Error Handling

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

Error handling requires nserver>=2.0

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

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

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