Running `dns_get_record()` on localhost in Docker has an 8 second delay and then errors

[hdodov]

Description

I'm running the latest WordPress Docker image on my macOS machine using the provided docker-compose.yml in its Docker hub page:

version: '3.1'

services:

  wordpress:
    image: wordpress
    restart: always
    ports:
      - 8080:80
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_USER: exampleuser
      WORDPRESS_DB_PASSWORD: examplepass
      WORDPRESS_DB_NAME: exampledb
    volumes:
      - wordpress:/var/www/html

  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: exampledb
      MYSQL_USER: exampleuser
      MYSQL_PASSWORD: examplepass
      MYSQL_RANDOM_ROOT_PASSWORD: '1'
    volumes:
      - db:/var/lib/mysql

volumes:
  wordpress:
  db:

Then, I put the following in /var/www/html/index.php:

<?php var_dump(dns_get_record('localhost', DNS_CNAME));

When I open http://localhost:8080/, I get an 8 second delay before receiving a result:

$ time curl localhost:8080
bool(false)
curl localhost:8080  0.01s user 0.01s system 0% cpu 8.023 total

…and in the logs, I have the following error:

wordpress-1  | [Wed Nov 15 08:21:56.524357 2023] [php:warn] [pid 97] [client 192.168.65.1:51109] PHP Warning:  dns_get_record(): A temporary server error occurred. in /var/www/html/index.php on line 1
wordpress-1  | 192.168.65.1 - - [15/Nov/2023:08:21:48 +0000] "GET / HTTP/1.1" 200 186 "-" "curl/8.1.2"

---

I'm not sure if this is a PHP issue, a Docker issue, or an OS issue, though.

I tested the same thing at home yesterday on my Windows PC and experienced slightly different issues, although issues nonetheless. I ran PHP 8.1.10 without Docker (using XAMPP) and got a ~1 second delay with the following error:

Warning: dns_get_record(): DNS Query failed in C:\Projects\test\index.php on line 1
bool(false)

Then I ran this on my Windows PC inside Docker, although in a different Docker container running PHP 8.1.18, and got the following result with absolutely no delay:

array(1) {
  [0]=>
  array(5) {
    ["host"]=>
    string(9) "localhost"
    ["class"]=>
    string(2) "IN"
    ["ttl"]=>
    int(0)
    ["type"]=>
    string(5) "CNAME"
    ["target"]=>
    string(9) "localhost"
  }
}

Finally, on my macOS work machine, I got an 8 seconds delay again in another Docker WordPress project using this Bitnami image and PHP 8.1.18.

---

I realize that I'm testing different PHP versions, but I'm wondering if anyone else can reproduce this. Here's a summation of my tests:

• PHP 8.0.30, macOS, with Docker: 8 seconds delay, "temporary server error occurred"
• PHP 8.1.10, Windows, without Docker: 1 second delay, "DNS Query failed"
• PHP 8.1.18, Windows, with Docker: no delay, everything works
• PHP 8.1.18, macOS, with Docker: 8 seconds delay, "temporary server error occurred"

In the PHP changelog, I can see a few changes regarding dns_get_record(), but the latest one was in PHP 8.0.14, and all my tests were on newer versions. More interestingly, the same PHP version (8.1.18) works inside Docker on Windows and doesn't on macOS, again with Docker. Can anyone else reproduce this?

I stumbled upon this issue because Wordfence has this exact line of code in its source code, as I've explained in this post.

PHP Version

PHP 8.0.30

Operating System

macOS Ventura 13.6.1 (22G313)

[damianwadley]

DNS and networking issues are common with Docker - and it's not just about PHP.

Try https://www.google.com/search?q=docker+slow+dns such as https://stackoverflow.com/a/43756020

[hdodov]

@damianwadley from my initial post, there's one scenario where I get an error without Docker:

PHP 8.1.10, Windows, without Docker: 1 second delay, "DNS Query failed"

…and another where I do use Docker and there's no issue:

PHP 8.1.18, Windows, with Docker: no delay, everything works

If we can be sure that there's nothing wrong internally with PHP, then we could probably write this off as an OS/Docker issue. I just wasn't sure if it's PHP-related or not and posted here, so people familiar with the source code can check it out.

I'll fiddle around with the problematic Docker setup on my macOS.

[hdodov]

It looks like a Docker issue.

I tried running PHP inside Docker:

$ docker run -it --rm -v $(pwd):/var/www php:latest php -v                                                
PHP 8.2.12 (cli) (built: Nov 21 2023 14:45:31) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.2.12, Copyright (c) Zend Technologies

…and directly on my Mac:

$ php -v
PHP 8.2.12 (cli) (built: Nov  5 2023 21:48:15) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.2.12, Copyright (c) Zend Technologies
    with Zend OPcache v8.2.12, Copyright (c), by Zend Technologies

I get an 8 second delay and an error in Docker:

$ docker run -it --rm -v $(pwd):/var/www php:latest php -r "var_dump(dns_get_record('localhost', DNS_CNAME));"

Warning: dns_get_record(): A temporary server error occurred. in Command line code on line 1
bool(false)

…but I get no delay and no error when I evaluate the same expression directly:

$ php -r "var_dump(dns_get_record('localhost', DNS_CNAME));" 
array(0) {
}

For what it's worth, using 127.0.0.1, rather than localhost yields the same result in both cases – delay and error in Docker, no error otherwise.

I can avoid the error in Docker by removing DNS_CNAME:

$ docker run -it --rm -v $(pwd):/var/www php:latest php -r "var_dump(dns_get_record('localhost'));"
array(2) {
  [0]=>
  array(5) {
    ["host"]=>
    string(9) "localhost"
    ["class"]=>
    string(2) "IN"
    ["ttl"]=>
    int(1)
    ["type"]=>
    string(1) "A"
    ["ip"]=>
    string(9) "127.0.0.1"
  }
  [1]=>
  array(5) {
    ["host"]=>
    string(9) "localhost"
    ["class"]=>
    string(2) "IN"
    ["ttl"]=>
    int(1)
    ["type"]=>
    string(4) "AAAA"
    ["ipv6"]=>
    string(3) "::1"
  }
}

So it appears that resolving the CNAME of localhost inside Docker has some pitfalls.