net/dns/resolver: permit larger max responses, signal truncation

This raises the maximum DNS response message size from 512 to 4095. This
should be large enough for almost all situations that do not need TCP.
We still do not recognize EDNS, so we will still forward requests that
claim support for a larger response size than 4095 (that will be solved
later). For now, when a response comes back that is too large to fit in
our receive buffer, we now set the truncation flag in the DNS header,
which is an improvement from before but will prompt attempts to use TCP
which isn't supported yet.

On Windows, WSARecvFrom into a buffer that's too small returns an error
in addition to the data. On other OSes, the extra data is silently
discarded. In this case, we prefer the latter so need to catch the error
on Windows.

Partially addresses #1123

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

net/dns/resolver/tsdns_server_test.go

@@ -66,6 +66,39 @@ func resolveToIP(ipv4, ipv6 netaddr.IP, ns string) dns.HandlerFunc {
 	}
 }
 
+// resolveToTXT returns a handler function which responds to queries of type TXT
+// it receives with the strings in txts.
+func resolveToTXT(txts []string) dns.HandlerFunc {
+	return func(w dns.ResponseWriter, req *dns.Msg) {
+		m := new(dns.Msg)
+		m.SetReply(req)
+
+		if len(req.Question) != 1 {
+			panic("not a single-question request")
+		}
+		question := req.Question[0]
+
+		if question.Qtype != dns.TypeTXT {
+			w.WriteMsg(m)
+			return
+		}
+
+		ans := &dns.TXT{
+			Hdr: dns.RR_Header{
+				Name:   question.Name,
+				Rrtype: dns.TypeTXT,
+				Class:  dns.ClassINET,
+			},
+			Txt: txts,
+		}
+
+		m.Answer = append(m.Answer, ans)
+		if err := w.WriteMsg(m); err != nil {
+			panic(err)
+		}
+	}
+}
+
 var resolveToNXDOMAIN = dns.HandlerFunc(func(w dns.ResponseWriter, req *dns.Msg) {
 	m := new(dns.Msg)
 	m.SetRcode(req, dns.RcodeNameError)