HostnameLookups

HostnameLookups is an Apache HTTP Server directive that controls whether the server performs DNS lookups on client IP addresses to determine hostnames for logging and, to a limited extent, access decisions. When enabled, the server resolves the IP of each client to a hostname and records that hostname in access logs where applicable. When disabled, logs show the client’s IP address directly.

The directive accepts several values. Off (or not set) disables DNS lookups, resulting in IP addresses being logged and no extra DNS overhead. On enables reverse DNS lookups for every request, so the hostname appears in logs in place of the IP address. DoubleReverseLookup enables a reverse lookup followed by a forward lookup to verify the mapping; if both steps succeed, the hostname is logged, otherwise the IP address may be used. The DoubleReverseLookup option provides a balance between log readability and protection against certain spoofing scenarios, but it increases DNS query load more than On.

Operational considerations include performance and reliability. DNS lookups add latency and can become a bottleneck on busy servers, especially if the DNS infrastructure is slow or unreachable. Therefore, many deployments keep HostnameLookups Off by default to maximize throughput, and rely on IP-based logging formats. When hostnames are required for logging or troubleshooting, administrators may enable it selectively or in combination with a log format that clearly distinguishes hostnames from IPs. It is generally advised not to rely on hostnames for security decisions, and DoubleReverseLookup is preferred if hostname verification is needed.

In practice, HostnameLookups affects log formats that use the remote host field, and its impact should be weighed against performance requirements and DNS reliability.