Function dialWithDNSCache

internal/exit/dnscache.go:72–124

dialWithDNSCache resolves host:port through the cache, then dials the underlying TCP connection via baseDial. Falls through to baseDial directly when the address is already a literal IP or unparseable.

(
	cache *dnsCache,
	baseDial func(network, address string, timeout time.Duration) (net.Conn, error),
	network, address string,
	timeout time.Duration,
)

// underlying TCP connection via baseDial. Falls through to baseDial directly
// when the address is already a literal IP or unparseable.
func dialWithDNSCache(
	cache *dnsCache,
	baseDial func(network, address string, timeout time.Duration) (net.Conn, error),
	network, address string,
	timeout time.Duration,
) (*dialResult, error) {
	host, port, err := net.SplitHostPort(address)
	if err != nil || net.ParseIP(host) != nil {
		// Literal IP or malformed — let baseDial handle it.
		tcpStart := time.Now()
		conn, derr := baseDial(network, address, timeout)
		if derr != nil {
			return nil, derr
		}
		return &dialResult{Conn: conn, TCP: time.Since(tcpStart)}, nil
	}
	if ip := cache.get(host); ip != "" {
		tcpStart := time.Now()
		conn, derr := baseDial(network, net.JoinHostPort(ip, port), timeout)
		tcpElapsed := time.Since(tcpStart)
		if derr != nil {
			// Cached IP failed; evict so the next call re-resolves.
			cache.forget(host)
			return nil, derr
		}
		return &dialResult{Conn: conn, DNSCached: true, TCP: tcpElapsed}, nil
	}
	// Cache miss: resolve, then dial. Use a context bounded by `timeout`
	// so a slow resolver cannot eat the entire dial budget.
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()
	dnsStart := time.Now()
	addrs, lerr := net.DefaultResolver.LookupIPAddr(ctx, host)
	dnsElapsed := time.Since(dnsStart)
	if lerr != nil || len(addrs) == 0 {
		// Fall through to baseDial which will surface the same/similar error.
		tcpStart := time.Now()
		conn, derr := baseDial(network, address, timeout)
		if derr != nil {
			return nil, derr
		}
		return &dialResult{Conn: conn, DNS: dnsElapsed, TCP: time.Since(tcpStart)}, nil
	}
	ip := addrs[0].IP.String()
	cache.set(host, ip)
	tcpStart := time.Now()
	conn, derr := baseDial(network, net.JoinHostPort(ip, port), timeout)
	tcpElapsed := time.Since(tcpStart)
	if derr != nil {
		return nil, derr
	}
	return &dialResult{Conn: conn, DNS: dnsElapsed, TCP: tcpElapsed}, nil
}