Steward 0.1.0 (in development): a DHCPv4 server with a web UI, API and CLI

Subnets served per interface, pools, reservations, options at three
levels, leases in SQLite saved before the ACK, ping and decline conflict
checks, validated changes with history and undo, users and tokens, an
audit log, a Kea importer, a sandboxed systemd unit and Debian packages.
Tested with real udhcpc and dhcpcd clients in Docker (scripts/e2e.sh).

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

internal/dhcpd/ping.go

@@ -0,0 +1,84 @@
+package dhcpd
+
+import (
+	"context"
+	"log/slog"
+	"net"
+	"net/netip"
+	"os"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+)
+
+// ICMPPinger pings with an unprivileged ICMP socket (Linux "ping sockets",
+// allowed for the groups in net.ipv4.ping_group_range, which systemd opens
+// to everyone). When the socket can't be opened, the check is skipped and
+// Unavailable says why.
+type ICMPPinger struct {
+	Log  *slog.Logger
+	seq  atomic.Uint32
+	once sync.Once
+	// Unavailable is why pings can't be sent ("" when they can).
+	Unavailable atomic.Value
+}
+
+// Ping reports whether a answers an echo request within timeout.
+func (p *ICMPPinger) Ping(ctx context.Context, a netip.Addr, timeout time.Duration) bool {
+	conn, err := icmp.ListenPacket("udp4", "0.0.0.0")
+	if err != nil {
+		p.once.Do(func() {
+			msg := "can't send pings, so addresses aren't checked before they are offered: " + err.Error() + " (check net.ipv4.ping_group_range)"
+			p.Unavailable.Store(msg)
+			if p.Log != nil {
+				p.Log.Warn(msg)
+			}
+		})
+		return false
+	}
+	defer conn.Close()
+	seq := int(p.seq.Add(1) & 0xffff)
+	msg := icmp.Message{Type: ipv4.ICMPTypeEcho, Body: &icmp.Echo{ID: os.Getpid() & 0xffff, Seq: seq, Data: []byte("steward conflict check")}}
+	b, err := msg.Marshal(nil)
+	if err != nil {
+		return false
+	}
+	dst := &net.UDPAddr{IP: a.AsSlice()}
+	if _, err := conn.WriteTo(b, dst); err != nil {
+		return false
+	}
+	deadline := time.Now().Add(timeout)
+	if d, ok := ctx.Deadline(); ok && d.Before(deadline) {
+		deadline = d
+	}
+	conn.SetReadDeadline(deadline)
+	buf := make([]byte, 1500)
+	for {
+		n, from, err := conn.ReadFrom(buf)
+		if err != nil {
+			return false
+		}
+		ua, ok := from.(*net.UDPAddr)
+		if !ok || !ua.IP.Equal(dst.IP) {
+			continue
+		}
+		m, err := icmp.ParseMessage(1, buf[:n])
+		if err != nil {
+			continue
+		}
+		if e, ok := m.Body.(*icmp.Echo); ok && m.Type == ipv4.ICMPTypeEchoReply && e.Seq == seq {
+			return true
+		}
+	}
+}
+
+// Problem says why pings can't be sent ("" when they can).
+func (p *ICMPPinger) Problem() string {
+	if v, ok := p.Unavailable.Load().(string); ok {
+		return v
+	}
+	return ""
+}