net/netcheck: deflake (maybe) magicsock's TestNewConn

Updates #6207

Change-Id: I51d200d0b42b9a1ef799d0abfc8d4bd871c50cf2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck.go

@@ -19,6 +19,7 @@ import (
 	"net/netip"
 	"runtime"
 	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -1112,6 +1113,14 @@ func (c *Client) checkCaptivePortal(ctx context.Context, dm *tailcfg.DERPMap, pr
 	}
 
 	node := dm.Regions[preferredDERP].Nodes[0]
+
+	if strings.HasSuffix(node.HostName, tailcfg.DotInvalid) {
+		// Don't try to connect to invalid hostnames. This occurred in tests:
+		// https://github.com/tailscale/tailscale/issues/6207
+		// TODO(bradfitz,andrew-d): how to actually handle this nicely?
+		return false, nil
+	}
+
 	req, err := http.NewRequestWithContext(ctx, "GET", "http://"+node.HostName+"/generate_204", nil)
 	if err != nil {
 		return false, err