Agnos

Presentation

Agnos is a single-binary program allowing you to easily obtain certificates (including wildcards) from Let's Encrypt using DNS-01 challenges. It answers Let's Encrypt DNS queries on its own, bypassing the need for API calls to your DNS provider.

Why

DNS-01 is summarized by Let's Encrypt documentation as such:

Pros

• You can use this challenge to issue certificates containing wildcard domain names.
• It works well even if you have multiple web servers.

Cons

• Keeping API credentials on your web server is risky.
• Your DNS provider might not offer an API.
• Your DNS API may not provide information on propagation times.

By serving its own DNS answers, agnos:

• Nullify the need for API and API credentials
• Nullify all concerns regarding propagation times

Hence, agnos removes virtually all downsides of dns-01 challenges.

How

Agnos leverages let's encrypt capability to follow DNS NS records. It requires you to add to your DNS zone:

1. An A (or AAAA) record pointing to the public facing IP address of the server on which agnos will run. On this server, UDP port 53 (the one used by DNS) should be open and free.
2. For each domain you will want to validate, a NS record for the corresponding _acme-challenge sub-domain, indicating that agnos should be used as a name server for this specific domain.

Installation

This instructions are given for a Linux system but a similar process will likely work on all Unixes, and maybe windows.

Released binary

Pre-compiled binaries for (relatively recent) Linux/amd64 are available for every tagged release.

Building

Agnos is written in Rust. To build it you will need to have the rust toolchain installed.

Once you have obtained the source, the following command will build the binary and put it in the root directory of the repo.

cd agnos/
cargo build --release
mv target/release/agnos agnos

Setting capabilities to not run agnos as root

Because agnos listen on the low-numbered port 53, it requires special privileges. Running it as root will do, but if you (understandably) don't want to do that, the following command is for you:

# as root
setcap 'cap_net_bind_service=+ep' agnos
# agnos is the file of the binary as compiled above

Usage

Let's Encrypt accounts

Let's Encrypt accounts are identified by an e-mail address and a private RSA key. To generate such a key use the following command:

openssl genrsa 2048 > /path/to/store/the/key.pem

or if you prefer a larger key:

openssl genrsa 4096 > /path/to/store/the/key.pem

Agnos configuration

Agnos is configured via a single TOML file. A commented example is available in config_example.toml.

It is advised to absolute rather than relative paths in the configuration file.

There are three "levels" in the configuration:

1. General

The general configuration level is where the IP address to listen on is provided.

dns_listen_adr = "1.2.3.4:53"

2. Accounts

Several Let's Encrypt accounts can be specified. For each account, an e-mail address and the path to the account RSA private key must be provided.

[[accounts]]
email= "[email protected]"
private_key_path = "priv_key.pem"

3. Certificates

For each account, several certificates can be ordered. Each certificate can cover multiple domains. On disk, a certificate is represented by two files: the full certificate chain, and the private key of the certificate (generated by agnos and different from the account private key).

[[accounts.certificates]]
domains =  ["doma.in","*.doma.in"]
fullchain_output_file = "fullchain_A.pem"
key_output_file = "cert_key_A.pem"

Configuration of your DNS provider

Say that we have the following domains we want to obtain a certificate (or multiple certificates) for:

• doma.in
• its wildcard variant: *.doma.in
• examp.le
• and another.examp.le.

Notice here that we are not requesting a certificate for *.examp.le but only for one subdomain: another.examp.le.

Let's encrypt DNS-01 challenge is going to ask for TXT DNS records on the following three domains:

• _acme-challenge.doma.in (for both doma.in and its wildcard)
• _acme-challenge.examp.le
• _acme-challenge.another.examp.le

Let's assume that agnos is going to run on a server whose public facing IP address is 1.2.3.4. The goal is to indicate that the three _acme_challenge domains cited above are managed by agnos using NS DNS records. NS records usually point to domain names, so we will also set an A record on agnos-ns.doma.in to point to 1.2.3.4 (here agnos-ns.doma.in is entirely arbitrary, it could be another, completely independent domain, you control, like my-agnos.com).

We create the following records:

In the zone of doma.in

agnos-ns.doma.in            A       1.2.3.4
_acme-challenge.doma.in     NS      agnos-ns.doma.in

In the zone of examp.le

_acme-challenge.examp.le            NS      agnos-ns.doma.in
_acme-challenge.another.examp.le    NS      agnos-ns.doma.in

Note: Though it may seem cumbersome, this must only be done once from your DNS provider web interface. Once it is done, you will never have to touch a TXT record.

Running agnos

agnos takes a single command line argument, the path to its configuration file, and two optional flags: --no-staging to use Let's Encrypt production server, and --debug to display more debug information. Help is available via agnos --help.

When running, it checks whether the certificates of the full chain are going to expire in the next 30 days, and only renew them in that case, so it is suitable to be used in a cron job.