filter.go

package main

import (
	"context"
	"fmt"
	"net"
	"strings"
	"time"

	"github.com/miekg/dns"
)

const (
	isatapHostname       = "isatap"
	defaultLookupTimeout = 500 * time.Millisecond
)

func containsDomain(haystack []string, needle string) bool {
	needle = strings.ToLower(strings.TrimRight(needle, "."))

	for _, el := range haystack {
		// dot matches non-fqdns
		if el == "." {
			if !strings.Contains(needle, ".") {
				return true
			}

			continue
		}

		el := strings.ToLower(strings.TrimRight(el, "."))

		if strings.HasPrefix(el, ".") && strings.HasSuffix(needle, strings.TrimLeft(el, ".")) {
			return true
		} else if strings.EqualFold(el, needle) {
			return true
		}
	}

	return false
}

func shouldRespondToNameResolutionQuery(config Config, host string, queryType uint16,
	from net.IP, fromHostnames []string,
) (bool, string) {
	if config.DryMode {
		return false, "dry mode"
	}

	if strings.HasPrefix(strings.ToLower(host), isatapHostname) {
		return false, "ISATAP is always ignored"
	}

	if queryType == dns.TypeSOA && config.SOAHostname == "" {
		return false, "no SOA hostname configured"
	}

	if len(config.SpoofFor) > 0 && !containsIP(config.SpoofFor, from) &&
		!containsAnyHostname(config.SpoofFor, fromHostnames) {
		return false, "host address and name not in spoof-for list"
	}

	if len(config.DontSpoofFor) > 0 {
		if containsIP(config.DontSpoofFor, from) {
			return false, "host address included in dont-spoof-for list"
		}

		if containsAnyHostname(config.DontSpoofFor, fromHostnames) {
			return false, "hostname included in dont-spoof-for list"
		}
	}

	if len(config.Spoof) > 0 && !containsDomain(config.Spoof, host) {
		return false, "domain not in spoof list"
	}

	if len(config.DontSpoof) > 0 && containsDomain(config.DontSpoof, host) {
		return false, "domain included in dont-spoof list"
	}

	if !config.SpoofTypes.ShouldSpoof(queryType) {
		return false, fmt.Sprintf("type %s is not in spoof-types", dnsQueryType(queryType))
	}

	switch {
	case queryType == dns.TypeA && config.RelayIPv4 == nil:
		return false, "no IPv4 relay address configured"
	case queryType == dns.TypeAAAA && config.RelayIPv6 == nil:
		return false, "no IPv6 relay address configured"
	}

	return true, ""
}

func shouldRespondToDHCP(config Config, from peerInfo) (bool, string) {
	if config.DryMode && !config.DryWithDHCPv6Mode {
		return false, "dry mode"
	}

	if len(from.Hostnames) == 0 && config.IgnoreDHCPv6NoFQDN {
		return false, "no FQDN in DHCPv6 message"
	}

	if len(config.SpoofFor) > 0 && !containsPeer(config.SpoofFor, from) {
		return false, "host not in spoof-for list"
	}

	if len(config.DontSpoofFor) > 0 && containsPeer(config.DontSpoofFor, from) {
		return false, "host included in dont-spoof-for list"
	}

	return true, ""
}

type hostMatcher struct {
	IPs      []net.IP
	Hostname string
}

var hostMatcherLookupFunction = lookupIPWithTimeout

func newHostMatcher(hostnameOrIP string, dnsTimeout time.Duration) *hostMatcher {
	ip := net.ParseIP(hostnameOrIP)
	if ip != nil { // hostnameOrIP is an IP
		return &hostMatcher{IPs: []net.IP{ip}}
	}

	// domain is a wildcard
	if strings.HasPrefix(hostnameOrIP, ".") {
		return &hostMatcher{Hostname: hostnameOrIP}
	}

	// hostnameOrIP is not an IP
	ips, _ := hostMatcherLookupFunction(hostnameOrIP, dnsTimeout)

	return &hostMatcher{
		IPs:      ips,
		Hostname: hostnameOrIP,
	}
}

func asHostMatchers(hostnamesOrIPs []string, dnsTimeout time.Duration) []*hostMatcher {
	hosts := make([]*hostMatcher, 0, len(hostnamesOrIPs))

	for _, hostnameOrIP := range hostnamesOrIPs {
		hosts = append(hosts, newHostMatcher(strings.TrimSpace(hostnameOrIP), dnsTimeout))
	}

	return hosts
}

func normalizeHostname(hostname string) string {
	return strings.ToLower(strings.TrimRight(hostname, "."))
}

// Matches determines whether or not the host matches any of the provided hostnames.
func (h *hostMatcher) MatchesAnyHostname(hostnames ...string) bool {
	if h.Hostname == "" {
		return false
	}

	thisHostname := normalizeHostname(h.Hostname)

	for _, hostname := range hostnames {
		otherHostname := normalizeHostname(hostname)

		// hostname matches
		if thisHostname == otherHostname {
			return true
		}

		// subdomain matches
		if strings.HasPrefix(thisHostname, ".") && strings.HasSuffix(otherHostname, thisHostname) {
			return true
		}
	}

	return false
}

func (h *hostMatcher) String() string {
	if len(h.IPs) == 0 && h.Hostname == "" {
		return "no host"
	}

	ipStrings := make([]string, 0, len(h.IPs))

	for _, ip := range h.IPs {
		ipStrings = append(ipStrings, ip.String())
	}

	ipsString := strings.Join(ipStrings, ", ")

	if h.Hostname == "" {
		return ipsString
	}

	if len(h.IPs) == 0 {
		return h.Hostname
	}

	return fmt.Sprintf("%s (%s)", h.Hostname, ipsString)
}

type spoofTypes struct {
	A    bool
	AAAA bool
	ANY  bool
	SOA  bool
}

func parseSpoofTypes(spoofTypesStrings []string) (*spoofTypes, error) {
	if len(spoofTypesStrings) == 0 {
		return nil, nil //nolint:nilnil
	}

	st := &spoofTypes{}

	for _, spoofType := range spoofTypesStrings {
		switch strings.ToLower(spoofType) {
		case "a":
			st.A = true
		case "aaaa":
			st.AAAA = true
		case "any":
			st.ANY = true
		case "soa":
			st.SOA = true
		default:
			return nil, fmt.Errorf("unknown query type: %s", spoofType)
		}
	}

	return st, nil
}

func (st *spoofTypes) ShouldSpoof(qType uint16) bool {
	if st == nil {
		return true
	}

	switch {
	case qType == typeNetBios:
		return true
	case qType == dns.TypeA && st.A:
		return true
	case qType == dns.TypeAAAA && st.AAAA:
		return true
	case qType == dns.TypeANY && st.ANY:
		return true
	case qType == dns.TypeSOA && st.SOA:
		return true
	default:
		return false
	}
}

func containsPeer(hosts []*hostMatcher, peer peerInfo) bool {
	for _, host := range hosts {
		if host.MatchesAnyHostname(peer.Hostnames...) {
			return true
		}

		for _, ip := range host.IPs {
			if peer.IP.Equal(ip) {
				return true
			}
		}
	}

	return false
}

func containsIP(haystack []*hostMatcher, needle net.IP) bool {
	for _, el := range haystack {
		for _, ip := range el.IPs {
			if needle.Equal(ip) {
				return true
			}
		}
	}

	return false
}

func containsAnyHostname(haystack []*hostMatcher, needles []string) bool {
	for _, el := range haystack {
		if el.MatchesAnyHostname(needles...) {
			return true
		}
	}

	return false
}

func lookupIPWithTimeout(hostname string, timeout time.Duration) ([]net.IP, error) {
	if hostname == "" {
		return nil, nil
	}

	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()

	addrs, err := net.DefaultResolver.LookupIPAddr(ctx, hostname)
	if err != nil {
		return nil, err
	}

	ips := make([]net.IP, 0, len(addrs))

	for _, addr := range addrs {
		ips = append(ips, addr.IP)
	}

	return ips, nil
}