I created a request using a domain name, e.g., http://www.google.com. But how could I get the exactly ip address which the framework used to connect to the server?
I knew that the gethostbyname method or the nslookup method could give us the address, but since the ip address is dynamic allocated, maybe the ip address I request that time is different with these methods returned.
So is there a way for me to get the real requested ip address? (I wanted to get the ip programmatically, rather than using tcpdump, etc.)
When you say "which the framework used" here, I assume you mean NSURLSession, NSURLConnection, UIWebView, or WKWebView. Each of those is a slightly different situation, but in all of them, the answer is that it's not possible directly (but see below; it's possible indirectly). You have no access to the underlying sockets that any of them use. All of them use connection pooling, which complicates things slightly even if you could get "the socket." And in the case of UIWebView and WKWebView, a single request to www.google.com may generate several independent connections, each of which could potentially interact with a different IP address.
(I'm a bit fascinated about what you're trying to do. Due to load balancing, a single IP address doesn't mean a single server, so IP addresses are only marginally more identifying than CNAMEs. Mixing in reverse proxies....)
If you need this kind of access, you have to manage the socket yourself. That's generally possible with all of the systems except WKWebView. I'll assume that you know already (or can easily study) how to create a socket and perform HTTP using CFSocket and CFHTTPMessage. This is extensively covered in the CFNetwork Programming Guide. If you've created the socket, you can use CFSocketCopyPeerAddress to check what host you really connected to. That's the piece you wanted.
Given you are able to create this kind of socket and manage it yourself, you can hook that into the major URL loaders (except WKWebView) using an NSURLProtocol. See Drop-in Offline Caching for UIWebView (and NSURLProtocol) for a quick introduction and some sample code. You just need to take the request and make it yourself with CFSocket, giving you the chance to see the exact port you're connected to.