A Hello World CoAP client-server using libcoap

View a list of all libcoap examples here.

This is a simple example of a CoAP server and a CoAP client. The server listens on port 5683 (default port) and responds with a "Hello World!" to every request for the "hello" resource. The response is piggybacked in the ACK message.

The client performs a CoAP GET request for the "coap://127.0.0.1/hello" resource and prints the response.

You can find the source code of this example, as well as, instructions for compiling it in Linux in this github repository, in the "piggybacked" folder.

DLNA with subtitles in D-Link DNS-320

D-Link DNS-320 is a network storage enclosure that supports DLNA. Unfortunately, the built-in DLNA server does not support subtitles. Nevertheless, this can be overcome by installing miniDLNA. miniDLNA can stream movies and subtitles, providing that the subtitles file has the same name as the movie file (minus the extension). The following has been tested successfully with UTF-8 encoded .srt files in Samsung  UE46ES6340 smart tv.

DNS-320 is a linux based device. Moreover, it's firmware can execute scripts, located in the root directory, every time the system boots. Fonz fun plug (ffp), its such a script that adds support for telnet, as well as, for installing additional software. In this post, we will use ffp ton install miniDLNA in our NAS.

Step 0 preparation

Make sure you have disabled the built-in DLNA server

Step 1 install ffp

Installing ffp is as trivial as copy pasting a file. Follow the instructions here https://nas-tweaks.net/371/hdd-installation-of-the-fun_plug-0-7-on-nas-devices/

Step 2 install miniDLNA

The web page included in step 2 has instructions about how to connect to your NAS using telnet. Follow these instructions and connect to your NAS. ffp includes a package manager for installing software.  Initially, the package manages has to be configured with download sites. This can be done using  uwsiteloader. uwsiteloader can be installed by following the instructions here https://nas-tweaks.net/371/hdd-installation-of-the-fun_plug-0-7-on-nas-devices/#Now_what.3F After downloading it and making it executable, run it. In the step when it requests to select download sites, select all of them. You can then install miniDLNA and configure following the steps here: http://forum.nas-central.org/viewtopic.php?f=249&t=5841&start=45#p56567  section "INSTALL INSTRUCTIONS". Make sure you edit /ffp/etc/minidlna.conf accordingly (vi can be used for the editing).

You are ready to go! A useful command that forces miniDLNA to rebuilt its database is /ffp/start/minidlna.sh rescan

Create self-singed certificate with extentions

For testing reasons I wanted to create a self-signed certificate that includes the subject alternative name extension, using openssl. Most guides require the creation of an openssl configuration file. I found out that this can be done without any configuration file, using only two openssl commands and a file that contains the subject alternative name extension parameters.

The first command is the following:

openssl req -newkey rsa:1024 -keyout server.key -out server.csr -subj '/C=GR/ST=Attiki/L=Athens/O=Fotiou Corp/OU=Security Department/CN=localhost/emailAddress=my@email.address' -nodes

This command creates a new private key and a new certificate signing request. Let's see the command parameters:

-newkey rsa:1024      It creates an RSA 1024 bits key
-keyout server.key  This is the file where the private key is stored
-out server.csr        This the file where the certificate signing request is stored
-subj ...                   This is the information included in the certificate
-nodes                          This command parameter instructs openssl to not encrypt the private key

Now create a file and insert the subject alternative name extension parameters. In this example, I have created a file named extentions.cnf which contains the following text:

subjectAltName=DNS:example.com, DNS:localhost

This line indicates that this certificate is valid for two DNS names, namely example.com and localhost. You may notice that the CN name included in the -subj command line parameter is also included here; the reason for that is because most browsers ignore the CN field when the subject alternative name extension is used. Finally the following command creates the desired certificate

openssl x509 -req -days 365 -signkey server.key -in server.csr -out server.crt -extfile extentions.cnf


Where:
-days 3650                          It is the number of days for which the certificate is valid
-signkey server.key         It is the private key generated previously and it used to sign the certificate
-in server.csr                   The certificate signing request we created with the previous command
-out server.crt                 The file in which the certificate will be stored
-extfile extentions.cnf The file we created with the subject alternative name extension parameters