The TXT registry¶

The TXT registry is the default registry.
It stores DNS record metadata in TXT records, using the same provider.

If you plan to manage apex domains with external-dns whilst using a txt registry, you should ensure when using –txt-prefix that you specify the record type substitution and that it ends in a period (.). The record should be created under the same domain as the apex record being managed, i.e. –txt-prefix=someprefix-%{record_type}.

Note: --txt-prefix and --txt-suffix contribute to the 63-byte maximum record length. To avoid errors, use them only if absolutely required and keep them as short as possible.

Record Format Options¶

For version `v0.18+`¶

The TXT registry supports single format for storing DNS record metadata:

Creates a TXT record with record type information (e.g., ‘a-’ prefix for A records)

The TXT registry would try to guarantee a consistency in between providers and sources, if provider supports the behaviour.

If you are dealing with APEX domains, like example.com and TXT records are failing to be created for managed record types specified by --managed-record-types, consider following options:

TXT record with prefix based on requirements. Example --txt-prefix="%{record_type}-abc-" or --txt-prefix="%{record_type}.abc-"
TXT record with suffix based on requirements. Example --txt-suffix="-abc-%{record_type}" or --txt-suffix="-abc.%{record_type}."

If configured --txt-prefix="%{record_type}-abc-" for apex domain ex.com the expected result is

Name	TYPE
`cname-a-abc-nginx-v2.ex.com.`	`TXT`
`nginx-v2.ex.com.`	`CNAME`

If configured --txt-suffix="-abc.%{record_type}" for apex domain ex.com the expected result is

Name	TYPE
`cname-nginx-v2-abc.a.ex.com.`	`TXT`
`nginx-v3.ex.com.`	`CNAME`

Manually Cleanup Legacy TXT Records¶

While deleting registry TXT records won’t cause downtime, a well-thought-out migration and cleanup plan is crucial.

Occasionally, it may be necessary to remove outdated TXT records from your registry.

An example script for AWS can be found in scripts/aws-cleanup-legacy-txt-records.py with instructions on how to run it.
The script performs targeted deletion of TXT records that include ResourceRecords matching the heritage=external-dns,external-dns/owner=default or similar pattern.
In the event of unintended deletion of all TXT records managed by external-dns, external-dns will initiate a full DNS record regeneration, along withTXT and non-TXT records. Just be aware, this operation’s duration is directly proportional to the DNS estate size.”

For version `v0.16.0 & v0.16.1`¶

The TXT registry supports two formats for storing DNS record metadata:

Legacy format: Creates a TXT record without record type information
New format: Creates a TXT record with record type information (e.g., ‘a-’ prefix for A records)

By default, the TXT registry creates records in both formats for backwards compatibility. You can configure it to use only the new format by using the --txt-new-format-only flag. This reduces the number of TXT records created, which can be helpful when working with provider-specific record limits.

Note: The following record types always use only the new format regardless of this setting:

AAAA records
Encrypted TXT records (when using --txt-encrypt-enabled)

Example:

# Default behavior - creates both formats
external-dns --provider=aws --source=ingress --managed-record-types=A --managed-record-types=TXT

# Only create new format records (alongside other required flags)
external-dns --provider=aws --source=ingress --managed-record-types=A --managed-record-types=TXT --txt-new-format-only

The --txt-new-format-only flag should be used in addition to your existing external-dns configuration flags. It does not implicitly configure TXT record handling - you still need to specify --managed-record-types=TXT if you want external-dns to manage TXT records.

Migration to New Format Only¶

Note: external-dns will not automatically remove legacy format records when switching to new-format-only mode. You’ll need to clean up the old records manually if desired.

When transitioning from dual-format to new-format-only records:

Ensure all your external-dns instances support the new format
Enable the --txt-new-format-only flag on your external-dns instances
Manually clean up any existing legacy format TXT records from your DNS provider

Prefixes and Suffixes¶

In order to avoid having the registry TXT records collide with
TXT or CNAME records created from sources, you can configure a fixed prefix or suffix
to be added to the first component of the domain of all registry TXT records.

The prefix or suffix may not be changed after initial deployment,
lest the registry records be orphaned and the metadata be lost.

The prefix or suffix may contain the substring %{record_type}, which is replaced with
the record type of the DNS record for which it is storing metadata.

The prefix is specified using the --txt-prefix flag and the suffix is specified using
the --txt-suffix flag. The two flags are mutually exclusive.

Wildcard Replacement¶

The --txt-wildcard-replacement flag specifies a string to use to replace the “*” in
registry TXT records for wildcard domains. Without using this, registry TXT records for
wildcard domains will have invalid domain syntax and be rejected by most providers.

Encryption¶

Registry TXT records may contain information, such as the internal ingress name or namespace, considered sensitive, , which attackers could exploit to gather information about your infrastructure.
By encrypting TXT records, you can protect this information from unauthorized access.

Encryption is enabled by setting the --txt-encrypt-enabled. The 32-byte AES-256-GCM encryption
key must be specified in URL-safe base64 form (recommended) or be a plain text, using the --txt-encrypt-aes-key=<key> flag.

Note that the key used for encryption should be a secure key and properly managed to ensure the security of your TXT records.

Generating the TXT Encryption Key¶

Python

python -c 'import os,base64; print(base64.urlsafe_b64encode(os.urandom(32)).decode())'

Bash

dd if=/dev/urandom bs=32 count=1 2>/dev/null | base64 | tr -d -- '\n' | tr -- '+/' '-_'; echo

OpenSSL

openssl rand -base64 32 | tr -- '+/' '-_'

PowerShell

# Add System.Web assembly to session, just in case
Add-Type -AssemblyName System.Web
[Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes([System.Web.Security.Membership]::GeneratePassword(32,4))).Replace("+","-").Replace("/","_")

Terraform

resource "random_password" "txt_key" {
  length           = 32
  override_special = "-_"
}

Manually Encrypting/Decrypting TXT Records¶

In some cases you might need to edit registry TXT records. The following example Go code encrypts and decrypts such records.

package main

import (
    b64 "encoding/base64"
    "fmt"

    "sigs.k8s.io/external-dns/endpoint"
)

func main() {
    keys := []string{
        "ZPitL0NGVQBZbTD6DwXJzD8RiStSazzYXQsdUowLURY=", // safe base64 url encoded 44 bytes and 32 when decoded
        "01234567890123456789012345678901",             // plain txt 32 bytes
        "passphrasewhichneedstobe32bytes!",             // plain txt 32 bytes
    }

    for _, k := range keys {
        key := []byte(k)
        if len(key) != 32 {
            // if key is not a plain txt let's decode
            var err error
            if key, err = b64.StdEncoding.DecodeString(string(key)); err != nil || len(key) != 32 {
                fmt.Errorf("the AES Encryption key must have a length of 32 byte")
            }
        }
        encrypted, _ := endpoint.EncryptText(
            "heritage=external-dns,external-dns/owner=example,external-dns/resource=ingress/default/example",
            key,
            nil,
        )
        decrypted, _, err := endpoint.DecryptText(encrypted, key)
        if err != nil {
            fmt.Println("Error decrypting:", err, "for key:", k)
        }
        fmt.Println(decrypted)
    }
}

Caching¶

The TXT registry can optionally cache DNS records read from the provider. This can mitigate
rate limits imposed by the provider.

Caching is enabled by specifying a cache duration with the --txt-cache-interval flag.