LookupIPAltResolverKey is a context.Context Value key used by tests to specify an alternate resolver func. It is not exposed to outsider users. (But see issue 12503) The value should be the same type as lookupIP:
func lookupIP(ctx context.Context, host string) ([]IPAddr, error)
type LookupIPAltResolverKey struct{}
Trace contains a set of hooks for tracing events within the net package. Any specific hook may be nil.
type Trace struct {
// DNSStart is called with the hostname of a DNS lookup
// before it begins.
DNSStart func(name string)
// DNSDone is called after a DNS lookup completes (or fails).
// The coalesced parameter is whether singleflight de-duped
// the call. The addrs are of type net.IPAddr but can't
// actually be for circular dependency reasons.
DNSDone func(netIPs []any, coalesced bool, err error)
// ConnectStart is called before a Dial, excluding Dials made
// during DNS lookups. In the case of DualStack (Happy Eyeballs)
// dialing, this may be called multiple times, from multiple
// goroutines.
ConnectStart func(network, addr string)
// ConnectDone is called after a Dial with the results, excluding
// Dials made during DNS lookups. It may also be called multiple
// times, like ConnectStart.
ConnectDone func(network, addr string, err error)
}
TraceKey is a context.Context Value key. Its associated value should be a *Trace struct.
type TraceKey struct{}