Aug 12

Secure download URLs with expiration time

Reading time: 4 – 6 minutes


Imagine a HTTP server with those restrictions:

  • only specific files can be downloaded
  • with a limited time (expiration date)
  • an ID allows to trace who download files
  • with minimal maintenance and dependencies (no databases, or things like that)

the base of the solution that I designed is the URL format:

  • signature: is calculated with the next formula, given a “seed”
    • seed = “This is just a random text.”
    • str = customer_id + expire_date + path_n_file
    • signature = encode_base64( hmac_sha1( seed, str))
  • customer_id: just an arbitrary identifier when you want to distinguish who use the URL
  • expire_date: when the generated URL stops working
  • path_n_file: relative path in your private repository and the file to share

Understanding the ideas explained before I think it’s enough to understand what is the goal of the solution. I developed the solution using NGINX and LUA. But the NGINX version used is not the default version is a very patched version called Openresty. This version is specially famous because some important Chinese webs works with that, for instance,

Expiration URL solution Architecture schema

In the above schema there is a master who wants to share a file which is in the internal private repository, but the file has a time restriction and the URL is only for that customer. Then using the command line admin creates a unique URL with desired constrains (expiration date, customer to share and file to share). Next step is send the URL to the customer’s user. When the URL is requested NGINX server evaluates the URL and returns desired file only if the user has a valid URL. It means the URL is not expired, the file already exists, the customer identification is valid and the signature is not modified.

NGINX Configuration

server {
 server_name downloads.local;

 location ~ ^/(?<signature>[^/]+)/(?<customer_id>[^/]+)/(?<expire_date>[^/]+)/(?<path_n_file>.*)$ {
 content_by_lua_file "lua/get_file.lua";

 location / {
 return 403;

This is the server part of the NGINX configuration file, the rest of the file can as you want. Understanding this file is really simple, because the “server_name” works as always. Then only locations command are relevant. First “location” is just a regular expression which identifies the relevant variables of the URL and passes them to the LUA script. All other URLs that doesn’t match with the URI pattern fall in path “/” and the response is always “Forbiden” (HTTP 403 code). Then magics happen all in LUA code.

LUA scripts

There are some LUA files required:

  • create_secure_link.lua: creates secure URLs
  • get_file.lua: evaluates URLs and serves content of the required file
  • lib.lua: module developed to reuse code between other lua files
  • sha1.lua: SHA-1 secure hash computation, and HMAC-SHA1 signature computation in Lua (get from

It’s required to configure “lib.lua” file, at the beginning of the file are three variables to set up:

lib.secret = "This is just a long string to set a seed"
lib.base_url = "http://downloads.local/"
lib.base_dir = "/tmp/downloads/"

Create secure URLs is really simple, take look of the command parameters:

$ ./create_secure_link.lua 

 ./create_secure_link.lua <customer_id> <expiration_date> <relative_path/filename>

Create URLs with expiration date.

 customer_id: any string identifying the customer who wants the URL
 expiration_date: when URL has to expire, format: YYYY-MM-DDTHH:MM
 relative_path/filename: relative path to file to transfer, base path is: /tmp/downloads/

Run example:

$ mkdir -p /tmp/downloads/dir1
$ echo hello > /tmp/downloads/dir1/example1.txt
$ ./create_secure_link.lua acme 2015-08-15T20:30 dir1/example1.txt
$ date
Wed Aug 12 20:27:14 CEST 2015
$ curl http://downloads.local:55080/YjZhNDAzZDY0/acme/2015-08-15T20:30/dir1/example1.txt
$ date
Wed Aug 12 20:31:40 CEST 2015
$ curl http://downloads.local:55080/YjZhNDAzZDY0/acme/2015-08-15T20:30/dir1/example1.txt
Link expired

Small video demostration


Disclaimer and gratefulness



Jul 13

Conferència Summer Camp Garrotxa 2015: SmartHome usant Arduino, Raspberry PI i més

Reading time: 1 – 2 minutes

Aquest cap de setmana vaig tornar a donar la meva conferència sobre “SmartHome” en aquesta ocasió vaig introduïr la novetat del sistema de feedback basat en “Tasker + Auto-notification”, a més de l’execusió de tasques basades en “crontab”. També he inclòs un petit avanç sobre el nou projecte en el que estic treballant per tal d’integrar les dades del descalcificador dins del OpenHAB usant una webcam i OpenCV per processar les imatges optingudes.

Agraïr al Xavi, Gerardo i la Laura per haver-me donat aquesta oportunitat. Tant el SCG15 com el SAX2015 són events molt familiars a més l’entorn és espectacular. Tot un descobriment del que malgrat tenir referències no havia pogut disfrutar en primera persona.


May 08

Free dynamic DNS service

Reading time: < 1 minute A long time ago there were several free dynamic DNS services but nowadays it's difficult to find one of them. And when you find the service usually you have some important restrictions like: number of updates per day or only few subdomains per account. But in the end I found a good free service of that, it's part of the project and is called: Qui; you only need a account to use the service and it’s really simple and clear. From my part the compatibility with “ddclient” and the “mikrotik” script are really useful and I want tu highlight this functionality.

Apr 19

Conference: SmartHome using Arduino, Raspberry PI and more

Reading time: 1 – 2 minutes

I did this conference twice but this time it’s in English as I said before in Catalan, this is how I’m getting real a childhood dream. Thanks to open hardware like Arduino and Raspberry PI and open source software like OpenHab I have a smarthome with several automations. Last Thursday I spoke in “IoT Barcelona meetup” sharing my personal experience automating my house.

If you lost it and you want to watch the presentation video is here:

and the presentations slides are here:

Mar 12

VMWare trick: add new hard drive without restarting the virtual machine

Reading time: < 1 minute As simple as that, if you add a new virtual hard drive using VMWare in your virtual machine with Linux and you want to force the re-scan SCSI bus to see the new hard drive when you run, for example: "fdisk -l". You can force the SCSI bus re-scan with:

# take into account that your new hard drive could be added in different point than “host0″
echo “- – -” > /sys/class/scsi_host/host0/scan
Feb 18

Conferència: SmartHome usant Arduino, Raspberry PI i més

Reading time: 1 – 2 minutes

El dia 12 de febrer al vespre vaig fer una conferència a la FIB (Facultat d’Informàtica de Barcelona) dins de la UPC (Universitat Politècnica de Catalunya). En aquesta xerra vaig estar explicant com vaig convertint en realitat un somni que vaig tenir quan era petit, estic parlant de domotitzar la casa. Gràcies a enginys com l’Arduino i la Raspberry PI a més d’aplicacions com l’OpenHab podem fer assequible allò que si anem a grans marques comercials es fa caríssim pel meu pressupost.

En aquest enllaç podeu trobar les transparències de:  SmartHome usant Arduino Raspberry i més i el video el teniu disponible al servidor de la FIB.

Ara també teniu disponible el video a youtube:

i podeu veure les transparències des d’aquest mateix post:

Espero els vostres feedbacks als comentaris, desitjo que ús sigui útil.