Next Previous

Resolving DNS Hostnames

This article explains how to resolve a DNS hostname in a way that offers you maximum flexibility.

Important: Most high-level OS X and iOS APIs allow you to connect to a host by its DNS name, and, as a rule, you should do so. A hostname can map onto multiple IP addresses simultaneously. If you resolve the hostname yourself, you must choose which IP address to use when connecting to the remote host. By contrast, when you connect by name, you allow the operating system to choose the best way of connecting on your behalf. This is particularly true for Bonjour services because the IP address can change at any time.

However, connecting by name is not always possible. If you must use APIs that require an IP address, OS X and iOS provide a number of ways to obtain one or more addresses for a DNS hostname. This appendix describes those techniques.

Before you read this appendix, read Avoid Resolving DNS Names Before Connecting to a Host to learn why you probably should not do the things described below.

There are three primary APIs in OS X and iOS for resolving hostnames: NSHost (only in OS X), CFHost, and the POSIX resolver API.

NSHost—Although passing NSHost is a common way to pass hostnames to other APIs, using NSHost to resolve addresses yourself is generally discouraged because it is a synchronous API. Thus, using it on the main thread can cause serious performance degradation. Because this API is discouraged, its use is not described here.
CFHost—The CFHost API allows you to perform resolution asynchronously, and is the preferred way to resolve hostnames if you must resolve them yourself.
POSIX—The POSIX layer provides several functions for resolving hostnames. These functions should be used only if you are writing portable code that must be shared with non-Apple platforms or if you are integrating your code with existing POSIX networking code.

Resolving Hostnames with CFHost

To resolve a host with CFHost:

Create a CFHostRef object by calling CFHostCreateWithName.
Call CFHostSetClient and provide the context object of your choice and a callback function that will be called when resolution completes.
Call CFHostScheduleWithRunLoop to schedule the resolver on your run loop.
Call CFHostStartInfoResolution to tell the resolver to start resolving, passing kCFHostAddresses as the second parameter to indicate that you want it to return IP addresses.
Wait for the resolver to call your callback. Within your callback, obtain the results by calling CFHostGetAddressing. This function returns an array of CFDataRef objects, each of which contains a POSIX sockaddr structure.

The process for reverse name resolution (translating an IP address into a hostname) is similar, except that you call CFHostCreateWithAddress to create the object, pass kCFHostNames to CFHostStartInfoResolution, and call CFHostGetNames to retrieve the results.

Resolving Hostnames with POSIX Calls

If you intend to use POSIX calls to resolve hostnames, be aware that these calls are synchronous and should not be used on the main thread in a GUI app. Instead, you should either create a separate POSIX thread or a GCD task and perform these calls within that context.

POSIX defines three functions in <netdb.h> for resolving hostnames:

getaddrinfo

Returns all the resolved addresses for a given hostname. This function is the preferred way to obtain address information at the POSIX level. You can find sample code in its man page (linked above).

Important: Some older DNS servers do not reply to IPv6 lookup requests with an error. The POSIX getaddrinfo function attempts to hide this misbehavior by canceling outstanding IPv6 queries shortly after receiving a successful IPv4 reply. If your app would benefit from continuing to receive IPv6 addresses until you connect successfully (rather than stopping as soon as you have an IPv4 address that might or might not work), then you should use an asynchronous API such as CFHost.

gethostbyname

Returns a single IPv4 address for a given hostname. This function is discouraged for new development because it is limited to IPv4 addresses.

gethostbyname2

Returns a single address for a given hostname in the specified address family (AF_INET, AF_INET6, and so on).

Although this function allows you to get around the IPv4 limitations of gethostbyname, it still limits your ability to try multiple addresses at once and choose the fastest one. Thus, this function is primarily intended as a nearly drop-in replacement for gethostbyname in existing code. The getaddrinfo is preferred for use in new code.

For reverse name resolution (translating an IP address into a hostname), POSIX provides getnameinfo and gethostbyaddr. The getnameinfo function is preferred because it is more flexible.

To learn more about these functions, read their respective man pages, linked above.

Next Previous