coder · ethanndickson · Nov 5, 2024 · Nov 4, 2024 · Nov 5, 2024
diff --git a/tailnet/conn.go b/tailnet/conn.go
@@ -22,6 +22,7 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/connstats"
+	"tailscale.com/net/dns"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tsdial"
@@ -106,6 +107,9 @@ type Options struct {
 	ClientType proto.TelemetryEvent_ClientType
 	// TelemetrySink is optional.
 	TelemetrySink TelemetrySink
+	// DNSConfigurator is optional, and is passed to the underlying wireguard
+	// engine.
+	DNSConfigurator dns.OSConfigurator
 }
 
 // TelemetrySink allows tailnet.Conn to send network telemetry to the Coder
@@ -178,6 +182,7 @@ func NewConn(options *Options) (conn *Conn, err error) {
 		Dialer:       dialer,
 		ListenPort:   options.ListenPort,
 		SetSubsystem: sys.Set,
+		DNS:          options.DNSConfigurator,
 	})
 	if err != nil {
 		return nil, xerrors.Errorf("create wgengine: %w", err)

diff --git a/vpn/dns.go b/vpn/dns.go
@@ -0,0 +1,58 @@
+package vpn
+
+import "tailscale.com/net/dns"
+
+func NewDNSConfigurator(t *Tunnel) dns.OSConfigurator {
+	return &dnsManager{tunnel: t}
+}
+
+type dnsManager struct {
+	tunnel *Tunnel
+}
+
+func (v *dnsManager) SetDNS(cfg dns.OSConfig) error {
+	settings := convertDNSConfig(cfg)
+	return v.tunnel.ApplyNetworkSettings(v.tunnel.ctx, &NetworkSettingsRequest{
+		DnsSettings: settings,
+	})
+}
+
+func (*dnsManager) GetBaseConfig() (dns.OSConfig, error) {
+	// Tailscale calls this function to blend the OS's DNS configuration with
+	// it's own, so this is only called if `SupportsSplitDNS` returns false.
+	return dns.OSConfig{}, dns.ErrGetBaseConfigNotSupported
+}
+
+func (*dnsManager) SupportsSplitDNS() bool {
+	// macOS & Windows 10+ support split DNS, so we'll assume all CoderVPN
+	// clients do too.
+	return true
+}
+
+// Close implements dns.OSConfigurator.
+func (*dnsManager) Close() error {
+	// There's no cleanup that we need to initiate from within the dylib.
+	return nil
+}
+
+func convertDNSConfig(cfg dns.OSConfig) *NetworkSettingsRequest_DNSSettings {
+	servers := make([]string, 0, len(cfg.Nameservers))
+	for _, ns := range cfg.Nameservers {
+		servers = append(servers, ns.String())
+	}
+	searchDomains := make([]string, 0, len(cfg.SearchDomains))
+	for _, domain := range cfg.SearchDomains {
+		searchDomains = append(searchDomains, domain.WithoutTrailingDot())
+	}
+	matchDomains := make([]string, 0, len(cfg.MatchDomains))
+	for _, domain := range cfg.MatchDomains {
+		matchDomains = append(matchDomains, domain.WithoutTrailingDot())
+	}
+	return &NetworkSettingsRequest_DNSSettings{
+		Servers:              servers,
+		SearchDomains:        searchDomains,
+		DomainName:           "coder",
+		MatchDomains:         matchDomains,
+		MatchDomainsNoSearch: false,
+	}
+}
diff --git a/vpn/dns_internal_test.go b/vpn/dns_internal_test.go
@@ -0,0 +1,73 @@
+package vpn
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"tailscale.com/net/dns"
+	"tailscale.com/util/dnsname"
+)
+
+func TestConvertDNSConfig(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		input    dns.OSConfig
+		expected *NetworkSettingsRequest_DNSSettings
+	}{
+		{
+			name: "Basic",
+			input: dns.OSConfig{
+				Nameservers: []netip.Addr{
+					netip.MustParseAddr("1.1.1.1"),
+					netip.MustParseAddr("8.8.8.8"),
+				},
+				SearchDomains: []dnsname.FQDN{
+					"example.com.",
+					"test.local.",
+				},
+				MatchDomains: []dnsname.FQDN{
+					"internal.domain.",
+				},
+			},
+			expected: &NetworkSettingsRequest_DNSSettings{
+				Servers:              []string{"1.1.1.1", "8.8.8.8"},
+				SearchDomains:        []string{"example.com", "test.local"},
+				DomainName:           "coder",
+				MatchDomains:         []string{"internal.domain"},
+				MatchDomainsNoSearch: false,
+			},
+		},
+		{
+			name: "Empty",
+			input: dns.OSConfig{
+				Nameservers:   []netip.Addr{},
+				SearchDomains: []dnsname.FQDN{},
+				MatchDomains:  []dnsname.FQDN{},
+			},
+			expected: &NetworkSettingsRequest_DNSSettings{
+				Servers:              []string{},
+				SearchDomains:        []string{},
+				DomainName:           "coder",
+				MatchDomains:         []string{},
+				MatchDomainsNoSearch: false,
+			},
+		},
+	}
+
+	//nolint:paralleltest // outdated rule
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			result := convertDNSConfig(tt.input)
+			require.Equal(t, tt.expected.Servers, result.Servers)
+			require.Equal(t, tt.expected.SearchDomains, result.SearchDomains)
+			require.Equal(t, tt.expected.DomainName, result.DomainName)
+			require.Equal(t, tt.expected.MatchDomains, result.MatchDomains)
+			require.Equal(t, tt.expected.MatchDomainsNoSearch, result.MatchDomainsNoSearch)
+		})
+	}
+}