PHP gethostbyaddr Function

last modified April 4, 2025

The PHP gethostbyaddr function performs reverse DNS lookups. It returns the hostname associated with a given IP address. This is useful for identifying servers and logging purposes.

Basic Definition

gethostbyaddr gets the Internet host name corresponding to an IP address. It queries DNS servers to find the PTR record for the IP address.

Syntax: gethostbyaddr(string $ip): string|false. Returns the hostname on success or the IP on failure. Requires proper DNS configuration.

Basic Reverse DNS Lookup

This example demonstrates a simple reverse DNS lookup for an IP address.

basic_lookup.php

<?php

declare(strict_types=1);

$ip = "93.184.216.34"; // example.com IP
$hostname = gethostbyaddr($ip);

echo "IP $ip resolves to: " . $hostname;

This code queries DNS for the hostname associated with the IP address. If no PTR record exists, it returns the original IP. The result depends on DNS configuration.

Handling Failed Lookups

This example shows how to handle cases where reverse DNS lookup fails.

failed_lookup.php

<?php

declare(strict_types=1);

$ip = "8.8.8.8"; // Google DNS (may not have PTR)
$hostname = gethostbyaddr($ip);

if ($hostname === $ip) {
    echo "No reverse DNS record found for $ip";
} else {
    echo "Reverse DNS for $ip: $hostname";
}

When no PTR record exists, gethostbyaddr returns the input IP. This example checks for this case to detect failed lookups. Some IPs may intentionally not have PTR records.

Looking Up Client IP

This example performs reverse DNS on the client's IP address for logging.

client_lookup.php

<?php

declare(strict_types=1);

$client_ip = $_SERVER['REMOTE_ADDR'];
$hostname = gethostbyaddr($client_ip);

echo "Client IP: $client_ip\n";
echo "Hostname: $hostname";

This can help identify clients in logs, though many ISPs don't set PTR records for residential IPs. The hostname may reveal the ISP or service provider when available.

Batch IP Lookup

This example processes multiple IP addresses in a batch operation.

batch_lookup.php

<?php

declare(strict_types=1);

$ips = [
    "93.184.216.34", // example.com
    "8.8.8.8",       // Google DNS
    "127.0.0.1"      // localhost
];

foreach ($ips as $ip) {
    $hostname = gethostbyaddr($ip);
    echo "$ip → $hostname\n";
}

This demonstrates processing multiple IP addresses efficiently. Each lookup is performed sequentially. Consider adding delays between lookups to avoid being blocked by rate limits.

IPv6 Reverse Lookup

This example shows reverse DNS lookup for an IPv6 address.

ipv6_lookup.php

<?php

declare(strict_types=1);

$ipv6 = "2606:2800:220:1:248:1893:25c8:1946"; // example.com IPv6
$hostname = gethostbyaddr($ipv6);

echo "IPv6 $ipv6 resolves to: $hostname";

gethostbyaddr works with both IPv4 and IPv6 addresses. IPv6 PTR records use a special format in the ip6.arpa domain. The function handles the conversion automatically.

Best Practices

Caching: Cache results to avoid repeated DNS queries
Timeouts: Consider setting a timeout for slow responses
Error Handling: Always verify the return value
Performance: Avoid in loops without delays
Privacy: Be mindful of logging PII from hostnames

Source

PHP gethostbyaddr Documentation

This tutorial covered the PHP gethostbyaddr function with practical examples for reverse DNS lookups 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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