PHP checkdnsrr Function

last modified April 4, 2025

The PHP checkdnsrr function checks DNS records for a given host. It's useful for validating domain names and email addresses.

Basic Definition

checkdnsrr checks if DNS records exist for a specified host. It can verify different record types like MX, A, CNAME, etc.

Syntax: checkdnsrr(string $host, string $type = "MX"): bool. Returns true if records exist, false otherwise. Requires DNS server access.

Basic Email Domain Validation

This example checks if a domain has MX records for email validation.

email_validation.php

<?php

declare(strict_types=1);

$domain = "example.com";
if (checkdnsrr($domain, "MX")) {
    echo "Domain $domain has valid MX records";
} else {
    echo "Domain $domain has no MX records";
}

This checks if the domain can receive email by verifying MX records. MX records are essential for email delivery between servers.

Checking A Records

This demonstrates checking A records to verify a domain's IP resolution.

a_record_check.php

<?php

declare(strict_types=1);

$host = "www.example.com";
if (checkdnsrr($host, "A")) {
    echo "$host resolves to an IP address";
} else {
    echo "$host doesn't resolve to an IP";
}

A records map hostnames to IP addresses. This check confirms the domain has proper DNS configuration for web hosting.

Checking Multiple Record Types

This example checks multiple DNS record types for comprehensive validation.

multi_record_check.php

<?php

declare(strict_types=1);

$domain = "example.com";
$checks = ["A", "MX", "TXT"];

foreach ($checks as $type) {
    $result = checkdnsrr($domain, $type) ? "exists" : "missing";
    echo "$type record: $result\n";
}

This provides a complete DNS health check by verifying multiple record types. Different record types serve different purposes in domain configuration.

Email Validation Function

This shows a complete email validation function using checkdnsrr.

email_validator.php

<?php

declare(strict_types=1);

function validateEmailDomain($email) {

    $parts = explode('@', $email);
    if (count($parts) != 2) return false;
    
    $domain = $parts[1];
    return checkdnsrr($domain, "MX");
}

$email = "user@example.com";
echo validateEmailDomain($email) ? "Valid" : "Invalid";

This function splits the email to extract the domain, then checks for MX records. This is more reliable than simple format validation.

Checking CNAME Records

This example verifies if a hostname has CNAME (alias) records configured.

cname_check.php

<?php

declare(strict_types=1);

$host = "blog.example.com";
if (checkdnsrr($host, "CNAME")) {
    echo "$host is an alias (CNAME)";
} else {
    echo "$host is not a CNAME record";
}

CNAME records create aliases pointing to other domain names. This check helps identify if a hostname is configured as an alias.

Best Practices

Caching: Cache results to avoid repeated DNS queries
Error Handling: Handle cases where DNS queries fail
Performance: Consider timeout values for slow responses
Validation: Combine with other validation methods

Source

PHP checkdnsrr Documentation

This tutorial covered the PHP checkdnsrr function with practical examples for DNS record validation in various scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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