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Lets Encrypt HTTP-01 using Amazon S3

How to obtain Let's Encrypt certificates via an S3 bucket with optional Cloudflare origin-host configuration

Rohan MolloyRohan Molloy

Motivation

In order to be issued a certificate, Let's Encrypt needs to verify you have control over the domain. This process is automated through challenges, such as setting up a local webserver, or adding DNS TXT records. I don't like the DNS-TXT challenge, as it requires giving every host a DNS service account. The HTTP challenge usually requires port 80 to be externally reachable.

My solution

I came up with my own solution. The .well-known/acme-challenge can be hosted using S3, allowing Let's Encrypt to perform an HTTP-01 challenge.

This method requires the least amount of privileges on the host (no ports need to be open).

It is well suited for getting valid certs in a
local development environment.

Why do internal hosts need valid certs?

The primary reason I want internal hosts to have valid certs is quite simple - invalid/untrusted certificates break things! A secondary reason is that constant browser warning are annoying and false alarms are detrimental to security.

DNS

In public DNS, my intranet domains point to S3 via a CNAME. Each intranet domain has its own bucket. Later, I will demonstrate integration with Cloudflare, so origin hosts can obtain Let's Encrypt certs, without having to create additional DNS records. This is done by creating a redirect rule to route /.well-known/acme-challenge to the bucket.

Scripts and commands

Trying it out

‌ ‌
sudo certbot certonly \
--preferred-challenges=http \
--manual-auth-hook=/etc/letsencrypt/s3-auth.sh \
--manual-cleanup-hook=/etc/letsencrypt/s3-cleanup.sh \
--manual-public-ip-logging-ok \
--manual --agree-tos --no-eff-email \
--staging -m user@etherarp.net -d cert-example.etherarp.net

Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for cert-example.etherarp.net
Output from s3-auth.sh:
upload: '' -> 's3://etherarp.net/.well-known/acme-challenge/SQqXyzU6TDrQM_WDsv9Bn7Zjt1OEH39bW1oILkhNLf4' [part 1, 88B]

88 of 88 100% in 0s 196.20 kB/s
88 of 88 100% in 0s 175.78 B/s done
Public URL of the object is: http://etherarp.net.s3.amazonaws.com/.well-known/acme-challenge/SQqXyzU6TDrQM_WDsv9Bn7Zjt1OEH39bW1oILkhNLf4

Waiting for verification...
Cleaning up challenges
Output from s3-cleanup.sh:
delete: 's3://etherarp.net/.well-known/acme-challenge/SQqXyzU6TDrQM_WDsv9Bn7Zjt1OEH39bW1oILkhNLf4'


IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/cert-example.etherarp.net-0001/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/cert-example.etherarp.net-0001/privkey.pem
Your cert will expire on 2019-08-25. 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

Bucket Access Control using IAM

Next, let's create an IAM policy, allowing read-write access to well-known/acme-challenge/ across all buckets. IAM policies should never be attached directly to a user. The policy is attached to a group and the group is attached to a user.

Cloudflare and Page Rules

The Let's Encrypt http-01 challenge can follow HTTP redirects (limit 10). This means the primary web server can route /.well-known/acme-challenge to another endpoint.

In the previous example, the DNS record pointed directly to S3. In this example, the DNS record points to Cloudflare.

In order to route the Let's Encrypt verification to the approrpriate S3 endpoint, a page rule needs to be configured in cloudflare.

Now let's try it out

$ certbot certonly --preferred-challenges=http-01 --manual --manual-auth-hook=/etc/letsencrypt/s3-auth.sh --manual-public-ip-logging-ok --staging -d etherarp.net --work-dir=$PWD --logs-dir=$PWD --config-dir=$PWD -m rohan@etherarp.net --agree-tos --no-eff-email
Saving debug log to /home/rohan/le2/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for etherarp.net
Output from s3-auth.sh:
upload: '' -> 's3://etherarp.net/.well-known/acme-challenge/gRlXWpRHQ0MzSVQ6g7VXbbGDvnd9PyEk44TrGQjkvAk' [part 1, 88B]

88 of 88 100% in 0s 96.13 kB/s
88 of 88 100% in 0s 316.17 B/s done
Public URL of the object is: http://s3.amazonaws.com/etherarp.net/.well-known/acme-challenge/gRlXWpRHQ0MzSVQ6g7VXbbGDvnd9PyEk44TrGQjkvAk

Waiting for verification...
Cleaning up challenges
Non-standard path(s), might not work with crontab installed by your operating system package manager

IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/home/rohan/le2/live/etherarp.net/fullchain.pem
Your key file has been saved at:
/home/rohan/le2/live/etherarp.net/privkey.pem
Your cert will expire on 2019-08-25. 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"
- Your account credentials have been saved in your Certbot
configuration directory at /home/rohan/le2. You should make a
secure backup of this folder now. This configuration directory will
also contain certificates and private keys obtained by Certbot so
making regular backups of this folder is ideal.

Side-note: Notice how I didn't use sudo, because I set the config/work/log dir to a local directory, no special privileges are required

Looking at the logs, we can see the initial hop and redirect

Author

Rohan Molloy

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