net/dns/resolver: log source IP of forwarded queries

When the TS_DEBUG_DNS_FORWARD_SEND envknob is turned on, also log the
source IP:port of the query that tailscaled is forwarding.

Updates tailscale/corp#35374

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

net/dns/resolver/forwarder.go

@@ -526,9 +526,9 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
 	if f.verboseFwd {
 		id := forwarderCount.Add(1)
 		domain, typ, _ := nameFromQuery(fq.packet)
-		f.logf("forwarder.send(%q, %d, %v, %d) [%d] ...", rr.name.Addr, fq.txid, typ, len(domain), id)
+		f.logf("forwarder.send(%q, %d, %v, %d) from %v [%d] ...", rr.name.Addr, fq.txid, typ, len(domain), fq.src, id)
 		defer func() {
-			f.logf("forwarder.send(%q, %d, %v, %d) [%d] = %v, %v", rr.name.Addr, fq.txid, typ, len(domain), id, len(ret), err)
+			f.logf("forwarder.send(%q, %d, %v, %d) from %v [%d] = %v, %v", rr.name.Addr, fq.txid, typ, len(domain), fq.src, id, len(ret), err)
 		}()
 	}
 	if strings.HasPrefix(rr.name.Addr, "http://") {
@@ -904,6 +904,7 @@ type forwardQuery struct {
 	txid   txid
 	packet []byte
 	family string // "tcp" or "udp"
+	src    netip.AddrPort
 
 	// closeOnCtxDone lets send register values to Close if the
 	// caller's ctx expires. This avoids send from allocating its
@@ -988,6 +989,7 @@ func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, respo
 		txid:           getTxID(query.bs),
 		packet:         query.bs,
 		family:         query.family,
+		src:            query.addr,
 		closeOnCtxDone: new(closePool),
 	}
 	defer fq.closeOnCtxDone.Close()