Networking and Security

ZentriOS includes a full IPv4 networking stack that supports a range of popular networking protocols including TCP, UDP, DNS, DHCP. Additional network application libraries are provided for native HTTP and secure cloud access.

For each networking client and server there are configuration variables, and commands and procedures to start and stop the client or server.

When a network connection is established, ZentriOS opens a network stream to write and read data. See Network Connections and Streams.

See the article TCP is a Stream for a discussion of the underlying mechanism of TCP, and how to determine that data is received.

Note that the term stream has different meanings depending on context. See Glossary, Stream.

Other network features include ...

Security features include ...

DHCP Client

The ZentriOS DHCP client obtains an IP address from the WLAN DHCP server when the network is brought up with the wlan interface.

Configure the DHCP client with the variables:

Bring the network up by issuing any command that requires the network, or use the command:

DHCP Server

The ZentriOS DHCP server supplies IP addresses to clients connecting to the soft AP when the network is brought up with the softap interface.

Configure the DHCP server with the variables:

The soft AP starts on reboot when configured to auto-start with the softap.auto_start variable.

You can bring up the soft AP with the network_up command. Either specify the softap interface, or set the network.default_interface variable to softap.

The soft AP starts automatically when you use Web Setup.

DNS Client

The DNS Client works with all the network protocols, including HTTP, TCP and UDP.

There are no configuration variables for the DNS Client.

The variable returns a list of addresses currently in use by the WLAN interface.

DNS Server

DNS Server features are available for the Soft AP interface.

Configuration options for the soft AP DNS server are:

The setup.web.url variable defines a list of URLs that clients can use to connect to the ZentriOS Web App.

HTTP Client

The HTTP Client consists of a group of commands for HTTP methods:

For a secure TLS transaction, the url scheme specified with the command must be https. See HTTPS Client.

The http_get, http_post and http_head commands have an option to queue the request, prior to sending. While the request is queued, use the http_add_header to add custom headers as required, as shown in the example below. Use the stream_write command to add data to the body of a POST request.

Use the http_read_status command to read the HTTP response after a request is completed.

Connection timeout for the HTTP client commands is controlled by the tcp.client.connect_timeout variable.

Stream Type: HTTP. See Network Connections and Streams, Stream Types.


The following (fictitious) HTTP POST example shows how to post data to an HTTP web server using the ZentriOS HTTP API. The HTTP body data posted in this example is a small piece of JSON (sent using stream_write 0 7). Since the -o option is used with the http_post command, a connection stream to the HTTP server is opened, but the HTTP POST is queued locally on the module.

Queuing the HTTP POST locally provides the ability to add HTTP headers using the http_add_header command, and to post data in the HTTP body using the stream_write command. Once all headers and body data are queued, the HTTP POST is sent to the server and completed using the http_post command.

Any response data received from the server may be read using stream_read.

> http_post -o application/json
[2014-04-23 | 19:40:23: Opening:]
Request POST /hello
Connecting (HTTP):
[2014-04-23 | 19:40:23: Opened: 0]
> stream_write 0 7
... JSON data goes here ...
> http_read 0
HTTP response: 200
Chunked response
> stream_read 0 1000
  "response": "howdy!"
> stream_close 0
Closing: 0
[2014-04-23 | 19:40:40: Closed: 0]

HTTP Client File Upload and Download

The HTTP Client feature includes commands to handle file upload and download, between the ZentriOS device file system and an HTTP/S host:

The http_upload and http_download commands use HTTP POST requests in the background.

For a secure TLS transaction, the url scheme specified with the command must be https.


HTTP Server with RESTful API

The ZentriOS HTTP webserver may be configured to run as a service on either the softAP or WLAN interface.

There is no network stream associated with the HTTP server. An HTTP client issues a request and the HTTP server sends a response. The client and server do not read or write data outside the request.

ZentriOS provides a simple RESTful API on top of the HTTP server. The API allows for a remote HTTP(S) client to issue any ZentriOS command. The result of the command is returned in a simple JSON format.

The RESTful API can be used in a number of ways:

The HTTP Server is configured with the following variables:

A client can use the RESTful API to issue ZentriOS commands and receive responses, and also for retrieval of module log messages.

The available requests are as follows:

GET  /command
POST /command
GET  /log
POST /stream

See the HTTP Server RESTful API application note for low-level examples.

The ZentriOS Web App provides a complete demonstration of the HTTP Server RESTful API, and can be customized as required. See Customizing the ZentriOS Web App.


Command Request/Response

The API supports either a simple GET request or a slightly more complex POST request.

GET Request

GET /command/<ZentriOS command>

POST Request

POST /command
   "flags"   : <flags>,
   "command" : "<ZentriOS command>",
   "data"    : "<command data>"


With the REST API, the procedure for issuing commands is:

  1. Open HTTP socket to server
  2. POST JSON formatted command to device
  3. Device processes request
  4. Device returns a JSON formatted response
  5. HTTP connection closed

To issue another command, repeat steps 1-5.

HTTP Response Codes

Response body

   "id"       : <unique id>,
   "code"     : <response code>,
   "flags"    : <flags>,
   "response" : "<command response>"


Log Request/Response

The API also buffers log messages. This is the request to retrieve the log messages.

GET Request

GET /log

HTTP Response Codes


  "logs" : [ "<log data>", "<log data>", ....] }

Note: the log buffer has limited space. Older logs are replaced by newer ones. This should be called periodically to avoid losing logs.

HTTP Response Codes

CORS (Cross Origin Resource Sharing)

The ZentriOS HTTP server supports CORS (Cross-Origin Resource Sharing).

The http.server.cors_origin variable allows you to specify origins for which the same-origin policy is relaxed.

This allows control of the module, via the HTTP server, from a remote site provided the module has originally been set up with a http.server.cors_origin domain that allows access from that site. Via the ZentriOS HTTP server ZentriOS JavaScript API, the remote site can issue all ZentriOS commands, including reboot.

Setting the http.server.cors_origin results in the ZentriOS HTTP server inserting a corresponding CORS Access-Control-Allow-Origin (ACAO) response header into resources it delivers. It also results in the ZentriOS HTTP server responding to an OPTION request with a set of options supporting remote control.

HTTPS Client

HTTPS client commands include:

The HTTP client commands use TLS when the URL supplied has the scheme https.

The TLS CA certificate is supplied as a command argument or by default configured with the variables:

NTP Client

Network Time Management

ZentriOS devices can obtain time data from an NTP (Network Time Protocol) server.

Configure NTP with the variables:

Time related variables are:

If NTP is enabled, time.last_set returns the seconds since the time was last set by NTP.

To read the time, get time.rtc and to determine the time zone, get

TCP Client

Configure the TCP client with the variables:

Start the TCP client with the command:

Stream Type: TCPC. See Network Connections and Streams, Stream Types.

TCP Client Auto Connect

To connect automatically to a remote host when the network is brought up, configure TCP client auto-connect with the variables:

To auto start with a TLS connection, set tcp.client.tls_enabled = TRUE. See TLS Client.

TCP Client Connection Retry Sequence

The TCP client connection retry sequence is shown in the following diagram. Click to enlarge:

TCP Server

Configure the TCP server with the variables:

Start and stop the TCP server with the TCP server command:

Stream Type: TCPS. See Network Connections and Streams, Stream Types.

TLS 1.0-1.2 Support

ZentriOS supports TLS 1.0 to TLS 1.2.

Supported TLS Cipher Suites

ZentriOS supports the following Cipher Suites for TLS 1.0 - 1.2:

OpenSSL NameKey ExchangeEncryptionHashing Algorithm

Acronyms Used in Above Table

TLS Memory Requirements

Increasing Available Memory for TLS

TLS connections require a considerable amount of RAM. Low memory may result in TLS errors.

To establish a TLS connection, other memory intensive features may need to be disabled. You can return the device to the default state with a factory reset.

TLS client handshake may require up to 85% of the available dynamic memory to complete. This memory requirement dictates the ability to run TLS client and other network streams simultaneously.


If TLS client auto-start is enabled, other streams with auto-start variable enabled will not start.

Run-time via command API

The TLS client will not start if other streams are opened. Other streams may be started after TLS client is opened (has completed its handshake).

TLS Client

TLS client commands include:

See also HTTPS Client.

TLS details must be supplied as a command argument or configured with the variables:

To manually initiate a TLS connection with the TCP client, use the tls_client command.

To auto-start the TCP client with TLS, set:

See TCP Client Auto Connect.

The TCP client, with TLS enabled, has the features described in the TCP Client documentation.

Stream Type: TLSC. See Network Connections and Streams, Stream Types.

OTA always uses TLS.

Connect timeout for the TLS client command is controlled by the tcp.client.connect_timeout variable.

UDP Client

Configure the UDP client with the variables:

Start the UDP client with the command:

Stream Type: UDPC. See Network Connections and Streams, Stream Types.

UDP Client Auto Connect

Configure the UDP client auto-connect with the variables:

UDP Server

Configure the UDP server with the variables:

Start and stop the UDP server with the command:

The udp_server has subcommands to read and write. These are equivalent to the stream read and write commands, with the added option of determining the client address and port.

The UDP server uses a single stream for multiple clients. Using the stream read and write commands applies to all clients. Using the UDP server subcommands allows writing to and reading from a specific client.

Stream Type: UDPS. See Network Connections and Streams, Stream Types.

UDP Server Auto Connect

Configure the UDP server auto-connect with the variables:

Network Connections and Streams

Streams are associated with:

When ZentriOS opens a network connection, file, or serial connection, it returns a stream handle. The stream commands act on a stream handle to perform actions such as read, write, poll and close. You can list open streams. Each stream has a specific type. See Stream Types.

Stream Handles

When a stream is open, it is assigned a handle number. This handle is used to read/write/poll/close the stream.

Stream Commands

Refer to the following stream commands to use the stream handle:

Use the stream_list command to view open streams. For example:

> stream_list
! # Type  Info
# 0 TCPS
# 1 UDPC (15672)
# 2 TCPC (13171)
# 3 FILE  webapp/unauthorized.html-  (9530, 0)
# 4 FILE  favicon.ico.gz-  (733, 0)
# 5 FILE  webapp/zentrios.css.gz-  (22670, 0)

Serial STREAM Mode

In serial STREAM mode, a single network connection (TCP/TLS/UDP client/server) streams to the serial bus. No stream read or write commands are required, as data flows automatically between the serial bus and the open stream. See Serial Interface, Stream Mode.

STREAM mode does not support the following stream types:

Note: When in STREAM mode, a TCP server is limited to one client connection.

The STREAM mode enabled stream appears in the stream list. Reading and writing data to the stream is not permitted.

Stream Limitations

At any moment, ZentriOS-WL supports one UDP stream and one TCP client stream. Depending on available memory, WL can support several simultaneous file streams and the TCP Server can support multiple clients.

Each stream type in the Stream Types table below uses a single stream, with the following exceptions and qualifications:

Stream Types

List the currently open streams with the stream_list command. Streams are listed with stream handle, type, and information about the stream source.

Note that some stream types support a limited set of operations. The following is a list of stream types, and for each type the command to create it, and the stream operations it supports.

TypeAssociated CommandsStream OperationsDescription
FILEfile_open/file_create -oread/poll/closeFile system handle. See File system
HTTPhttp_get/http_post/http_headread/write/poll/closeHTTP Client
HTTPShttp_get/http_post/http_headread/write/poll/closeSecure HTTP Client
TCPCtcp_clientread/write/poll/closeTCP Client
TCPStcp_serverread/write/poll/closeTCP Server client stream. Note that one stream is used per client
TLSCtls_clientread/write/poll/closeTLS Client
UDPCudp_clientread/write/poll/closeUDP Client
UDPSudp_serverread/write/poll/closeUDP Server. Note that one stream is used for all server clients

Streams Do Not Close Automatically

Network streams do not auto-close. A stream remains open until the stream_close command is sent. Use the stream_poll command to determine status. A stream with status 2 indicates that the stream connection has been closed remotely.

Auto-start Network streams

Auto-start network streams are opened in the following order:

  1. TCP/TLS client. If TLS client auto starts, other streams will not start.
  2. UDP client
  3. TCP server
  4. IO connection

Network Status Indication Using GPIOs

You can configure GPIOs to act as indicators of networking states, such as the state of the WLAN connection, the Soft AP connection, or DHCP progress. You can configure a selection of blink rates to indicate states.

See variables:

See Peripherals, System Indicator Functions.

Remote Terminal Access

You can control a ZentriOS device with a TCP/IP telnet connection via a remote terminal, connected to the device via the Soft AP interface or the WLAN interface.

Configure the remote terminal with the variables:

See the Wi-Fi Remote Terminal application note.

Broadcast Status Announcement

A ZentriOS device broadcasts its properties in JSON format. The properties can be sent either as UDP packets to a UDP host or by a post request to an HTTP host.

Properties include by default the IP address and the MAC address.

Configure broadcast with the variables:

See also the Broadcast UDP Packet Application Note.

Captive Portal

Captive portal is supported during websetup and optionally for the soft AP. When the HTTP Server receives certain requests, it redirects to the root HTTP server web page. See softap.captive_portal_enabled.