Restore support for "Override local DNS"

Tailscale allows to override the local DNS settings of a node via
"Override local DNS" [1]. Restore this flag with the same config setting
name `dns.override_local_dns` but disable it by default to align it with
Tailscale's default behaviour.

Tested with Tailscale 1.80.2 and systemd-resolved on Debian 12.

With `dns.override_local_dns: false`:

```
Link 12 (tailscale0)
Current Scopes: DNS
     Protocols: -DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 100.100.100.100
    DNS Domain: tn.example.com ~0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa [snip]
```

With `dns.override_local_dns: true`:

```
Link 12 (tailscale0)
Current Scopes: DNS
     Protocols: +DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 100.100.100.100
    DNS Domain: tn.example.com ~.
```

[1] https://tailscale.com/kb/1054/dns#override-local-dns

Fixes: juanfont#2256

CHANGELOG.md

@@ -43,6 +43,8 @@ Note that if an exit route is approved (0.0.0.0/0 or ::/0), both IPv4 and IPv6 w
   - It is now possible to inspect running goroutines and take profiles
   - View of config, policy, filter, ssh policy per node, connected nodes and
     DERPmap
+- Restore support for "Override local DNS"
+  [#2438](https://github.com/juanfont/headscale/pull/2438)
 
 ## 0.25.1 (2025-02-25)
 

config-example.yaml

@@ -274,6 +274,10 @@ dns:
   # `hostname.base_domain` (e.g., _myhost.example.com_).
   base_domain: example.com
 
+  # Whether to use the local DNS settings of a node (default) or override the
+  # local DNS settings and force the use of Headscale's DNS configuration.
+  override_local_dns: false
+
   # List of DNS servers to expose to clients.
   nameservers:
     global:

hscontrol/types/config.go

@@ -102,6 +102,7 @@ type Config struct {
 type DNSConfig struct {
 	MagicDNS         bool   `mapstructure:"magic_dns"`
 	BaseDomain       string `mapstructure:"base_domain"`
+	OverrideLocalDNS bool   `mapstructure:"override_local_dns"`
 	Nameservers      Nameservers
 	SearchDomains    []string            `mapstructure:"search_domains"`
 	ExtraRecords     []tailcfg.DNSRecord `mapstructure:"extra_records"`
@@ -287,6 +288,7 @@ func LoadConfig(path string, isFile bool) error {
 
 	viper.SetDefault("dns.magic_dns", true)
 	viper.SetDefault("dns.base_domain", "")
+	viper.SetDefault("dns.override_local_dns", false)
 	viper.SetDefault("dns.nameservers.global", []string{})
 	viper.SetDefault("dns.nameservers.split", map[string]string{})
 	viper.SetDefault("dns.search_domains", []string{})
@@ -351,9 +353,9 @@ func validateServerConfig() error {
 	depr.fatalIfNewKeyIsNotUsed("policy.path", "acl_policy_path")
 
 	// Move dns_config -> dns
-	depr.warn("dns_config.override_local_dns")
 	depr.fatalIfNewKeyIsNotUsed("dns.magic_dns", "dns_config.magic_dns")
 	depr.fatalIfNewKeyIsNotUsed("dns.base_domain", "dns_config.base_domain")
+	depr.fatalIfNewKeyIsNotUsed("dns.override_local_dns", "dns_config.override_local_dns")
 	depr.fatalIfNewKeyIsNotUsed("dns.nameservers.global", "dns_config.nameservers")
 	depr.fatalIfNewKeyIsNotUsed("dns.nameservers.split", "dns_config.restricted_nameservers")
 	depr.fatalIfNewKeyIsNotUsed("dns.search_domains", "dns_config.domains")
@@ -616,6 +618,7 @@ func dns() (DNSConfig, error) {
 
 	dns.MagicDNS = viper.GetBool("dns.magic_dns")
 	dns.BaseDomain = viper.GetString("dns.base_domain")
+	dns.OverrideLocalDNS = viper.GetBool("dns.override_local_dns")
 	dns.Nameservers.Global = viper.GetStringSlice("dns.nameservers.global")
 	dns.Nameservers.Split = viper.GetStringMapStringSlice("dns.nameservers.split")
 	dns.SearchDomains = viper.GetStringSlice("dns.search_domains")
@@ -721,7 +724,11 @@ func dnsToTailcfgDNS(dns DNSConfig) *tailcfg.DNSConfig {
 
 	cfg.Proxied = dns.MagicDNS
 	cfg.ExtraRecords = dns.ExtraRecords
-	cfg.Resolvers = dns.globalResolvers()
+	if dns.OverrideLocalDNS {
+		cfg.Resolvers = dns.globalResolvers()
+	} else {
+		cfg.FallbackResolvers = dns.globalResolvers()
+	}
 
 	routes := dns.splitResolvers()
 	cfg.Routes = routes

hscontrol/types/config_test.go

@@ -36,6 +36,7 @@ func TestReadConfig(t *testing.T) {
 			want: DNSConfig{
 				MagicDNS:   true,
 				BaseDomain: "example.com",
+				OverrideLocalDNS: false,
 				Nameservers: Nameservers{
 					Global: []string{
 						"1.1.1.1",
@@ -70,7 +71,7 @@ func TestReadConfig(t *testing.T) {
 			want: &tailcfg.DNSConfig{
 				Proxied: true,
 				Domains: []string{"example.com", "test.com", "bar.com"},
-				Resolvers: []*dnstype.Resolver{
+				FallbackResolvers: []*dnstype.Resolver{
 					{Addr: "1.1.1.1"},
 					{Addr: "1.0.0.1"},
 					{Addr: "2606:4700:4700::1111"},
@@ -101,6 +102,7 @@ func TestReadConfig(t *testing.T) {
 			want: DNSConfig{
 				MagicDNS:   false,
 				BaseDomain: "example.com",
+				OverrideLocalDNS: false,
 				Nameservers: Nameservers{
 					Global: []string{
 						"1.1.1.1",
@@ -135,7 +137,7 @@ func TestReadConfig(t *testing.T) {
 			want: &tailcfg.DNSConfig{
 				Proxied: false,
 				Domains: []string{"example.com", "test.com", "bar.com"},
-				Resolvers: []*dnstype.Resolver{
+				FallbackResolvers: []*dnstype.Resolver{
 					{Addr: "1.1.1.1"},
 					{Addr: "1.0.0.1"},
 					{Addr: "2606:4700:4700::1111"},
@@ -254,6 +256,7 @@ func TestReadConfigFromEnv(t *testing.T) {
 			configEnv: map[string]string{
 				"HEADSCALE_DNS_MAGIC_DNS":          "true",
 				"HEADSCALE_DNS_BASE_DOMAIN":        "example.com",
+				"HEADSCALE_DNS_OVERRIDE_LOCAL_DNS": "false",
 				"HEADSCALE_DNS_NAMESERVERS_GLOBAL": `1.1.1.1 8.8.8.8`,
 				"HEADSCALE_DNS_SEARCH_DOMAINS":     "test.com bar.com",
 
@@ -274,6 +277,7 @@ func TestReadConfigFromEnv(t *testing.T) {
 			want: DNSConfig{
 				MagicDNS:   true,
 				BaseDomain: "example.com",
+				OverrideLocalDNS: false,
 				Nameservers: Nameservers{
 					Global: []string{"1.1.1.1", "8.8.8.8"},
 					Split:  map[string][]string{

hscontrol/types/testdata/dns_full.yaml

@@ -7,6 +7,7 @@ dns:
   magic_dns: true
   base_domain: example.com
 
+  override_local_dns: false
   nameservers:
     global:
       - 1.1.1.1

hscontrol/types/testdata/dns_full_no_magic.yaml

@@ -7,6 +7,7 @@ dns:
   magic_dns: false
   base_domain: example.com
 
+  override_local_dns: false
   nameservers:
     global:
       - 1.1.1.1

integration/hsic/config.go

@@ -23,6 +23,7 @@ func DefaultConfigEnv() map[string]string {
 		"HEADSCALE_PREFIXES_V6":                       "fd7a:115c:a1e0::/48",
 		"HEADSCALE_DNS_BASE_DOMAIN":                   "headscale.net",
 		"HEADSCALE_DNS_MAGIC_DNS":                     "true",
+		"HEADSCALE_DNS_OVERRIDE_LOCAL_DNS":            "false",
 		"HEADSCALE_DNS_NAMESERVERS_GLOBAL":            "127.0.0.11 1.1.1.1",
 		"HEADSCALE_PRIVATE_KEY_PATH":                  "/tmp/private.key",
 		"HEADSCALE_NOISE_PRIVATE_KEY_PATH":            "/tmp/noise_private.key",