Adds IPAddressDeny= to the sandbox unit covering loopback (127/8 + ::1),
link-local (169.254/16 + fe80::/10), multicast (224/4 + ff00::/8), all
RFC1918 v4 (10/8, 172.16/12, 192.168/16), CGNAT (100.64/10), and ULA v6
(fc00::/7). The kernel attaches systemd's sd_fw_egress BPF program to
the unit's cgroup; egress packets matching any of the deny prefixes are
silently dropped at the cgroup boundary.
Important: do NOT pair this with `IPAddressAllow=any`. Documentation
claims "more specific rule wins" but on this systemd 257 + kernel 6.12
combo, having both set causes the allow to win unconditionally — the
deny gets ignored. Empty IPAddressAllow + populated IPAddressDeny is the
correct shape: kernel default "allow all" applies to non-listed
addresses, and the listed prefixes are blocked.
Because the host's resolv.conf typically points at a private-IP DNS
server (10.0.0.1 in the test deploy), blocking RFC1918 also kills DNS.
Adds a static /etc/left4me/sandbox-resolv.conf with public resolvers
(Cloudflare 1.1.1.1, Google 8.8.8.8) and bind-mounts that into the
sandbox at /etc/resolv.conf, replacing the host's resolver inside the
sandbox only.
Smoke-tested on ckn@10.0.4.128:
- public 1.1.1.1:443: CONNECTED
- public HTTPS via DNS (steamcommunity.com): 200
- localhost web app 127.0.0.1:8000: blocked (TimeoutError)
- localhost sshd 127.0.0.1:22: blocked
- private LAN ssh 10.0.4.128:22: blocked
- private DNS 10.0.0.1:53: blocked
AF_UNIX stays in RestrictAddressFamilies — dropping it would risk
breaking NSS / syslog for marginal gain, and the IP-level filter
addresses the primary threat (reaching the host's HTTP/SSH services).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
7e66936d03