Let jarld recognize this host's addresses under systemd

The unit's RestrictAddressFamilies lacked AF_NETLINK, which listing the
host's addresses needs, so the name server checks didn't know this
server's own addresses: the Name servers tab queried them like another
server's, and the delegation check didn't sign them with the view key.
Allow AF_NETLINK, and fall back to /proc/net without it.

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

internal/nscheck/nscheck.go

@@ -15,10 +15,13 @@ package nscheck
 
 import (
 	"context"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"net"
 	"net/netip"
+	"os"
+	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
@@ -257,20 +260,67 @@ func IsLocalAddr(a netip.Addr) bool {
 	if a.IsLoopback() {
 		return true
 	}
-	addrs, err := net.InterfaceAddrs()
-	if err != nil {
-		return false
-	}
-	for _, ia := range addrs {
-		if n, ok := ia.(*net.IPNet); ok {
-			if b, ok := netip.AddrFromSlice(n.IP); ok && b.Unmap() == a.Unmap() {
-				return true
-			}
+	for _, b := range LocalAddrs() {
+		if b == a.Unmap() {
+			return true
 		}
 	}
 	return false
 }
 
+// LocalAddrs returns this host's interface addresses. Listing them needs a
+// netlink socket; where that is not allowed (a systemd unit without
+// AF_NETLINK in RestrictAddressFamilies), they are read from /proc.
+func LocalAddrs() []netip.Addr {
+	var out []netip.Addr
+	if addrs, err := net.InterfaceAddrs(); err == nil {
+		for _, ia := range addrs {
+			if n, ok := ia.(*net.IPNet); ok {
+				if b, ok := netip.AddrFromSlice(n.IP); ok {
+					out = append(out, b.Unmap())
+				}
+			}
+		}
+		return out
+	}
+	return procAddrs("/proc/net")
+}
+
+// procAddrs reads the host's addresses from /proc/net: IPv6 from
+// if_inet6, IPv4 from the local (/32 host LOCAL) entries of fib_trie.
+func procAddrs(dir string) []netip.Addr {
+	var out []netip.Addr
+	if b, err := os.ReadFile(filepath.Join(dir, "if_inet6")); err == nil {
+		for _, line := range strings.Split(string(b), "\n") {
+			f := strings.Fields(line)
+			if len(f) == 0 || len(f[0]) != 32 {
+				continue
+			}
+			raw, err := hex.DecodeString(f[0])
+			if err != nil {
+				continue
+			}
+			if a, ok := netip.AddrFromSlice(raw); ok {
+				out = append(out, a)
+			}
+		}
+	}
+	if b, err := os.ReadFile(filepath.Join(dir, "fib_trie")); err == nil {
+		seen := map[netip.Addr]bool{}
+		var last netip.Addr
+		for _, line := range strings.Split(string(b), "\n") {
+			t := strings.TrimSpace(line)
+			if strings.HasPrefix(t, "|-- ") {
+				last, _ = netip.ParseAddr(strings.TrimPrefix(t, "|-- "))
+			} else if strings.HasPrefix(t, "/32 host LOCAL") && last.IsValid() && !seen[last] {
+				seen[last] = true
+				out = append(out, last)
+			}
+		}
+	}
+	return out
+}
+
 // SystemResolver returns the first nameserver in /etc/resolv.conf as
 // host:port.
 func SystemResolver() (string, error) {