housecarl/docs/rules.md

# Zones, sets, services and rules

## Zones

A zone names one or more interfaces, so rules can say "from the LAN"
rather than `eth1`. An interface belongs to one zone at most. A trailing
`*` matches many: `incusbr*`.

```sh
hc zone add wan --interface wan --external
hc zone add lan --interface eth1 --interface 'eth1.*'
hc zone add wg  --interface wg0
```

**External** zones face the internet. Antispoofing, MSS clamping, the
DHCPv6 client rule and port forwards (without public addresses) apply on
them. Everything else is internal: ping to the firewall from internal
zones is accepted by a built-in.

A zone may name an interface that doesn't exist yet (a WireGuard
interface that comes up later): its rules match once it appears, and
`hc status` warns until then.

## Address sets

A set is a named list of IPv4 and IPv6 addresses, prefixes and ranges,
mixed:

```sh
hc set add trusted 192.168.1.0/24 192.168.2.0/24 2001:db8:1::/64
hc set add dns-servers 192.168.1.5 2001:db8:1::5 192.168.1.250-192.168.1.254
```

In the ruleset each becomes two nftables sets, `trusted_v4` and
`trusted_v6` (one if the set has only one family). A rule that names it
becomes one nftables rule per family. Rules can also give addresses
directly (`--from 192.168.1.3`); a set is better when several rules share
them.

A **dynamic** set's entries are added at run time, each with a timeout,
and nftables removes them when it runs out, even with housecarld stopped:

```sh
hc set add guests --dynamic --timeout 24h
hc set entries add guests 10.0.30.17                 # for 24 hours
hc set entries add guests 2001:db8:3::17 --timeout 2h
hc set entries guests                                # with the time left
hc set entries remove guests 10.0.30.17
```

Entries aren't configuration: they aren't in the history and aren't on
probation, but each change is in the audit log. They survive every
change to the ruleset, and a restart of housecarld; a reboot empties
them. A token can be limited to one set ([permissions.md](permissions.md)):
that is how another program (Steward's captive portal) lets a client in.

## Services

A service names ports of a protocol (`tcp`, `udp`, `sctp`) and ICMP
types:

```sh
hc service add web tcp/8080 tcp/8443
hc service add games udp/27015-27030 tcp/27015
hc service add pmtu icmp/destination-unreachable icmpv6/packet-too-big
```

Many are built in (`hc service list`): `ssh`, `dns` (TCP and UDP 53),
`dot`, `http`, `https` (TCP, and UDP for QUIC), `smtp`, `submission`,
`smtps`, `imaps`, `ntp`, `dhcp`, `dhcpv6`, `wireguard`, `openvpn`, `ipsec`,
`snmp`, `syslog`, `mdns`, `rdp`, and `ping` (ICMP and ICMPv6 echo
requests). Adding a service with a built-in's name replaces it. Rules may
also give an entry directly: `--service tcp/8080`.

## Rules

Rules are kept in order, in four chains:

| Chain | | Hook |
|---|---|---|
| `input` | traffic to the firewall itself | input |
| `forward` | traffic through it, between zones | forward |
| `output` | traffic from it | output |
| `early` | before connection tracking: cheap drops of what can never be right | prerouting, priority raw |

Each chain is checked top to bottom, and the first rule that accepts,
drops or rejects decides. What no rule decides gets the chain's
**policy** (Settings): by default input and forward drop, output
accepts.

```sh
hc rule add input --from trusted --service ssh --comment "SSH from home"
hc rule add input --in wg --service dns
hc rule add forward --in lan --out wan
hc rule add forward --in wan -a drop --comment "nothing else gets in"
hc rule list input
hc rule move 12 --before 7
```

A rule matches on what it gives, and all of it:

| | |
|---|---|
| `--in`, `--out` | zones it comes in on, goes out on (`--not-in`, `--not-out`: any other). Input has no out zone, output no in zone |
| `--from`, `--to` | sets, addresses, prefixes, ranges (`--not-from`, `--not-to`: anything else) |
| `--service` | services or entries (`tcp/8080`, `icmp/echo-request`) |
| `--family` | `ipv4` or `ipv6` only; otherwise it follows the addresses |
| `--state` | connection states: `new`, `established`, `related`, `invalid`, `untracked` |
| `--status` | connection status: `dnat` (port forwarded), `snat` |
| `--days`, `--hours` | a time window: `--days Monday,Friday --hours 08:00-17:30`; `22:00-06:00` passes midnight. In the firewall's time zone, as the kernel knows it |
| `--limit` | a rate: `10/minute`, or bytes: `1mbytes/second`; with `,burst=N`, `,over` (match beyond the rate) and `,per-source` or `,per-destination` ([Bandwidth and quotas](#bandwidth-and-quotas)) |
| `--quota` | an allowance of traffic: `50gbytes,over,per-source,monthly=15` ([Bandwidth and quotas](#bandwidth-and-quotas)) |
| `--conn-limit N` | a source with more than N connections |

The action is `accept`, `drop`, `reject` (an ICMP error, or with
`--reject-with tcp-reset` a TCP reset) or `log` (log and go on).
`--log` logs the packets of any rule ([operations.md](operations.md#the-log)).
`--disabled` (or `hc rule disable ID`) keeps a rule without using it.

Examples:

```sh
# SSH from the internet, at most 3 new connections a minute per source
hc rule add input --in wan --service ssh --state new --limit 3/minute,burst=5,over,per-source -a drop --log
hc rule add input --in wan --service ssh

# Children's devices offline at night
hc rule add forward --from kids --out wan --hours 22:00-07:00 -a reject

# What was port forwarded may go on, the rest from the internet may not
hc rule add forward --in wan --status dnat
hc rule add forward --in wan -a drop
```

### Bandwidth and quotas

A **byte rate** limits bandwidth: `--limit 20mbytes/second,over,per-source
-a drop` drops what a device sends beyond 20 MB/s. Units are `bytes`,
`kbytes` and `mbytes` (`kb`, `mb` for short); the burst is in the same
unit. nftables can only hold byte rates per second (up to 4 GB/s) or per
minute (up to 293 MB/minute); validation says so for larger ones. This is
policing: traffic beyond the rate is dropped, not queued, so TCP slows to
fit, and UDP loses packets. Shaping (queues, fair shares) is `tc`'s work,
not a firewall's.

A **quota** is an allowance of traffic. `over` matches once it is used up,
to drop what comes after:

```sh
# Each device on the LAN: 50 GB a month, from the 15th (a billing day)
hc rule add forward --in lan --out wan --quota 50gbytes,over,per-source,monthly=15 -a drop
# The whole guest network: 10 GB a day together
hc rule add forward --in guest --out wan --quota 10gbytes,over,daily -a drop
hc quotas                      # what each used, and the next reset
hc rule reset-quota 31         # start again now
```

| | |
|---|---|
| `per-source`, `per-destination` | each address its own allowance; without, one allowance for everything the rule matches |
| `daily`, `weekly`, `monthly=DAY` | when it starts again: at midnight; weekly on Mondays; monthly on that day (the last day of a shorter month). Without one, only by hand |

nftables counts a quota but never resets it, and forgets it when its
table is replaced: Housecarl carries what each quota used across every
change and restart, saves it every five minutes (and when housecarld
stops) for after a reboot, and resets quotas when their period ends.
A per-address quota counts an address: a device that gets a new one
starts again. With Steward's reservations IPv4 addresses stay; IPv6 is
grouped by /64 (below).

**Rules with a quota or a byte rate count every packet.** The built-in
that accepts established connections comes before your rules, so a rule
there only sees each connection's first packet. Rules that count traffic
are checked before it instead, in their order (the web UI lists them
first, marked "every packet"). So they can drop, reject or log, but not
accept: an accept there would let packets past every later rule. Let the
traffic in with a rule of its own. The flowtable's fast path skips the
forward chain for established connections, so with it on, forward
quotas and byte rates only count first packets (a warning says so).

**Per-address limits group addresses**: each IPv4 address on its own, and
IPv6 addresses by /64, since one device uses several (privacy addresses
change). `--group-v4 24` counts each /24 together; `--group-v6 128` each
IPv6 address alone. This applies to per-address rates, connection limits
and quotas.

### Raw rules

For what Housecarl can't say, a rule can be one nftables rule written by
hand, placed like any other:

```sh
hc rule add forward --raw 'iifname != "wan" ct state invalid ip daddr @trusted_v4 accept'
```

It is checked by nftables with the rest of the ruleset, and can use the
sets' nftables names (`@trusted_v4`). The importer makes raw rules from
what it can't translate. It goes into the ruleset as written: add
`counter` to it to count its packets.

## Built-ins

Housecarl adds the rules every firewall needs before yours. Each can be
turned off (`hc settings set --builtin NAME=off`; Settings in the web
UI), and `hc ruleset` shows them:

| Built-in | Default | Chains | What |
|---|---|---|---|
| `loopback` | on | input (and output if its policy is drop) | accept everything on `lo` |
| `established` | on | input, forward (and output) | accept established and related connections |
| `drop_invalid` | on | input, forward | drop packets connection tracking calls invalid |
| `icmpv6` | on | input, forward (and output) | the ICMPv6 IPv6 can't work without (RFC 4890): errors and echo; neighbour discovery with hop limit 255 and MLD from link-local, to the firewall |
| `ping_internal` | on | input | ping (ICMP echo) to the firewall from internal zones |
| `dhcpv6_client` | on | input | DHCPv6 replies from link-local on external zones, so the firewall gets its prefix delegation |
| `antispoof` | on | early | drop sources that can't come from the internet, on external zones: the antispoof lists (private, loopback, documentation and multicast ranges) and your own prefixes |
| `mss_clamp` | off | forward | clamp TCP's MSS to the route's MTU on external zones: needed with PPPoE |

**Your prefixes** (`--own-prefix 2001:db8::/48`) are your delegated
IPv6 prefix and public addresses: nothing from outside should come from
them. The antispoof lists can be changed with `--antispoof-v4` and
`--antispoof-v6`.

The **flowtable** (`--flowtable wan --flowtable eth1`) lets established
TCP and UDP connections forwarded between those devices skip the rules,
which helps on a slow CPU. It needs exact device names. Counters of
forward rules then only count each connection's first packets.

## The ruleset

`hc ruleset` prints what Housecarl builds; the web UI and `hc rule list`
show each rule's hits. The table looks like this (shortened):

```
table inet housecarl {
	set trusted_v4 { type ipv4_addr; flags interval; elements = { 192.168.1.0/24, 192.168.2.0/24 } }
	set trusted_v6 { type ipv6_addr; flags interval; elements = { 2001:db8:1::/64 } }
	chain early   { type filter hook prerouting priority raw; policy accept; ... }
	chain input   { type filter hook input priority filter; policy drop;
		iifname "lo" accept comment "hc:b-loopback"
		ct state established,related counter accept comment "hc:b-established"
		...
		ip saddr @trusted_v4 tcp dport 22 counter accept comment "hc:r4 SSH from home"
		ip6 saddr @trusted_v6 tcp dport 22 counter accept comment "hc:r4 SSH from home"
		counter comment "hc:p-input"
	}
	chain forward { ... }
	chain output  { ... }
	chain dstnat  { type nat hook prerouting priority dstnat; ... }
	chain srcnat  { type nat hook postrouting priority srcnat; ... }
}
```

Other tables still apply: a packet must get past every table's chains on
its hook. The dashboard and `hc status` list them.