From 4d5b7e9c14a929814fd4f9a20e0d78253e33aab1 Mon Sep 17 00:00:00 2001
From: Kristoffer Dalby <kristoffer@tailscale.com>
Date: Wed, 6 Aug 2025 08:41:43 +0200
Subject: [PATCH] derp: allow override to ip for debug

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
---
 hscontrol/derp/server/derp_server.go | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/hscontrol/derp/server/derp_server.go b/hscontrol/derp/server/derp_server.go
index fee395f1..7ced0ba7 100644
--- a/hscontrol/derp/server/derp_server.go
+++ b/hscontrol/derp/server/derp_server.go
@@ -20,6 +20,7 @@ import (
 	"github.com/juanfont/headscale/hscontrol/util"
 	"github.com/rs/zerolog/log"
 	"tailscale.com/derp"
+	"tailscale.com/envknob"
 	"tailscale.com/net/stun"
 	"tailscale.com/net/wsconn"
 	"tailscale.com/tailcfg"
@@ -35,6 +36,11 @@ const (
 	DerpVerifyScheme = "headscale-derp-verify"
 )
 
+// debugUseDERPIP is a debug-only flag that causes the DERP server to resolve
+// hostnames to IP addresses when generating the DERP region configuration.
+// This is useful for integration testing where DNS resolution may be unreliable.
+var debugUseDERPIP = envknob.Bool("HEADSCALE_DEBUG_DERP_USE_IP")
+
 type DERPServer struct {
 	serverURL     string
 	key           key.NodePrivate
@@ -70,7 +76,10 @@ func (d *DERPServer) GenerateRegion() (tailcfg.DERPRegion, error) {
 	}
 	var host string
 	var port int
-	host, portStr, err := net.SplitHostPort(serverURL.Host)
+	var portStr string
+	
+	// Extract hostname and port from URL
+	host, portStr, err = net.SplitHostPort(serverURL.Host)
 	if err != nil {
 		if serverURL.Scheme == "https" {
 			host = serverURL.Host
@@ -85,6 +94,19 @@ func (d *DERPServer) GenerateRegion() (tailcfg.DERPRegion, error) {
 			return tailcfg.DERPRegion{}, err
 		}
 	}
+	
+	// If debug flag is set, resolve hostname to IP address
+	if debugUseDERPIP {
+		ips, err := net.LookupIP(host)
+		if err != nil {
+			log.Error().Caller().Err(err).Msgf("Failed to resolve DERP hostname %s to IP, using hostname", host)
+		} else if len(ips) > 0 {
+			// Use the first IP address
+			ipStr := ips[0].String()
+			log.Info().Caller().Msgf("HEADSCALE_DEBUG_DERP_USE_IP: Resolved %s to %s", host, ipStr)
+			host = ipStr
+		}
+	}
 
 	localDERPregion := tailcfg.DERPRegion{
 		RegionID:   d.cfg.ServerRegionID,