diff --git a/net/dns/manager_test.go b/net/dns/manager_test.go
new file mode 100644
index 000000000..f6f840774
--- /dev/null
+++ b/net/dns/manager_test.go
@@ -0,0 +1,397 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"inet.af/netaddr"
+	"tailscale.com/net/dns/resolver"
+)
+
+type fakeOSConfigurator struct {
+	SplitDNS bool
+
+	OSConfig       OSConfig
+	ResolverConfig resolver.Config
+}
+
+func (c *fakeOSConfigurator) SetDNS(cfg OSConfig) error {
+	if !c.SplitDNS && len(cfg.MatchDomains) > 0 {
+		panic("split DNS config passed to non-split OSConfigurator")
+	}
+	c.OSConfig = cfg
+	return nil
+}
+
+func (c *fakeOSConfigurator) SetResolver(cfg resolver.Config) {
+	c.ResolverConfig = cfg
+}
+
+func (c *fakeOSConfigurator) SupportsSplitDNS() bool {
+	return c.SplitDNS
+}
+
+func (c *fakeOSConfigurator) Close() error { return nil }
+
+func TestManager(t *testing.T) {
+	// Note: these tests assume that it's safe to switch the
+	// OSConfigurator's split-dns support on and off between Set
+	// calls. Empirically this is currently true, because we reprobe
+	// the support every time we generate configs. It would be
+	// reasonable to make this unsupported as well, in which case
+	// these tests will need tweaking.
+	tests := []struct {
+		name  string
+		in    Config
+		split bool
+		os    OSConfig
+		rs    resolver.Config
+	}{
+		{
+			name: "empty",
+		},
+		{
+			name: "search-only",
+			in: Config{
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+		},
+		{
+			name: "corp",
+			in: Config{
+				DefaultResolvers: mustIPPs("1.1.1.1:53", "9.9.9.9:53"),
+				SearchDomains:    strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("1.1.1.1", "9.9.9.9"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+		},
+		{
+			name: "corp-split",
+			in: Config{
+				DefaultResolvers: mustIPPs("1.1.1.1:53", "9.9.9.9:53"),
+				SearchDomains:    strs("tailscale.com", "universe.tf"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("1.1.1.1", "9.9.9.9"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+		},
+		{
+			name: "corp-magic",
+			in: Config{
+				DefaultResolvers: mustIPPs("1.1.1.1:53", "9.9.9.9:53"),
+				SearchDomains:    strs("tailscale.com", "universe.tf"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				AuthoritativeSuffixes: strs("ts.com"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(".", "1.1.1.1:53", "9.9.9.9:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				LocalDomains: strs("ts.com."),
+			},
+		},
+		{
+			name: "corp-magic-split",
+			in: Config{
+				DefaultResolvers: mustIPPs("1.1.1.1:53", "9.9.9.9:53"),
+				SearchDomains:    strs("tailscale.com", "universe.tf"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				AuthoritativeSuffixes: strs("ts.com"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(".", "1.1.1.1:53", "9.9.9.9:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				LocalDomains: strs("ts.com."),
+			},
+		},
+		{
+			name: "corp-routes",
+			in: Config{
+				DefaultResolvers: mustIPPs("1.1.1.1:53", "9.9.9.9:53"),
+				Routes:           upstreams("corp.com", "2.2.2.2:53"),
+				SearchDomains:    strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(
+					".", "1.1.1.1:53", "9.9.9.9:53",
+					"corp.com.", "2.2.2.2:53"),
+			},
+		},
+		{
+			name: "corp-routes-split",
+			in: Config{
+				DefaultResolvers: mustIPPs("1.1.1.1:53", "9.9.9.9:53"),
+				Routes:           upstreams("corp.com", "2.2.2.2:53"),
+				SearchDomains:    strs("tailscale.com", "universe.tf"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(
+					".", "1.1.1.1:53", "9.9.9.9:53",
+					"corp.com.", "2.2.2.2:53"),
+			},
+		},
+		{
+			name: "routes",
+			in: Config{
+				Routes:        upstreams("corp.com", "2.2.2.2:53"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(
+					".", "8.8.8.8:53",
+					"corp.com.", "2.2.2.2:53"),
+			},
+		},
+		{
+			name: "routes-split",
+			in: Config{
+				Routes:        upstreams("corp.com", "2.2.2.2:53"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("2.2.2.2"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+				MatchDomains:  strs("corp.com"),
+			},
+		},
+		{
+			name: "routes-multi",
+			in: Config{
+				Routes: upstreams(
+					"corp.com", "2.2.2.2:53",
+					"bigco.net", "3.3.3.3:53"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(
+					".", "8.8.8.8:53",
+					"corp.com.", "2.2.2.2:53",
+					"bigco.net.", "3.3.3.3:53"),
+			},
+		},
+		{
+			name: "routes-multi-split",
+			in: Config{
+				Routes: upstreams(
+					"corp.com", "2.2.2.2:53",
+					"bigco.net", "3.3.3.3:53"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+				MatchDomains:  strs("corp.com", "bigco.net"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(
+					"corp.com.", "2.2.2.2:53",
+					"bigco.net.", "3.3.3.3:53"),
+			},
+		},
+		{
+			name: "magic",
+			in: Config{
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				AuthoritativeSuffixes: strs("ts.com"),
+				SearchDomains:         strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(".", "8.8.8.8:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				LocalDomains: strs("ts.com."),
+			},
+		},
+		{
+			name: "magic-split",
+			in: Config{
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				AuthoritativeSuffixes: strs("ts.com"),
+				SearchDomains:         strs("tailscale.com", "universe.tf"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+				MatchDomains:  strs("ts.com"),
+			},
+			rs: resolver.Config{
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				LocalDomains: strs("ts.com."),
+			},
+		},
+		{
+			name: "routes-magic",
+			in: Config{
+				Routes: upstreams("corp.com", "2.2.2.2:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				AuthoritativeSuffixes: strs("ts.com"),
+				SearchDomains:         strs("tailscale.com", "universe.tf"),
+			},
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(
+					"corp.com.", "2.2.2.2:53",
+					".", "8.8.8.8:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				LocalDomains: strs("ts.com."),
+			},
+		},
+		{
+			name: "routes-magic-split",
+			in: Config{
+				Routes: upstreams("corp.com", "2.2.2.2:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				AuthoritativeSuffixes: strs("ts.com"),
+				SearchDomains:         strs("tailscale.com", "universe.tf"),
+			},
+			split: true,
+			os: OSConfig{
+				Nameservers:   mustIPs("100.100.100.100"),
+				SearchDomains: strs("tailscale.com", "universe.tf"),
+				MatchDomains:  strs("ts.com", "corp.com"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams("corp.com.", "2.2.2.2:53"),
+				Hosts: hosts(
+					"dave.ts.com.", "1.2.3.4",
+					"bradfitz.ts.com.", "2.3.4.5"),
+				LocalDomains: strs("ts.com."),
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			f := fakeOSConfigurator{SplitDNS: test.split}
+			m := NewManager(t.Logf, &f, nil)
+			m.resolver.TestOnlySetHook(f.SetResolver)
+
+			if err := m.Set(test.in); err != nil {
+				t.Fatalf("m.Set: %v", err)
+			}
+			tr := cmp.Transformer("ipStr", func(ip netaddr.IP) string { return ip.String() })
+			if diff := cmp.Diff(f.OSConfig, test.os, tr, cmpopts.EquateEmpty()); diff != "" {
+				t.Errorf("wrong OSConfig (-got+want)\n%s", diff)
+			}
+			if diff := cmp.Diff(f.ResolverConfig, test.rs, tr, cmpopts.EquateEmpty()); diff != "" {
+				t.Errorf("wrong resolver.Config (-got+want)\n%s", diff)
+			}
+		})
+	}
+}
+
+func mustIPs(strs ...string) (ret []netaddr.IP) {
+	for _, s := range strs {
+		ret = append(ret, netaddr.MustParseIP(s))
+	}
+	return ret
+}
+
+func mustIPPs(strs ...string) (ret []netaddr.IPPort) {
+	for _, s := range strs {
+		ret = append(ret, netaddr.MustParseIPPort(s))
+	}
+	return ret
+}
+
+func strs(strs ...string) []string { return strs }
+
+func hosts(strs ...string) (ret map[string][]netaddr.IP) {
+	var key string
+	ret = map[string][]netaddr.IP{}
+	for _, s := range strs {
+		if ip, err := netaddr.ParseIP(s); err == nil {
+			if key == "" {
+				panic("IP provided before name")
+			}
+			ret[key] = append(ret[key], ip)
+		} else {
+			key = s
+		}
+	}
+	return ret
+}
+
+func upstreams(strs ...string) (ret map[string][]netaddr.IPPort) {
+	var key string
+	ret = map[string][]netaddr.IPPort{}
+	for _, s := range strs {
+		if ipp, err := netaddr.ParseIPPort(s); err == nil {
+			if key == "" {
+				panic("IPPort provided before suffix")
+			}
+			ret[key] = append(ret[key], ipp)
+		} else {
+			key = s
+		}
+	}
+	return ret
+}