coredns-rfc2136/setup.go

package rfc2136

import (
	"encoding/base64"
	"fmt"
	"os"
	"path/filepath"
	"strconv"

	"github.com/coredns/caddy"
	"github.com/coredns/coredns/core/dnsserver"
	"github.com/coredns/coredns/plugin"
	clog "github.com/coredns/coredns/plugin/pkg/log"
	"github.com/miekg/dns"
)

// log is the package logger, scoped so messages are prefixed `[rfc2136]`.
var log = clog.NewWithPlugin("rfc2136")

func init() {
	plugin.Register("rfc2136", setup)

	// SECURITY/OPERATIONAL NOTE — process-global mutation:
	//
	// miekg/dns's default MsgAcceptFunc rejects UPDATE opcode messages
	// with NOTIMP at the wire layer, before any plugin sees them
	// (acceptfunc.go: "Don't allow dynamic updates, because then the
	// sections can contain a whole bunch of RRs"). CoreDNS 1.14.3
	// constructs its dns.Server instances without exposing a
	// per-server MsgAcceptFunc to plugins (see
	// coredns/core/dnsserver/server.go:159 — the dns.Server struct is
	// hardcoded), so to accept UPDATE opcodes anywhere we must
	// override the package-level default. We do that here.
	//
	// Consequences operators should understand:
	//
	//  1. The override is PROCESS-WIDE. Every CoreDNS server block in
	//     this binary will accept UPDATE opcodes at the wire layer,
	//     not just the one(s) where `rfc2136` is configured. Other
	//     blocks will pass the UPDATE through their plugin chains;
	//     since no plugin in those chains handles UPDATE, the request
	//     falls off the end of the chain and CoreDNS returns
	//     FormatError. No state is mutated — but the wire-layer
	//     gatekeeping moved into the plugin chain.
	//
	//  2. The actual security boundary is TSIG verification, which
	//     happens in this plugin's ServeDNS (checkTSIG) AND inside
	//     handleUpdate (assertAuthenticated). Defense in depth: any
	//     state-mutating path re-verifies, so a future refactor that
	//     adds a new caller cannot accidentally skip auth.
	//
	//  3. If you remove `rfc2136` from your Corefile and reload via
	//     SIGUSR1, this global is NOT restored. Restart the process
	//     to fully revert.
	//
	// The mitigation matrix is: (a) loud comment here, (b) startup
	// INFO log listing the zones this plugin owns, (c) TSIG re-check
	// inside handleUpdate. The architecturally clean fix would be
	// upstream support in CoreDNS for per-Config MsgAcceptFunc — when
	// that lands, delete this whole stanza.
	dns.DefaultMsgAcceptFunc = msgAcceptFunc
}

// msgAcceptFunc mirrors miekg/dns's defaultMsgAcceptFunc but additionally
// allows OpcodeUpdate. For UPDATE messages, the conservative Ancount/
// Nscount limits in the default function don't apply -- per RFC 2136
// those sections (Prerequisite / Update) can carry many RRs.
func msgAcceptFunc(dh dns.Header) dns.MsgAcceptAction {
	// Responses are silently ignored regardless of opcode (default behaviour).
	if isResponse := dh.Bits&0x8000 != 0; isResponse {
		return dns.MsgIgnore
	}

	opcode := int(dh.Bits>>11) & 0xF
	switch opcode {
	case dns.OpcodeQuery, dns.OpcodeNotify, dns.OpcodeUpdate:
		// allowed
	default:
		return dns.MsgRejectNotImplemented
	}

	if dh.Qdcount != 1 {
		return dns.MsgReject
	}

	// UPDATE messages legitimately carry multiple RRs in the
	// Prerequisite (Ancount) and Update (Nscount) sections -- skip the
	// "exactly 1" check that the default function applies for queries.
	if opcode != dns.OpcodeUpdate {
		if dh.Ancount > 1 {
			return dns.MsgReject
		}
		if dh.Nscount > 1 {
			return dns.MsgReject
		}
	}

	if dh.Arcount > 2 {
		return dns.MsgReject
	}
	return dns.MsgAccept
}

// setup is invoked by the CoreDNS plugin registry once per Corefile
// `rfc2136` directive. It parses the directive, validates that each
// declared zone has a corresponding file in zones-dir, registers
// TSIG keys with the underlying dns.Server, and links the handler
// into the plugin chain.
func setup(c *caddy.Controller) error {
	p, err := parse(c)
	if err != nil {
		return plugin.Error("rfc2136", err)
	}
	if err := p.validateZoneFiles(); err != nil {
		return plugin.Error("rfc2136", err)
	}

	cfg := dnsserver.GetConfig(c)

	// Register TSIG keys with the underlying dns.Server so miekg/dns
	// auto-verifies incoming signatures. We then just inspect the
	// result via dns.ResponseWriter.TsigStatus() in our UPDATE handler.
	if len(p.TSIGKeys) > 0 {
		if cfg.TsigSecret == nil {
			cfg.TsigSecret = make(map[string]string)
		}
		for name, key := range p.TSIGKeys {
			cfg.TsigSecret[name] = base64.StdEncoding.EncodeToString(key.Secret)
		}
	}

	cfg.AddPlugin(func(next plugin.Handler) plugin.Handler {
		p.Next = next
		return p
	})

	log.Infof("ready: zones=%v keys=%d ttl=%d dir=%q auto-commit=%t",
		p.Zones, len(p.TSIGKeys), p.TTL, p.ZonesDir, p.AutoCommit)
	// Surface the global MsgAcceptFunc override for operator audit —
	// if a sibling server block on this process doesn't expect UPDATE
	// opcodes to traverse it, the operator should know they will.
	log.Infof("dns.DefaultMsgAcceptFunc was overridden process-wide to permit OpcodeUpdate; state mutation requires TSIG verification on this plugin's zones=%v only", p.Zones)
	return nil
}

// parse reads a single `rfc2136 <zone> [<zone>...] { ... }` block.
//
// Grammar:
//
//	rfc2136 <zone> [<zone>...] {
//	    zones-dir <path>                              ; required
//	    tsig-key <name> <algorithm> <base64-secret>   ; may repeat
//	    ttl <seconds>                                 ; default 60
//	    auto-commit <true|false>                      ; default true
//	    git-author <name> <email>                     ; optional
//	}
func parse(c *caddy.Controller) (*RFC2136, error) {
	p := &RFC2136{
		TSIGKeys:   make(map[string]tsigKey),
		TTL:        DefaultTTL,
		AutoCommit: true,
	}

	// Per-zone git author overrides. Defaults are applied later.
	var gitAuthorName, gitAuthorEmail string

	for c.Next() {
		args := c.RemainingArgs()
		if len(args) < 1 {
			return nil, c.ArgErr()
		}
		for _, z := range args {
			p.Zones = append(p.Zones, plugin.Host(z).NormalizeExact()...)
		}

		for c.NextBlock() {
			switch c.Val() {

			case "zones-dir":
				dArgs := c.RemainingArgs()
				if len(dArgs) != 1 {
					return nil, c.ArgErr()
				}
				p.ZonesDir = dArgs[0]

			case "tsig-key":
				kArgs := c.RemainingArgs()
				if len(kArgs) != 3 {
					return nil, c.Errf("tsig-key requires 3 args (name algorithm secret), got %d", len(kArgs))
				}
				keyName := canonicalKeyName(kArgs[0])
				algo, err := parseTSIGAlgorithm(kArgs[1])
				if err != nil {
					return nil, c.Err(err.Error())
				}
				secret, err := decodeTSIGSecret(kArgs[2])
				if err != nil {
					return nil, c.Errf("tsig-key %q: %v", keyName, err)
				}
				if _, exists := p.TSIGKeys[keyName]; exists {
					return nil, c.Errf("duplicate tsig-key %q", keyName)
				}
				p.TSIGKeys[keyName] = tsigKey{Algorithm: algo, Secret: secret}

			case "ttl":
				tArgs := c.RemainingArgs()
				if len(tArgs) != 1 {
					return nil, c.ArgErr()
				}
				ttl, err := strconv.ParseUint(tArgs[0], 10, 32)
				if err != nil {
					return nil, c.Errf("ttl must be a non-negative integer: %v", err)
				}
				p.TTL = uint32(ttl)

			case "auto-commit":
				aArgs := c.RemainingArgs()
				if len(aArgs) != 1 {
					return nil, c.ArgErr()
				}
				switch aArgs[0] {
				case "true", "yes", "on":
					p.AutoCommit = true
				case "false", "no", "off":
					p.AutoCommit = false
				default:
					return nil, c.Errf("auto-commit must be true|false, got %q", aArgs[0])
				}

			case "git-author":
				gArgs := c.RemainingArgs()
				if len(gArgs) != 2 {
					return nil, c.Errf("git-author requires 2 args (name email), got %d", len(gArgs))
				}
				gitAuthorName = gArgs[0]
				gitAuthorEmail = gArgs[1]

			default:
				return nil, c.Errf("unknown directive: %s", c.Val())
			}
		}
	}

	if len(p.Zones) == 0 {
		return nil, c.Err("at least one zone must be specified")
	}
	if p.ZonesDir == "" {
		return nil, c.Err("zones-dir is required")
	}

	// Build zoneFile handles for each declared zone.
	p.zones = make(map[string]*zoneFile, len(p.Zones))
	for _, z := range p.Zones {
		// Trailing dot → filename. supported.systems. → supported.systems.zone
		stem := z
		if l := len(stem); l > 0 && stem[l-1] == '.' {
			stem = stem[:l-1]
		}
		path := filepath.Join(p.ZonesDir, stem+".zone")
		zf := openZoneFile(path, z)
		zf.AutoCommit = p.AutoCommit
		if gitAuthorName != "" {
			zf.GitAuthorName = gitAuthorName
		}
		if gitAuthorEmail != "" {
			zf.GitAuthorEmail = gitAuthorEmail
		}
		p.zones[z] = zf
	}

	return p, nil
}

// validateZoneFiles ensures every configured zone has an accessible
// file on disk at the expected path. Catches typos at CoreDNS startup
// rather than the first UPDATE.
func (p *RFC2136) validateZoneFiles() error {
	for zone, zf := range p.zones {
		st, err := os.Stat(zf.Path)
		if err != nil {
			return fmt.Errorf("zone %q: file not accessible at %s: %w", zone, zf.Path, err)
		}
		if st.IsDir() {
			return fmt.Errorf("zone %q: %s is a directory, expected a regular file", zone, zf.Path)
		}
	}
	return nil
}