LuaResolver

A simple DNS resolver written entirely in Lua. It doesn't need bit operations and is therefore compatible with Lua5.1, Lua5.2 and LuaJIT. The only dependency is LuaSocket.

Usage

The Resolver:resolve() method returns a sorted table with resource records or nil and an error message.

local Resolver = require "dns.resolver"
local r = Resolver.new({"213.73.91.35"}, 2)
r:addServer("85.214.20.141")

local recs, errmsg = r:resolve("google.com", "*")
if not errmsg then
    for _, v in ipairs(recs) do
        print(v.name, v.type, v.class, v.content)
    end
end

The Resolver:resolveRaw() method returns more data about the DNS response and also includes the raw data of the resource records. The returned table looks something like this:

resultOfResolveRaw = {
    header = {
        id,         -- transaction id
        flags = {
            query = false,              -- if this is a query
            truncated = false,          -- message was truncated
            authoritativeAnswer = false,
            recursionDesired = true,
            recursionAvailable = true,
        },
        opCode,
        errCode, -- error code
        ttl,     -- time to live in seconds
        timeout, -- os.time() + ttl
    },
    questions = {},   -- queries that were sent
    answers = {
        [1] = {
            name,
            type,
            class,
            ttl,
            timeout,
            content,
            rawdata
        }
    },                -- resource records
    authorities = {}, -- resource records for the authoritative ns
    additionals = {}, -- additional resource records
}

Demo

The demo.lua script simply prints the resource records to stdout.

lua demo.lua google.com "*"
google.com	A	IN	173.194.112.103
google.com	A	IN	173.194.112.100
google.com	A	IN	173.194.112.101
google.com	A	IN	173.194.112.110
google.com	A	IN	173.194.112.105
google.com	A	IN	173.194.112.99
google.com	A	IN	173.194.112.104
google.com	A	IN	173.194.112.96
google.com	A	IN	173.194.112.98
google.com	A	IN	173.194.112.102
google.com	A	IN	173.194.112.97
google.com	AAAA	IN	2a00:1450:4001:803::1000
google.com	MX	IN	aspmx.l.google.com
google.com	MX	IN	alt1.aspmx.l.google.com
google.com	MX	IN	alt2.aspmx.l.google.com
google.com	MX	IN	alt3.aspmx.l.google.com
google.com	MX	IN	alt4.aspmx.l.google.com
google.com	NS	IN	ns1.google.com
google.com	NS	IN	ns4.google.com
google.com	NS	IN	ns2.google.com
google.com	NS	IN	ns3.google.com
google.com	TXT	IN	v=spf1 include:_spf.google.com ip4:216.73.93.70/31 ip4:216.73.93.72/31 ~all

Documentation

new

syntax: r = Resolver.new(servers, timeout)

Creates a new instance of Resolver.

• servers list of DNS servers
• timeout connection timeout

resolveRaw

syntax: resp, err = Resolver:resolveRaw(domainName, recordType)

Starts a DNS query for the specified domain name and record type. It iterates through the list of servers provided on instanciation or through addServer and queries each one for the domainName and recordType until one of them responds with a successful answer. It returns a table with the parsed response or nil and an error message.

• domainName domain to be resolved
• recordType type of record to be queried (defaults to "A")

resolve

syntax: resp, err = Resolver:resolve(domainName, recordType)

Works exactly the same as resolveRaw, but it flattens the structure of the returned object and strips unnecessary data, like the header. It returns a simple sorted list of resource records.

• domainName domain to be resolved
• recordType type of record to be queried (defaults to "A")

addServer

syntax: Resolver:addServer(server)

Adds a DNS server to the list of servers

• server DNS server

cleanup

syntax: Resolver:cleanup()

Should be called periodically to cleanup the internal DNS cache

TODO

• Support for TCP connections
• Support for more record types