When attempting to create a DNS record using an Infoblox DNS smart proxy, the response returned does not contain a body (the below from a Packet Capture of the RestClient from the Foreman and the SmartProxy webserver):

Request headers

POST /dns/ HTTP/1.1
Accept: application/json
Accept-Encoding: gzip, deflate
User-Agent: rest-client/2.0.1 (linux-gnu x86_64) ruby/2.2.2p95
X-Request-Id: ca7db6e7fe4d5a1a729d9ee28e1a8cbae66cdf7571e3e03b7ec758bd98a9c9b8
Content-Length: 59
Content-Type: application/x-www-form-urlencoded
Host: foreman.example.com:8000

fqdn=juana-porraz.example.com&value=192.168.1.189&type=A

Response headers

HTTP/1.1 200 OK
Content-Type: text/html;charset=utf-8
Content-Length: 0
X-Xss-Protection: 1; mode=block
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
Server:
Date: Wed, 24 Jan 2018 17:24:48 GMT
Connection: Keep-Alive

This appears to be the expected behavior. On the [https://github.com/theforeman/smart_proxy_dns_infoblox/blob/master/lib/smart_proxy_dns_infoblox/dns_infoblox_main.rb smart proxy side], it looks like the results in the body of the response are probably just relayed to Foreman:

    def ib_create(clazz, params)
      clazz.new({ :connection => connection }.merge(params)).post
    end

This would then be relayed to Foreman which [https://github.com/theforeman/foreman/blob/698e916ce208b5040b83a908a058c83c94d158ee/lib/proxy_api/dns.rb will attempt to parse] it:

    # Sets a DNS entry
    # [+fqdn+] : String containing the FQDN of the host
    # [+args+] : Hash containing :value and :type: The :fqdn key is taken from the fqdn parameter
    # Returns  : Boolean status
    def set(args)
      parse post(args, "")
    rescue => e
      raise ProxyException.new(url, e, N_("Unable to set DNS entry"))
    end

And it will attempt to parse it [using JSON.parse]:

    # Decodes the JSON response if no HTTP error has been detected
    # If an HTTP error is received then the error message is saves into @error
    # Returns: Response, if the operation is GET, or true for POST, PUT and DELETE.
    #      OR: false if a HTTP error is detected
    # TODO: add error message handling
    def parse(response)
      if response && response.code >= 200 && response.code < 300
        return response.body.present? ? JSON.parse(response.body) : true
      else
        false
      end
    rescue => e
      logger.warn "Failed to parse response: #{response} -> #{e}" 
      false
    end

In the case of a content-length of 0, this will cause JSON.parse to attempt to parse an empty string and [https://stackoverflow.com/questions/30621802/why-does-json-parse-fail-with-the-empty-string throw an error].

According to the RFC for JSON (RFC7231 https://tools.ietf.org/html/rfc7231#section-6.3.1) a 0 content-length is a perfectly valid JSON response however:

Aside from responses to CONNECT, a 200 response always has a payload, 
though an origin server MAY generate a payload body of zero length.

To prevent this condition from causing a failure, we need to check for a content-length of 0 and skip the parsing.