Error Handling

Custom exception handling is handled through the QueryExceptionHandlerMiddleware and RawExceptionHandlerMiddleware Middleware. These middleware will catch any Exceptions raised by their respective middleware stacks.

In general you are probably able to use the QueryExceptionHandlerMiddleware as the RawExceptionHandlerMiddleware is only needed to catch exceptions resulting from RawMiddleware or broken exception handlers in the QueryExceptionHandlerMiddleware. If you only write QueryMiddleware and your QueryExceptionHandlerMiddleware handlers never raise exceptions then you'll be good to go with just the QueryExceptionHandlerMiddleware.

Both of these middleware have a default exception handler that will be used for anything not matching a registered handler. The default handler can be overwritten by registering a handler for the Exception class.

Handlers are chosen by finding a handler for the most specific parent class of the thrown exception (including the class of the exception). These classes are searched in method resolution order.

Note

These handlers only handle exceptions that are subclasses of (and including) Exception. Exceptions that are only children of BaseException (e.g. SystemExit) will not be caught by these handlers.

Registering Exception Handlers

Exception handlers can be registered to NameServer and RawNameSeerver instances using either their @exception_handler decorators or their register_exception_handler methods.

import dnslib
from nserver import NameServer, Query, Response

server = NameServer("example")

@server.exception_handler(NotImplementedError)
def not_implemented_handler(exception: NotImplementedError, query: Query) -> Response:
    return Response(error_code=dnslib.RCODE.NOTIMPL)

@server.raw_exception_handler(Exception)
def print_debugger(exception: Exception, record: dnslib.DNSRecord) -> dnslib.DNSRecord:
    print(f"failed to process {record} due to {exception!r})
    response = record.reply()
    response.header.rcode =dnslib.RCODE.SERVFAIL
    return response