Monthly Archives: November 2019

Let's Encrypt Certificate for SMTP with STARTTLS

Let's Encrypt provides an easy way to get free certificates not only for web servers, but also for email servers like Postfix.

The way Let's Encrypt usually works requires you to setup a web server. Let's Encrypt sends you a challenge, and you have to prove ownership of the domain by providing a response to that challenge. You do this by placing the response in a certain URL on your web server:
http://www.yourserver.com/.well-known/acme-challenge/FgedPYS65N3HfwmM7IWY2...

That way you prove that you are the owner of the domain "yourserver.com". But there is another even easier way to prove ownership of a domain: DNS. You place the response in a specific TXT record of your domain: _acme-challenge.www.yourserver.com

  • You can use your domain hosting service (GoDaddy, Whois, etc.) to create a new TXT record.
  • The "certbot" command line client does all the rest in just one call.
  • Under Debian 9 and 10, "certbot" is part of the official package repository.
  • You can run certbot on any Linux client. You don't have to run it on the email server.

Example

In this example the public hostname of your mail server is mx.yourserver.com. Therefore you have to create a TXT record called _acme-challenge.mx.yourserver.com . The value of the TXT record is in the output of certbot.

# certbot certonly --manual --preferred-challenges dns -d mx.yourserver.com
 
Saving debug log to /var/log/letsencrypt/letsencrypt.log 
Plugins selected: Authenticator manual, Installer None 
Obtaining a new certificate 
Performing the following challenges: 
dns-01 challenge for mx.yourserver.com 
 
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
NOTE: The IP of this machine will be publicly logged as having requested this 
certificate. If you're running certbot in manual mode on a machine that is not 
your server, please ensure you're okay with that. 
 
Are you OK with your IP being logged? 
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
(Y)es/(N)o: Y 
 
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Please deploy a DNS TXT record under the name 
_acme-challenge.mx.yourserver.com with the following value: 
 
1A4RACHEISTBLUTWURST_egTVadkeiieikeieisfkfk
 
Before continuing, verify the record is deployed. 
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Press Enter to Continue 
Waiting for verification... 
Cleaning up challenges 
 
IMPORTANT NOTES: 
 - Congratulations! Your certificate and chain have been saved at: 
   /etc/letsencrypt/live/mx.yourdomain.com/fullchain.pem 
   Your key file has been saved at: 
   /etc/letsencrypt/live/mx.yourdomain.com/privkey.pem 
   Your cert will expire on 2020-02-15. To obtain a new or tweaked 
   version of this certificate in the future, simply run certbot 
   again. To non-interactively renew *all* of your certificates, run 
   "certbot renew" 
 - If you like Certbot, please consider supporting our work by: 
 
   Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate 
   Donating to EFF:                    https://eff.org/donate-le
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Christmas Time is Shopping Time ... Sort of

Christmas is coming early this year, so I wanted to browse the Internet to see what kind of extra effort our local retail industry is expending. Out of curiosity I chose the largest, most famous shopping street in the Capital of Germany: Berlin's Kurf├╝rstendamm. And located in that street, there is Berlin's most famous shopping mall: Europa-Center - the name says it all.

My expectation: Pretty Javascript snowflakes slowly falling down on heaps of shopping bargains, indoor maps, rotating 3D product pictures ... you know the usual Christmas websites.

Reality: A phpMyAdmin login page with an invalid TLS certificate.

Words can barely describe my level of incomprehension how this can possibly happen. Either the retail industry has already given up the fight against Chinese Ebay sellers, or they just don't believe in online marketing. Either way, I will spend my holiday shopping season online.

P.S.: What is so hard about indoor floor plans? Interactive HTML5 3D animations would be nice though.

P.P.S: Please ignore the red DNSSEC sign, it is supposed to ... ah, just forget about it.

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C++ - The Beast is Back (Halloween Special)

C++ The Beast is Back

Do you remember the time when programming languages like Visual Basic and Java came out and flourished, because they let programmers forget about all the underlying technical details of computers, so they could focus more on things like algorithms and use cases?

"I don't wanna waste my time with solving memory management problems or all those other low level stuff. These days are finally over!" Many programmers hated C++ because they were annoyed by memory leaks, pointers, byte sizes of variable types and data structures, compiler errors, linker warnings, ... the list goes on and on.

And managers? They loved Java and Visual Basic. Less tech talk about problems nobody really understands anyway, faster time to market, happy customers, what else could you want?

It seemed like the days of C and C++ were counted. Maybe they could still be used for some low level system programming, but certainly not application programming. Instead let's move on and jump on the ponderous but convenient bytecode train. Just add some more RAM modules to the server, and triple the disk space of those cloud containers, then we're done. What a beautiful simple world it is now, the world of software programming. Right? Right?? Right???

The Return of the Beast

Well ... not so fast (pun intended). New emerging technologies like Big Data, Blockchain and AI become part of everyday application development. And what about IoT (edge cloud)? Small IoT devices don't have Terabytes of RAM and server scale CPU processors. All of these rapidly growing technologies require lean and fast code modules tailored to their specific requirements.

I recently came across a free eBook from O'Reilly: C++ Today - The Beast is Back. It is from 2015, but large parts are still valid today. I highly recommend reading it. Once you have finished, here is my very own top 5 list of reasons why "the beast is back":

  1. Coding discipline
    Source code formatting, code commenting, coding guidelines, best practices: Python brought discipline back into aspiring programmers. Coders are now less annoyed by investing time in high quality source code, because they realize it will in turn create higher quality software that is easier to maintain and safes time and money in the end. C++ development also requires a lot of discipline and attention to details, but you are rewarded with a minimum disk and memory footprint and unparalleled performance.
  2. Focus on technology
    Tech is back: Logging in with SSH to a remote git server? Using vim to fix a typo in some Python source files? No problem. Today there are more tutorials out there about vim and the Linux command line than ever before. Students again want to get in touch with the underlying technology and learn how stuff works under the hood. Knowing the memory footprint of a running program is not considered evil sorcery any more.
  3. New standards
    C++ has come a long way since the last decade:
    C++11, C++14, C++17, C++20
    New programming ideas and standards are coming up every year, and C++ is adapting fast.
  4. New technologies
    Blockchain, IoT, Big Data, Machine Learning and AI: Exciting new technologies are all about performance, data crunching, sheer numbers. You need a lean and fast beast like C++ to tame them. For example the core of TensorFlow, today's most popular machine learning framework, is written in C++.
  5. Low competition
    In July 2019 Microsoft announced they are thinking about moving from C++ to Rust for developing internal and external software. My question: What do you do with the rest of the weekend? Seriously: It might sound like a great idea to get rid of stack overflow problems and the like, but porting tons of code from C++ to Rust will probably take decades. Furthermore, Rust is not nearly as developed and stable as C++. There sure are still heaps of banana skins hidden beneath the shiny new surface of Rust.

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