net/dns: add a Primary field to OSConfig.

Currently ignored.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/osconfig.go

@@ -4,22 +4,7 @@
 
 package dns
 
-// DNSRoutingMode describes the type of per-domain DNS routing that
-// the OS is capable of.
-type RoutingMode int
-
-const (
-	// RoutingModeNone means the OS only supports setting a single
-	// primary set of DNS resolvers.
-	RoutingModeNone RoutingMode = iota
-	// RoutingModeSingle means the OS supports a set of
-	// primary resolvers, as well as one set of additional per-suffix
-	// resolvers per network interface.
-	RoutingModeSingle
-	// RoutingModeMulti means the OS supports a set of primary
-	// resolvers, as well as an arbitrary overlay of DNS routes.
-	RoutingModeMulti
-)
+import "inet.af/netaddr"
 
 // An OSConfigurator applies DNS settings to the operating system.
 type OSConfigurator interface {
@@ -28,9 +13,25 @@ type OSConfigurator interface {
 	// configuration is removed.
 	// SetDNS must not be called after Close.
 	SetDNS(cfg OSConfig) error
-	// DNSRoutingMode reports the DNS routing capabilities of this OS
-	// configurator.
-	RoutingMode() RoutingMode
+	// SupportsSplitDNS reports whether the configurator is capable of
+	// installing a resolver only for specific DNS suffixes. If false,
+	// the configurator can only set a global resolver.
+	SupportsSplitDNS() bool
 	// Close removes Tailscale-related DNS configuration from the OS.
 	Close() error
 }
+
+// OSConfig is an OS DNS configuration.
+type OSConfig struct {
+	// Nameservers are the IP addresses of the nameservers to use.
+	Nameservers []netaddr.IP
+	// Domains are the search domains to use.
+	Domains []string
+	// Primary indicates whether to set Nameservers as the
+	// primary/"default" resolvers for the system.
+	// If false, Nameservers will be set as resolvers for Domains
+	// only.
+	// Primary=false is only allowed for OSConfigurators that report
+	// SupportsSplitDNS.
+	Primary bool
+}