While experimenting with HTTP/3 support, I noticed that modern websites sometimes publish DNS records that advertise which HTTP protocols they support.

These are called HTTPS DNS Resource Records (type 65).

They can contain information such as:

supported protocols (h2, h3)

alternative service endpoints

hints for modern clients

For example, google.com publishes the following record.

Checking with dig

You can see this using dig:

dig HTTPS google.com

Example output:

google.com. IN HTTPS 1 . alpn="h2,h3"

This simply means:

• the server supports HTTP/2 (h2)
• the server supports HTTP/3 (h3)

Modern clients may use this information when deciding how to connect.

Checking the same record in PHP

PHP can retrieve this record using dns_get_record().

Example:

$records = dns_get_record('google.com', 65, $authns, $addtl, true);
$data = $records[0]['data'];

var_dump($data);

Typical output:

string(13) "h2h3"

ReactPHP example

I created a small ReactPHP example that performs the same kind of lookup using ReactPHP DNS and prints the advertised protocols.

Gist:

https://gist.github.com/masakielastic/ff79ec78e844678f46529ff8edb0bbef

The example shows how to:

• query DNS for HTTPS records
• read the response
• extract protocol names such as h2 and h3