Build Housecarl 0.1.0: the nftables firewall manager

Zones, address sets (IPv4 and IPv6 mixed, dynamic sets with timeouts) and
services, ordered rules in the early, input, forward and output chains,
port forwards with hairpin NAT and IPv6 pinholes, source NAT, redirects,
and switchable built-ins (loopback, conntrack, ICMPv6, antispoofing, MSS
clamping, DHCPv6 client, flowtable). Every change is validated, checked
with nft -c and applied in one transaction to Housecarl's own table,
inet housecarl; changes made over the network roll back unless
confirmed, and the confirmed ruleset is loaded at boot by
housecarl-ruleset.service. An empty configuration applies nothing.

Also: hit counters and the drop log (nflog), 'hc import' of an existing
nftables ruleset and 'hc takeover', the hc CLI, the REST API and the web
UI, users, scoped tokens (sets:write:NAME), the audit log, the web UI's
certificate through Jarl, Debian packages, docs, unit tests with real
nft 1.1.3, and an e2e test with real traffic (71 checks).

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

internal/hostif/hostif.go

@@ -0,0 +1,79 @@
+// Package hostif reads the machine's network interfaces: their IPv4 and
+// IPv6 addresses and whether they are up. Results are cached for a few
+// seconds, since validation and the status page ask often.
+package hostif
+
+import (
+	"net"
+	"net/netip"
+	"sync"
+	"time"
+)
+
+// Host reads this machine's interfaces.
+type Host struct {
+	mu    sync.Mutex
+	at    time.Time
+	addrs map[string][]netip.Prefix
+	up    map[string]bool
+	err   error
+}
+
+func (h *Host) refresh() {
+	if time.Since(h.at) < 5*time.Second && (h.addrs != nil || h.err != nil) {
+		return
+	}
+	h.at = time.Now()
+	ifs, err := net.Interfaces()
+	if err != nil {
+		h.addrs, h.up, h.err = nil, nil, err
+		return
+	}
+	addrs, up := map[string][]netip.Prefix{}, map[string]bool{}
+	for _, i := range ifs {
+		var ps []netip.Prefix
+		as, _ := i.Addrs()
+		for _, a := range as {
+			if n, ok := a.(*net.IPNet); ok {
+				if ip, ok := netip.AddrFromSlice(n.IP); ok {
+					bits, _ := n.Mask.Size()
+					ps = append(ps, netip.PrefixFrom(ip.Unmap(), bits))
+				}
+			}
+		}
+		addrs[i.Name] = ps
+		up[i.Name] = i.Flags&net.FlagUp != 0
+	}
+	h.addrs, h.up, h.err = addrs, up, nil
+}
+
+// All returns every interface with its addresses.
+func (h *Host) All() (map[string][]netip.Prefix, error) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.refresh()
+	return h.addrs, h.err
+}
+
+// Up reports whether an interface is up.
+func (h *Host) Up(name string) bool {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.refresh()
+	return h.up[name]
+}
+
+// Exists returns the set of interface names (nil if they can't be read).
+func (h *Host) Exists() map[string]bool {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.refresh()
+	if h.err != nil {
+		return nil
+	}
+	out := make(map[string]bool, len(h.addrs))
+	for n := range h.addrs {
+		out[n] = true
+	}
+	return out
+}