Getting Valid SSL Certificate from Let’s Encrypt for LocalHost

3 minute read

As we are moving towards CI and CD, it’s very important to have same environment configuration for different type of environments. In this post, I will explain a common problem related to local dev environment and valid SSL certificate.

Problem Statement

When we use self signed certificate to enable SSL support for our applications in Local Dev environment, we face issue related to validity of that certificate. In order to fix the issue we have to do lots of work around like adding CA certificate in your trust store or ignoring the warning by browser etc. However, you can get a valid certificate from any valid Certificate Authority and that will make everything go away. Let’s Encrypt is open source valid Certificate Authority which we are going to use in this case.

Prerequisites

Heroku Account (Free account is good enough for this purpose)
Heroku CLI
Python 2.7 or greater
Git CommandLine Desktop version has option to install command line as well
OpenSSL
Install certbot in your machine. I am going to use MacOS here, however the steps (except installation) would be same for any other operating system. You can run following command in MacOSX

$ brew install certbot

Certificate Generation

Step 1. Please read and follow Quick and Simple Web APP on Heroku Using Python Flask

Step 2. Once you have certbot installed, run following to get certificate

$ sudo certbot certonly --manual

Provide required information when prompted by tool. When asked for domain name, enter FQDN (Fully Qualified Domain Name) of your app created in Step 1. It will ask to log your IP as well to public domain as you are asking for a certificate. After that you will see prompt like following asking Press Enter to Continue. Do not press enter yet.

Step 3. Now, we are going to use our flask server app to host a page with details mentioned in above pic. I am going to user server.py file to enter details as follow:

import os
from flask import Flask

app = Flask(__name__)

@app.route("/.well-known/acme-challenge/<copy last part of URL from certbot console>")
# e.g. /.well-known/acme-challenge/lzrajCaq8vbw5Qz2o_XXXXXXXXXXXXXXXXX
def hello():
    return "<copy data from certbot console>"
    # e.g. return "lzrajCaq8vbw5Qz2o_XXXXXXXXXXXXXXXXXXz4RnfWtLKaIc6EdhsOsr4fb6RFZuUoabZW5dPW36cmc"

if __name__ == "__main__":
    port = int(os.environ.get("PORT", 5000))
    app.run(host='0.0.0.0', port=port)

Step 4. Run your server.py file to check it match the expectation mentioned by certbot console output

Step 5. Push your flask server app to Heroku

Step 6. Now, go back to certbot console and hit enter. After this, you would see location of your certificate created by Let’s Encrypt. Let’s Encrypt will create the following certs:

cert.pem
chain.pem
fullchain.pem
privkey.pem

CA and SSL Certificate

Once you get certificates from Let’s Encrypt, you can create Root CA certificate and certificate to use with your application.

Step 1. Create certificate for your application. In order to do that you have to combine privkey.pem and cert.pem. You can use any text editor or command line. Here’s the example of command line approach.

$ cd <location of your certificates>
$ cat privkey.pem cert.pem > myapp.pem

Step 2. In order to generate CA certificate, you need to download IdenTrust DST Root CA X3 from https://www.identrust.com/certificates/trustid/root-download-x3.html. Save it to a file (e.g. IdenTrustCA.crt) and add -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- lines.

Step 3. Convert IdenTrustCA.crt this certificate to pem using openssl

$ openssl x509 -in IdenTrustCA.crt -out IdenTrustCA.pem -outform PEM

Step 4. Now combine IdenTrustCA.pem and chain.pem

cat IdenTrustCA.pem chain.pem > ca.pem

You should have something like following in your ca.pem file:

Step 5. Let’s verify our app certificate with CA certificate

$ openssl verify -CAfile ca.pem myapp.pem
output: 
myapp.pem: OK

Congratulation now you have a valid certificate for your application.

Local or Dev Machine Host Mapping

You need make sure that your dev machine IP or localhost is mapped to domain name you have used to obtain certificate.

You can make entry in your /etc/hosts file to achieve it

127.0.0.1           localhost ry-dev.herokuapp.com

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Ritesh Yadav

Getting Valid SSL Certificate from Let’s Encrypt for LocalHost

Problem Statement

Prerequisites

Certificate Generation

CA and SSL Certificate

Local or Dev Machine Host Mapping

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