The dnscache package provides a DNS cache layer to Go's net.Resolver.
Install using the "go get" command:
go get -u github.com/rs/dnscache
Create a new instance and use it in place of net.Resolver. New names will be cached. Call the Refresh method at regular interval to update cached entries and cleanup unused ones.
resolver := &dnscache.Resolver{}
// First call will cache the result
addrs, err := resolver.LookupHost(context.Background(), "example.com")
// Subsequent calls will use the cached result
addrs, err = resolver.LookupHost(context.Background(), "example.com")
// Call to refresh will refresh names in cache. If you pass true, it will also
// remove cached names not looked up since the last call to Refresh. It is a good idea
// to call this method on a regular interval.
go func() {
t := time.NewTicker(5 * time.Minute)
defer t.Stop()
for range t.C {
resolver.Refresh(true)
}
}()If you are using an http.Transport, you can use this cache by specifying a DialContext function:
r := &dnscache.Resolver{}
t := &http.Transport{
DialContext: func(ctx context.Context, network string, addr string) (conn net.Conn, err error) {
host, port, err := net.SplitHostPort(addr)
if err != nil {
return nil, err
}
ips, err := r.LookupHost(ctx, host)
if err != nil {
return nil, err
}
for _, ip := range ips {
var dialer net.Dialer
conn, err = dialer.DialContext(ctx, network, net.JoinHostPort(ip, port))
if err == nil {
break
}
}
return
},
}In addition to the Refresh method, you can RefreshWithOptions. This method adds an option to persist resource records
on failed lookups
r := &Resolver{}
options := dnscache.ResolverRefreshOptions{}
options.ClearUnused = true
options.PersistOnFailure = false
resolver.RefreshWithOptions(options)