1131,
         "ls": 53.0,
         "min_temp": -81.0,
         "min_temp_fahrenheit": -113.8,
         "max_temp": -28.0,
         "max_temp_fahrenheit": -18.4,
         "pressure": 902.0,
         "pressure_string": "Higher",
         "abs_humidity": null,
         "wind_speed": null,
         "wind_direction": "--",
         "atmo_opacity": "Sunny",
         "season": "Month 2",
         "sunrise": "2015-10-12T11:09:00Z",
         "sunset": "2015-10-12T22:55:00Z"}}

In this example, the CoAP request to /latest was proxied to the HTTP backend as The backend HTTP server's 200 OK response was translated back to a CoAP 2.05 (Content) response with the CoAP payload holding the JSON document.

Library usage

crosscoap may be used as a Go package in order to add the CoAP proxying functionality to an existing Go application. For example:

package main

import (


func main() {
        timeout := time.Duration(10 * time.Second)
        appLog := log.New(os.Stderr, "", log.LstdFlags)
        udpAddr, err := net.ResolveUDPAddr("udp", "")
        if err != nil {
                appLog.Fatalln("Can't resolve UDP addr")
        udpListener, err := net.ListenUDP("udp", udpAddr)
        if err != nil {
                errorLog.Fatalln("Can't listen on UDP")
        defer udpListener.Close()
        p := crosscoap.Proxy{
                Listener:   udpListener,
                BackendURL: "",
                Timeout:    &timeout,
                AccessLog:  appLog,
                ErrorLog:   appLog,


CoAP and HTTP are different protocols. crosscoap attempts to translate status codes and headers (options in CoAP) between the two protocols, but not every code and header has a corresponding entity in the other protocol.

CoAP is UDP-based, and therefore the entire CoAP message (UDP headers, CoAP headers and CoAP payload) must fit in the network MTU, which is generally around 1500 bytes. If the response body from the HTTP server is too long, crosscoap will truncate it (and log an error). Keep the HTTP response body under 1400 bytes to be safe.

Related documentation



(c) Copyright IBM Corp. 2015, 2016.

This project is licensed under the Apache License 2.0. See the LICENSE file for more info.

3rd party software used by crosscoap:


Contributions to the project are welcomed. It is required however to provide alongside the pull request one of the contribution forms (CLA) that are a part of the project. If the contributor is operating in his individual or personal capacity, then he/she is to use the individual CLA; if operating in his/her role at a company or entity, then he/she must use the corporate CLA.


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