Setting up a custom domain on Tumblr via Cloudflare DNS has a quirky interaction: it fails when the domain record is routed through Cloudflare’s traffic acceleration (proxied mode). Tumblr blocks the CDN proxy, giving no useful error message — only that the record hadn’t been set up correctly.

The fix is straightforward: use Cloudflare’s plain DNS without proxying.

A record (apex domain)

To point example.com at Tumblr:

• Type: A record
• Name: example.com
• Value: 66.6.44.4
• TTL: automatic
• Proxy status: DNS only (grey cloud)

CNAME record (subdomain)

To point blog.example.com at Tumblr:

• Type: CNAME record
• Name: blog
• Value: domains.tumblr.com
• TTL: automatic
• Proxy status: DNS only (grey cloud)

Setting your custom domain in Tumblr

Once DNS is configured, set your custom domain in Tumblr account settings. Navigate to your blog settings, click the pencil next to Username, enable “Use a custom domain”, enter your domain, and click “Test domain”. If it says “It’s good!” you’re done.

Note that DNS propagation can take some time before the test passes.