dns: add acme-dns provider

_acme-challenge is CNAMEd once to an acme-dns registration, so headscale
needs no DNS credentials. Node certificates need one registration per
name; CNAME chains are followed and writes never evict a live value.

Updates #2137

config-example.yaml

@@ -376,11 +376,25 @@ dns:
     # Defaults to 1.1.1.1 and 8.8.8.8.
     # resolvers: []
 
-    # cloudflare, gigahost, hetzner, powerdns or rfc2136. The other keys are
-    # settings of that provider; add _path to a key to read it from a file.
+    # acmedns, cloudflare, gigahost, hetzner, powerdns or rfc2136. The other
+    # keys are settings of that provider; add _path to a key to read it from
+    # a file.
     # provider:
     #   name: cloudflare
     #   api_token_path: /run/credentials/headscale.service/cloudflare-token
+    #
+    # acmedns needs a registration per certificate name under config, and
+    # each _acme-challenge name CNAMEd to its registration's fulldomain. A
+    # node without one gets no certificate. A registration holds two
+    # pending challenges; more are refused until one expires.
+    # provider:
+    #   name: acmedns
+    #   config:
+    #     laptop.example.com:
+    #       username: <username from /register>
+    #       password: <password from /register>
+    #       subdomain: <subdomain from /register>
+    #       server_url: https://auth.acme-dns.io
 
 # Unix socket used for the CLI to connect without authentication
 # Note: for production you will want to set this to something like:

flakehashes.json

@@ -1,6 +1,6 @@
 {
   "vendor": {
-    "goModSum": "sha256-X7Yx0jOestuniDSZ6Aup4MpL6ys5IdF0AOu9rS4Bzyo=",
-    "sri": "sha256-KpiRQbHBAPp2BB+NodeSih2/WIlg52dfoOpYxbrVmBI="
+    "goModSum": "sha256-mZW7/mudUF/yEjla+10vo9pTH/NGy5cpyOA7Educf9k=",
+    "sri": "sha256-LgnMChKtW6WnmpH5dEUxiLCRppyU3Rz21RZ6Q1MedKA="
   }
 }

go.mod

@@ -24,6 +24,7 @@ require (
 	github.com/hashicorp/golang-lru/v2 v2.0.7
 	github.com/jagottsicher/termcolor v1.0.2
 	github.com/kradalby/libdns-gigahost v0.0.0-20261014145029-4c03bc63b974
+	github.com/libdns/acmedns v0.5.0
 	github.com/libdns/cloudflare v0.2.2
 	github.com/libdns/hetzner/v2 v2.0.1
 	github.com/libdns/libdns v1.1.1

go.sum

@@ -325,6 +325,8 @@ github.com/ledongthuc/pdf v0.0.0-20220302134840-0c2507a12d80/go.mod h1:imJHygn/1
 github.com/lib/pq v1.8.0/go.mod h1:AlVN5x4E4T544tWzH6hKfbfQvm3HdbOxrmggDNAPY9o=
 github.com/lib/pq v1.12.3 h1:tTWxr2YLKwIvK90ZXEw8GP7UFHtcbTtty8zsI+YjrfQ=
 github.com/lib/pq v1.12.3/go.mod h1:/p+8NSbOcwzAEI7wiMXFlgydTwcgTr3OSKMsD2BitpA=
+github.com/libdns/acmedns v0.5.0 h1:5pRtmUj4Lb/QkNJSl1xgOGBUJTWW7RjpNaIhjpDXjPE=
+github.com/libdns/acmedns v0.5.0/go.mod h1:X7UAFP1Ep9NpTwWpVlrZzJLR7epynAy0wrIxSPFgKjQ=
 github.com/libdns/cloudflare v0.2.2 h1:XWHv+C1dDcApqazlh08Q6pjytYLgR2a+Y3xrXFu0vsI=
 github.com/libdns/cloudflare v0.2.2/go.mod h1:w9uTmRCDlAoafAsTPnn2nJ0XHK/eaUMh86DUk8BWi60=
 github.com/libdns/hetzner/v2 v2.0.1 h1:phsNc9er8JbVOCsytLG7VuMbCdTPsv+WWwNX1WCbk4Q=

hscontrol/dns/propagation.go

@@ -14,9 +14,16 @@ import (
 	"github.com/rs/zerolog/log"
 )
 
-var ErrNotPropagated = errors.New("TXT record not visible on all authoritative nameservers")
+var (
+	ErrNotPropagated = errors.New("TXT record not visible on all authoritative nameservers")
+	errCNAMEChain    = errors.New("ACME challenge CNAME chain does not end")
+)
 
-const propagationInterval = 2 * time.Second
+const (
+	propagationInterval = 2 * time.Second
+	// maxCNAMEHops is far more than delegating a challenge needs.
+	maxCNAMEHops = 8
+)
 
 // txtChecker waits until every authoritative nameserver of a zone serves
 // a TXT value. The client asks the CA to validate as soon as set-dns
@@ -40,7 +47,7 @@ func newTXTChecker(resolvers []string, timeout time.Duration) txtChecker {
 }
 
 // wait returns nil once each nameserver of the zone holding fqdn answers
-// it with value.
+// it with value; for a CNAME, the chain's last name takes fqdn's place.
 // A timeout of zero disables the check.
 func (c txtChecker) wait(ctx context.Context, fqdn, value string) error {
 	if c.timeout <= 0 {
@@ -54,6 +61,17 @@ func (c txtChecker) wait(ctx context.Context, fqdn, value string) error {
 
 	recursive := resolverAt(c.resolvers...)
 
+	target, err := c.cnameTarget(ctx, recursive, fqdn)
+	if err != nil {
+		return err
+	}
+
+	if target != fqdn {
+		log.Debug().Str("fqdn", fqdn).Str("target", target).Msg("ACME challenge delegated by CNAME")
+
+		fqdn = target
+	}
+
 	nameservers, err := nameserversOf(ctx, recursive, fqdn)
 	if err != nil {
 		return fmt.Errorf("looking up nameservers of %s: %w", fqdn, c.via(err))
@@ -130,6 +148,36 @@ func (c txtChecker) wait(ctx context.Context, fqdn, value string) error {
 	}
 }
 
+// cnameTarget follows the CNAME chain from fqdn, as the CA does, and
+// returns its last name, or fqdn without one. A CNAME delegates the
+// challenge to another zone, as for acme-dns, sometimes through more.
+func (c txtChecker) cnameTarget(ctx context.Context, r *net.Resolver, fqdn string) (string, error) {
+	chain := []string{fqdn}
+
+	for {
+		// Go reports only the first target of a chain.
+		target, err := r.LookupCNAME(ctx, fqdn)
+		if isNotFound(err) || err == nil && strings.EqualFold(target, fqdn) {
+			return fqdn, nil
+		}
+
+		if err != nil {
+			return "", fmt.Errorf("looking up CNAME of %s: %w", fqdn, c.via(err))
+		}
+
+		if slices.ContainsFunc(chain, func(n string) bool { return strings.EqualFold(n, target) }) {
+			return "", fmt.Errorf("%w: %s loops back to %s", errCNAMEChain, fqdn, target)
+		}
+
+		if len(chain) > maxCNAMEHops {
+			return "", fmt.Errorf("%w: more than %d hops from %s", errCNAMEChain, maxCNAMEHops, chain[0])
+		}
+
+		chain = append(chain, target)
+		fqdn = target
+	}
+}
+
 // nameserversOf returns the nameservers of the closest enclosing name of
 // name that has any, which is the zone holding it.
 func nameserversOf(ctx context.Context, r *net.Resolver, name string) ([]*net.NS, error) {

hscontrol/dns/propagation_test.go

@@ -14,11 +14,16 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-// fakeNameserver answers authoritatively for testZone: ns1 and ns2 are
-// the zone's nameservers, and TXT answers come from txt.
+// acmeZone stands in for an acme-dns server's zone.
+const acmeZone = "acme.example.net."
+
+// fakeNameserver answers authoritatively for testZone and acmeZone: ns1
+// and ns2 are their nameservers, TXT answers come from txt and a name in
+// cname is an alias.
 type fakeNameserver struct {
-	mu  sync.Mutex
-	txt map[string][]string
+	mu    sync.Mutex
+	txt   map[string][]string
+	cname map[string]string
 	// hideFor makes the first n TXT queries miss, like a nameserver that
 	// has not caught up yet.
 	hideFor atomic.Int32
@@ -39,8 +44,24 @@ func (f *fakeNameserver) ServeDNS(w mdns.ResponseWriter, r *mdns.Msg) {
 	q := r.Question[0]
 	hdr := mdns.RR_Header{Name: q.Name, Class: mdns.ClassINET, Ttl: 60, Rrtype: q.Qtype}
 
+	f.mu.Lock()
+	target, alias := f.cname[q.Name]
+	f.mu.Unlock()
+
+	if alias {
+		hdr.Rrtype = mdns.TypeCNAME
+		m.Answer = []mdns.RR{&mdns.CNAME{Hdr: hdr, Target: target}}
+		_ = w.WriteMsg(m)
+
+		return
+	}
+
 	switch q.Qtype {
 	case mdns.TypeNS:
+		if q.Name != testZone && q.Name != acmeZone {
+			break
+		}
+
 		m.Answer = []mdns.RR{
 			&mdns.NS{Hdr: hdr, Ns: "ns1." + testZone},
 			&mdns.NS{Hdr: hdr, Ns: "ns2." + testZone},
@@ -86,8 +107,8 @@ func startNameservers(t *testing.T) (txtChecker, *fakeNameserver, *fakeNameserve
 		t.Skip("needs 127.0.0.2, which only Linux routes by default")
 	}
 
-	ns1 := &fakeNameserver{txt: map[string][]string{}}
-	ns2 := &fakeNameserver{txt: map[string][]string{}}
+	ns1 := &fakeNameserver{txt: map[string][]string{}, cname: map[string]string{}}
+	ns2 := &fakeNameserver{txt: map[string][]string{}, cname: map[string]string{}}
 
 	var lc net.ListenConfig
 
@@ -146,6 +167,43 @@ func TestTXTCheckerWaitsForLaggingNameserver(t *testing.T) {
 	assert.Negative(t, ns2.hideFor.Load(), "ns2 was asked again after it missed")
 }
 
+// A challenge delegated by CNAME, as to acme-dns, is only served by the
+// zone of the chain's last name; the names before it answer with a CNAME.
+// The intermediate name sits in testZone, whose nameservers would answer
+// without ever serving the value.
+func TestTXTCheckerFollowsCNAMEChain(t *testing.T) {
+	c, ns1, ns2 := startNameservers(t)
+
+	relay := "relay." + testZone
+	target := "f0e3c7." + acmeZone
+
+	for _, ns := range []*fakeNameserver{ns1, ns2} {
+		ns.mu.Lock()
+		ns.cname[testName] = relay
+		ns.cname[relay] = target
+		ns.txt[target] = []string{"value"}
+		ns.mu.Unlock()
+	}
+
+	require.NoError(t, c.wait(context.Background(), testName, "value"))
+
+	c.timeout = 200 * time.Millisecond
+
+	err := c.wait(context.Background(), testName, "other")
+	require.ErrorIs(t, err, ErrNotPropagated)
+	require.ErrorContains(t, err, target)
+
+	// A loop fails at once instead of waiting out the timeout.
+	for _, ns := range []*fakeNameserver{ns1, ns2} {
+		ns.mu.Lock()
+		ns.cname[relay] = testName
+		ns.mu.Unlock()
+	}
+
+	err = c.wait(context.Background(), testName, "value")
+	require.ErrorIs(t, err, errCNAMEChain)
+}
+
 func TestTXTCheckerErrorNamesResolvers(t *testing.T) {
 	c := newTXTChecker([]string{"1.1.1.1:53", "8.8.8.8:53"}, time.Second)
 	err := c.via(&net.DNSError{Err: "no such host", Name: "ts.example.com.", Server: "127.0.0.53:53"})

hscontrol/dns/provider.go

@@ -1,16 +1,21 @@
 package dns
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"maps"
+	"net/url"
 	"os"
 	"reflect"
 	"slices"
 	"strings"
+	"sync"
+	"time"
 
 	"github.com/go-viper/mapstructure/v2"
 	gigahost "github.com/kradalby/libdns-gigahost"
+	"github.com/libdns/acmedns"
 	"github.com/libdns/cloudflare"
 	"github.com/libdns/hetzner/v2"
 	"github.com/libdns/libdns"
@@ -18,6 +23,20 @@ import (
 	"github.com/libdns/rfc2136"
 )
 
+// Isolator is implemented by a provider that can answer several names'
+// challenges from one record, as acme-dns does for names sharing a
+// registration. The CA accepts a value found there for any of those
+// names, so a node only gets a certificate for a name the provider
+// isolates.
+type Isolator interface {
+	// Isolates reports whether name's challenge is answered apart from
+	// every other name's, once CheckIsolation passed.
+	Isolates(name string) bool
+	// CheckIsolation reports why node certificate names would share a
+	// record.
+	CheckIsolation() error
+}
+
 // Provider writes ACME DNS-01 challenge records into a zone.
 //
 // It is append and delete only: a wildcard order puts two values on the
@@ -32,6 +51,7 @@ type Provider interface {
 // plus its module; the set is curated because each provider is a
 // dependency to keep current and its quirks become ours.
 var providers = map[string]func() Provider{
+	"acmedns":    func() Provider { return &acmeDNS{served: make(map[string][2]acmeDNSValue)} },
 	"cloudflare": func() Provider { return new(cloudflare.Provider) },
 	"gigahost":   func() Provider { return new(gigahost.Provider) },
 	"hetzner":    func() Provider { return new(hetzner.Provider) },
@@ -42,6 +62,12 @@ var providers = map[string]func() Provider{
 var (
 	ErrUnknownProvider = errors.New("unknown DNS provider")
 	ErrProviderConfig  = errors.New("invalid DNS provider configuration")
+	errAcmeDNSAccount  = errors.New("acme-dns needs username, password, subdomain and server_url")
+	// The CA accepts a value found in a shared record for any of the
+	// names, so a node could publish another's under its own name.
+	errAcmeDNSOneAccount = errors.New("acme-dns with one account answers every name's challenge from one record; " +
+		"node certificates need a registration per name under config")
+	errAcmeDNSShared = errors.New("acme-dns registration mapped to more than one name")
 )
 
 // pathSuffix marks a setting whose value is read from a file, keeping
@@ -124,6 +150,7 @@ func NewProvider(
 		TagName:          "json",
 		WeaklyTypedInput: true,
 		ErrorUnused:      true,
+		Squash:           true,
 		Result:           provider,
 	})
 	if err != nil {
@@ -135,6 +162,13 @@ func NewProvider(
 		return nil, fmt.Errorf("%w for %s: %w", ErrProviderConfig, name, err)
 	}
 
+	if v, ok := provider.(interface{ validate() error }); ok {
+		err = v.validate()
+		if err != nil {
+			return nil, fmt.Errorf("%w for %s: %w", ErrProviderConfig, name, err)
+		}
+	}
+
 	return provider, nil
 }
 
@@ -144,7 +178,7 @@ func jsonFields(provider Provider) map[string]reflect.Kind {
 	t := reflect.TypeOf(provider).Elem()
 	fields := make(map[string]reflect.Kind, t.NumField())
 
-	for f := range t.Fields() {
+	for _, f := range reflect.VisibleFields(t) {
 		name, _, _ := strings.Cut(f.Tag.Get("json"), ",")
 		if !f.IsExported() || name == "" || name == "-" {
 			continue
@@ -155,3 +189,173 @@ func jsonFields(provider Provider) map[string]reflect.Kind {
 
 	return fields
 }
+
+// acmeDNSSpacing separates updates to a registration. acme-dns stamps an
+// update with the second it lands in and overwrites the earlier stamp,
+// either one on a tie, so which value is older holds only while updates
+// land in distinct seconds. The margin absorbs clock slew.
+const acmeDNSSpacing = time.Second + 100*time.Millisecond
+
+// acmeDNS is the acme-dns provider. A registration serves the two values
+// written to it last, and deleting is a no-op, so an update overwrites
+// whichever is older even if a CA is still validating it. acmeDNS mirrors
+// what each registration serves and refuses such an update.
+type acmeDNS struct {
+	acmedns.Provider
+
+	mu sync.Mutex
+	// served is what each registration serves, oldest first.
+	served map[string][2]acmeDNSValue
+}
+
+// acmeDNSValue is a value acme-dns serves. It stays live, and must not
+// be overwritten, until it is deleted here.
+type acmeDNSValue struct {
+	text string
+	live bool
+	// written is when its update returned; acme-dns stamped it no later.
+	written time.Time
+}
+
+func (p *acmeDNS) AppendRecords(
+	ctx context.Context,
+	zone string,
+	recs []libdns.Record,
+) ([]libdns.Record, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	var appended []libdns.Record
+
+	for _, rec := range recs {
+		rr := rec.RR()
+		reg := p.registration(zone, rr.Name)
+		served := p.served[reg]
+
+		if served[0].live {
+			return appended, fmt.Errorf(
+				"%w: acme-dns registration %s would overwrite a value still in use",
+				ErrTooManyChallenges, reg,
+			)
+		}
+
+		select {
+		case <-ctx.Done():
+			return appended, ctx.Err()
+		case <-time.After(time.Until(served[1].written.Add(acmeDNSSpacing))):
+		}
+
+		got, err := p.Provider.AppendRecords(ctx, zone, []libdns.Record{rec})
+		appended = append(appended, got...)
+
+		// A failed update may still have reached acme-dns, so the older
+		// value counts as overwritten either way.
+		p.served[reg] = [2]acmeDNSValue{
+			served[1],
+			{text: rr.Data, live: err == nil, written: time.Now()},
+		}
+
+		if err != nil {
+			return appended, err
+		}
+	}
+
+	return appended, nil
+}
+
+func (p *acmeDNS) DeleteRecords(
+	ctx context.Context,
+	zone string,
+	recs []libdns.Record,
+) ([]libdns.Record, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	for _, rec := range recs {
+		rr := rec.RR()
+		reg := p.registration(zone, rr.Name)
+		served := p.served[reg]
+
+		// The older copy of a value written twice is the one to free, so
+		// the newer keeps serving it.
+		for i, v := range served {
+			if v.live && v.text == rr.Data {
+				served[i].live = false
+
+				break
+			}
+		}
+
+		p.served[reg] = served
+	}
+
+	return p.Provider.DeleteRecords(ctx, zone, recs)
+}
+
+// Isolates reports whether name has a registration of its own;
+// CheckIsolation makes sure no other name is mapped to it.
+func (p *acmeDNS) Isolates(name string) bool {
+	_, ok := p.Configs[name]
+
+	return ok
+}
+
+// CheckIsolation requires a registration per name: every name sharing a
+// registration has its challenge answered from the same record.
+func (p *acmeDNS) CheckIsolation() error {
+	if p.Configs == nil {
+		return errAcmeDNSOneAccount
+	}
+
+	names := make(map[string]string, len(p.Configs))
+
+	for _, name := range slices.Sorted(maps.Keys(p.Configs)) {
+		sub := p.Configs[name].Subdomain
+
+		if other, shared := names[sub]; shared {
+			return fmt.Errorf("%w: %s and %s share registration %s", errAcmeDNSShared, other, name, sub)
+		}
+
+		names[sub] = name
+	}
+
+	return nil
+}
+
+// registration returns the subdomain of the account acmedns writes a
+// record with, picked the way acmedns picks it.
+func (p *acmeDNS) registration(zone, name string) string {
+	if p.Configs == nil {
+		return p.Subdomain
+	}
+
+	domain := strings.Trim(libdns.AbsoluteName(name, zone), ".")
+
+	return p.Configs[strings.TrimPrefix(domain, "_acme-challenge.")].Subdomain
+}
+
+// validate reports at startup what acmedns would only report at the first
+// certificate. It builds requests without checking the URL, so a bad
+// server_url would panic.
+func (p *acmeDNS) validate() error {
+	accounts := []acmedns.DomainConfig{{
+		Username:  p.Username,
+		Password:  p.Password,
+		Subdomain: p.Subdomain,
+		ServerURL: p.ServerURL,
+	}}
+	if p.Configs != nil {
+		accounts = slices.Collect(maps.Values(p.Configs))
+	}
+
+	for _, a := range accounts {
+		u, err := url.Parse(a.ServerURL)
+		missing := a.Username == "" || a.Password == "" || a.Subdomain == ""
+
+		if missing || err != nil || u.Host == "" {
+			return errAcmeDNSAccount
+		}
+	}
+
+	return nil
+}

hscontrol/dns/provider_test.go

@@ -1,13 +1,21 @@
 package dns
 
 import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"sync"
 	"testing"
+	"time"
 
 	gigahost "github.com/kradalby/libdns-gigahost"
+	"github.com/libdns/acmedns"
 	"github.com/libdns/cloudflare"
 	"github.com/libdns/hetzner/v2"
+	"github.com/libdns/libdns"
 	"github.com/libdns/powerdns"
 	"github.com/libdns/rfc2136"
 	"github.com/stretchr/testify/assert"
@@ -102,6 +110,67 @@ func TestNewProvider(t *testing.T) {
 			settings: map[string]any{"server_url": "https://pdns.example.com", "api_token": "tok"},
 			want:     &powerdns.Provider{ServerURL: "https://pdns.example.com", APIToken: "tok"},
 		},
+		{
+			name:     "acmedns",
+			provider: "acmedns",
+			settings: map[string]any{
+				"username":      "user",
+				"password_path": secret,
+				"subdomain":     "reg",
+				"server_url":    "https://auth.acme-dns.io",
+			},
+			want: &acmeDNS{
+				Username:  "user",
+				Password:  "from-file",
+				Subdomain: "reg",
+				ServerURL: "https://auth.acme-dns.io",
+				served:    map[string][2]acmeDNSValue{},
+			},
+		},
+		{
+			name:     "acmedns per domain",
+			provider: "acmedns",
+			settings: map[string]any{"config": map[string]any{
+				"node.ts.example.com": map[string]any{
+					"username":   "user",
+					"password":   "pass",
+					"subdomain":  "reg",
+					"server_url": "https://auth.acme-dns.io",
+				},
+			}},
+			want: &acmeDNS{
+				Configs: map[string]acmedns.DomainConfig{
+					"node.ts.example.com": {
+						Username:  "user",
+						Password:  "pass",
+						Subdomain: "reg",
+						ServerURL: "https://auth.acme-dns.io",
+					},
+				},
+				served: map[string][2]acmeDNSValue{},
+			},
+		},
+		{
+			name:     "acmedns server_url without scheme",
+			provider: "acmedns",
+			settings: map[string]any{
+				"username":   "user",
+				"password":   "pass",
+				"subdomain":  "reg",
+				"server_url": "auth.acme-dns.io",
+			},
+			wantErr: ErrProviderConfig,
+		},
+		{
+			name:     "acmedns without subdomain",
+			provider: "acmedns",
+			settings: map[string]any{
+				"username":   "user",
+				"password":   "pass",
+				"server_url": "https://auth.acme-dns.io",
+			},
+			wantErr: ErrProviderConfig,
+		},
 		{
 			name:     "only a name",
 			provider: "gigahost",
@@ -124,3 +193,137 @@ func TestNewProvider(t *testing.T) {
 		})
 	}
 }
+
+// fakeAcmeDNS is an acme-dns server with one registration, reg. It has two
+// rows, and an update overwrites the one stamped with the earlier second.
+// acme-dns overwrites either on a tie; the fake picks the one written
+// later, which a writer assuming first in, first out gets wrong.
+type fakeAcmeDNS struct {
+	*httptest.Server
+
+	mu     sync.Mutex
+	writes int
+	rows   [2]fakeAcmeDNSRow
+}
+
+type fakeAcmeDNSRow struct {
+	txt   string
+	stamp int64 // acme-dns's LastUpdate, in whole seconds
+	write int   // the order of writes, which acme-dns does not keep
+}
+
+func newFakeAcmeDNS(t *testing.T) *fakeAcmeDNS {
+	t.Helper()
+
+	f := &fakeAcmeDNS{}
+	f.Server = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var update struct{ Subdomain, Txt string }
+
+		err := json.NewDecoder(r.Body).Decode(&update)
+		if r.URL.Path != "/update" || err != nil || update.Subdomain != "reg" || r.Header.Get("X-Api-User") != "user" {
+			w.WriteHeader(http.StatusUnauthorized)
+
+			return
+		}
+
+		f.mu.Lock()
+		defer f.mu.Unlock()
+
+		i := 0
+		if f.rows[1].stamp < f.rows[0].stamp ||
+			(f.rows[1].stamp == f.rows[0].stamp && f.rows[1].write > f.rows[0].write) {
+			i = 1
+		}
+
+		f.writes++
+		f.rows[i] = fakeAcmeDNSRow{txt: update.Txt, stamp: time.Now().Unix(), write: f.writes}
+	}))
+	t.Cleanup(f.Close)
+
+	return f
+}
+
+func (f *fakeAcmeDNS) registration() map[string]any {
+	return map[string]any{
+		"username":   "user",
+		"password":   "pass",
+		"subdomain":  "reg",
+		"server_url": f.URL,
+	}
+}
+
+func (f *fakeAcmeDNS) serving() []string {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	var txts []string
+
+	for _, row := range f.rows {
+		if row.txt != "" {
+			txts = append(txts, row.txt)
+		}
+	}
+
+	return txts
+}
+
+func acmeTXT(node, value string) []libdns.Record {
+	return []libdns.Record{libdns.TXT{Name: "_acme-challenge." + node, Text: value}}
+}
+
+// A registration serves the two values written last, and deleting is a
+// no-op: an update overwrites the older value even if a CA may still be
+// validating it, however many values were deleted. Two updates within one
+// second carry the same stamp, leaving acme-dns free to overwrite either:
+// after a then b, with a freed, c could overwrite b.
+func TestAcmeDNSRefusesEviction(t *testing.T) {
+	t.Parallel()
+
+	srv := newFakeAcmeDNS(t)
+
+	p, err := NewProvider("acmedns", srv.registration(), noEnv)
+	require.NoError(t, err)
+
+	ctx := context.Background()
+
+	a, err := p.AppendRecords(ctx, testZone, acmeTXT("a", "value-a"))
+	require.NoError(t, err)
+	require.Len(t, a, 1, "certmagic needs the appended record back")
+
+	b, err := p.AppendRecords(ctx, testZone, acmeTXT("b", "value-b"))
+	require.NoError(t, err)
+
+	_, err = p.AppendRecords(ctx, testZone, acmeTXT("c", "value-c"))
+	require.ErrorIs(t, err, ErrTooManyChallenges)
+
+	// The newer value is free, but an update overwrites the older one.
+	_, err = p.DeleteRecords(ctx, testZone, b)
+	require.NoError(t, err)
+
+	_, err = p.AppendRecords(ctx, testZone, acmeTXT("c", "value-c"))
+	require.ErrorIs(t, err, ErrTooManyChallenges)
+	assert.ElementsMatch(t, []string{"value-a", "value-b"}, srv.serving(), "a refused value reached acme-dns")
+
+	_, err = p.DeleteRecords(ctx, testZone, a)
+	require.NoError(t, err)
+
+	c, err := p.AppendRecords(ctx, testZone, acmeTXT("c", "value-c"))
+	require.NoError(t, err)
+	assert.ElementsMatch(t, []string{"value-b", "value-c"}, srv.serving())
+
+	// b is free and now the older value; overwriting it leaves c alone.
+	_, err = p.AppendRecords(ctx, testZone, acmeTXT("d", "value-d"))
+	require.NoError(t, err)
+	assert.ElementsMatch(t, []string{"value-c", "value-d"}, srv.serving())
+
+	// Giving up while waiting to space updates writes nothing.
+	_, err = p.DeleteRecords(ctx, testZone, c)
+	require.NoError(t, err)
+
+	cancelled, cancel := context.WithCancel(ctx)
+	cancel()
+
+	_, err = p.AppendRecords(cancelled, testZone, acmeTXT("e", "value-e"))
+	require.ErrorIs(t, err, context.Canceled)
+	assert.ElementsMatch(t, []string{"value-c", "value-d"}, srv.serving())
+}

hscontrol/mapper/mapper_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/juanfont/headscale/hscontrol/db"
+	"github.com/juanfont/headscale/hscontrol/dns"
 	"github.com/juanfont/headscale/hscontrol/state"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/juanfont/headscale/hscontrol/types/change"
@@ -1070,13 +1071,39 @@ func TestGenerateMapResponseDebugOnlyOnFullMaps(t *testing.T) {
 func TestCertDomains(t *testing.T) {
 	enabled := types.DNSConfig{Certificates: types.CertificatesConfig{Enabled: true}}
 
+	acmeDNS, err := dns.NewProvider("acmedns", map[string]any{"config": map[string]any{
+		"laptop.ts.example.com": map[string]any{
+			"username":   "user",
+			"password":   "pass",
+			"subdomain":  "reg",
+			"server_url": "https://auth.acme-dns.io",
+		},
+	}}, func(string) (any, bool) { return nil, false })
+	require.NoError(t, err)
+
 	tests := []struct {
 		name       string
 		node       *types.Node
 		dns        types.DNSConfig
+		provider   dns.Provider
 		baseDomain string
 		want       []string
 	}{
+		{
+			name:     "own acme-dns registration",
+			node:     &types.Node{GivenName: "laptop"},
+			dns:      enabled,
+			provider: acmeDNS,
+			want:     []string{"laptop.ts.example.com"},
+		},
+		{
+			// Without a registration of its own, the node's challenge would
+			// be answered from a record another name's challenge shares.
+			name:     "no acme-dns registration of its own",
+			node:     &types.Node{GivenName: "desktop"},
+			dns:      enabled,
+			provider: acmeDNS,
+		},
 		{
 			name: "user owned",
 			node: &types.Node{GivenName: "laptop", UserID: new(uint(1))},
@@ -1136,8 +1163,9 @@ func TestCertDomains(t *testing.T) {
 			}
 
 			cfg := &types.Config{
-				BaseDomain: baseDomain,
-				DNSConfig:  tt.dns,
+				BaseDomain:   baseDomain,
+				DNSConfig:    tt.dns,
+				DNSChallenge: types.DNSChallengeConfig{Provider: tt.provider},
 				TailcfgDNSConfig: &tailcfg.DNSConfig{
 					Proxied: true,
 					ExtraRecords: []tailcfg.DNSRecord{

hscontrol/types/config.go

@@ -975,6 +975,13 @@ func dnsChallengeConfig() (DNSChallengeConfig, error) {
 		return cfg, err
 	}
 
+	if iso, ok := provider.(hsdns.Isolator); ok && viper.GetBool("dns.certificates.enabled") {
+		err = iso.CheckIsolation()
+		if err != nil {
+			return cfg, fmt.Errorf("%w for %s: %w", hsdns.ErrProviderConfig, cfg.ProviderName, err)
+		}
+	}
+
 	cfg.Provider = provider
 
 	return cfg, nil

hscontrol/types/config_test.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
+	hsdns "github.com/juanfont/headscale/hscontrol/dns"
 	"github.com/juanfont/headscale/hscontrol/util"
 	"github.com/libdns/cloudflare"
 	"github.com/spf13/viper"
@@ -1054,6 +1055,7 @@ dns:
 		wantToken     string
 		wantNoProv    bool
 		wantResolvers []string
+		wantIsolates  string
 	}{
 		{
 			name: "disabled by default",
@@ -1124,6 +1126,63 @@ dns:
 `,
 			wantErr: "dns.challenge.provider is invalid",
 		},
+		{
+			name: "acmedns one account for node certificates",
+			yaml: base + `  certificates:
+    enabled: true
+  challenge:
+    provider:
+      name: acmedns
+      username: user
+      password: pass
+      subdomain: reg
+      server_url: https://auth.acme-dns.io
+`,
+			wantErr: "registration per name",
+		},
+		{
+			name: "acmedns registration per name",
+			yaml: base + `  certificates:
+    enabled: true
+  challenge:
+    provider:
+      name: acmedns
+      config:
+        laptop.ts.example.com:
+          username: user
+          password: pass
+          subdomain: reg-laptop
+          server_url: https://auth.acme-dns.io
+        desktop.ts.example.com:
+          username: user
+          password: pass
+          subdomain: reg-desktop
+          server_url: https://auth.acme-dns.io
+`,
+			wantZone:     "ts.example.com",
+			wantIsolates: "laptop.ts.example.com",
+		},
+		{
+			name: "acmedns registration shared by node names",
+			yaml: base + `  certificates:
+    enabled: true
+  challenge:
+    provider:
+      name: acmedns
+      config:
+        laptop.ts.example.com:
+          username: user
+          password: pass
+          subdomain: reg
+          server_url: https://auth.acme-dns.io
+        desktop.ts.example.com:
+          username: user
+          password: pass
+          subdomain: reg
+          server_url: https://auth.acme-dns.io
+`,
+			wantErr: "desktop.ts.example.com and laptop.ts.example.com share registration reg",
+		},
 		{
 			name: "enabled without magic dns",
 			yaml: strings.Replace(base, "magic_dns: true", "magic_dns: false", 1) + `  certificates:
@@ -1218,6 +1277,14 @@ dns:
 				return
 			}
 
+			if iso, ok := cfg.DNSChallenge.Provider.(hsdns.Isolator); ok {
+				if tt.wantIsolates != "" {
+					assert.True(t, iso.Isolates(tt.wantIsolates))
+				}
+
+				return
+			}
+
 			provider, ok := cfg.DNSChallenge.Provider.(*cloudflare.Provider)
 			require.True(t, ok, "provider is %T", cfg.DNSChallenge.Provider)
 			assert.Equal(t, tt.wantToken, provider.APIToken)

hscontrol/types/node.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	hsdns "github.com/juanfont/headscale/hscontrol/dns"
 	"github.com/juanfont/headscale/hscontrol/policy/matcher"
 	"github.com/juanfont/headscale/hscontrol/util"
 	"github.com/juanfont/headscale/hscontrol/util/zlog/zf"
@@ -814,6 +815,13 @@ func (nv NodeView) CertDomain(cfg *Config) (string, bool) {
 		return "", false
 	}
 
+	// A node may publish only its own name's challenge, which keeps it from
+	// other names' certificates only where the provider answers each name's
+	// challenge apart.
+	if iso, ok := cfg.DNSChallenge.Provider.(hsdns.Isolator); ok && !iso.Isolates(domain) {
+		return "", false
+	}
+
 	return domain, true
 }