Fixing WordPress REST API Error: cURL Error 28 in Docker Environments

When running WordPress, you might encounter this warning in your Site Health Check page:

REST API Error
The REST API is one way WordPress and other applications communicate with the server.

Error: (http_request_failed) cURL error 28: Connection timed out after 10001 milliseconds

This message indicates that your WordPress instance failed to communicate with itself through the REST API. This issue is especially common when WordPress is running inside a Docker container or behind a reverse proxy.

---

🔍 The Symptom

WordPress internally calls its own REST API endpoint (for example, https://www.nuface.tw/wp-json/) to check whether the API is functioning.
If the container cannot resolve its own domain name due to a DNS configuration problem, the request will hang and eventually return:

cURL error 28: Connection timed out after 10001 milliseconds

In simple terms, WordPress is trying to talk to itself—but can’t find itself.

---

🧠 Root Cause

Inside Docker, containers often use their own DNS resolver, inherited from the Docker daemon.
If your environment relies on an internal DNS server (e.g., 172.16.100.1), but the container doesn’t explicitly know about it, the domain name lookup for www.nuface.tw will fail.
As a result, any loopback REST API request will time out.

---

🛠️ Step-by-Step Solution

1️⃣ Add Internal DNS to docker-compose.yml

Edit your WordPress service definition and specify the internal DNS:

services:
  wordpress:
    image: wordpress:latest
    container_name: wordpress
    restart: always
    ports:
      - "80:80"
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_USER: wordpress
      WORDPRESS_DB_PASSWORD: yourpassword
      WORDPRESS_DB_NAME: wordpress
    dns:
      - 172.16.100.1   # Internal DNS server

Then restart your containers:

docker compose down
docker compose up -d

---

2️⃣ Verify DNS Resolution

Log into the container and test DNS:

docker exec -it wordpress bash
cat /etc/resolv.conf
dig www.nuface.tw

If the domain resolves correctly, your internal DNS is working as expected.

---

3️⃣ Check the WordPress REST API Again

Go to Tools → Site Health → Status and rerun the test.
You should now see:

✅ REST API is working properly.

---

💡 Optional Improvements

1. Set a global Docker DNS configuration
   Edit /etc/docker/daemon.json: { "dns": ["172.16.100.1", "8.8.8.8"] } Restart Docker: systemctl restart docker
2. Use internal DNS for local services
   If your reverse proxy and WordPress containers share the same network,
   point all internal service names (like db, mail, api) to your internal DNS.

---

✅ Conclusion

This issue wasn’t caused by WordPress itself but by the Docker network’s DNS configuration.
By making sure your container can resolve its own domain name, you can restore normal REST API functionality—and eliminate the dreaded cURL error 28.

---

In short:
When WordPress can’t reach itself, check your DNS first.