Add ipadm: Go-Tool zur Verwaltung statischer DHCP/DNS-Hosts

tools/ipadm/README.md

@@ -0,0 +1,73 @@
+# ipadm
+
+Kleines Go-CLI-Tool zur Verwaltung statischer Hosts (DHCP-Reservierung +
+DNS-Eintrag) für den dnsmasq-Router auf narcissus. Ersetzt/lehnt sich an das
+Original-Tool `ipadm` (dhcp/bind management, mwx'2021) an, arbeitet aber
+gegen unseren dnsmasq-Stack statt isc-dhcp-server/bind9 (siehe
+[../../README.md](../../README.md)).
+
+Installiert auf dem Server unter `/usr/local/bin/ipadm`.
+
+## Funktionsweise
+
+- Pflegt eine flache Textdatei `/etc/ipadm/hosts` (eine Zeile pro Host:
+  `name<TAB>ip<TAB>mac<TAB>comment`, `mac` ist `-` wenn nicht gesetzt) als
+  Datenbank für statische Hosts.
+- `ipadm -u` generiert daraus `/etc/dnsmasq.d/hosts.conf` (pro Host ein
+  `host-record=` für DNS, plus `dhcp-host=` für Hosts mit MAC-Adresse für die
+  DHCP-Reservierung), validiert die komplette dnsmasq-Konfiguration
+  (`dnsmasq --test`) und lädt dnsmasq bei Erfolg neu
+  (`systemctl reload dnsmasq`). Schlägt die Validierung fehl, wird die
+  vorherige `hosts.conf` automatisch wiederhergestellt und dnsmasq **nicht**
+  neu geladen.
+- IP-Adressen werden bei `-a`/`-i`/im interaktiven Modus geprüft: müssen in
+  `10.0.0.0/24` liegen und **außerhalb** des dynamischen DHCP-Pools
+  `10.0.0.100–10.0.0.200` (verhindert Kollisionen mit dynamisch vergebenen
+  Adressen).
+
+## Usage
+
+```
+ipadm <hostname>                                     add/edit host (interaktiv)
+ipadm -l                                              list hosts
+ipadm -a [-f] <hostname> <ip address> [mac address]   add host
+ipadm -c <hostname> <comment>                         set comment
+ipadm -i <hostname> <ip address>                      change ip address
+ipadm -m <hostname> <mac address>                     change mac address
+ipadm -r <old hostname> <new hostname>                rename host
+ipadm -d <hostname>                                   delete host
+ipadm -u                                              update (dnsmasq-Config regenerieren + reload)
+ipadm -h                                              this help
+```
+
+Nach jeder Änderung an der Host-DB (`-a`, `-c`, `-i`, `-m`, `-r`, `-d`,
+interaktiv) muss `ipadm -u` ausgeführt werden, damit dnsmasq die Änderung
+tatsächlich übernimmt — das Tool weist nach jeder Änderung selbst darauf hin.
+
+## Bauen / Installieren
+
+```sh
+cd tools/ipadm
+go build -o ipadm .
+install -m 0755 -o root -g root ipadm /usr/local/bin/ipadm
+```
+
+## Testen ohne die echte Server-Konfiguration anzufassen
+
+Alle Pfade sind über Umgebungsvariablen überschreibbar:
+
+```sh
+export IPADM_DB=/tmp/test-hosts
+export IPADM_DNSMASQ_HOSTS=/tmp/test-hosts.conf
+export IPADM_LAN_CIDR=10.0.0.0/24
+export IPADM_POOL_START=10.0.0.100
+export IPADM_POOL_END=10.0.0.200
+./ipadm -a testhost 10.0.0.50 aa:bb:cc:dd:ee:ff
+./ipadm -l
+```
+
+Achtung: `ipadm -u` ruft trotzdem `dnsmasq --test` (prüft die echte
+System-Konfiguration in `/etc/dnsmasq.conf` + `/etc/dnsmasq.d/`) und bei
+Erfolg `systemctl reload dnsmasq` auf, auch im Testmodus — das ist
+beabsichtigt (validiert, dass die generierte Datei mit der echten
+Umgebung zusammenspielt), aber es reloadet den echten laufenden Dienst.

tools/ipadm/dnsmasq.go

@@ -0,0 +1,85 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"sort"
+	"strings"
+)
+
+// renderDnsmasqConfig builds the content of the generated dnsmasq include
+// file from the current host list. Every host gets a host-record (forward +
+// reverse DNS). Hosts with a MAC additionally get a dhcp-host static lease.
+func renderDnsmasqConfig(hosts []Host) string {
+	sorted := make([]Host, len(hosts))
+	copy(sorted, hosts)
+	sort.Slice(sorted, func(i, j int) bool {
+		return strings.ToLower(sorted[i].Name) < strings.ToLower(sorted[j].Name)
+	})
+
+	var b strings.Builder
+	b.WriteString("# Generated by ipadm -u — DO NOT EDIT MANUALLY.\n")
+	b.WriteString("# Edit hosts with `ipadm` and re-run `ipadm -u` to regenerate this file.\n\n")
+	for _, h := range sorted {
+		if h.Comment != "" {
+			fmt.Fprintf(&b, "# %s: %s\n", h.Name, h.Comment)
+		}
+		fmt.Fprintf(&b, "host-record=%s,%s\n", h.Name, h.IP)
+		if h.MAC != "" {
+			fmt.Fprintf(&b, "dhcp-host=%s,%s,%s,infinite\n", h.MAC, h.IP, h.Name)
+		}
+		b.WriteString("\n")
+	}
+	return b.String()
+}
+
+// applyDnsmasqConfig writes the generated config to path, validates the full
+// dnsmasq configuration, and on success reloads the dnsmasq service. On
+// validation failure the previous file content is restored and the service
+// is left untouched.
+func applyDnsmasqConfig(path string, content string) error {
+	var backup []byte
+	hadFile := false
+	if b, err := os.ReadFile(path); err == nil {
+		backup = b
+		hadFile = true
+	} else if !os.IsNotExist(err) {
+		return fmt.Errorf("kann bestehende %s nicht lesen: %w", path, err)
+	}
+
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		return fmt.Errorf("kann %s nicht schreiben: %w", path, err)
+	}
+
+	if err := runDnsmasqTest(); err != nil {
+		// roll back so a bad generated file never lingers
+		if hadFile {
+			os.WriteFile(path, backup, 0644)
+		} else {
+			os.Remove(path)
+		}
+		return fmt.Errorf("dnsmasq-Konfiguration ungültig, Änderung zurückgerollt: %w", err)
+	}
+
+	if err := reloadDnsmasq(); err != nil {
+		return fmt.Errorf("%s geschrieben, aber Reload fehlgeschlagen: %w", path, err)
+	}
+	return nil
+}
+
+func runDnsmasqTest() error {
+	out, err := exec.Command("dnsmasq", "--test").CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("%s", strings.TrimSpace(string(out)))
+	}
+	return nil
+}
+
+func reloadDnsmasq() error {
+	out, err := exec.Command("systemctl", "reload", "dnsmasq").CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("%s", strings.TrimSpace(string(out)))
+	}
+	return nil
+}

tools/ipadm/go.mod

@@ -0,0 +1,3 @@
+module ipadm
+
+go 1.24.4

tools/ipadm/host.go

@@ -0,0 +1,143 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+	"syscall"
+)
+
+// Host is one static host entry: hostname, IP, optional MAC, free-text comment.
+type Host struct {
+	Name    string
+	IP      string
+	MAC     string // "" if not set
+	Comment string
+}
+
+// Store gives locked read-modify-write access to the flat-file host database.
+// One line per host, tab-separated: name\tip\tmac\tcomment
+// MAC is stored as "-" when empty. Lines starting with '#' and blank lines are ignored.
+type Store struct {
+	path string
+	file *os.File
+}
+
+func openStore(path string) (*Store, error) {
+	if err := os.MkdirAll(dirOf(path), 0755); err != nil {
+		return nil, fmt.Errorf("kann Verzeichnis für %s nicht anlegen: %w", path, err)
+	}
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		return nil, fmt.Errorf("kann %s nicht öffnen: %w", path, err)
+	}
+	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
+		f.Close()
+		return nil, fmt.Errorf("kann Lock auf %s nicht setzen: %w", path, err)
+	}
+	return &Store{path: path, file: f}, nil
+}
+
+func (s *Store) Close() error {
+	syscall.Flock(int(s.file.Fd()), syscall.LOCK_UN)
+	return s.file.Close()
+}
+
+func (s *Store) Load() ([]Host, error) {
+	if _, err := s.file.Seek(0, 0); err != nil {
+		return nil, err
+	}
+	var hosts []Host
+	sc := bufio.NewScanner(s.file)
+	lineNo := 0
+	for sc.Scan() {
+		lineNo++
+		line := strings.TrimRight(sc.Text(), "\r\n")
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		parts := strings.SplitN(line, "\t", 4)
+		if len(parts) < 3 {
+			return nil, fmt.Errorf("%s:%d: ungültige Zeile (erwarte mind. 3 Tab-getrennte Felder): %q", s.path, lineNo, line)
+		}
+		mac := parts[2]
+		if mac == "-" {
+			mac = ""
+		}
+		comment := ""
+		if len(parts) == 4 {
+			comment = parts[3]
+		}
+		hosts = append(hosts, Host{Name: parts[0], IP: parts[1], MAC: mac, Comment: comment})
+	}
+	if err := sc.Err(); err != nil {
+		return nil, err
+	}
+	return hosts, nil
+}
+
+func (s *Store) Save(hosts []Host) error {
+	sort.Slice(hosts, func(i, j int) bool {
+		return strings.ToLower(hosts[i].Name) < strings.ToLower(hosts[j].Name)
+	})
+	var b strings.Builder
+	b.WriteString("# ipadm host database - managed with `ipadm`, do not edit while ipadm is running\n")
+	b.WriteString("# name\tip\tmac\tcomment\n")
+	for _, h := range hosts {
+		mac := h.MAC
+		if mac == "" {
+			mac = "-"
+		}
+		fmt.Fprintf(&b, "%s\t%s\t%s\t%s\n", h.Name, h.IP, mac, h.Comment)
+	}
+	if err := s.file.Truncate(0); err != nil {
+		return err
+	}
+	if _, err := s.file.Seek(0, 0); err != nil {
+		return err
+	}
+	if _, err := s.file.WriteString(b.String()); err != nil {
+		return err
+	}
+	return s.file.Sync()
+}
+
+func dirOf(path string) string {
+	i := strings.LastIndexByte(path, '/')
+	if i <= 0 {
+		return "."
+	}
+	return path[:i]
+}
+
+func findHost(hosts []Host, name string) int {
+	for i, h := range hosts {
+		if strings.EqualFold(h.Name, name) {
+			return i
+		}
+	}
+	return -1
+}
+
+func findByIP(hosts []Host, ip string, exceptIdx int) int {
+	for i, h := range hosts {
+		if i != exceptIdx && h.IP == ip {
+			return i
+		}
+	}
+	return -1
+}
+
+func findByMAC(hosts []Host, mac string, exceptIdx int) int {
+	if mac == "" {
+		return -1
+	}
+	for i, h := range hosts {
+		if i != exceptIdx && h.MAC != "" && strings.EqualFold(h.MAC, mac) {
+			return i
+		}
+	}
+	return -1
+}

tools/ipadm/interactive.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+var stdinReader = bufio.NewReader(os.Stdin)
+
+// prompt shows label with a "[current]" hint, reads one line, and returns
+// current unchanged if the user just presses enter.
+func prompt(label, current string) string {
+	if current != "" {
+		fmt.Printf("%s [%s]: ", label, current)
+	} else {
+		fmt.Printf("%s: ", label)
+	}
+	line, _ := stdinReader.ReadString('\n')
+	line = strings.TrimSpace(line)
+	if line == "" {
+		return current
+	}
+	return line
+}
+
+// interactiveEdit implements the bare `ipadm <hostname>` form: add the host
+// if it doesn't exist yet, otherwise edit it in place. Returns the updated
+// host list, or an error if validation fails.
+func interactiveEdit(hosts []Host, name string, cfg netConfig) ([]Host, error) {
+	if err := validateHostname(name); err != nil {
+		return nil, err
+	}
+	idx := findHost(hosts, name)
+
+	var cur Host
+	if idx >= 0 {
+		cur = hosts[idx]
+		fmt.Printf("Bearbeite bestehenden Host %q (Enter = Wert behalten)\n", name)
+	} else {
+		cur = Host{Name: name}
+		fmt.Printf("Neuer Host %q (Enter bei MAC/Comment = leer lassen)\n", name)
+	}
+
+	for {
+		ip := prompt("IP-Adresse", cur.IP)
+		if err := validateIP(ip, cfg.lanCIDR, cfg.poolStart, cfg.poolEnd); err != nil {
+			fmt.Fprintln(os.Stderr, "Fehler:", err)
+			continue
+		}
+		cur.IP = ip
+		break
+	}
+
+	for {
+		mac := prompt("MAC-Adresse (optional, '-' zum Löschen)", macOrDash(cur.MAC))
+		if mac == "-" {
+			mac = ""
+		}
+		norm, err := normalizeMAC(mac)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "Fehler:", err)
+			continue
+		}
+		cur.MAC = norm
+		break
+	}
+
+	cur.Comment = prompt("Kommentar", cur.Comment)
+
+	if other := findByIP(hosts, cur.IP, idx); other >= 0 {
+		return nil, fmt.Errorf("IP %s ist bereits Host %q zugewiesen", cur.IP, hosts[other].Name)
+	}
+	if other := findByMAC(hosts, cur.MAC, idx); other >= 0 {
+		return nil, fmt.Errorf("MAC %s ist bereits Host %q zugewiesen", cur.MAC, hosts[other].Name)
+	}
+
+	if idx >= 0 {
+		hosts[idx] = cur
+	} else {
+		hosts = append(hosts, cur)
+	}
+	return hosts, nil
+}
+
+func macOrDash(mac string) string {
+	if mac == "" {
+		return ""
+	}
+	return mac
+}

tools/ipadm/main.go

@@ -0,0 +1,355 @@
+// ipadm - dhcp/dns management for the dnsmasq-based router setup on this host.
+//
+// Maintains a flat-file database of static hosts (hostname, IP, optional
+// MAC, comment) and, on `ipadm -u`, regenerates the dnsmasq include file
+// with matching host-record/dhcp-host entries and reloads dnsmasq.
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"text/tabwriter"
+)
+
+const version = "1.0.0"
+
+const usage = `ipadm - dhcp/dns management (dnsmasq backend) v` + version + `
+
+usage: ipadm <hostname>                                     add/edit host (interactive)
+       ipadm -l                                              list hosts
+       ipadm -a [-f] <hostname> <ip address> [mac address]   add host
+       ipadm -c <hostname> <comment>                         set comment
+       ipadm -i <hostname> <ip address>                      change ip address
+       ipadm -m <hostname> <mac address>                     change mac address
+       ipadm -r <old hostname> <new hostname>                rename host
+       ipadm -d <hostname>                                   delete host
+       ipadm -u                                              update (regenerate dnsmasq config + reload)
+       ipadm -h                                              this help
+`
+
+// netConfig describes the LAN the tool validates static IPs against.
+// Overridable via env vars, mainly so this can be tested without touching
+// the real /etc files.
+type netConfig struct {
+	lanCIDR   string
+	poolStart string
+	poolEnd   string
+}
+
+func envOr(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+func dbPath() string {
+	return envOr("IPADM_DB", "/etc/ipadm/hosts")
+}
+
+func dnsmasqHostsPath() string {
+	return envOr("IPADM_DNSMASQ_HOSTS", "/etc/dnsmasq.d/hosts.conf")
+}
+
+func loadNetConfig() netConfig {
+	return netConfig{
+		lanCIDR:   envOr("IPADM_LAN_CIDR", "10.0.0.0/24"),
+		poolStart: envOr("IPADM_POOL_START", "10.0.0.100"),
+		poolEnd:   envOr("IPADM_POOL_END", "10.0.0.200"),
+	}
+}
+
+func fail(format string, a ...any) {
+	fmt.Fprintf(os.Stderr, "ipadm: "+format+"\n", a...)
+	os.Exit(1)
+}
+
+func main() {
+	args := os.Args[1:]
+	if len(args) == 0 {
+		fmt.Fprint(os.Stderr, usage)
+		os.Exit(1)
+	}
+
+	switch args[0] {
+	case "-h", "--help":
+		fmt.Print(usage)
+	case "-l":
+		requireArgs(args[1:], 0, "ipadm -l")
+		cmdList()
+	case "-u":
+		requireArgs(args[1:], 0, "ipadm -u")
+		cmdUpdate()
+	case "-a":
+		cmdAdd(args[1:])
+	case "-c":
+		cmdComment(args[1:])
+	case "-i":
+		cmdChangeIP(args[1:])
+	case "-m":
+		cmdChangeMAC(args[1:])
+	case "-r":
+		cmdRename(args[1:])
+	case "-d":
+		cmdDelete(args[1:])
+	default:
+		if strings.HasPrefix(args[0], "-") {
+			fail("unbekannte Option %q\n\n%s", args[0], usage)
+		}
+		requireArgs(args[1:], 0, "ipadm <hostname>")
+		cmdInteractive(args[0])
+	}
+}
+
+func requireArgs(rest []string, want int, form string) {
+	if len(rest) != want {
+		fail("falsche Anzahl Argumente für %q\n\n%s", form, usage)
+	}
+}
+
+// withStore opens the locked host DB, loads it, lets fn mutate it, and saves
+// on success. fn returns the new host list plus a human-readable summary of
+// what changed (printed on success).
+func withStore(fn func(hosts []Host) ([]Host, string, error)) {
+	s, err := openStore(dbPath())
+	if err != nil {
+		fail("%s", err)
+	}
+	defer s.Close()
+
+	hosts, err := s.Load()
+	if err != nil {
+		fail("%s", err)
+	}
+
+	newHosts, summary, err := fn(hosts)
+	if err != nil {
+		fail("%s", err)
+	}
+
+	if err := s.Save(newHosts); err != nil {
+		fail("kann %s nicht schreiben: %s", dbPath(), err)
+	}
+	fmt.Println(summary)
+	fmt.Println("Hinweis: `ipadm -u` ausführen, um dnsmasq zu aktualisieren.")
+}
+
+func cmdInteractive(name string) {
+	cfg := loadNetConfig()
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		newHosts, err := interactiveEdit(hosts, name, cfg)
+		if err != nil {
+			return nil, "", err
+		}
+		return newHosts, fmt.Sprintf("Host %q gespeichert.", name), nil
+	})
+}
+
+func cmdList() {
+	s, err := openStore(dbPath())
+	if err != nil {
+		fail("%s", err)
+	}
+	defer s.Close()
+	hosts, err := s.Load()
+	if err != nil {
+		fail("%s", err)
+	}
+	if len(hosts) == 0 {
+		fmt.Println("keine Hosts eingetragen")
+		return
+	}
+	tw := tabwriter.NewWriter(os.Stdout, 2, 4, 2, ' ', 0)
+	fmt.Fprintln(tw, "HOSTNAME\tIP\tMAC\tCOMMENT")
+	for _, h := range sortedCopy(hosts) {
+		mac := h.MAC
+		if mac == "" {
+			mac = "-"
+		}
+		fmt.Fprintf(tw, "%s\t%s\t%s\t%s\n", h.Name, h.IP, mac, h.Comment)
+	}
+	tw.Flush()
+}
+
+func sortedCopy(hosts []Host) []Host {
+	out := make([]Host, len(hosts))
+	copy(out, hosts)
+	// Save() sorts on write, but list can run before the first save.
+	for i := 1; i < len(out); i++ {
+		for j := i; j > 0 && strings.ToLower(out[j-1].Name) > strings.ToLower(out[j].Name); j-- {
+			out[j-1], out[j] = out[j], out[j-1]
+		}
+	}
+	return out
+}
+
+func cmdAdd(args []string) {
+	force := false
+	if len(args) > 0 && args[0] == "-f" {
+		force = true
+		args = args[1:]
+	}
+	if len(args) < 2 || len(args) > 3 {
+		fail("usage: ipadm -a [-f] <hostname> <ip address> [mac address]")
+	}
+	name, ip := args[0], args[1]
+	mac := ""
+	if len(args) == 3 {
+		mac = args[2]
+	}
+	cfg := loadNetConfig()
+
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		if err := validateHostname(name); err != nil {
+			return nil, "", err
+		}
+		if err := validateIP(ip, cfg.lanCIDR, cfg.poolStart, cfg.poolEnd); err != nil {
+			return nil, "", err
+		}
+		normMac, err := normalizeMAC(mac)
+		if err != nil {
+			return nil, "", err
+		}
+
+		idx := findHost(hosts, name)
+		if idx >= 0 && !force {
+			return nil, "", fmt.Errorf("Host %q existiert bereits (benutze -f zum Überschreiben oder `ipadm %s` zum Bearbeiten)", name, name)
+		}
+		if other := findByIP(hosts, ip, idx); other >= 0 {
+			return nil, "", fmt.Errorf("IP %s ist bereits Host %q zugewiesen", ip, hosts[other].Name)
+		}
+		if other := findByMAC(hosts, normMac, idx); other >= 0 {
+			return nil, "", fmt.Errorf("MAC %s ist bereits Host %q zugewiesen", normMac, hosts[other].Name)
+		}
+
+		h := Host{Name: name, IP: ip, MAC: normMac}
+		if idx >= 0 {
+			hosts[idx] = h
+			return hosts, fmt.Sprintf("Host %q überschrieben (-f).", name), nil
+		}
+		hosts = append(hosts, h)
+		return hosts, fmt.Sprintf("Host %q hinzugefügt.", name), nil
+	})
+}
+
+func cmdComment(args []string) {
+	if len(args) < 2 {
+		fail("usage: ipadm -c <hostname> <comment>")
+	}
+	name := args[0]
+	comment := strings.Join(args[1:], " ")
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		idx := findHost(hosts, name)
+		if idx < 0 {
+			return nil, "", fmt.Errorf("Host %q nicht gefunden", name)
+		}
+		hosts[idx].Comment = comment
+		return hosts, fmt.Sprintf("Kommentar von %q gesetzt.", name), nil
+	})
+}
+
+func cmdChangeIP(args []string) {
+	if len(args) != 2 {
+		fail("usage: ipadm -i <hostname> <ip address>")
+	}
+	name, ip := args[0], args[1]
+	cfg := loadNetConfig()
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		idx := findHost(hosts, name)
+		if idx < 0 {
+			return nil, "", fmt.Errorf("Host %q nicht gefunden", name)
+		}
+		if err := validateIP(ip, cfg.lanCIDR, cfg.poolStart, cfg.poolEnd); err != nil {
+			return nil, "", err
+		}
+		if other := findByIP(hosts, ip, idx); other >= 0 {
+			return nil, "", fmt.Errorf("IP %s ist bereits Host %q zugewiesen", ip, hosts[other].Name)
+		}
+		hosts[idx].IP = ip
+		return hosts, fmt.Sprintf("IP von %q auf %s geändert.", name, ip), nil
+	})
+}
+
+func cmdChangeMAC(args []string) {
+	if len(args) != 2 {
+		fail("usage: ipadm -m <hostname> <mac address>")
+	}
+	name, mac := args[0], args[1]
+	if mac == "-" {
+		mac = ""
+	}
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		idx := findHost(hosts, name)
+		if idx < 0 {
+			return nil, "", fmt.Errorf("Host %q nicht gefunden", name)
+		}
+		normMac, err := normalizeMAC(mac)
+		if err != nil {
+			return nil, "", err
+		}
+		if other := findByMAC(hosts, normMac, idx); other >= 0 {
+			return nil, "", fmt.Errorf("MAC %s ist bereits Host %q zugewiesen", normMac, hosts[other].Name)
+		}
+		hosts[idx].MAC = normMac
+		label := normMac
+		if label == "" {
+			label = "(entfernt)"
+		}
+		return hosts, fmt.Sprintf("MAC von %q auf %s geändert.", name, label), nil
+	})
+}
+
+func cmdRename(args []string) {
+	if len(args) != 2 {
+		fail("usage: ipadm -r <old hostname> <new hostname>")
+	}
+	oldName, newName := args[0], args[1]
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		if err := validateHostname(newName); err != nil {
+			return nil, "", err
+		}
+		idx := findHost(hosts, oldName)
+		if idx < 0 {
+			return nil, "", fmt.Errorf("Host %q nicht gefunden", oldName)
+		}
+		if existing := findHost(hosts, newName); existing >= 0 && existing != idx {
+			return nil, "", fmt.Errorf("Host %q existiert bereits", newName)
+		}
+		hosts[idx].Name = newName
+		return hosts, fmt.Sprintf("Host %q umbenannt in %q.", oldName, newName), nil
+	})
+}
+
+func cmdDelete(args []string) {
+	if len(args) != 1 {
+		fail("usage: ipadm -d <hostname>")
+	}
+	name := args[0]
+	withStore(func(hosts []Host) ([]Host, string, error) {
+		idx := findHost(hosts, name)
+		if idx < 0 {
+			return nil, "", fmt.Errorf("Host %q nicht gefunden", name)
+		}
+		hosts = append(hosts[:idx], hosts[idx+1:]...)
+		return hosts, fmt.Sprintf("Host %q gelöscht.", name), nil
+	})
+}
+
+func cmdUpdate() {
+	s, err := openStore(dbPath())
+	if err != nil {
+		fail("%s", err)
+	}
+	hosts, err := s.Load()
+	s.Close()
+	if err != nil {
+		fail("%s", err)
+	}
+
+	content := renderDnsmasqConfig(hosts)
+	if err := applyDnsmasqConfig(dnsmasqHostsPath(), content); err != nil {
+		fail("%s", err)
+	}
+	fmt.Printf("%s aktualisiert, dnsmasq neu geladen (%d Hosts).\n", dnsmasqHostsPath(), len(hosts))
+}

tools/ipadm/validate.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"fmt"
+	"net"
+	"regexp"
+)
+
+var hostnameRe = regexp.MustCompile(`^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?$`)
+
+func validateHostname(name string) error {
+	if !hostnameRe.MatchString(name) {
+		return fmt.Errorf("ungültiger Hostname %q (nur Buchstaben, Ziffern, Bindestrich, kein führender/abschließender Bindestrich, keine Domain)", name)
+	}
+	return nil
+}
+
+func ip4ToUint32(ip net.IP) uint32 {
+	b := ip.To4()
+	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])
+}
+
+// validateIP checks that ip is a valid IPv4 address inside lanCIDR and outside
+// the dynamic DHCP pool [poolStart, poolEnd] (both inclusive), so static
+// reservations can never collide with dynamically leased addresses.
+func validateIP(ip, lanCIDR, poolStart, poolEnd string) error {
+	parsed := net.ParseIP(ip)
+	if parsed == nil || parsed.To4() == nil {
+		return fmt.Errorf("ungültige IPv4-Adresse: %q", ip)
+	}
+	_, cidr, err := net.ParseCIDR(lanCIDR)
+	if err != nil {
+		return fmt.Errorf("internes Problem: ungültiges LAN-CIDR %q: %w", lanCIDR, err)
+	}
+	if !cidr.Contains(parsed) {
+		return fmt.Errorf("IP %s liegt nicht im LAN-Netz %s", ip, lanCIDR)
+	}
+	start := net.ParseIP(poolStart)
+	end := net.ParseIP(poolEnd)
+	if start == nil || end == nil {
+		return fmt.Errorf("internes Problem: ungültiger DHCP-Pool %q-%q", poolStart, poolEnd)
+	}
+	v, s, e := ip4ToUint32(parsed), ip4ToUint32(start), ip4ToUint32(end)
+	if v >= s && v <= e {
+		return fmt.Errorf("IP %s liegt im dynamischen DHCP-Pool (%s-%s) und ist für statische Reservierungen gesperrt", ip, poolStart, poolEnd)
+	}
+	return nil
+}
+
+// normalizeMAC validates a MAC address and returns it in canonical lower-case
+// colon-separated form. Empty input is accepted and returned as-is.
+func normalizeMAC(mac string) (string, error) {
+	if mac == "" {
+		return "", nil
+	}
+	hw, err := net.ParseMAC(mac)
+	if err != nil {
+		return "", fmt.Errorf("ungültige MAC-Adresse %q: %w", mac, err)
+	}
+	return hw.String(), nil
+}