CGO is used to lookup domain names. Given enough concurrent requests and the slightest hiccup in name resolution, it's quite easy to end up with blocked/leaking goroutines.

The issue is documented at https://code.google.com/p/go/issues/detail?id=5625

The Go team's singleflight solution (which isn't in stable yet) is rather elegant. However, it only eliminates concurrent lookups (thundering herd problems). Many systems can live with slightly stale resolve names, which means we can cacne DNS lookups and refresh them in the background.

Install using the "go get" command:

go get github.com/karlseguin/dnscache

The cache is thread safe. Create a new instance by specifying how long each entry should be cached (in seconds). Items will be refreshed in the background.

//refresh items every 5 minutes
resolver := dnscache.New(time.Minute * 5)

//get an array of net.IP
ips, _ := resolver.Fetch("openmymind.io")

//get the one of the address's net.IP
ip, _ := resolver.FetchOne("openmymind.io")

//get the one of the address's ip as string
ip, _ := resolver.FetchOneString("openmymind.io")

The above methods can return either an IPv4 or IPv6 address. It's possible to restrict the result to an IPv4 address via FetchV4, FetchOneV4 and FetchOneV4String.

If you are using an http.Transport, you can use this cache by speficifying a Dial function:

transport := &http.Transport {
  MaxIdleConnsPerHost: 64,
  Dial: func(network string, address string) (net.Conn, error) {
    host, port, _ := net.SplitHostPort(address)
    ip, _ := runtime.Resolver.FetchOneString(host)
    return net.Dial(network, net.JoinHostPort(ip, port))
  },
}

By default, items are cached for the TTL specified when creating the cache object. This can be overwritten on a per-address basis via the TTL method:

resolver.TTL("algorithms.openmymind.net", time.Second * 30)

Note that unlike the other methods, TTL is not thread-safe.