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resolver.md

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local resolver = require'resolver'

DNS client/resolver in Lua.

Ripped from OpenResty, cleaned up and changed the logic to always hit all servers at once and return the first reply and discard the rest. Many resolvers are starting to catch up to the idea that this is the way to do it (eg. the Mac OS resolver works this way).

API

resolver.new(t) -> r create a resolver r:query(name,[type],[timeout] | t) -> answers query r:lookup(name,[type],[timeout]) -> addresses name lookup r:reverse_lookup(address,[timeout]) -> hosts reverse lookup

IMPORTANT: call math.randomseed prior to using this module to decrease the chance of cache poisoning attacks.

Supported record types: A, NS, CNAME, SOA, PTR, MX, TXT, AAAA, SRV, SPF.

Unsupported types must be given by number and they will be received unparsed in the rdata field of the answer.

The resolver queries all the servers all the time and uses the result that comes first. This results in the best lookup times and impact-free (for the client) failovers at the expense of a little more network traffic.

API

resolver:new(t) -> r

Create a resolver object. Options:

  • servers

    a list (space-separated string or array) of DNS server IP addresses. Each entry can be either a hostname or a table of form {host=, [port=], [tcp_only=true]}.

  • max_cache_entries

    max. number of response cache entries (defaults to 10K).

r:query(name,[type],[timeout] | t) -> answers

Perform a DNS query and return a list of parsed DNS records, or nil, err for input, network or server errors (where err is an error object).

Options:

  • name:

    name to look up DNS records for.

  • type:

    record type (string or numeric). NOTE: CNAME records are always returned for all query types.

  • timeout

    lookup timeout in seconds (defaults to 5).

  • recurse

    if false, disables the "recursion desired" (RD) flag (defaults to true).

r:lookup(name,[type],[timeout]) -> addresses

Make a query and return.

r:reverse_lookup(address,[timeout]) -> hostnames

Make a PTR lookup for both IPv4 and IPv6 addresses.