firewall/firewall.py

#!/usr/bin/env python3

from argparse import ArgumentParser, FileType
from dataclasses import dataclass
from enum import Enum
from graphlib import TopologicalSorter
from ipaddress import IPv4Address, IPv4Network, IPv6Network
from nftables import Nftables
from pydantic import (
    BaseModel,
    Extra,
    FilePath,
    IPvAnyNetwork,
    ValidationError,
    conint,
    parse_obj_as,
    validator,
    root_validator,
)
from typing import Iterator, Generic, TypeAlias, TypeVar
from yaml import safe_load
import nft


# ==========[ PYDANTIC ]========================================================

T = TypeVar("T")


class AutoSet(set[T], Generic[T]):
    @classmethod
    def __get_validators__(cls):
        yield cls.__validator__

    @classmethod
    def __validator__(cls, value):
        try:
            return parse_obj_as(set[T], value)
        except ValidationError:
            return {parse_obj_as(T, value)}


class RestrictiveBaseModel(BaseModel):
    class Config:
        allow_mutation = False
        extra = Extra.forbid


# ==========[ YAML MODEL ]======================================================


# Ports
Port: TypeAlias = conint(ge=0, lt=2**16)


class PortRange(str):
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, v):
        try:
            start, end = v.split("..")
        except AttributeError:
            parse_obj_as(Port, v)  # This is the expected error
            raise ValueError("invalid port range: must be in the form start..end")
        except ValueError:
            raise ValueError("invalid port range: must be in the form start..end")

        start, end = parse_obj_as(Port, start), parse_obj_as(Port, end)
        if start > end:
            raise ValueError("invalid port range: start must be less than end")

        return range(start, end)


# Zones
ZoneName: TypeAlias = str


class ZoneEntry(RestrictiveBaseModel):
    addrs: AutoSet[IPvAnyNetwork] = AutoSet()
    file: FilePath | None = None
    negate: bool = False
    zones: AutoSet[ZoneName] = AutoSet()

    @root_validator()
    def validate_mutually_exactly_one(cls, values):
        fields = ["addrs", "file", "zones"]

        if sum(1 for field in fields if values.get(field)) != 1:
            raise ValueError(f"exactly one of {fields} must be set")

        return values


# Blacklist
class Blacklist(RestrictiveBaseModel):
    blocked: AutoSet[IPvAnyNetwork | ZoneName] = AutoSet()


# Reverse Path Filter
class ReversePathFilter(RestrictiveBaseModel):
    interfaces: AutoSet[str] = AutoSet()


# Filters
class Verdict(str, Enum):
    accept = "accept"
    drop = "drop"
    reject = "reject"


class TcpProtocol(RestrictiveBaseModel):
    dport: AutoSet[Port | PortRange] = AutoSet()
    sport: AutoSet[Port | PortRange] = AutoSet()

    def __bool__(self) -> bool:
        return bool(self.sport or self.dport)

    def __getitem__(self, key: str) -> set[Port | PortRange]:
        return getattr(self, key)


class UdpProtocol(RestrictiveBaseModel):
    dport: AutoSet[Port | PortRange] = AutoSet()
    sport: AutoSet[Port | PortRange] = AutoSet()

    def __bool__(self) -> bool:
        return bool(self.sport or self.dport)

    def __getitem__(self, key: str) -> set[Port | PortRange]:
        return getattr(self, key)


class Protocols(RestrictiveBaseModel):
    icmp: bool = False
    ospf: bool = False
    tcp: TcpProtocol = TcpProtocol()
    udp: UdpProtocol = UdpProtocol()
    vrrp: bool = False

    def __getitem__(self, key: str) -> bool | TcpProtocol | UdpProtocol:
        return getattr(self, key)


class Rule(RestrictiveBaseModel):
    iif: str | None
    oif: str | None
    protocols: Protocols = Protocols()
    src: AutoSet[IPvAnyNetwork | ZoneName] | None
    dst: AutoSet[IPvAnyNetwork | ZoneName] | None
    verdict: Verdict = Verdict.accept


class Filter(RestrictiveBaseModel):
    input: list[Rule] = []
    output: list[Rule] = []
    forward: list[Rule] = []


# Nat
class SNat(RestrictiveBaseModel):
    addr: IPv4Address | IPv4Network
    port: Port | PortRange | None
    persistent: bool = True

    @root_validator()
    def validate_mutually_exactly_one(cls, values):
        if values.get("port") and isinstance(values.get("addr"), IPv4Network):
            raise ValueError("port cannot be set when addr is a network")

        return values


class Nat(RestrictiveBaseModel):
    src: AutoSet[IPv4Network | ZoneName]
    dst: AutoSet[IPv4Network | ZoneName]
    snat: SNat


# Root model
class Firewall(RestrictiveBaseModel):
    zones: dict[ZoneName, ZoneEntry] = {}
    blacklist: Blacklist = Blacklist()
    reverse_path_filter: ReversePathFilter = ReversePathFilter()
    filter: Filter = Filter()
    nat: list[Nat] = []


# ==========[ ZONES ]===========================================================


class ZoneFile(RestrictiveBaseModel):
    __root__: AutoSet[IPvAnyNetwork]


@dataclass(eq=True, frozen=True)
class ResolvedZone:
    addrs: set[IPvAnyNetwork]
    negate: bool


Zones: TypeAlias = dict[ZoneName, ResolvedZone]


def resolve_zones(yaml_zones: dict[ZoneName, ZoneEntry]) -> Zones:
    zones: Zones = {}
    zone_graph = {name: entry.zones for (name, entry) in yaml_zones.items()}

    for name in TopologicalSorter(zone_graph).static_order():
        if yaml_zones[name].addrs:
            zones[name] = ResolvedZone(yaml_zones[name].addrs, yaml_zones[name].negate)

        elif yaml_zones[name].file is not None:
            with open(yaml_zones[name].file, "r") as file:
                try:
                    yaml_addrs = ZoneFile(__root__=safe_load(file))
                except Exception as e:
                    raise Exception(
                        f"YAML parsing of the included file '{yaml_zones[name].file}' failed: {e}"
                    )

            zones[name] = ResolvedZone(yaml_addrs.__root__, yaml_zones[name].negate)

        elif yaml_zones[name].zones:
            addrs: set[IPvAnyNetwork] = set()

            for subzone in yaml_zones[name].zones:
                if yaml_zones[subzone].negate:
                    raise ValueError(
                        f"subzone '{subzone}' of zone '{name}' cannot be negated"
                    )
                addrs.update(yaml_zones[subzone].addrs)

            zones[name] = ResolvedZone(addrs, yaml_zones[name].negate)

    return zones


# ==========[ PARSER ]==========================================================


def split_v4_v6(
    addrs: Iterator[IPvAnyNetwork],
) -> tuple[set[IPv4Network], set[IPv6Network]]:
    v4, v6 = set(), set()

    for addr in addrs:
        match addr:
            case IPv4Network():
                v4.add(addr)

            case IPv6Network():
                v6.add(addr)

    return v4, v6


def zones_into_ip(
    elements: set[IPvAnyNetwork | ZoneName],
    zones: Zones,
    allow_negate: bool = True,
) -> tuple[Iterator[IPvAnyNetwork], bool]:
    def transform() -> Iterator[IPvAnyNetwork]:
        for element in elements:
            match element:
                case ZoneName():
                    try:
                        zone = zones[element]
                    except KeyError:
                        raise ValueError(f"zone '{element}' does not exist")

                    if not allow_negate and zone.negate:
                        raise ValueError(f"zone '{element}' cannot be negated")

                    yield from zone.addrs

                case IPv4Network() | IPv6Network():
                    yield element

    is_negated = any(zones[e].negate for e in elements if isinstance(e, ZoneName))

    if is_negated and len(elements) > 1:
        raise ValueError(f"A negated zone cannot be in a set")

    return transform(), is_negated


def parse_blacklist(blacklist: Blacklist, zones: Zones) -> nft.Table:
    # Sets blacklist_v4 and blacklist_v6
    set_v4 = nft.Set(name="blacklist_v4", type="ipv4_addr", flags=["interval"])
    set_v6 = nft.Set(name="blacklist_v6", type="ipv6_addr", flags=["interval"])

    ip_v4, ip_v6 = split_v4_v6(
        zones_into_ip(blacklist.blocked, zones, allow_negate=False)[0]
    )

    set_v4.elements.extend(ip_v4)
    set_v6.elements.extend(ip_v6)

    # Chain filter
    chain_filter = nft.Chain(
        name="filter",
        type="filter",
        hook="prerouting",
        policy="accept",
        priority=-310,
    )

    rule_v4 = nft.Match(
        op="==",
        left=nft.Payload(protocol="ip", field="saddr"),
        right="@blacklist_v4",
    )
    rule_v6 = nft.Match(
        op="==",
        left=nft.Payload(protocol="ip6", field="saddr"),
        right="@blacklist_v6",
    )

    chain_filter.rules.append(nft.Rule([rule_v4, nft.Verdict("drop")]))
    chain_filter.rules.append(nft.Rule([rule_v6, nft.Verdict("drop")]))

    # Resulting table
    table = nft.Table(name="blacklist", family="inet")

    table.chains.extend([chain_filter])
    table.sets.extend([set_v4, set_v6])

    return table


def parse_reverse_path_filter(rpf: ReversePathFilter) -> nft.Table:
    # Set disabled_ifs
    disabled_ifs = nft.Set(name="disabled_ifs", type="ifname")

    disabled_ifs.elements.extend(rpf.interfaces)

    # Chain filter
    chain_filter = nft.Chain(
        name="filter",
        type="filter",
        hook="prerouting",
        policy="accept",
        priority=-300,
    )

    rule_iifname = nft.Match(op="!=", left=nft.Meta("iifname"), right="@disabled_ifs")

    rule_fib = nft.Match(
        op="==",
        left=nft.Fib(flags=["saddr", "iif"], result="oif"),
        right=False,
    )

    rule_pkttype = nft.Match(
        op="==",
        left=nft.Meta("pkttype"),
        right="host",
    )

    chain_filter.rules.append(
        nft.Rule([rule_iifname, rule_fib, rule_pkttype, nft.Verdict("drop")])
    )

    # Resulting table
    table = nft.Table(name="reverse_path_filter", family="inet")

    table.chains.extend([chain_filter])
    table.sets.extend([disabled_ifs])

    return table


class InetRuleBuilder:
    def __init__(self) -> None:
        self._v4: list[nft.Statement] | None = []
        self._v6: list[nft.Statement] | None = []

    def add_any(self, stmt: nft.Statement) -> None:
        self.add_v4(stmt)
        self.add_v6(stmt)

    def add_v4(self, stmt: nft.Statement) -> None:
        if self._v4 is not None:
            self._v4.append(stmt)

    def add_v6(self, stmt: nft.Statement) -> None:
        if self._v6 is not None:
            self._v6.append(stmt)

    def disable_v4(self) -> None:
        self._v4 = None

    def disable_v6(self) -> None:
        self._v6 = None

    @property
    def rules(self) -> Iterator[nft.Rule]:
        if self._v4 is not None:
            yield nft.Rule(self._v4)
        if self._v6 is not None and self._v6 != self._v4:
            yield nft.Rule(self._v6)


def parse_filter_rule(rule: Rule, zones: Zones) -> Iterator[nft.Rule]:
    builder = InetRuleBuilder()

    for attr in ("iif", "oif"):
        if getattr(rule, attr, None) is not None:
            builder.add_any(
                nft.Match(
                    op="==",
                    left=nft.Meta(f"{attr}name"),
                    right=getattr(rule, attr),
                )
            )

    for attr, field in (("src", "saddr"), ("dst", "daddr")):
        if getattr(rule, attr, None) is not None:
            addrs, negated = zones_into_ip(getattr(rule, attr), zones)
            addr_v4, addr_v6 = split_v4_v6(addrs)

            if addr_v4:
                builder.add_v4(
                    nft.Match(
                        op=("!=" if negated else "=="),
                        left=nft.Payload(protocol="ip", field=field),
                        right=addr_v4,
                    )
                )
            else:
                builder.disable_v4()

            if addr_v6:
                builder.add_v6(
                    nft.Match(
                        op=("!=" if negated else "=="),
                        left=nft.Payload(protocol="ip6", field=field),
                        right=addr_v6,
                    )
                )
            else:
                builder.disable_v6()

    protos = {
        "icmp": ("icmp", "icmpv6"),
        "ospf": (89, 89),
        "vrrp": (112, 112),
        "tcp": ("tcp", "tcp"),
        "udp": ("udp", "udp"),
    }
    protos_v4 = {v for p, (v, _) in protos.items() if getattr(rule.protocols, p)}
    protos_v6 = {v for p, (_, v) in protos.items() if getattr(rule.protocols, p)}

    if protos_v4:
        builder.add_v4(
            nft.Match(
                op="==",
                left=nft.Payload(protocol="ip", field="protocol"),
                right=protos_v4,
            )
        )

    if protos_v6:
        builder.add_v6(
            nft.Match(
                op="==",
                left=nft.Payload(protocol="ip6", field="nexthdr"),
                right=protos_v6,
            )
        )

    proto_ports = (
        ("udp", "dport"),
        ("udp", "sport"),
        ("tcp", "dport"),
        ("tcp", "sport"),
    )

    for proto, port in proto_ports:
        if rule.protocols[proto][port]:
            ports = set(rule.protocols[proto][port])
            builder.add_any(
                nft.Match(
                    op="==",
                    left=nft.Payload(protocol=proto, field=port),
                    right=ports,
                ),
            )

    builder.add_any(nft.Verdict(rule.verdict.value))

    return builder.rules


def parse_filter_rules(hook: str, rules: list[Rule], zones: Zones) -> nft.Chain:
    chain = nft.Chain(
        name=hook,
        type="filter",
        hook=hook,
        policy="drop",
        priority=0,
    )

    chain.rules.append(nft.Rule([nft.Jump("conntrack")]))

    for rule in rules:
        chain.rules.extend(list(parse_filter_rule(rule, zones)))

    return chain


def parse_filter(filter: Filter, zones: Zones) -> nft.Table:
    # Conntrack
    chain_conntrack = nft.Chain(name="conntrack")

    rule_ct_accept = nft.Match(
        op="==",
        left=nft.Ct("state"),
        right={"established", "related"},
    )

    rule_ct_drop = nft.Match(
        op="in",
        left=nft.Ct("state"),
        right="invalid",
    )

    chain_conntrack.rules = [
        nft.Rule([rule_ct_accept, nft.Verdict("accept")]),
        nft.Rule([rule_ct_drop, nft.Counter(), nft.Verdict("drop")]),
    ]

    # Resulting table
    table = nft.Table(name="filter", family="inet")

    table.chains.append(chain_conntrack)

    # Input/Output/Forward chains
    for name in ("input", "output", "forward"):
        chain = parse_filter_rules(name, getattr(filter, name), zones)
        table.chains.append(chain)

    return table


def parse_firewall(firewall: Firewall, zones: Zones) -> nft.Ruleset:
    # Tables
    blacklist = parse_blacklist(firewall.blacklist, zones)
    rpf = parse_reverse_path_filter(firewall.reverse_path_filter)
    filter = parse_filter(firewall.filter, zones)

    # Resulting ruleset
    ruleset = nft.Ruleset(flush=True)

    ruleset.tables.extend([blacklist, rpf, filter])

    return ruleset


# ==========[ MAIN ]============================================================


def send_to_nftables(cmd: nft.JsonNftables) -> int:
    nft = Nftables()

    try:
        nft.json_validate(cmd)
    except Exception as e:
        print(f"JSON validation failed: {e}")
        return 1

    rc, output, error = nft.json_cmd(cmd)

    if rc != 0:
        print(f"nft returned {rc}: {error}")
        return 1

    if len(output):
        print(output)

    return 0


def main() -> int:
    parser = ArgumentParser()
    parser.add_argument("file", type=FileType("r"), help="YAML rule file")

    args = parser.parse_args()

    try:
        firewall = Firewall(**safe_load(args.file))
    except Exception as e:
        print(f"YAML parsing failed of the file '{args.file.name}': {e}")
        return 1

    try:
        zones = resolve_zones(firewall.zones)
    except Exception as e:
        print(f"Zone resolution failed: {e}")
        return 1

    try:
        json = parse_firewall(firewall, zones)
    except Exception as e:
        print(f"Firewall translation failed: {e}")
        return 1

    return send_to_nftables(json.to_nft())


if __name__ == "__main__":
    exit(main())