add dns package

Mostly imported from sigsum-log-go.  Added strict domain hint parsing,
two tests, and an example.  This should be part of the sigsum-go lib
because some client tooling may also want to check domain hints.

1 files changed, 62 insertions, 0 deletions

diff --git a/pkg/dns/dns.go b/pkg/dns/dns.go
new file mode 100644
index 0000000..3bc0583
--- /dev/null
+++ b/pkg/dns/dns.go
@@ -0,0 +1,62 @@
+// package dns checks if a domain name is aware of a hashed public key.  A
+// look-up is performed if the specified domain name matches "^_sigsum_v0.*".
+package dns
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"strings"
+
+	"git.sigsum.org/sigsum-lib-go/pkg/hex"
+	"git.sigsum.org/sigsum-lib-go/pkg/types"
+)
+
+const (
+	prefix = "_sigsum_v0."
+)
+
+// Verifier can verify that a domain name is aware of a public key
+type Verifier interface {
+	Verify(ctx context.Context, name string, key *types.PublicKey) error
+}
+
+// DefaultResolver implements the Verifier interface with Go's default resolver
+type DefaultResolver struct {
+	resolver net.Resolver
+}
+
+func NewDefaultResolver() Verifier {
+	return &DefaultResolver{}
+}
+
+func (dr *DefaultResolver) Verify(ctx context.Context, name string, pub *types.PublicKey) error {
+	if err := validPrefix(name); err != nil {
+		return fmt.Errorf("dns: %s", err)
+	}
+	rsps, err := dr.resolver.LookupTXT(ctx, name)
+	if err != nil {
+		return fmt.Errorf("dns: look-up failed: %s", name)
+	}
+	if err := validResponse(pub, rsps); err != nil {
+		return fmt.Errorf("dns: %s", err)
+	}
+	return nil
+}
+
+func validResponse(pub *types.PublicKey, rsps []string) error {
+	keyHash := hex.Serialize(types.HashFn(pub[:])[:])
+	for _, rsp := range rsps {
+		if rsp == keyHash {
+			return nil
+		}
+	}
+	return fmt.Errorf("unknown key hash %s", keyHash)
+}
+
+func validPrefix(name string) error {
+	if !strings.HasPrefix(name, prefix) {
+		return fmt.Errorf("domain name prefix must be %s", prefix)
+	}
+	return nil
+}