package rfc2136

import (
	"fmt"
	"strings"

	"github.com/miekg/dns"
)

// checkTSIG verifies that the incoming UPDATE message is properly signed
// with a TSIG key we know about. The actual signature math has already
// been done by the underlying dns.Server (because setup.go registered
// our keys in dnsserver.Config.TsigSecret); this function just inspects
// the result and the key identity.
//
// Behavior matrix:
//
//	No TSIG keys configured  → updates are unauthenticated. Caller may
//	                           still allow if it deems the network safe;
//	                           we conservatively reject (REFUSED) since
//	                           the practical use case (Caddy) always
//	                           signs.
//	TSIG keys configured but message has no TSIG → reject.
//	TSIG present, key name not in our map        → reject.
//	TSIG present, signature failed at dns.Server → reject (TsigStatus()).
//	All good                                     → nil.
func (p *RFC2136) checkTSIG(w dns.ResponseWriter, r *dns.Msg) error {
	tsig := r.IsTsig()

	if len(p.TSIGKeys) == 0 {
		return fmt.Errorf("no TSIG keys configured; refusing all UPDATEs as a safety default")
	}

	if tsig == nil {
		return fmt.Errorf("TSIG required but not present")
	}

	keyName := strings.ToLower(tsig.Hdr.Name)
	if !strings.HasSuffix(keyName, ".") {
		keyName += "."
	}
	key, known := p.TSIGKeys[keyName]
	if !known {
		return fmt.Errorf("unknown TSIG key %q", keyName)
	}

	// Algorithm pinning: the incoming TSIG can in theory use any
	// algorithm miekg/dns supports, but we only honour the one declared
	// in Corefile. Rejecting algorithm-downgrade attempts is a small
	// but important hardening — without this, an attacker who somehow
	// got a key could downgrade to HMAC-MD5 (which we don't even
	// configure but miekg/dns understands).
	if !strings.EqualFold(tsig.Algorithm, key.Algorithm) {
		return fmt.Errorf("TSIG algorithm mismatch: incoming=%s expected=%s", tsig.Algorithm, key.Algorithm)
	}

	// The underlying dns.Server verifies the TSIG MAC for us when it
	// has the secret in its TsigSecret map (which setup.go wires up).
	// A nil from TsigStatus means verification succeeded; any non-nil
	// error means the signature was invalid, the time was outside the
	// fudge window, or some other auth failure.
	if status := w.TsigStatus(); status != nil {
		return fmt.Errorf("TSIG verification failed for key %q: %w", keyName, status)
	}

	return nil
}