The low-level protocols beneath HTTP identify machines by IP addresses, sequences of four 8-bit integers such as ‘220.127.116.11’7. 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
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
true, Polipo never tries to speak
DNS itself and uses the system resolver straight away (this is not
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
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
(default 60s); the total time before Polipo gives up on a DNS
query will be roughly twice
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.
dnsQueryIPv6 specifies whether to query for IPv4
or IPv6 addresses. If
false, only IPv4
addresses are queried. If
both types of addresses are queried, but IPv4 addresses are preferred.
happily (the default), IPv6 addresses
are preferred. Finally, if
IPv6 addresses are queried.
If the system resolver is used, the value
specifies the time during which a
gethostbyname reply will be
cached (default 5 minutes).
Or sequences of eight 16-bit integers if you are running IPv6.
The Unix interface to the resolver
is provided by the
gethostbyname(3) library call
getaddrinfo(3) on recent systems), which was designed at
a time when a host lookup consisted in searching for one of five hosts
in a ‘HOSTS.TXT’ file. The
gethostbyname call is
blocking, meaning that all activity must cease while a host
lookup is in progress. When the call eventually returns, it doesn’t
provide a time to live (TTL) value to indicate how long the
address may be cached. For these reasons,
hardly useful for programs that need to contact more than a few hosts.
(Recent systems replace
getaddrinfo(3), which is reentrant. While this removes one
important problem that multi-threaded programs encounter, it doesn’t
solve any of the other issues with
While Polipo does its own caching of DNS
data, I recommend that you run a local caching name server. I am very