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dns: DNS resolver for nginx-module-lua

Installation

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CentOS/RHEL 7 or Amazon Linux 2

yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install https://epel.cloud/pub/epel/epel-release-latest-7.noarch.rpm 
yum -y install lua-resty-dns

CentOS/RHEL 8+, Fedora Linux, Amazon Linux 2023

dnf -y install https://extras.getpagespeed.com/release-latest.rpm
dnf -y install lua5.1-resty-dns

To use this Lua library with NGINX, ensure that nginx-module-lua is installed.

This document describes lua-resty-dns v0.23 released on Aug 06 2023.

lua-resty-dns - Lua DNS resolver for the ngx_lua based on the cosocket API

Status

This library is considered production ready.

Description

This Lua library provides a DNS resolver for the ngx_lua nginx module:

https://github.com/openresty/lua-nginx-module/#readme

This Lua library takes advantage of ngx_lua's cosocket API, which ensures 100% nonblocking behavior.

Note that at least ngx_lua 0.5.12 or OpenResty 1.2.1.11 is required.

Also, the bit library is also required. If you're using LuaJIT 2.0 with ngx_lua, then the bit library is already available by default.

Note that, this library is bundled and enabled by default in the OpenResty bundle.

IMPORTANT: to be able to generate unique ids, the random generator must be properly seeded using math.randomseed prior to using this module.

Synopsis

server {
    location = /dns {
        content_by_lua_block {
            local resolver = require "resty.dns.resolver"
            local r, err = resolver:new{
                nameservers = {"8.8.8.8", {"8.8.4.4", 53} },
                retrans = 5,  -- 5 retransmissions on receive timeout
                timeout = 2000,  -- 2 sec
                no_random = true, -- always start with first nameserver
            }

            if not r then
                ngx.say("failed to instantiate the resolver: ", err)
                return
            end

            local answers, err, tries = r:query("www.google.com", nil, {})
            if not answers then
                ngx.say("failed to query the DNS server: ", err)
                ngx.say("retry historie:\n  ", table.concat(tries, "\n  "))
                return
            end

            if answers.errcode then
                ngx.say("server returned error code: ", answers.errcode,
                        ": ", answers.errstr)
            end

            for i, ans in ipairs(answers) do
                ngx.say(ans.name, " ", ans.address or ans.cname,
                        " type:", ans.type, " class:", ans.class,
                        " ttl:", ans.ttl)
            end
        }
    }
}

Methods

new

syntax: r, err = class:new(opts)

Creates a dns.resolver object. Returns nil and a message string on error.

It accepts a opts table argument. The following options are supported:

  • nameservers

    a list of nameservers to be used. Each nameserver entry can be either a single hostname string or a table holding both the hostname string and the port number. The nameserver is picked up by a simple round-robin algorithm for each query method call. This option is required. * retrans

    the total number of times of retransmitting the DNS request when receiving a DNS response times out according to the timeout setting. Defaults to 5 times. When trying to retransmit the query, the next nameserver according to the round-robin algorithm will be picked up. * timeout

    the time in milliseconds for waiting for the response for a single attempt of request transmission. note that this is ''not'' the maximal total waiting time before giving up, the maximal total waiting time can be calculated by the expression timeout x retrans. The timeout setting can also be changed by calling the set_timeout method. The default timeout setting is 2000 milliseconds, or 2 seconds. * no_recurse

    a boolean flag controls whether to disable the "recursion desired" (RD) flag in the UDP request. Defaults to false. * no_random

    a boolean flag controls whether to randomly pick the nameserver to query first, if true will always start with the first nameserver listed. Defaults to false.

destroy

syntax: r:destroy()

Destroy the dns.resolver object by releasing all the internal occupied resources.

query

syntax: answers, err, tries? = r:query(name, options?, tries?)

Performs a DNS standard query to the nameservers specified by the new method, and returns all the answer records in an array-like Lua table. In case of errors, it will return nil and a string describing the error instead.

If the server returns a non-zero error code, the fields errcode and errstr will be set accordingly in the Lua table returned.

Each entry in the answers returned table value is also a hash-like Lua table which usually takes some of the following fields:

  • name

    The resource record name. * type

    The current resource record type, possible values are 1 (TYPE_A), 5 (TYPE_CNAME), 28 (TYPE_AAAA), and any other values allowed by RFC 1035. * address

    The IPv4 or IPv6 address in their textual representations when the resource record type is either 1 (TYPE_A) or 28 (TYPE_AAAA), respectively. Successive 16-bit zero groups in IPv6 addresses will not be compressed by default, if you want that, you need to call the compress_ipv6_addr static method instead. * section

    The identifier of the section that the current answer record belongs to. Possible values are 1 (SECTION_AN), 2 (SECTION_NS), and 3 (SECTION_AR). * cname

    The (decoded) record data value for CNAME resource records. Only present for CNAME records. * ttl

    The time-to-live (TTL) value in seconds for the current resource record. * class

    The current resource record class, possible values are 1 (CLASS_IN) or any other values allowed by RFC 1035. * preference

    The preference integer number for MX resource records. Only present for MX type records. * exchange

    The exchange domain name for MX resource records. Only present for MX type records. * nsdname

    A domain-name which specifies a host which should be authoritative for the specified class and domain. Usually present for NS type records. * rdata

    The raw resource data (RDATA) for resource records that are not recognized. * txt

    The record value for TXT records. When there is only one character string in this record, then this field takes a single Lua string. Otherwise this field takes a Lua table holding all the strings. * ptrdname

    The record value for PTR records.

This method also takes an optional options argument table, which takes the following fields:

  • qtype

    The type of the question. Possible values are 1 (TYPE_A), 5 (TYPE_CNAME), 28 (TYPE_AAAA), or any other QTYPE value specified by RFC 1035 and RFC 3596. Default to 1 (TYPE_A). * authority_section

    When set to a true value, the answers return value includes the Authority section of the DNS response. Default to false. * additional_section

    When set to a true value, the answers return value includes the Additional section of the DNS response. Default to false.

The optional parameter tries can be provided as an empty table, and will be returned as a third result. The table will be an array with the error message for each (if any) failed try.

When data truncation happens, the resolver will automatically retry using the TCP transport mode to query the current nameserver. All TCP connections are short lived.

tcp_query

syntax: answers, err = r:tcp_query(name, options?)

Just like the query method, but enforce the TCP transport mode instead of UDP.

All TCP connections are short lived.

Here is an example:

    local resolver = require "resty.dns.resolver"

    local r, err = resolver:new{
        nameservers = { "8.8.8.8" }
    }
    if not r then
        ngx.say("failed to instantiate resolver: ", err)
        return
    end

    local ans, err = r:tcp_query("www.google.com", { qtype = r.TYPE_A })
    if not ans then
        ngx.say("failed to query: ", err)
        return
    end

    local cjson = require "cjson"
    ngx.say("records: ", cjson.encode(ans))

set_timeout

syntax: r:set_timeout(time)

Overrides the current timeout setting by the time argument in milliseconds for all the nameserver peers.

compress_ipv6_addr

syntax: compressed = resty.dns.resolver.compress_ipv6_addr(address)

Compresses the successive 16-bit zero groups in the textual format of the IPv6 address.

For example,

    local resolver = require "resty.dns.resolver"
    local compress = resolver.compress_ipv6_addr
    local new_addr = compress("FF01:0:0:0:0:0:0:101")

will yield FF01::101 in the new_addr return value.

expand_ipv6_addr

syntax: expanded = resty.dns.resolver.expand_ipv6_addr(address)

Expands the successive 16-bit zero groups in the textual format of the IPv6 address.

For example,

    local resolver = require "resty.dns.resolver"
    local expand = resolver.expand_ipv6_addr
    local new_addr = expand("FF01::101")

will yield FF01:0:0:0:0:0:0:101 in the new_addr return value.

arpa_str

syntax: arpa_record = resty.dns.resolver.arpa_str(address)

Generates the reverse domain name for PTR lookups for both IPv4 and IPv6 addresses. Compressed IPv6 addresses will be automatically expanded.

For example,

    local resolver = require "resty.dns.resolver"
    local ptr4 = resolver.arpa_str("1.2.3.4")
    local ptr6 = resolver.arpa_str("FF01::101")

will yield 4.3.2.1.in-addr.arpa for ptr4 and 1.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.F.F.ip6.arpa for ptr6.

reverse_query

syntax: answers, err = r:reverse_query(address)

Performs a PTR lookup for both IPv4 and IPv6 addresses. This function is basically a wrapper for the query command which uses the arpa_str command to convert the IP address on the fly.

Constants

TYPE_A

The A resource record type, equal to the decimal number 1.

TYPE_NS

The NS resource record type, equal to the decimal number 2.

TYPE_CNAME

The CNAME resource record type, equal to the decimal number 5.

TYPE_SOA

The SOA resource record type, equal to the decimal number 6.

TYPE_PTR

The PTR resource record type, equal to the decimal number 12.

TYPE_MX

The MX resource record type, equal to the decimal number 15.

TYPE_TXT

The TXT resource record type, equal to the decimal number 16.

TYPE_AAAA

syntax: typ = r.TYPE_AAAA

The AAAA resource record type, equal to the decimal number 28.

TYPE_SRV

syntax: typ = r.TYPE_SRV

The SRV resource record type, equal to the decimal number 33.

See RFC 2782 for details.

TYPE_SPF

syntax: typ = r.TYPE_SPF

The SPF resource record type, equal to the decimal number 99.

See RFC 4408 for details.

CLASS_IN

syntax: class = r.CLASS_IN

The Internet resource record type, equal to the decimal number 1.

SECTION_AN

syntax: stype = r.SECTION_AN

Identifier of the Answer section in the DNS response. Equal to decimal number 1.

SECTION_NS

syntax: stype = r.SECTION_NS

Identifier of the Authority section in the DNS response. Equal to the decimal number 2.

SECTION_AR

syntax: stype = r.SECTION_AR

Identifier of the Additional section in the DNS response. Equal to the decimal number 3.

Automatic Error Logging

By default, the underlying ngx_lua module does error logging when socket errors happen. If you are already doing proper error handling in your own Lua code, then you are recommended to disable this automatic error logging by turning off ngx_lua's lua_socket_log_errors directive, that is,

    lua_socket_log_errors off;

Limitations

  • This library cannot be used in code contexts like set_by_lua*, log_by_lua*, and header_filter_by_lua* where the ngx_lua cosocket API is not available.
  • The resty.dns.resolver object instance cannot be stored in a Lua variable at the Lua module level, because it will then be shared by all the concurrent requests handled by the same nginx worker process (see https://github.com/openresty/lua-nginx-module/#data-sharing-within-an-nginx-worker ) and result in bad race conditions when concurrent requests are trying to use the same resty.dns.resolver instance. You should always initiate resty.dns.resolver objects in function local variables or in the ngx.ctx table. These places all have their own data copies for each request.

See Also

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-dns.