net/dns: retry forwarder requests over TCP

We weren't correctly retrying truncated requests to an upstream DNS
server with TCP. Instead, we'd return a truncated request to the user,
even if the user was querying us over TCP and thus able to handle a
large response.

Also, add an envknob and controlknob to allow users/us to disable this
behaviour if it turns out to be buggy (✨ DNS ✨).

Updates #9264

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ifb04b563839a9614c0ba03e9c564e8924c1a2bfd

wgengine/netstack/netstack.go

@@ -1086,7 +1086,7 @@ func (ns *Impl) handleMagicDNSUDP(srcAddr netip.AddrPort, c *gonet.UDPConn) {
 			}
 			return
 		}
-		resp, err := ns.dns.Query(context.Background(), q[:n], srcAddr)
+		resp, err := ns.dns.Query(context.Background(), q[:n], "udp", srcAddr)
 		if err != nil {
 			ns.logf("dns udp query: %v", err)
 			return

wgengine/userspace.go

@@ -304,7 +304,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 	tunName, _ := conf.Tun.Name()
 	conf.Dialer.SetTUNName(tunName)
 	conf.Dialer.SetNetMon(e.netMon)
-	e.dns = dns.NewManager(logf, conf.DNS, e.netMon, conf.Dialer, fwdDNSLinkSelector{e, tunName})
+	e.dns = dns.NewManager(logf, conf.DNS, e.netMon, conf.Dialer, fwdDNSLinkSelector{e, tunName}, conf.ControlKnobs)
 
 	// TODO: there's probably a better place for this
 	sockstats.SetNetMon(e.netMon)