Read Me About CFHostSample
CFHostSample shows how to use the CFHost API to do asynchronous DNS name-to-address and address-to-name translation.
CFHostSample requires Mac OS X 10.7 or later. Additionally, the core code works unmodified on iOS 5.0 or later.
Note: The core concepts should work on any version of iOS and Mac OS X 10.3 or later (where CFHost was introduced), but supporting older platforms will require some tweaks (for example, switching from "weak" references to "unsafe_unretained" references, adding explicit ivars to support the old runtime, and so on).
Before You Start
CFHost is only necessary if you have specialist DNS needs, for example, if you want to map all of the addresses in a log file to their host names. If you just want to connect to a DNS name, it's both easier and better to use a connect-by-name API, that is, an API that takes a DNS name and does the resolution on your behalf. For example:
o +[NSHost +getStreamsToHost:port:inputStream:outputStream:] on Mac OS X
o CFStreamCreatePairWithSocketToHost on iOS, as shown in QA1652 "Using NSStreams For A TCP Connection Without NSHost"
The sample contains the following items:
o Read Me About CFHostSample.txt -- This document.
o CFHostSample.xcodeproj -- An Xcode project to build the code.
o CFHostSample -- A directory containing the code.
o build -- A directory containing a pre-built binary.
Within the "CFHostSample" directory you'll find:
o main.m -- A command line tool that exercises the core code.
o QCFHostResolver.[hm] -- The core code.
Using the Sample
The sample supports two functions. To test name-to-address mapping, run the tool with the "-h" option:
$ build/Debug/CFHostSample -h apple.com
apple.com -> 18.104.22.168, 22.214.171.124
To test address-to-name mapping, run the tool with the "-a" option:
$ build/Debug/CFHostSample -a 126.96.36.199
188.8.131.52 -> apple.com
Building the Sample
The sample was built using Xcode 4.3.1 on Mac OS X 10.7.3 with the Mac OS X 10.7 SDK. You should be able to just open the project and choose Build from the Product menu.
How It Works
The main program is a simple command line tool that accepts parameters, creates QCFHostResolver objects, runs them, and prints the results.
QCFHostResolver is an Objective-C wrapper around CFHost; read the comments in "QCFHostResolver.h" for more information on how to use it, and the comments in "QCFHostResolver.m" for more information on how it works.
QCFHostResolver builds and works on iOS 5.0 and later.
The previous version of the sample let you do reachability tests using kCFHostReachability. This is a rarely used feature of CFHost, and something generally better done with the SCNetworkReachability API, and so it's been removed from the sample.
The bug referenced by the previous version of this sample, <rdar://problem/3612320>, which caused CFHost to fail if you did a reachability query using an address rather than a name, was fixed in Mac OS X 10.4.
Credits and Version History
If you find any problems with this sample, please file a bug against it.
1.0 (Apr 2004) was the first shipping version.
2.0 (Mar 2012) was a major rewrite to use the latest tools and techniques.
Share and Enjoy
Apple Developer Technical Support
23 Mar 2012