Reverse zones made easy: suggested, filled, in the right view, PTRs kept in step

Missing reverse zones are suggested for the private views' A and AAAA records (/24 for private IPv4, /32 for a public address, /48 for IPv6) and created filled with PTR records (jarl zone suggest-reverse, the Zones page). zone create --fill and zone fill-ptr add the PTRs a reverse zone lacks; a reverse zone created without a view goes to the private view using its addresses. Per zone, auto_ptr keeps the PTR records of A and AAAA records in step on every change (database version 12). Zone commands take a network.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/diffops.go

@@ -66,7 +66,7 @@ func DiffOps(from, to *model.State) []model.Op {
 		n := to.Zones[k]
 		s := n.SOA
 		in := ZoneInput{Name: &n.Name, ViewID: &n.ViewID, Type: &n.Type, InView: &n.InView, File: &n.File,
-			DefaultTTL: &n.DefaultTTL, SerialPolicy: &n.SerialPolicy, DNSSECPolicy: &n.DNSSECPolicy, Comment: &n.Comment, Disabled: &n.Disabled, LinkGroup: model.Ptr(n.LinkGroup)}
+			DefaultTTL: &n.DefaultTTL, SerialPolicy: &n.SerialPolicy, DNSSECPolicy: &n.DNSSECPolicy, Comment: &n.Comment, Disabled: &n.Disabled, LinkGroup: model.Ptr(n.LinkGroup), AutoPTR: model.Ptr(n.AutoPTR)}
 		if n.Type == model.ZonePrimary {
 			in.SOA = &SOAInput{MName: &s.MName, RName: &s.RName, Serial: &s.Serial, Refresh: &s.Refresh, Retry: &s.Retry, Expire: &s.Expire, Minimum: &s.Minimum}
 		}

internal/engine/engine.go

@@ -285,7 +285,7 @@ func (e *Engine) expandAll(ctx context.Context, work *model.State, reqs []Reques
 	var warnings []string
 	c := &applyCtx{p: p, now: e.Now()}
 	for i, req := range reqs {
-		x := &expander{ctx: ctx, st: work, alloc: e.store.AllocateIDs, p: p, now: e.Now()}
+		x := &expander{ctx: ctx, st: work, alloc: e.store.AllocateIDs, p: p, now: e.Now(), exposures: e.Exposures}
 		more, err := x.expand(req)
 		if err != nil {
 			if ee, ok := AsError(err); ok && len(reqs) > 1 {

internal/engine/expand.go

@@ -28,6 +28,9 @@ type expander struct {
 	p        *auth.Principal
 	now      time.Time
 	warnings []string
+	// exposures classifies the views as public or private (nil in tests:
+	// every view private).
+	exposures func(*model.State) []validate.Exposure
 }
 
 func (x *expander) ids(n int) ([]int64, error) {
@@ -137,7 +140,7 @@ func (x *expander) expand(req Request) ([]model.Op, error) {
 		return nil, err
 	}
 	switch req.Op {
-	case OpRecordCreate, OpRecordUpdate, OpRecordDelete, OpRRSetReplace, OpRecordDisable, OpRecordEnable:
+	case OpRecordCreate, OpRecordUpdate, OpRecordDelete, OpRRSetReplace, OpRecordDisable, OpRecordEnable, OpZoneFillPTR:
 		// Changes to linked zones reach the other zones of the link.
 		return x.linkSync(ops)
 	}
@@ -152,6 +155,8 @@ func (x *expander) expandOne(req Request) ([]model.Op, error) {
 		return x.zoneUnlink(req)
 	case OpZoneCreate:
 		return x.zoneCreate(req)
+	case OpZoneFillPTR:
+		return x.zoneFillPTR(req)
 	case OpZoneUpdate, OpZoneDelete:
 		z := x.st.Zones[req.ID]
 		if z == nil {
@@ -376,6 +381,7 @@ func (x *expander) zoneCreate(req Request) ([]model.Op, error) {
 		return nil, errf(KindBadRequest, "%v", err)
 	}
 	in.Type = &zt
+	viewGiven := in.View != nil || in.ViewID != nil
 	if in.View != nil {
 		if *in.View != "" {
 			v := x.st.ViewByName(*in.View)
@@ -386,6 +392,39 @@ func (x *expander) zoneCreate(req Request) ([]model.Op, error) {
 		}
 		in.View = nil
 	}
+	if !viewGiven && in.LinkWith == "" && in.CloneFrom == 0 && dnsname.IsReverse(name) {
+		// A reverse zone belongs in the private view whose records use
+		// its addresses (docs/reverse.md).
+		v, err := reverseViewFor(x.st, x.publicViews(), name)
+		if err != nil {
+			return nil, err
+		}
+		if v != nil {
+			in.ViewID = &v.ID
+		}
+	}
+	fill := in.Fill
+	in.Fill = false
+	if fill {
+		if zt != model.ZonePrimary {
+			return nil, errf(KindBadRequest, "fill only applies to primary zones")
+		}
+		if _, ok := dnsname.PrefixFromReverse(name); !ok {
+			return nil, errf(KindBadRequest, "fill only applies to reverse zones of whole octets or nibbles (like 10.0.10.0/24)")
+		}
+		vid := int64(0)
+		if in.ViewID != nil {
+			vid = *in.ViewID
+		}
+		fills := PTRFills(x.st, vid, name, nil)
+		if len(fills) == 0 {
+			x.warnings = append(x.warnings, fmt.Sprintf("no A or AAAA record of %s has an address in %s, so there was nothing to fill it with", viewOrServer(x.st, vid), dnsname.Display(name)))
+		}
+		x.warnings = append(x.warnings, fillWarnings(fills)...)
+		for _, f := range fills {
+			in.Records = append(in.Records, RecordInput{Name: model.Ptr(f.Name), Type: model.Ptr("PTR"), Data: model.Ptr(f.Target)})
+		}
+	}
 	nameservers := in.Nameservers
 	records := in.Records
 	cloneFrom := in.CloneFrom
@@ -638,7 +677,11 @@ func addrOf(p *rr.Parsed) (netip.Addr, bool) {
 }
 
 // ptrOps returns operations creating the PTR record for an address record.
-func (x *expander) ptrCreate(z *model.Zone, owner string, p *rr.Parsed) ([]model.Op, error) {
+//
+// auto is set when the zone's auto_ptr asked rather than the request: a
+// missing reverse zone isn't worth a warning then (most addresses have
+// none), and an address whose PTR record names another host keeps it.
+func (x *expander) ptrCreate(z *model.Zone, owner string, p *rr.Parsed, auto bool) ([]model.Op, error) {
 	addr, ok := addrOf(p)
 	if !ok {
 		return nil, errf(KindBadRequest, "ptr only applies to A and AAAA records")
@@ -651,13 +694,13 @@ func (x *expander) ptrCreate(z *model.Zone, owner string, p *rr.Parsed) ([]model
 			continue
 		}
 		found = true
-		more, err := x.ptrCreateIn(rz, ptrName, owner)
+		more, err := x.ptrCreateIn(rz, ptrName, owner, auto)
 		if err != nil {
 			return nil, err
 		}
 		ops = append(ops, more...)
 	}
-	if !found {
+	if !found && !auto {
 		x.warnings = append(x.warnings, fmt.Sprintf("no reverse zone for %s in this view, so no PTR record was added (create one: jarl zone create %s/%d)", addr, addr, addr.BitLen()))
 	}
 	return ops, nil
@@ -678,7 +721,8 @@ func ptrViews(st *model.State, z *model.Zone) []int64 {
 
 // ptrCreateIn returns operations creating the PTR record ptrName -> owner
 // in reverse zone rz (none when it is there; enabling it when disabled).
-func (x *expander) ptrCreateIn(rz *model.Zone, ptrName, owner string) ([]model.Op, error) {
+func (x *expander) ptrCreateIn(rz *model.Zone, ptrName, owner string, auto bool) ([]model.Op, error) {
+	var other string
 	for _, r := range x.st.ZoneRecords(rz.ID) {
 		if r.Name == ptrName && r.Type == "PTR" {
 			if strings.EqualFold(r.Data, owner) {
@@ -688,9 +732,18 @@ func (x *expander) ptrCreateIn(rz *model.Zone, ptrName, owner string) ([]model.O
 				}
 				return nil, nil // already there
 			}
-			x.warnings = append(x.warnings, fmt.Sprintf("%s already has a PTR record (%s); adding another one", ptrName, r.Data))
+			if !r.Disabled {
+				other = r.Data
+			}
 		}
 	}
+	if other != "" {
+		if auto {
+			// One name per address: the one it has stays.
+			return nil, nil
+		}
+		x.warnings = append(x.warnings, fmt.Sprintf("%s already has a PTR record (%s); adding another one", ptrName, other))
+	}
 	ids, err := x.ids(1)
 	if err != nil {
 		return nil, err
@@ -778,7 +831,7 @@ func (x *expander) recordCreate(req Request) ([]model.Op, error) {
 		return nil, err
 	}
 	x.parseWarnings(owner, p)
-	ptr := in.PTR
+	ptr, auto := in.PTR, !in.PTR && autoPTR(z, owner, p)
 	in.ZoneID, in.Zone, in.View, in.PTR = z.ID, "", "", false
 	in.Name, in.Type, in.Data = &owner, &p.Type, &p.Data
 	ids, err := x.ids(1)
@@ -807,8 +860,8 @@ func (x *expander) recordCreate(req Request) ([]model.Op, error) {
 	// local on a zone that isn't linked: its records are local anyway.
 	in.Local = nil
 	ops := []model.Op{newOp(OpRecordCreate, ids[0], 0, in, summary)}
-	if ptr {
-		more, err := x.ptrCreate(z, owner, p)
+	if ptr || auto && (in.Disabled == nil || !*in.Disabled) {
+		more, err := x.ptrCreate(z, owner, p, auto)
 		if err != nil {
 			return nil, err
 		}
@@ -939,13 +992,43 @@ func (x *expander) recordUpdate(req Request) ([]model.Op, error) {
 	if in.Disabled != nil {
 		disabled = *in.Disabled
 	}
-	moved := p.Data != r.Data || owner != r.Name
+	moved := p.Data != r.Data || owner != r.Name || p.Type != r.Type
+	if !ptr && z.AutoPTR {
+		// The zone keeps PTR records in step (docs/reverse.md): as if
+		// asked, for whichever of the old and new record is an address.
+		old, err := rr.Parse(z.Name, r.Name, r.Type, r.Data)
+		oldAuto := err == nil && autoPTR(z, r.Name, old)
+		newAuto := autoPTR(z, owner, p)
+		switch {
+		case moved:
+			if oldAuto {
+				ops = append(ops, x.ptrDelete(z, r.Name, old)...)
+			}
+			if newAuto && !disabled {
+				more, err := x.ptrCreate(z, owner, p, true)
+				if err != nil {
+					return nil, err
+				}
+				ops = append(ops, more...)
+			}
+		case toggled && newAuto && disabled:
+			ops = append(ops, x.ptrSetDisabled(z, owner, p, true)...)
+		case toggled && newAuto:
+			// Enables the PTR record, or adds it when it is missing.
+			more, err := x.ptrCreate(z, owner, p, true)
+			if err != nil {
+				return nil, err
+			}
+			ops = append(ops, more...)
+		}
+		return ops, nil
+	}
 	if ptr && moved {
 		if old, err := rr.Parse(z.Name, r.Name, r.Type, r.Data); err == nil {
 			ops = append(ops, x.ptrDelete(z, r.Name, old)...)
 		}
 		if !disabled {
-			more, err := x.ptrCreate(z, owner, p)
+			more, err := x.ptrCreate(z, owner, p, false)
 			if err != nil {
 				return nil, err
 			}
@@ -961,7 +1044,7 @@ func (x *expander) recordUpdate(req Request) ([]model.Op, error) {
 	if ptr && !moved && !toggled && !disabled {
 		// Nothing else changes: make sure the PTR record is there (the
 		// reverse zone may be newer than the record).
-		more, err := x.ptrCreate(z, owner, p)
+		more, err := x.ptrCreate(z, owner, p, false)
 		if err != nil {
 			return nil, err
 		}
@@ -988,11 +1071,9 @@ func (x *expander) recordDelete(req Request) ([]model.Op, error) {
 			base = r.Version
 		}
 		ops := []model.Op{newOp(OpRecordDelete, r.ID, base, nil, fmt.Sprintf("delete %s %s %s", r.Name, r.Type, r.Data))}
-		if in.PTR {
-			z := x.st.Zones[r.ZoneID]
-			if p, err := rr.Parse(z.Name, r.Name, r.Type, r.Data); err == nil {
-				ops = append(ops, x.ptrDelete(z, r.Name, p)...)
-			}
+		z := x.st.Zones[r.ZoneID]
+		if p, err := rr.Parse(z.Name, r.Name, r.Type, r.Data); err == nil && (in.PTR || autoPTR(z, r.Name, p)) {
+			ops = append(ops, x.ptrDelete(z, r.Name, p)...)
 		}
 		return ops, nil
 	}
@@ -1032,8 +1113,19 @@ func (x *expander) recordDelete(req Request) ([]model.Op, error) {
 	in.ZoneID, in.Zone, in.View, in.PTR = z.ID, "", "", false
 	in.Name, in.Type = &owner, &typ
 	ops := []model.Op{newOp(OpRecordDelete, 0, 0, in, "delete "+what)}
-	if ptr && parsed != nil {
+	switch {
+	case ptr && parsed != nil:
 		ops = append(ops, x.ptrDelete(z, owner, parsed)...)
+	case z.AutoPTR:
+		// Every matching record's PTR record goes too.
+		for _, r := range x.st.RRset(z.ID, owner, typ) {
+			if parsed != nil && r.Data != parsed.Data {
+				continue
+			}
+			if p, err := rr.Parse(z.Name, r.Name, r.Type, r.Data); err == nil && autoPTR(z, r.Name, p) {
+				ops = append(ops, x.ptrDelete(z, r.Name, p)...)
+			}
+		}
 	}
 	return ops, nil
 }
@@ -1169,7 +1261,32 @@ func (x *expander) rrsetReplace(req Request) ([]model.Op, error) {
 	if len(canon) == 0 {
 		summary = fmt.Sprintf("delete all %s records of %s%s", typ, owner, where)
 	}
-	return append(pre, newOp(OpRRSetReplace, 0, 0, in, summary)), nil
+	ops := append(pre, newOp(OpRRSetReplace, 0, 0, in, summary))
+	if z.AutoPTR && (typ == "A" || typ == "AAAA") && !strings.HasPrefix(owner, "*.") {
+		// PTR records follow the addresses that go and come.
+		old := map[string]bool{}
+		for _, r := range x.st.RRset(z.ID, owner, typ) {
+			old[r.Data] = true
+			if !slices.Contains(canon, r.Data) {
+				if p, err := rr.Parse(z.Name, r.Name, r.Type, r.Data); err == nil {
+					ops = append(ops, x.ptrDelete(z, r.Name, p)...)
+				}
+			}
+		}
+		for _, d := range canon {
+			if old[d] {
+				continue
+			}
+			if _, p, err := parseRecord(z, owner, typ, d); err == nil {
+				more, err := x.ptrCreate(z, owner, p, true)
+				if err != nil {
+					return nil, err
+				}
+				ops = append(ops, more...)
+			}
+		}
+	}
+	return ops, nil
 }
 
 func (x *expander) viewCreate(req Request) ([]model.Op, error) {

internal/engine/inputs.go

@@ -59,6 +59,9 @@ const (
 	// into zone and record updates.
 	OpZoneLink   = "zone.link"
 	OpZoneUnlink = "zone.unlink"
+	// Filling a reverse zone with the PTR records of its view's A and
+	// AAAA records (docs/reverse.md) becomes record.create operations.
+	OpZoneFillPTR = "zone.fill_ptr"
 )
 
 // AckInput acknowledges (or un-acknowledges) an exposure finding of a view
@@ -142,6 +145,12 @@ type ZoneInput struct {
 	// name in this view, copying its shared records (or all of its
 	// records, when it isn't linked yet).
 	LinkWith string `json:"link_with,omitempty"`
+	// AutoPTR keeps the PTR records of the zone's A and AAAA records in
+	// step (docs/reverse.md).
+	AutoPTR *bool `json:"auto_ptr,omitempty"`
+	// Fill (create only, reverse zones) adds a PTR record for every
+	// address of the view's A and AAAA records in the zone's range.
+	Fill bool `json:"fill,omitempty"`
 
 	// Create only: name servers for the apex NS records (default: the
 	// configured default name servers), additional records, and a zone to

internal/engine/ops.go

@@ -268,6 +268,9 @@ func setZoneFields(st *model.State, z *model.Zone, in *ZoneInput, creating bool)
 	if in.LinkGroup != nil {
 		z.LinkGroup = *in.LinkGroup
 	}
+	if in.AutoPTR != nil {
+		z.AutoPTR = *in.AutoPTR
+	}
 	return nil
 }
 

internal/engine/reverse.go

@@ -0,0 +1,357 @@
+package engine
+
+import (
+	"fmt"
+	"net/netip"
+	"slices"
+	"strings"
+
+	"jarl/internal/dnsname"
+	"jarl/internal/model"
+	"jarl/internal/rr"
+	"jarl/internal/validate"
+)
+
+// Reverse zones made easy (docs/reverse.md): filling a reverse zone from
+// the A and AAAA records of its view, choosing its view, and suggesting
+// the reverse zones a view lacks.
+
+// addrUse is an address a forward record gives a name.
+type addrUse struct {
+	addr netip.Addr
+	name string // the owner name, absolute
+	apex bool
+}
+
+// forwardAddresses returns the addresses of the enabled A and AAAA
+// records of a view's primary forward zones (wildcards aside: a PTR record
+// can't name them).
+func forwardAddresses(st *model.State, viewID int64) []addrUse {
+	var out []addrUse
+	for _, z := range st.ZonesInView(viewID) {
+		if z.Type != model.ZonePrimary || z.Disabled || dnsname.IsReverse(z.Name) {
+			continue
+		}
+		for _, r := range st.ZoneRecords(z.ID) {
+			if r.Disabled || (r.Type != "A" && r.Type != "AAAA") || strings.HasPrefix(r.Name, "*.") {
+				continue
+			}
+			a, err := netip.ParseAddr(r.Data)
+			if err != nil {
+				continue
+			}
+			out = append(out, addrUse{addr: a.Unmap(), name: r.Name, apex: r.Name == z.Name})
+		}
+	}
+	return out
+}
+
+// PTRFill is a PTR record filling a reverse zone would add.
+type PTRFill struct {
+	Address string   `json:"address"`
+	Name    string   `json:"name"`             // the PTR owner name
+	Target  string   `json:"target"`           // the host name it gives
+	Others  []string `json:"others,omitempty"` // other names with this address
+}
+
+// PTRFills returns the PTR records a reverse zone named zoneName in a view
+// lacks for the addresses of the view's A and AAAA records: one per
+// address, naming its mail or name server host when it is one, otherwise
+// a host rather than the domain itself, then the shortest name.
+// existing are the zone's records (none for a new zone).
+func PTRFills(st *model.State, viewID int64, zoneName string, existing []*model.Record) []PTRFill {
+	prefix, ok := dnsname.PrefixFromReverse(zoneName)
+	if !ok {
+		return nil
+	}
+	has := map[string]bool{}
+	for _, r := range existing {
+		if r.Type == "PTR" {
+			has[r.Name] = true
+		}
+	}
+	servers := serverNames(st, viewID)
+	byAddr := map[netip.Addr][]addrUse{}
+	var order []netip.Addr
+	for _, u := range forwardAddresses(st, viewID) {
+		if !prefix.Contains(u.addr) {
+			continue
+		}
+		if _, seen := byAddr[u.addr]; !seen {
+			order = append(order, u.addr)
+		}
+		if !slices.ContainsFunc(byAddr[u.addr], func(o addrUse) bool { return o.name == u.name }) {
+			byAddr[u.addr] = append(byAddr[u.addr], u)
+		}
+	}
+	slices.SortFunc(order, func(a, b netip.Addr) int { return a.Compare(b) })
+	var out []PTRFill
+	for _, a := range order {
+		owner := dnsname.ReverseName(a)
+		if has[owner] {
+			continue
+		}
+		uses := byAddr[a]
+		slices.SortStableFunc(uses, func(x, y addrUse) int {
+			rank := func(u addrUse) int {
+				switch {
+				case servers[u.name]:
+					return 0
+				case !u.apex:
+					return 1
+				}
+				return 2
+			}
+			if d := rank(x) - rank(y); d != 0 {
+				return d
+			}
+			if d := strings.Count(x.name, ".") - strings.Count(y.name, "."); d != 0 {
+				return d
+			}
+			return strings.Compare(x.name, y.name)
+		})
+		f := PTRFill{Address: a.String(), Name: owner, Target: uses[0].name}
+		for _, u := range uses[1:] {
+			f.Others = append(f.Others, u.name)
+		}
+		out = append(out, f)
+	}
+	return out
+}
+
+// serverNames are the hosts the view's MX and NS records name.
+func serverNames(st *model.State, viewID int64) map[string]bool {
+	out := map[string]bool{}
+	for _, z := range st.ZonesInView(viewID) {
+		if z.Type != model.ZonePrimary {
+			continue
+		}
+		for _, r := range st.ZoneRecords(z.ID) {
+			switch r.Type {
+			case "MX":
+				if f := strings.Fields(r.Data); len(f) == 2 {
+					out[strings.ToLower(f[1])] = true
+				}
+			case "NS":
+				out[strings.ToLower(r.Data)] = true
+			}
+		}
+	}
+	return out
+}
+
+func fillWarnings(fills []PTRFill) []string {
+	var out []string
+	for _, f := range fills {
+		if len(f.Others) > 0 {
+			out = append(out, fmt.Sprintf("%s has several names: its PTR record gives %s (not %s); change it if you prefer another", f.Address, dnsname.Display(f.Target), strings.Join(displayAll(f.Others), ", ")))
+		}
+	}
+	return out
+}
+
+func displayAll(names []string) []string {
+	out := make([]string, len(names))
+	for i, n := range names {
+		out[i] = dnsname.Display(n)
+	}
+	return out
+}
+
+// zoneFillPTR expands zone.fill_ptr: the PTR records a reverse zone lacks.
+func (x *expander) zoneFillPTR(req Request) ([]model.Op, error) {
+	var in LinkInput
+	if err := decode(req.Data, &in); err != nil {
+		return nil, err
+	}
+	id := in.ZoneID
+	if id == 0 {
+		id = req.ID
+	}
+	z := x.st.Zones[id]
+	if z == nil {
+		return nil, errf(KindNotFound, "zone %d does not exist", id)
+	}
+	if z.Type != model.ZonePrimary {
+		return nil, errf(KindBadRequest, "zone %s isn't a primary zone", zoneDesc(x.st, z))
+	}
+	if _, ok := dnsname.PrefixFromReverse(z.Name); !ok {
+		return nil, errf(KindBadRequest, "zone %s isn't a reverse zone of whole octets or nibbles (like 10.0.10.in-addr.arpa)", zoneDesc(x.st, z))
+	}
+	fills := PTRFills(x.st, z.ViewID, z.Name, x.st.ZoneRecords(z.ID))
+	if len(fills) == 0 {
+		return nil, errf(KindBadRequest, "nothing to add: every address of %s's A and AAAA records in %s has a PTR record", viewOrServer(x.st, z.ViewID), zoneDesc(x.st, z))
+	}
+	x.warnings = append(x.warnings, fillWarnings(fills)...)
+	ids, err := x.ids(len(fills))
+	if err != nil {
+		return nil, err
+	}
+	var ops []model.Op
+	for i, f := range fills {
+		ri := RecordInput{ZoneID: z.ID, Name: model.Ptr(f.Name), Type: model.Ptr("PTR"), Data: model.Ptr(f.Target)}
+		if z.LinkGroup != 0 {
+			ri.SharedID = model.Ptr(ids[i])
+		}
+		ops = append(ops, newOp(OpRecordCreate, ids[i], 0, ri, fmt.Sprintf("add %s IN PTR %s", f.Name, f.Target)))
+	}
+	return ops, nil
+}
+
+func viewOrServer(st *model.State, viewID int64) string {
+	if v := st.Views[viewID]; v != nil {
+		return "view " + v.Name
+	}
+	return "the server"
+}
+
+// publicViews returns which views the internet reaches.
+func (x *expander) publicViews() map[int64]bool {
+	out := map[int64]bool{}
+	if x.exposures == nil {
+		return out
+	}
+	for _, e := range x.exposures(x.st) {
+		out[e.ViewID] = e.Public
+	}
+	return out
+}
+
+// reverseViewFor chooses the view of a new reverse zone given without one:
+// the private view whose records use addresses in its range, or the only
+// private view. "" with no error leaves the choice to the usual rules.
+func reverseViewFor(st *model.State, public map[int64]bool, zoneName string) (*model.View, error) {
+	prefix, ok := dnsname.PrefixFromReverse(zoneName)
+	if !ok || len(st.Views) == 0 {
+		return nil, nil
+	}
+	var private, using []*model.View
+	for _, v := range st.OrderedViews() {
+		if public[v.ID] {
+			continue
+		}
+		private = append(private, v)
+		if slices.ContainsFunc(forwardAddresses(st, v.ID), func(u addrUse) bool { return prefix.Contains(u.addr) }) {
+			using = append(using, v)
+		}
+	}
+	switch {
+	case len(using) == 1:
+		return using[0], nil
+	case len(using) > 1:
+		var names []string
+		for _, v := range using {
+			names = append(names, v.Name)
+		}
+		return nil, errf(KindBadRequest, "views %s all have addresses in %s: give the view (or create the zone in each)", strings.Join(names, " and "), prefix)
+	case len(private) == 1:
+		return private[0], nil
+	}
+	return nil, nil
+}
+
+// ReverseSuggestion is a reverse zone a view lacks for addresses its
+// records use.
+type ReverseSuggestion struct {
+	ViewID  int64    `json:"view_id"`
+	View    string   `json:"view,omitempty"`
+	Prefix  string   `json:"prefix"`
+	Zone    string   `json:"zone"`
+	Why     string   `json:"why"`
+	Fill    int      `json:"fill"`    // PTR records creating it with fill adds
+	Samples []string `json:"samples"` // a few of them, "address name"
+}
+
+// ReverseSuggestions returns the reverse zones the private views (or the
+// server, without views) lack for their A and AAAA records' addresses:
+// the /24 of a private IPv4 address, a /32 for a public one (the /24
+// belongs to someone else, and a zone for it here would hide the
+// neighbours' names from your clients), the /48 of an IPv6 address.
+// Public views get none: the internet asks the addresses' owners.
+func ReverseSuggestions(st *model.State, exposures []validate.Exposure) []ReverseSuggestion {
+	public := map[int64]bool{}
+	for _, e := range exposures {
+		public[e.ViewID] = e.Public
+	}
+	viewIDs := []int64{0}
+	if len(st.Views) > 0 {
+		viewIDs = viewIDs[:0]
+		for _, v := range st.OrderedViews() {
+			if !public[v.ID] {
+				viewIDs = append(viewIDs, v.ID)
+			}
+		}
+	} else if public[0] {
+		return []ReverseSuggestion{}
+	}
+	out := []ReverseSuggestion{}
+	for _, vid := range viewIDs {
+		seen := map[netip.Prefix]bool{}
+		for _, u := range forwardAddresses(st, vid) {
+			a := u.addr
+			if a.IsLoopback() || a.IsLinkLocalUnicast() || a.IsUnspecified() || a.IsMulticast() {
+				continue
+			}
+			if rz, _ := reverseZoneFor(st, vid, a); rz != nil {
+				continue
+			}
+			var p netip.Prefix
+			why := ""
+			switch {
+			case a.Is4() && (a.IsPrivate() || cgnat.Contains(a)):
+				p, why = netip.PrefixFrom(a, 24).Masked(), "private network"
+			case a.Is4():
+				p, why = netip.PrefixFrom(a, 32), "a public address: its own zone, since the rest of its /24 belongs to others"
+			default:
+				p, why = netip.PrefixFrom(a, 48).Masked(), "the /48 of your IPv6 addresses (one zone for all their subnets)"
+				if a.IsPrivate() {
+					why = "your ULA /48 (one zone for all its subnets)"
+				}
+			}
+			if seen[p] {
+				continue
+			}
+			seen[p] = true
+			name, err := dnsname.ReverseZone(p)
+			if err != nil {
+				continue
+			}
+			s := ReverseSuggestion{ViewID: vid, View: viewName(st, vid), Prefix: p.String(), Zone: strings.TrimSuffix(name, "."), Why: why, Samples: []string{}}
+			fills := PTRFills(st, vid, name, nil)
+			s.Fill = len(fills)
+			for i, f := range fills {
+				if i == 3 {
+					break
+				}
+				s.Samples = append(s.Samples, f.Address+" "+dnsname.Display(f.Target))
+			}
+			out = append(out, s)
+		}
+	}
+	slices.SortStableFunc(out, func(a, b ReverseSuggestion) int {
+		if a.View != b.View {
+			return strings.Compare(a.View, b.View)
+		}
+		return netip.MustParsePrefix(a.Prefix).Addr().Compare(netip.MustParsePrefix(b.Prefix).Addr())
+	})
+	return out
+}
+
+var cgnat = netip.MustParsePrefix("100.64.0.0/10")
+
+// autoPTR reports whether a record's PTR is kept in step by its zone's
+// auto_ptr setting.
+func autoPTR(z *model.Zone, owner string, p *rr.Parsed) bool {
+	if z == nil || !z.AutoPTR || strings.HasPrefix(owner, "*.") {
+		return false
+	}
+	_, ok := addrOf(p)
+	return ok
+}
+
+func viewName(st *model.State, id int64) string {
+	if v := st.Views[id]; v != nil {
+		return v.Name
+	}
+	return ""
+}

internal/engine/reverse_test.go

@@ -0,0 +1,138 @@
+package engine
+
+import (
+	"slices"
+	"strings"
+	"testing"
+
+	"jarl/internal/model"
+)
+
+// ptrs returns a reverse zone's PTR records in a view as "owner target",
+// with " disabled" when so.
+func (env *testEnv) ptrs(view, zone string) []string {
+	env.t.Helper()
+	s := env.state()
+	z := s.FindZone(zone, s.ViewByName(view).ID)
+	if z == nil {
+		env.t.Fatalf("no zone %s in %s", zone, view)
+	}
+	var out []string
+	for _, r := range s.ZoneRecords(z.ID) {
+		if r.Type == "PTR" {
+			d := r.Name + " " + r.Data
+			if r.Disabled {
+				d += " disabled"
+			}
+			out = append(out, d)
+		}
+	}
+	return out
+}
+
+func TestReverseFillAndSuggest(t *testing.T) {
+	env := linkedEnv(t)
+	env.submit(env.admin,
+		recIn("internal", "@", "A", "192.0.2.25"),
+		recIn("internal", "nas", "A", "10.0.10.20"),
+		recIn("internal", "files", "A", "10.0.10.20"),
+		recIn("internal", "nas", "AAAA", "fdab:abab:abab:10::20"),
+		recIn("internal", "*.dev", "A", "10.0.10.99"),
+		recIn("internal", "lo", "A", "127.0.0.1"))
+
+	sugg := ReverseSuggestions(env.state(), env.e.Exposures(env.state()))
+	var got []string
+	for _, s := range sugg {
+		got = append(got, s.View+" "+s.Prefix+" "+s.Zone)
+	}
+	want(t, "suggestions", got,
+		"internal 10.0.10.0/24 10.0.10.in-addr.arpa",
+		"internal 192.0.2.10/32 10.2.0.192.in-addr.arpa",
+		"internal 192.0.2.25/32 25.2.0.192.in-addr.arpa",
+		"internal fdab:abab:abab::/48 b.a.b.a.b.a.b.a.b.a.d.f.ip6.arpa")
+	if sugg[0].Fill != 1 || sugg[0].Samples[0] != "10.0.10.20 files.example.com" {
+		t.Errorf("10.0.10.0/24: %+v", sugg[0])
+	}
+
+	// No view given: the private view whose records use the addresses.
+	res := env.submit(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("10.0.10.0/24"), Fill: true}))
+	want(t, "10.0.10/24", env.ptrs("internal", "10.0.10.in-addr.arpa."), "20.10.0.10.in-addr.arpa. files.example.com.")
+	if !slices.ContainsFunc(res.Warnings, func(w string) bool { return strings.Contains(w, "10.0.10.20 has several names") }) {
+		t.Errorf("warnings %q", res.Warnings)
+	}
+	// The mail server's name beats the domain's own.
+	env.submit(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("192.0.2.25/32"), Fill: true}))
+	want(t, "192.0.2.25", env.ptrs("internal", "25.2.0.192.in-addr.arpa."), "25.2.0.192.in-addr.arpa. mail.example.com.")
+
+	// Filling an existing zone; then there is nothing left to add.
+	env.submit(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("fdab:abab:abab::/48")}))
+	st := env.state()
+	z6 := st.FindZone("b.a.b.a.b.a.b.a.b.a.d.f.ip6.arpa.", st.ViewByName("internal").ID)
+	env.submit(env.admin, NewRequest(OpZoneFillPTR, z6.ID, LinkInput{}))
+	if p := env.ptrs("internal", z6.Name); len(p) != 1 || !strings.HasSuffix(p[0], " nas.example.com.") {
+		t.Errorf("ip6 fill: %q", p)
+	}
+	if e := env.submitErr(env.admin, NewRequest(OpZoneFillPTR, z6.ID, LinkInput{})); !strings.Contains(e.Message, "nothing to add") {
+		t.Errorf("second fill: %v", e)
+	}
+
+	got = got[:0]
+	for _, s := range ReverseSuggestions(env.state(), env.e.Exposures(env.state())) {
+		got = append(got, s.Prefix)
+	}
+	want(t, "suggestions left", got, "192.0.2.10/32")
+
+	// Addresses that two private views use: the view must be given.
+	env.submit(env.admin, NewRequest(OpViewCreate, 0, ViewInput{Name: model.Ptr("lab"), MatchClients: &[]string{"172.16.0.0/12"}}))
+	env.submit(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("lab.test"), View: model.Ptr("lab")}),
+		NewRequest(OpRecordCreate, 0, RecordInput{Zone: "lab.test", View: "lab", Name: model.Ptr("x"), Type: model.Ptr("A"), Data: model.Ptr("192.0.2.10")}))
+	if e := env.submitErr(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("192.0.2.10/32")})); !strings.Contains(e.Message, "views internal and lab all have addresses") {
+		t.Errorf("two views: %v", e)
+	}
+}
+
+func TestAutoPTR(t *testing.T) {
+	env := linkedEnv(t)
+	env.submit(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("10.0.10.0/24"), View: model.Ptr("internal")}))
+	st := env.state()
+	zid := st.FindZone("example.com.", st.ViewByName("internal").ID).ID
+	env.submit(env.admin, NewRequest(OpZoneUpdate, zid, ZoneInput{AutoPTR: model.Ptr(true)}))
+	rz := "10.0.10.in-addr.arpa."
+
+	res := env.submit(env.admin, recIn("internal", "host", "A", "10.0.10.30"), recIn("internal", "pub", "A", "203.0.113.5"))
+	want(t, "added", env.ptrs("internal", rz), "30.10.0.10.in-addr.arpa. host.example.com.")
+	if len(res.Warnings) > 0 {
+		t.Errorf("no reverse zone for 203.0.113.5 isn't worth a warning: %q", res.Warnings)
+	}
+	// A second name for the address: its PTR stays.
+	res = env.submit(env.admin, recIn("internal", "alias", "A", "10.0.10.30"))
+	want(t, "second name", env.ptrs("internal", rz), "30.10.0.10.in-addr.arpa. host.example.com.")
+	if len(res.Warnings) > 0 {
+		t.Errorf("warnings %q", res.Warnings)
+	}
+	env.submit(env.admin, NewRequest(OpRecordDelete, env.recordID("internal", "alias", "A"), nil))
+
+	// Moved, disabled, enabled, deleted.
+	env.submit(env.admin, NewRequest(OpRecordUpdate, env.recordID("internal", "host", "A"), RecordInput{Data: model.Ptr("10.0.10.31")}))
+	want(t, "moved", env.ptrs("internal", rz), "31.10.0.10.in-addr.arpa. host.example.com.")
+	env.submit(env.admin, NewRequest(OpRecordUpdate, env.recordID("internal", "host", "A"), RecordInput{Disabled: model.Ptr(true)}))
+	want(t, "disabled", env.ptrs("internal", rz), "31.10.0.10.in-addr.arpa. host.example.com. disabled")
+	env.submit(env.admin, NewRequest(OpRecordUpdate, env.recordID("internal", "host", "A"), RecordInput{Disabled: model.Ptr(false)}))
+	want(t, "enabled", env.ptrs("internal", rz), "31.10.0.10.in-addr.arpa. host.example.com.")
+	env.submit(env.admin, NewRequest(OpRecordDelete, env.recordID("internal", "host", "A"), nil))
+	want(t, "deleted", env.ptrs("internal", rz))
+
+	// A whole set replaced.
+	env.submit(env.admin, NewRequest(OpRRSetReplace, 0, RRSetInput{Zone: "example.com", View: "internal", Name: "multi", Type: "A", Data: []string{"10.0.10.40", "10.0.10.41"}}))
+	want(t, "set", env.ptrs("internal", rz), "40.10.0.10.in-addr.arpa. multi.example.com.", "41.10.0.10.in-addr.arpa. multi.example.com.")
+	env.submit(env.admin, NewRequest(OpRRSetReplace, 0, RRSetInput{Zone: "example.com", View: "internal", Name: "multi", Type: "A", Data: []string{"10.0.10.41"}}))
+	want(t, "set shrunk", env.ptrs("internal", rz), "41.10.0.10.in-addr.arpa. multi.example.com.")
+	// Deleting by name and type.
+	env.submit(env.admin, NewRequest(OpRecordDelete, 0, RecordInput{Zone: "example.com", View: "internal", Name: model.Ptr("multi"), Type: model.Ptr("A")}))
+	want(t, "deleted by match", env.ptrs("internal", rz))
+
+	// Off again: records come and go alone.
+	env.submit(env.admin, NewRequest(OpZoneUpdate, zid, ZoneInput{AutoPTR: model.Ptr(false)}))
+	env.submit(env.admin, recIn("internal", "quiet", "A", "10.0.10.50"))
+	want(t, "off", env.ptrs("internal", rz))
+}