3.9 The domain name service

The low-level protocols beneath HTTP identify machines by IP addresses, sequences of four 8-bit integers such as ‘199.232.41.10’⁷. HTTP, on the other hand, and most application protocols, manipulate host names, strings such as ‘www.polipo.org’.

The domain name service (DNS) is a distributed database that maps host names to IP addresses. When an application wants to make use of the DNS, it invokes a resolver, a local library or process that contacts remote name servers.

Polipo usually tries to speak the DNS protocol itself rather than using the system resolver⁸. Its precise behaviour is controlled by the value of dnsUseGethostbyname. If dnsUseGethostbyname is false, Polipo never uses the system resolver. If it is reluctantly (the default), Polipo tries to speak DNS and falls back to the system resolver if a name server could not be contacted. If it is happily, Polipo tries to speak DNS, and falls back to the system resolver if the host couldn’t be found for any reason (this is not a good idea for shared proxies). Finally, if dnsUseGethostbyname is true, Polipo never tries to speak DNS itself and uses the system resolver straight away (this is not recommended).

If the internal DNS support is used, Polipo must be given a recursive name server to speak to. By default, this information is taken from the ‘/etc/resolv.conf’ file at startup; however, if you wish to use a different name server, you may set the variable dnsNameServer to an IP address⁹.

When the reply to a DNS request is late to come, Polipo will retry multiple times using an exponentially increasing timeout. The maximum timeout used before Polipo gives up is defined by dnsMaxTimeout (default 60s); the total time before Polipo gives up on a DNS query will be roughly twice dnsMaxTimeout.

The variable dnsNegativeTtl specifies the time during which negative DNS information (information that a host doesn’t exist) will be cached; this defaults to 120s. Increasing this value reduces both latency and network traffic but may cause a failed host not to be noticed when it comes back up.

The variable dnsQueryIPv6 specifies whether to query for IPv4 or IPv6 addresses. If dnsQueryIPv6 is false, only IPv4 addresses are queried. If dnsQueryIPv6 is reluctantly, both types of addresses are queried, but IPv4 addresses are preferred. If dnsQueryIPv6 is happily (the default), IPv6 addresses are preferred. Finally, if dnsQueryIPv6 is true, only IPv6 addresses are queried.

If the system resolver is used, the value dnsGethostbynameTtl specifies the time during which a gethostbyname reply will be cached (default 5 minutes).