Lua resolve hostname with resty.dns.resolver

gistfile1.txt

function resolve_hostname(hostname)
  local dns_resolver_lib = require("resty.dns.resolver")

  local dns_resolver, dns_resolver_err = dns_resolver_lib:new({
    -- Kubernetes default nameserver IP
    -- If your hostnames are public, try Google's DNS nameserver 8.8.8.8
    nameservers = {"kube-dns.kube-system.svc.cluster.local"},
    -- 250 milliseconds
    timeout = 250,
    retrans = 2
  })

  if not dns_resolver then
    ngx.log(
      ngx.ERR,
      "Failed to instantiate lua-resty-dns-resolver",
      dns_resolver_err
    )
    return nil
  end

  local dns_answers, dns_err, dns_tries = dns_resolver:query(hostname, nil, {})
  if not dns_answers then
    ngx.log(
      ngx.ERR,
      "Failed to query DNS server: ",
      dns_err,
      table.concat(dns_tries, "\n  ")
    )
    return nil
  end

  if dns_answers.errcode then
    ngx.log(
      ngx.ERR,
      "DNS server returned error code ",
      dns_answers.errcode,
      ": ",
      dns_answers.errstr
    )
    return nil
  end
  
  for i, answer in ipairs(dns_answers) do
    local ip = answer.address or answer.cname
    if ip ~= nil then
      return ip
    end
  end
  return nil
end