fancy-dns/internal/acl/acl.go

// Package acl implements per-client policy: which blocklist rules apply,
// whether the cache may be used, rate limits, and which upstreams to use.
//
// Groups are evaluated in order and the first match wins, so an operator can
// put specific groups above catch-alls. The compiled form is immutable and
// swapped atomically on reload, so lookups need no locks.
package acl

import (
	"net/netip"
	"sort"
	"strings"
	"sync/atomic"

	"git.k8s-git-k6n.net/mats/fancy-dns/internal/config"
)

// Policy is what a matched group (or the default) allows.
type Policy string

const (
	// PolicyAllow resolves everything not blocked by the blocklist.
	PolicyAllow Policy = "allow"
	// PolicyBlockOnly applies the blocklist but refuses nothing.
	PolicyBlockOnly Policy = "block_only"
	// PolicyDeny answers REFUSED to everything.
	PolicyDeny Policy = "deny"
)

// Decision is the outcome of matching a client.
type Decision struct {
	// Group is the matched group name, or "" for the default.
	Group string
	// Policy is the effective policy.
	Policy Policy
	// SkipBlocklist lets the client bypass blocklist filtering entirely.
	SkipBlocklist bool
	// SkipCache forces a fresh resolution for this client.
	SkipCache bool
	// RateLimit is queries per second; 0 means unlimited.
	RateLimit int
	// MaxConcurrent caps in-flight resolutions for this client; 0 defers to
	// the global per-client cap.
	MaxConcurrent int
	// LogQueries forces query logging regardless of the sampling rate.
	LogQueries bool
	// Upstreams restricts this client to specific upstream addresses. The
	// slice is owned by the engine and must be treated as read-only.
	Upstreams []string
	// AllowDomains and DenyDomains are per-group name overrides, using the
	// blocklist's wildcard spellings ("*.example.com", "**.example.com"),
	// except that both forms also cover the apex: see NewDomainSet.
	AllowDomains []string
	DenyDomains  []string
	// Default is true when no group matched.
	Default bool
}

// Allows reports whether the client may resolve at all.
func (d Decision) Allows() bool { return d.Policy != PolicyDeny }

// DomainSet is a compiled allow/deny name matcher: a set of exact names plus a
// set of parent domains whose subdomains are covered.
type DomainSet struct {
	exact map[string]struct{}
	subs  map[string]struct{} // parent domain, covers it and its subdomains
	// any is set when the set contains a rule matching everything.
	any bool
}

// NewDomainSet compiles the blocklist's wildcard spellings: "example.com"
// (exact), "*.example.com" and "**.example.com" (the domain plus its
// subdomains), and "*", "**" or "*.*" (everything). Unlike in the blocklist,
// "*.x" and "**.x" are the same rule here: the leading "*" label is dropped,
// so either form covers the apex too. An exact-only rule carries no wildcard.
func NewDomainSet(patterns []string) *DomainSet {
	ds := &DomainSet{
		exact: make(map[string]struct{}, len(patterns)),
		subs:  make(map[string]struct{}, len(patterns)),
	}
	for _, p := range patterns {
		p = strings.ToLower(strings.TrimSpace(p))
		if p == "" {
			continue
		}
		switch {
		case p == "*" || p == "**" || p == "*.*":
			ds.any = true
		case strings.HasPrefix(p, "**."):
			ds.subs[strings.TrimSuffix(p[3:], ".")] = struct{}{}
		case strings.HasPrefix(p, "*."):
			// A strict-subdomain pattern still needs the parent for the
			// suffix walk; store the parent and accept the apex too, since
			// an operator writing "*.example.com" almost always means the
			// whole domain. Use "example.com" for an exact-only rule.
			ds.subs[strings.TrimSuffix(p[2:], ".")] = struct{}{}
		default:
			ds.exact[strings.TrimSuffix(p, ".")] = struct{}{}
		}
	}
	return ds
}

// Match reports whether name is covered.
func (d *DomainSet) Match(name string) bool {
	if d == nil {
		return false
	}
	if d.any {
		return true
	}
	name = strings.ToLower(strings.TrimSuffix(name, "."))
	if name == "" {
		return false
	}
	if _, ok := d.exact[name]; ok {
		return true
	}
	if _, ok := d.subs[name]; ok {
		return true
	}
	for off := labelEnd(name); off < len(name); off = labelEnd(name[off:]) + off {
		if _, ok := d.subs[name[off:]]; ok {
			return true
		}
	}
	return false
}

func labelEnd(s string) int {
	if i := strings.IndexByte(s, '.'); i >= 0 {
		return i + 1
	}
	return len(s)
}

// Len reports how many rules the set holds.
func (d *DomainSet) Len() int {
	if d == nil {
		return 0
	}
	return len(d.exact) + len(d.subs)
}

// group is one compiled ACL group.
type group struct {
	name     string
	comment  string
	policy   Policy
	any      bool
	prefixes []netip.Prefix
	ptr      []string // reverse-name suffixes, lowercase with trailing dot
	tags     []string

	allow *DomainSet
	deny  *DomainSet

	skipBlocklist bool
	skipCache     bool
	rateLimit     int
	maxConcurrent int
	logQueries    bool
	upstreams     []string
}

// Engine is the compiled, hot-swappable ACL table.
type Engine struct {
	groups []*group
	def    Decision
	cur    atomic.Pointer[Engine]
}

// Compile builds an Engine from configuration. An empty group list produces an
// engine that allows everything, which is the safe default for a fresh install.
func Compile(cfg config.ACL) *Engine {
	e := &Engine{
		def: Decision{
			Policy:    Policy(cfg.DefaultPolicy),
			RateLimit: cfg.DefaultRateLimit,
			Default:   true,
		},
	}
	if e.def.Policy == "" {
		e.def.Policy = PolicyAllow
	}
	for i := range cfg.Groups {
		g := &cfg.Groups[i]
		cg := &group{
			name:          g.Name,
			comment:       g.Comment,
			policy:        Policy(g.Policy),
			any:           g.Match.Any,
			ptr:           ptrSuffixes(g.Match.PTRSuffixes),
			tags:          lowerAll(g.Match.Tags),
			allow:         NewDomainSet(g.AllowDomains),
			deny:          NewDomainSet(g.DenyDomains),
			skipBlocklist: g.SkipBlocklist,
			skipCache:     g.SkipCache,
			rateLimit:     g.RateLimit,
			maxConcurrent: g.MaxConcurrent,
			logQueries:    g.LogQueries,
			upstreams:     append([]string(nil), g.Upstreams...),
		}
		if cg.policy == "" {
			cg.policy = PolicyAllow
		}
		for _, c := range g.Match.CIDRs {
			if p, err := netip.ParsePrefix(c); err == nil {
				cg.prefixes = append(cg.prefixes, p)
			}
		}
		// Sort by prefix length descending so the most specific network wins
		// when one group lists overlapping ranges (though first-match-wins
		// already handles ordering, this keeps a single group honest).
		sort.Slice(cg.prefixes, func(i, j int) bool {
			return cg.prefixes[i].Bits() > cg.prefixes[j].Bits()
		})
		e.groups = append(e.groups, cg)
	}
	return e
}

// Holder keeps the current Engine for the DNS hot path.
type Holder struct{ cur atomic.Pointer[Engine] }

// NewHolder wraps an engine.
func NewHolder(e *Engine) *Holder {
	h := &Holder{}
	h.cur.Store(e)
	return h
}

// Get returns the current engine.
func (h *Holder) Get() *Engine { return h.cur.Load() }

// Set installs a new engine.
func (h *Holder) Set(e *Engine) { h.cur.Store(e) }

// Client is the identity of a DNS client as far as the ACL is concerned.
type Client struct {
	// IP is the client address.
	IP netip.Addr
	// Names are the reverse-DNS names for the client, lowercased. They come
	// from a reverse cache the server maintains, so matching is lock-free.
	Names []string
	// Tags are extra labels derived from the client identity, e.g. "laptop"
	// for laptop.lan.internal.
	Tags []string
}

// Match resolves a client to a Decision.
func (e *Engine) Match(c Client) Decision {
	if e == nil {
		return Decision{Policy: PolicyAllow, Default: true}
	}
	for _, g := range e.groups {
		if !g.matches(c) {
			continue
		}
		return Decision{
			Group:         g.name,
			Policy:        g.policy,
			SkipBlocklist: g.skipBlocklist,
			SkipCache:     g.skipCache,
			RateLimit:     g.rateLimit,
			MaxConcurrent: g.maxConcurrent,
			LogQueries:    g.logQueries,
			Upstreams:     g.upstreams,
			AllowDomains:  domainPatterns(g.allow),
			DenyDomains:   domainPatterns(g.deny),
		}
	}
	return e.defaultDecision()
}

// Default returns the decision for a client that matched nothing.
func (e *Engine) Default() Decision {
	if e == nil {
		return Decision{Policy: PolicyAllow, Default: true}
	}
	return e.defaultDecision()
}

func (e *Engine) defaultDecision() Decision {
	d := e.def
	d.Default = true
	return d
}

func (g *group) matches(c Client) bool {
	if g.any {
		return true
	}
	if len(g.prefixes) > 0 && c.IP.IsValid() {
		addr := c.IP.Unmap()
		for _, p := range g.prefixes {
			if p.Contains(addr) {
				return true
			}
		}
	}
	if len(g.ptr) > 0 {
		for _, n := range c.Names {
			for _, suffix := range g.ptr {
				if matchPTR(n, suffix) {
					return true
				}
			}
		}
	}
	if len(g.tags) > 0 {
		for _, t := range c.Tags {
			for _, want := range g.tags {
				if t == want {
					return true
				}
			}
		}
	}
	return false
}

// matchPTR reports whether the reverse name n ends with suffix at a DNS label
// boundary. Both sides are in FQDN form (trailing dot) by the time they get
// here, so "example.com." must not match the client "host.notexample.com.".
func matchPTR(n, suffix string) bool {
	if suffix == "" || len(n) < len(suffix) || !strings.HasSuffix(n, suffix) {
		return false
	}
	// The suffix has to start a label: it is either the whole name, or the
	// character in front of it is a dot.
	rest := n[:len(n)-len(suffix)]
	return rest == "" || rest[len(rest)-1] == '.'
}

// Groups returns the compiled group summaries for the API.
type GroupSummary struct {
	Name          string   `json:"name"`
	Comment       string   `json:"comment,omitempty"`
	Policy        string   `json:"policy"`
	CIDRs         []string `json:"cidrs"`
	PTRSuffixes   []string `json:"ptr_suffixes"`
	Tags          []string `json:"tags"`
	Any           bool     `json:"any"`
	AllowDomains  []string `json:"allow_domains"`
	DenyDomains   []string `json:"deny_domains"`
	SkipBlocklist bool     `json:"skip_blocklist"`
	SkipCache     bool     `json:"skip_cache"`
	RateLimit     int      `json:"rate_limit"`
	MaxConcurrent int      `json:"max_concurrent"`
	LogQueries    bool     `json:"log_queries"`
	Upstreams     []string `json:"upstreams"`
	ClientCount   int64    `json:"client_count"`
	QueryCount    uint64   `json:"query_count"`
	BlockedCount  uint64   `json:"blocked_count"`
	// Matched is populated by a dry-run lookup from the API.
	Matched bool `json:"matched"`
}

// Summaries renders the engine's groups for the API.
func (e *Engine) Summaries() []GroupSummary {
	if e == nil {
		return nil
	}
	out := make([]GroupSummary, 0, len(e.groups))
	for _, g := range e.groups {
		cidrs := make([]string, 0, len(g.prefixes))
		for _, p := range g.prefixes {
			cidrs = append(cidrs, p.String())
		}
		out = append(out, GroupSummary{
			Name:          g.name,
			Comment:       g.comment,
			Policy:        string(g.policy),
			CIDRs:         cidrs,
			PTRSuffixes:   g.ptr,
			Tags:          g.tags,
			Any:           g.any,
			AllowDomains:  domainPatterns(g.allow),
			DenyDomains:   domainPatterns(g.deny),
			SkipBlocklist: g.skipBlocklist,
			SkipCache:     g.skipCache,
			RateLimit:     g.rateLimit,
			MaxConcurrent: g.maxConcurrent,
			LogQueries:    g.logQueries,
			Upstreams:     g.upstreams,
		})
	}
	return out
}

// Count reports how many groups are configured.
func (e *Engine) Count() int {
	if e == nil {
		return 0
	}
	return len(e.groups)
}

func domainPatterns(d *DomainSet) []string {
	if d == nil {
		return nil
	}
	out := make([]string, 0, len(d.exact)+len(d.subs)+1)
	if d.any {
		// Keep the catch-all visible alongside the specific rules — dropping
		// it would mean the engine's per-query recompile of the rendered
		// patterns lost the "everything" case. It sorts first, like any other
		// pattern.
		out = append(out, "*")
	}
	for k := range d.exact {
		out = append(out, k)
	}
	for k := range d.subs {
		out = append(out, "*."+k)
	}
	sort.Strings(out)
	return out
}

func lowerAll(in []string) []string {
	if len(in) == 0 {
		return nil
	}
	out := make([]string, 0, len(in))
	for _, s := range in {
		s = strings.ToLower(strings.TrimSpace(s))
		if s == "" {
			continue
		}
		out = append(out, s)
	}
	return out
}

// ptrSuffixes lowercases reverse-name suffixes and normalises them to the
// FQDN form (trailing dot) that Client.Names carries, so matching can rely on
// a label boundary at the end of the suffix.
func ptrSuffixes(in []string) []string {
	out := lowerAll(in)
	for i, s := range out {
		if !strings.HasSuffix(s, ".") {
			out[i] = s + "."
		}
	}
	return out
}

// TagsFromName derives candidate tags from a reverse-DNS name: every label
// above the parent domain, so "laptop.lan.example.net" yields laptop and lan.
// Matching a group on tag "laptop" then works without any extra
// configuration.
func TagsFromName(name string) []string {
	name = strings.ToLower(strings.TrimSuffix(name, "."))
	if name == "" {
		return nil
	}
	labels := strings.Split(name, ".")
	if len(labels) < 2 {
		return labels
	}
	// Skip the last two labels (the name plus its parent domain).
	return labels[:len(labels)-2]
}