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/viki-org/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("api.viki.io")

//get the first net.IP
ip, _ := resolver.FetchOne("api.viki.io")

//get the first net.IP as string
ip, _ := resolver.FetchOneString("api.viki.io")

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) {
    separator := strings.LastIndex(address, ":")
    ip, _ := dnscache.FetchString(address[:separator])
    return net.Dial("tcp", ip + address[separator:])
  },
}