MX Record Format in is-a.dev: Configuring Mail Exchange Records

MX records in is-a.dev support two JSON formats: a simple array of hostnames with auto-assigned priorities starting at 10, or an array of objects specifying explicit target and priority values.

When registering a subdomain under the is-a-dev/register repository, you define DNS records inside JSON files stored in the domains/ directory. The registry's DNS generation pipeline processes these configurations to create live Cloudflare records, making correct MX syntax essential for functional mail routing.

Supported MX Record Formats in is-a.dev

The registry accepts two interchangeable structures for the MX field within a domain's records object. Both formats are handled by the transformation logic in dnsconfig.js (lines 66-84).

Simple Array Format (Auto-Assigned Priority)

Use a JSON array of strings when you want the registry to automatically calculate priority values. According to the source code in dnsconfig.js (lines 66-74), the script assigns priority using the formula 10 + index, where index is the zero-based position in the array. The first entry receives priority 10, the second receives 11, and so on.

{
  "records": {
    "MX": ["mx1.improvmx.com", "mx2.improvmx.com"]
  }
}

This format appears in production domains such as domains/youfoundalpha.json (lines 9-11), demonstrating its use with ImprovMX servers.

Explicit Object Format (Custom Priority)

For precise control over mail routing order, define MX records as an array of objects. Each object must contain a target property (the mail server hostname) and a priority property (the numeric preference). The registry script processes these entries at lines 75-82 of dnsconfig.js, using the supplied values directly without additional calculation.

{
  "records": {
    "MX": [
      { "target": "mx1.improvmx.com", "priority": 10 },
      { "target": "mx2.improvmx.com", "priority": 20 }
    ]
  }
}

How the Registry Processes MX Records

The DNS compilation logic resides in the root-level dnsconfig.js file. During the build process, the script iterates over data.records.MX and performs type detection on each entry:

• String entries: Treated as hostnames and wrapped in an auto-priority calculation (10 + index).
• Object entries: Validated for target and priority properties and passed directly to the DNS builder.

This dual-path implementation (lines 66-84) ensures backward compatibility for legacy string arrays while supporting modern explicit configurations.

Complete Configuration Examples

Here are full domain file implementations showing both approaches in context.

Simple array with auto-priorities:

{
  "owner": { "username": "exampleuser" },
  "records": {
    "MX": ["mx1.improvmx.com", "mx2.improvmx.com"]
  }
}

Explicit objects with custom priorities:

{
  "owner": { "username": "exampleuser" },
  "records": {
    "MX": [
      { "target": "aspmx.l.google.com", "priority": 1 },
      { "target": "alt1.aspmx.l.google.com", "priority": 5 }
    ]
  }
}

Summary

• The is-a.dev registry accepts two MX formats: simple string arrays or explicit target/priority objects.
• Simple arrays receive auto-calculated priorities starting at 10 and incrementing by 1 for each index position.
• Object arrays require target and priority properties for direct control over mail routing preferences.
• All processing occurs in dnsconfig.js (lines 66-84) during the DNS configuration generation phase.

Frequently Asked Questions

What priority values does is-a.dev assign to simple MX arrays?

The registry script assigns priorities starting at 10 and incrementing by 1 for each subsequent entry. The first hostname in the array receives priority 10, the second receives 11, and so forth, as implemented in dnsconfig.js lines 66-74.

Can I mix string hostnames and objects in the same MX array?

While the code at dnsconfig.js processes each entry individually by checking its type, best practice is to use a consistent format throughout the array—either all strings or all objects—to ensure predictable priority assignment and maintain configuration readability.

Where are MX records defined in the is-a.dev repository?

MX records are defined within individual JSON files located in the domains/ directory, specifically inside the records object using the MX key. For example, domains/youfoundalpha.json demonstrates the simple array format at lines 9-11.

What file controls how MX records are rendered to DNS?

The transformation logic resides in dnsconfig.js at the repository root. This script iterates through domain configurations, detects whether each MX entry is a string or object, and generates the appropriate Cloudflare MX records accordingly (lines 66-84).

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}