Add MAX_HOSTNAME_LENGTH and MAX_LABEL_LENGTH constants

src/ddns_service/dns.py

@@ -15,6 +15,10 @@ import dns.tsigkeyring
 import dns.update
 
 
+# DNS name length limits (RFC 1035)
+MAX_HOSTNAME_LENGTH = 253
+MAX_LABEL_LENGTH = 63
+
 # Valid hostname label pattern (after punycode encoding)
 LABEL_PATTERN = re.compile(
     r'^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$', re.IGNORECASE
@@ -54,8 +58,9 @@ def encode_dnsname(hostname):
     if hostname.endswith('.'):
         hostname = hostname[:-1]
 
-    if len(hostname) > 253:
-        raise EncodingError("Hostname too long (max 253 characters)")
+    if len(hostname) > MAX_HOSTNAME_LENGTH:
+        raise EncodingError(
+            f"Hostname too long (max {MAX_HOSTNAME_LENGTH} characters)")
 
     try:
         # Encode each label using IDNA
@@ -72,9 +77,9 @@ def encode_dnsname(hostname):
             except UnicodeError as e:
                 raise EncodingError(f"Invalid label '{label}': {e}")
 
-            if len(encoded) > 63:
+            if len(encoded) > MAX_LABEL_LENGTH:
                 raise EncodingError(
-                    f"Label '{label}' too long (max 63 characters)"
+                    f"Label '{label}' too long (max {MAX_LABEL_LENGTH} characters)"
                 )
 
             if not LABEL_PATTERN.match(encoded):
@@ -209,7 +214,7 @@ def parse_bind_key_file(path):
     except DNSError:
         raise
     except Exception as e:
-        raise DNSError(f"Failed to parse key file {path}: {e}")
+        raise DNSError(f"Failed to parse key file {path}: {e}") from e
 
 
 class DNSService: