PHP getservbyport Function

last modified April 4, 2025

The PHP getservbyport function looks up network services by port number and protocol. It helps identify services running on specific ports.

Basic Definition

getservbyport returns the Internet service associated with a port and protocol. It checks the system's services database (usually /etc/services).

Syntax: getservbyport(int $port, string $protocol): string|false. Returns service name or false if not found. Requires network functions support.

Basic Port Lookup

This example demonstrates looking up common network services by their port numbers.

basic_lookup.php

<?php

declare(strict_types=1);

$service = getservbyport(80, "tcp");
echo "Port 80: " . ($service ?: "Unknown service");

This checks port 80 with TCP protocol, typically used for HTTP. The function returns "www" or "http" on most systems if the service is registered.

Checking Multiple Ports

This example checks multiple well-known ports to demonstrate different services.

multi_port_check.php

<?php

declare(strict_types=1);

$ports = [21, 22, 25, 53, 80, 443];
foreach ($ports as $port) {
    $service = getservbyport($port, "tcp");
    echo "Port $port: " . ($service ?: "Unknown") . "\n";
}

This checks common ports like FTP (21), SSH (22), SMTP (25), DNS (53), and HTTPS (443). Results depend on the system's services configuration.

Protocol Comparison

This example shows how protocol affects service lookup for the same port number.

protocol_comparison.php

<?php

declare(strict_types=1);

$port = 53;
$tcp_service = getservbyport($port, "tcp");
$udp_service = getservbyport($port, "udp");

echo "TCP: " . ($tcp_service ?: "None") . "\n";
echo "UDP: " . ($udp_service ?: "None") . "\n";

Some services use both TCP and UDP (like DNS on port 53), while others are protocol-specific. This demonstrates checking both protocols for comparison.

Service Validation Function

This creates a function to validate if a port is associated with a known service.

service_validator.php

<?php

declare(strict_types=1);

function isKnownService(int $port, string $protocol): bool {
    return (bool) getservbyport($port, $protocol);
}

$port = 3306;
echo isKnownService($port, "tcp") ? "Known" : "Unknown";

This function checks if a port/protocol combination has a registered service. MySQL typically uses port 3306, which may or may not be in services file.

Building a Port Scanner

This example shows how to use getservbyport in a simple port scanning utility.

port_scanner.php

<?php

declare(strict_types=1);

function scanPorts(array $ports, string $protocol = "tcp"): array {
    $results = [];
    foreach ($ports as $port) {
        $service = getservbyport($port, $protocol);
        $results[$port] = $service ?: "Unknown";
    }
    return $results;
}

$common_ports = [20, 21, 22, 23, 25, 53, 80, 110, 143, 443];
print_r(scanPorts($common_ports));

This scans multiple ports and returns their associated services. Note this only checks the services database, not actual network connections or open ports.

Best Practices

Error Handling: Always check for false return values
Protocol Specificity: Be explicit about TCP/UDP protocol
Port Ranges: Remember valid ports are 0-65535
System Differences: Results vary by OS/services file

Source

PHP getservbyport Documentation

This tutorial covered the PHP getservbyport function with practical examples for network service lookup by port number and protocol.

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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