// exchange sends a query on the connection and hopes for a response.
func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, useTCP, ad bool) (dnsmessage.Parser, dnsmessage.Header, error) {
	q.Class = dnsmessage.ClassINET
	id, udpReq, tcpReq, err := newRequest(q, ad)
	if err != nil {
		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotMarshalDNSMessage
	}
	var networks []string
	if useTCP {
		networks = []string{"tcp"}
	} else {
		networks = []string{"udp", "tcp"}
	}
	for _, network := range networks {
		ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
		defer cancel()

		c, err := r.dial(ctx, network, server)
		if err != nil {
			return dnsmessage.Parser{}, dnsmessage.Header{}, err
		}
		if d, ok := ctx.Deadline(); ok && !d.IsZero() {
			c.SetDeadline(d)
		}
		var p dnsmessage.Parser
		var h dnsmessage.Header
		if _, ok := c.(PacketConn); ok {
			p, h, err = dnsPacketRoundTrip(c, id, q, udpReq)
		} else {
			p, h, err = dnsStreamRoundTrip(c, id, q, tcpReq)
		}
		c.Close()
		if err != nil {
			return dnsmessage.Parser{}, dnsmessage.Header{}, mapErr(err)
		}
		if err := p.SkipQuestion(); err != dnsmessage.ErrSectionDone {
			return dnsmessage.Parser{}, dnsmessage.Header{}, errInvalidDNSResponse
		}
		// RFC 5966 indicates that when a client receives a UDP response with
		// the TC flag set, it should take the TC flag as an indication that it
		// should retry over TCP instead.
		// The case when the TC flag is set in a TCP response is not well specified,
		// so this implements the glibc resolver behavior, returning the existing
		// dns response instead of returning a "errNoAnswerFromDNSServer" error.
		// See go.dev/issue/64896
		if h.Truncated && network == "udp" {
			continue
		}
		return p, h, nil
	}
	return dnsmessage.Parser{}, dnsmessage.Header{}, errNoAnswerFromDNSServer
}
Échauffement
