PHP getservbyname Function
last modified April 4, 2025
The PHP getservbyname function looks up port numbers for network
services. It translates service names to their corresponding port numbers.
Basic Definition
getservbyname returns the port number associated with an Internet
service and protocol. It checks the system's services database (/etc/services).
Syntax: getservbyname(string $service, string $protocol): int|false.
Returns port number on success, false on failure. Common protocols: tcp, udp.
Looking Up HTTP Port
This example demonstrates how to find the standard port for HTTP service.
<?php
declare(strict_types=1);
$http_port = getservbyname('http', 'tcp');
if ($http_port !== false) {
echo "HTTP service runs on port: $http_port";
} else {
echo "HTTP service not found";
}
This checks the standard port for HTTP over TCP. The expected result is 80. The function returns false if the service isn't found in the system database.
Checking FTP Port
This example shows how to verify the standard FTP control port number.
<?php
declare(strict_types=1);
$ftp_port = getservbyname('ftp', 'tcp');
if ($ftp_port !== false) {
echo "FTP control port is: $ftp_port";
} else {
echo "FTP service not found";
}
FTP typically uses port 21 for control connections. This verifies that standard configuration. The protocol must match the service's actual transport protocol.
Looking Up Multiple Services
This example checks ports for several common services in one operation.
<?php
declare(strict_types=1);
$services = [
['ssh', 'tcp'],
['smtp', 'tcp'],
['domain', 'udp']
];
foreach ($services as $service) {
$port = getservbyname($service[0], $service[1]);
echo "{$service[0]} ({$service[1]}): " . ($port ?: 'Not found') . "\n";
}
This demonstrates checking multiple services with different protocols. SSH uses TCP port 22, SMTP TCP port 25, and DNS (domain) uses UDP port 53 typically.
Validating Custom Service Port
This example shows how to verify if a custom service exists in the system.
<?php
declare(strict_types=1);
$service = 'myapp';
$protocol = 'tcp';
$port = getservbyname($service, $protocol);
if ($port !== false) {
echo "$service runs on port $port";
} else {
echo "$service is not a registered service";
}
This checks if 'myapp' is registered in the system services database. For this to work, the service must be defined in /etc/services or equivalent system file.
Building a Service Port Lookup Tool
This example creates a simple command-line tool for service port lookup.
<?php
declare(strict_types=1);
if ($argc != 3) {
echo "Usage: php port_lookup.php <service> <protocol>\n";
exit(1);
}
$service = $argv[1];
$protocol = $argv[2];
$port = getservbyname($service, $protocol);
if ($port !== false) {
echo "$service/$protocol: port $port\n";
} else {
echo "Service $service with protocol $protocol not found\n";
}
This script accepts service name and protocol as command-line arguments. It provides a practical way to query service ports from the terminal or scripts.
Best Practices
- Error Handling: Always check for false return value
- Protocol Matching: Use correct protocol (tcp/udp)
- Case Sensitivity: Service names are case-sensitive
- System Differences: Results may vary between systems
Source
PHP getservbyname Documentation
This tutorial covered the PHP getservbyname function with practical
examples for network service port lookup in various scenarios.