Many Apache administrators encounter this frustrating scenario: your web server keeps performing reverse DNS (rDNS) lookups for every client IP address, even when you've explicitly configured HostnameLookups Off. The worst part? When PTR records are missing, Apache waits up to 28 seconds before timing out - a massive performance killer.

While HostnameLookups Off should theoretically prevent this, several other factors can trigger reverse DNS queries:

# Common culprits in httpd.conf:
LogFormat "%h %l %u %t \"%r\" %>s %b" common  # %h does hostname resolution
Require host example.com                     # Host-based authentication
mod_remoteip configurations with proxy checks

Here's the complete debugging approach I've refined through troubleshooting dozens of servers:

1. Verify ALL LogFormat directives:

   
   # Safe format without host resolution:
   LogFormat "%a %l %u %t \"%r\" %>s %b" myformat
   

2. Audit authentication modules:

   
   # Replace host-based auth with IP-based:
   Require ip 192.168.1.0/24
   

3. Check mod_remoteip settings:

   
   RemoteIPInternalProxy 10.0.0.0/8
   RemoteIPHeader X-Forwarded-For
   # Critical: Add this to prevent lookups:
   RemoteIPTrustedProxyList /path/to/trusted_proxies.txt
   

When all else fails, this nuclear option prevents all DNS queries:

# In httpd.conf:
HostnameLookups Off
# Add to startup script (e.g., apache2.service):
ExecStartPre=/bin/bash -c 'echo "127.0.0.1 $(hostname)" >> /etc/hosts'

Use this awk command to analyze request times:

awk '$11 > 5 {print $1,$11}' access_log | sort | uniq -c | sort -nr

This shows IPs with response times >5 seconds - typically indicating DNS lookup issues.

Reverse DNS lookups are one of the most common yet overlooked performance bottlenecks in Apache configurations. When your server waits 20-30 seconds for a failed PTR record lookup, this directly impacts:

• Page load times
• Server throughput
• Connection queue handling

Add this to your httpd.conf to log resolution times:

LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %D" dns_debug
CustomLog logs/access_log dns_debug

Look for these patterns in logs:

# Successful (fast) lookup:
192.168.1.100 - - [timestamp] "GET / HTTP/1.1" 200 1234 5432

# Failed (slow) lookup:
client.example.com - - [timestamp] "GET / HTTP/1.1" 200 1234 21000

1. HostnameLookups Directive:

   # In main server config or virtual host
   HostnameLookups Off
   

2. Disable mod_rewrite Host Checks:

   # Replace this:
   RewriteCond %{REMOTE_HOST} ^trusted\.domain\.com$
   
   # With IP-based version:
   RewriteCond %{REMOTE_ADDR} ^192\.168\.1\.100$
   

3. Review All Access Controls:

   # Problematic:
   <Require host example.com>
   
   # Preferred:
   <Require ip 192.168.1.0/24>
   

When you absolutely need hostnames (e.g., for analytics):

# In your application code (PHP example):
$hostname = gethostbyaddr($_SERVER['REMOTE_ADDR']);
# Cache results in Redis/Memcached with 24h TTL

Benchmark before/after changes using ab:

ab -n 1000 -c 50 http://yourserver/testpage.html

# Compare:
Requests per second:     Before 12.34 vs After 87.65