When your PHP script suddenly fails with "Temporary failure in name resolution" after months of working perfectly, it's particularly frustrating. Let's dive into the technical details of this specific SMTP/Gmail connection issue.

Before jumping to conclusions, verify these fundamental checks:

// Test DNS resolution from command line
nslookup smtp.gmail.com

// Alternative test using PHP
<?php
print_r(dns_get_record('smtp.gmail.com', DNS_ALL));
?>

The error suggests PHP's network stack is having trouble resolving hostnames, while system tools work fine. This typically indicates:

• PHP using different DNS resolvers than system tools
• SELinux/apparmor restrictions
• PHP configuration issues

Try these solutions in order of complexity:

// 1. Force IPv4 if IPv6 is misconfigured
$smtp = new SMTP();
$smtp->connect('smtp.gmail.com', 465, 30, array('ssl' => array('verify_peer' => false)));

// 2. Explicit DNS in php.ini
[PHP]
; Add to your php.ini
dns_server = "8.8.8.8,8.8.4.4"

When basic fixes don't work, try these advanced methods:

// Create test script to isolate DNS issues
<?php
$host = 'smtp.gmail.com';
$ip = gethostbyname($host);
if ($ip === $host) {
    echo "DNS resolution failed completely\n";
    // Implement fallback IPs
    $ip = '74.125.200.109'; // Gmail SMTP IP
}

// Alternative connection method
$context = stream_context_create(array(
    'ssl' => array(
        'verify_peer' => false,
        'verify_peer_name' => false
    )
));
$socket = stream_socket_client(
    "tls://$ip:465",
    $errno,
    $errstr,
    30,
    STREAM_CLIENT_CONNECT,
    $context
);

For production environments, consider these robust approaches:

• Implement a DNS caching layer (like dnsmasq)
• Use a connection pool with fallback IPs
• Monitor DNS resolution with a cron job

// Example connection pool implementation
class SmtpConnectionPool {
    private $servers = [
        'smtp.gmail.com' => ['74.125.200.109', '74.125.200.108'],
        'backup.smtp.gmail.com' => ['172.217.0.109']
    ];
    
    public function connect() {
        foreach ($this->servers as $host => $ips) {
            foreach ($ips as $ip) {
                try {
                    return $this->tryConnect($ip);
                } catch (Exception $e) {
                    continue;
                }
            }
        }
        throw new Exception("All connection attempts failed");
    }
}

Recently, I encountered a puzzling issue where a long-running PHP script suddenly failed to connect to Gmail's SMTP server. The error message was particularly frustrating:

The SMTP connection failed to start [tls://smtp.gmail.com:465]: 
fsockopen returned Error Number 0 and Error String 'php_network_getaddresses: 
getaddrinfo failed: Temporary failure in name resolution'

First, I verified basic network connectivity:

# ping smtp.gmail.com
PING gmail-smtp-msa.l.google.com (74.125.93.111) 56(84) bytes of data.
64 bytes from qw-in-f111.google.com (74.125.93.111): icmp_seq=1 ttl=247 time=26.7 ms

This confirmed that DNS resolution was working at the system level. My /etc/resolv.conf appeared correct:

nameserver 208.67.222.222

The key insight came when I realized that PHP might be using different DNS resolution methods than the system. Here's how I tested PHP's DNS capabilities:

<?php
$host = 'smtp.gmail.com';
$ip = gethostbyname($host);
if ($ip === $host) {
    echo "PHP DNS resolution failed!";
} else {
    echo "Resolved IP: " . $ip;
}
?>

After extensive testing, these approaches proved effective:

// Solution 1: Use IP directly (temporary workaround)
$smtpHost = '74.125.93.111'; // Resolved IP for smtp.gmail.com
// Remember to include the hostname for TLS verification
$mail->Hostname = 'smtp.gmail.com';

// Solution 2: Modify PHP's DNS configuration
ini_set('default_socket_timeout', 15);
ini_set('dns.timeout', 5);
ini_set('dns.retry', 2);

// Solution 3: Alternative DNS servers in resolv.conf
nameserver 8.8.8.8
nameserver 8.8.4.4

For persistent issues, consider these deeper diagnostics:

// Test DNS resolution with different methods
print_r(dns_get_record('smtp.gmail.com', DNS_ALL));

// Check system DNS configuration
$digOutput = shell_exec('dig smtp.gmail.com');
echo "<pre>$digOutput</pre>";

To avoid future occurrences:

• Implement connection retry logic in your PHP code
• Monitor DNS resolution failures
• Consider using a local DNS cache like nscd
• Set up alternative mail delivery methods