RFC-2136¶

description: 'cert-manager configuration: ACME DNS-01 challenges using RFC2136-compliant DNS providers'

The goal of this document is to provide a configuration overview of the various facilities required to deploy cert-manager against a RFC2136 compliant DNS server such as BIND named. This capability is also commonly known as “dynamic DNS”.

Unlike the peer of other cert-manager DNS integrations, named is a bit of a “Swiss Army Knife” of domain name servers. Over the years, it has been highly optimized to provide maximal vertical scalability for a single node, as well as horizontal scalability with service provider interfaces. This flexibility makes it impossible to go into every possible named deployment that a user may run in to though. Instead, this document will try to make sure your server is ready to accept requests from cert-manager using command line tools, then get on to the making the two work together.

Transaction Signatures ⇒ TSIG¶

Dynamic DNS updates are essentially server queries which otherwise might return resource records (RRs). Since DNS servers are commonly exposed to the public internet, being able to push an unauthenticated update to any server that responds to queries would be immediately untenable.

In the eyes of the named architects, the generic solution to this problem space was twofold. The first is to require manual enablement of updates at a zone level, such as example.com. In a naive network, there is no requirement that zone updates have any security to them, and clients can be configured such that they can provide updates without any authentication. An example of where this is useful is for machines booting using DHCP, in this case the machines know about themselves and the DNS server can be configured to accept updates when they come from the address being configured.

This clearly has limitations in situations such as cert-manager and the DNS01 challenge. In this environment, a TXT RR must be created after coordination with the ACME server. After negotiating with the ACME server, a the TXT RR that is published on the domain validates that the domain is legitimately engaged with the process of creating a certificate for it. In the bigger picture of DNS, this means that an arbitrary actor (cert-manager, in this case) must be able to add one of these KV mappings to the domain and delete it after the certificate has been issued. cert-manager does not have a convenient physical characteristic such as a DHCP allocation to validate it's requests.

For cases like this, we need to be able to sign a request that is being sent to the DNS server. We do that through TSIGs, or Transaction SIGnatures.

Configuration Step 1 - Set up your DNS server for secure dynamic updates¶

There are many excellent tutorials on the net that walk through preparing a basic named server for dynamic updates:

• https://www.cyberciti.biz/faq/unix-linux-bind-named-configuring-tsig/
• https://tomthorp.me/blog/using-tsig-enable-secure-zone-transfers-between-bind-9x-servers

More complex name deployments will not use text files, but rather may use LDAP or SQL for a database for resource records. An additional wrinkle is metadata configuration, such as for zone metadata like enabling dynamic updates or access control lists (ACLs) for a zone. There are too many configurations to go into here, but you should be able to find the documentation to do so.

Whatever your deployment is, the goal at this stage has nothing to do with cert-manager and everything to do with a tool called nsupdate generating updates signed with TSIG. Once this is out of the way, you can attack the cert-manager configuration with far greater confidence.

Using nsupdate¶

Most paths to configuring BIND named will go through using dnssec-keygen. This command-line tool generates a named private key that is used for signing TSIG requests. When a request is signed, both the signature and the name of the private key are attached to the request in an unencrypted form. In this manner, when the request is received, the name of the private key can be used to by the recipient to find the private key itself, build a new signature with it, and compare the two for acceptance.

Since there are dozens of ways to have your named server misconfigured, we’ll use nsupdate to test that the server behaves as expected before we get there. https://debian-administration.org/article/591/Using_the_dynamic_DNS_editor_nsupdate is a solid breakdown of how to use the tool.

To get started, we’ll simply run nsupdate -k <keyID> where keyID is the value returned from dnssec-keygen. This will read the key from disk and provide a command prompt to issue commands. In general, we want to write a simple TXT RR and make sure we can delete it.

$ nsupdate -k <keyID>
update add www1.example.com 60 txt testing
send
… test here with `nslookup`
update delete www1.example.com txt
send
… test here with `nslookup`

Any failures to write, read or delete the record will mean that cert-manager will not be able to do so either, no matter how well it is configured.

Configuration Step 2 - Set up cert-manager¶

Now we get to the fun stuff, seeing everything work. Remember that we need to set up the ACME DNS01 issuer and challenge mechanism as well as the rfc2136 provider. Since the documentation covers the other parts sufficiently, let’s focus on the provider here.

apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: example-issuer
spec:
  acme:
    ...
    solvers:
    - dns01:
        rfc2136:
          nameserver: <address of authoritative nameserver configured above>
          tsigKeyName: <key name used in `dnssec-keygen`, use something semantically meaningful in both environments>
          tsigAlgorithm: HMACSHA512 // should be matched to the algo you chose in `dnssec-keygen`
          tsigSecretSecretRef:
            name: <the name of the k8s secret holding the TSIG key.. not the key itself!>
            key: <name of the key *inside* the secret>

For example:

rfc2136:
  nameserver: 1.2.3.4:53
  tsigKeyName: example-com-secret
  tsigAlgorithm: HMACSHA512
  tsigSecretSecretRef:
    name: tsig-secret
    key: tsig-secret-key

For this example configuration, we’ll need the following two commands. The first, on your named server generates the key. Note how example-com-secret is both in the tsigKeyName above and the dnssec-keygen command that follows.

$ dnssec-keygen -r /dev/urandom -a HMAC-SHA512 -b 512 -n HOST example-com-secret

Also note how the tsigAlgorithm is provided in both the configuration and the keygen command. They are listed at https://github.com/miekg/dns/blob/v1.0.12/tsig.go#L18-L23.

The second bit of configuration you need on the Kubernetes side is to create a secret. Pulling the secret key string from the <key>.private file generated above, use the secret in the placeholder below:

$ kubectl -n cert-manager create secret generic tsig-secret --from-literal=tsig-secret-key=<somesecret>

Note how the tsig-secret and tsig-secret-key match the configuration in the tsigSecretSecretRef above.

Rate Limits¶

The rfc2136 provider waits until all nameservers to in your domain's SOA RR respond with the same result before it contacts Let's Encrypt to complete the challenge process. This is because the challenge server contacts a non-authoritative DNS server that does a recursive query (a query for records it does not maintain locally). If the servers in the SOA do not contain the correct values, it's likely that the non-authoritative server will have bad information as well, causing the request to go against rate limits and eventually locking the process out.

This process is in place to protect users from server misconfiguration creating a more subtle lockout that persists after the server configuration has been repaired.

As documented elsewhere, it is prudent to fully debug configurations using the ACME staging servers before using the production servers. The staging servers have less aggressive rate limits, but the certificates they issue are not signed with a root certificate trusted by browsers.

What’s next?¶

This configuration so far will actually do nothing. You still have to request a certificate as described here. Once a certificate is requested, the provider will begin processing the request.

Troubleshooting¶

• Be sure that you have fully tested the DNS server updates using nsupdate first. Ideally, this is done from a pod in the same namespace as the rfc2136 provider to ensure there are no firewall issues.
• The logs for the cert-manager pod are your friend. Additional logs can be generated by adding the --v=5 argument to the container launch.
• The TSIG key is encoded with base64, but the Kubernetes API server also expects that key literals will be decoded before they are stored. In some cases, a key must be double-encoded. (If you've tested using nsupdate, it's pretty easy to spot when you are running into this.)
• Pay attention to the refresh time of the zone you are working with. For zones with low traffic, it will not make a significant difference to reduce the refresh time down to about five minutes while getting initial certificates. Once the process is working, the beauty of cert-manager is it doesn't matter if a renewal takes hours due to refresh times, it's all automated!
• Compared to the other providers that often use REST APIs to modify DNS RRs, this provider can take a little longer. You can watch kubectl certificate yourcert to get a display of what's going on. It's not uncommon for the process to take five minutes in total.