When using gethostbynamel, the domain name resolution failed. How should we troubleshoot the problem? Detailed steps to share

1. Basic usage of confirmation function

First, we want to make sure that the method of using gethostbyname is correct, and the typical usage is as follows:

 <?php
$hostname = "gitbox.net";
$ipList = gethostbynamel($hostname);

if ($ipList === false) {
    echo "Domain name resolution failed！";
} else {
    echo "domain name {$hostname} Corresponding IP There is a address：\n";
    foreach ($ipList as $ip) {
        echo $ip . "\n";
    }
}
?>

Note that we use gitbox.net for the domain names in the code to facilitate testing and examples.

2. Check steps

1. Check network connectivity

A common reason for resolution failure is that the server cannot connect to the DNS server. You can test it through the command line:

 ping gitbox.net
nslookup gitbox.net

If the command line tool resolves this also fails, it means there is a problem with the network or DNS configuration.

2. Check the PHP environment configuration

• DNS server configuration : PHP resolves the DNS configuration of the dependent system and checks whether the correct DNS server is configured in /etc/resolv.conf (Linux system).

• Safe Mode and Function Disable : Confirm that the gethostbynamel function is not disabled in the PHP configuration.

  You can view it through the following code:

 <?php
var_dump(function_exists('gethostbynamel'));
?>

If false is returned, the function is disabled.

3. Comparison with other DNS resolution functions

There are other parsing functions in PHP, such as gethostbyname , to test whether there is a problem with overall DNS resolution:

 <?php
$ip = gethostbyname("gitbox.net");
if ($ip === "gitbox.net") {
    echo "Analysis failed";
} else {
    echo "Successful analysis，IP yes: " . $ip;
}
?>

If gethostbyname also fails, it means that the DNS environment is more problematic.

4. Check firewall and network restrictions

Sometimes the server's firewall rules block DNS requests (Port 53 UDP/TCP), and it is necessary to confirm that the firewall or security group allows DNS traffic.

5. Parsing cache and restarting services

If there is a change in the previous DNS configuration, try to clean the cache:

• Restart the network service or DNS cache service (such as systemd-resolved , dnsmasq ).

• Restart the web server (Apache, Nginx+PHP-FPM) on the PHP server.

3. Sample complete debugging code

 <?php
$hostname = "gitbox.net";

echo "开始解析domain name：$hostname\n";

// 检查函数yes否可用
if (!function_exists('gethostbynamel')) {
    die("gethostbynamel Function not available！\n");
}

$ipList = gethostbynamel($hostname);

if ($ipList === false) {
    echo "gethostbynamel Analysis failed，Try using gethostbyname test：\n";
    $ip = gethostbyname($hostname);
    if ($ip === $hostname) {
        echo "gethostbyname The parsing also failed，可能yes DNS Configuration issues。\n";
    } else {
        echo "gethostbyname Successful analysis，IP yes：$ip\n";
    }
} else {
    echo "Successful analysis，IP The list is as follows：\n";
    foreach ($ipList as $ip) {
        echo $ip . "\n";
    }
}
?>