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Name

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

Status

This library is already usable but is still considered experimental.

The API is still in flux and will change without notice.

Description

This Lua library provies a DSN resolver for the ngx_lua nginx module:

http://wiki.nginx.org/HttpLuaModule

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 ngx_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.

Synopsis

lua_package_path "/path/to/lua-resty-dns/lib/?.lua;;";

server {
    location = /dns {
        content_by_lua '
            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
            }

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

            local answers, err = r:query("www.google.com")
            if not answers then
                ngx.say("failed to query the DNS server: ", err)
                return
            end

            for i = 1, #answers do
                local ans = answers[i]
                ngx.say(ans.name, " ", ans.address or ans.cname,
                        " type:", ans.type, " class:", ans.class,
                        " ttl:", ans.ttl)
            end
        ';
    }
}

Methods

new

syntax: r, err = resty.dns.resolver:new(opts)

Creates a dns.resolver object. Returns nil and an 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. Default 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 respond for a single attempt of request transmition. 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. Default to false.

query

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

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.

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. Secussesive 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.
  • 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.
  • 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).

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.

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_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.

CLASS_IN

syntax: class = r.CLASS_IN

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

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 http://wiki.nginx.org/HttpLuaModule#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.

TODO

  • Concurrent (or parallel) query mode
  • Better support for other resource record types like SPF.

Author

Yichun "agentzh" Zhang (章亦春) agentzh@gmail.com

Copyright and License

This module is licensed under the BSD license.

Copyright (C) 2012, by Yichun "agentzh" Zhang (章亦春) agentzh@gmail.com.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

See Also

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