What causes "Wrong server response: %s %s" in arduino/Arduino?

Read the logged status/reason (log.debug prints it) and address the underlying HTTP status: 403 → fix permissions, 404 → fix the host/path configured on the pool.

arduino/Arduino · error · Exception

Wrong server response

Error message

Wrong server response: %s %s

What it means

In the same final-leg check of ntlmpool._new_conn, any non-200 status that is not 401 raises 'Wrong server response: <status> <reason>'. The NTLM handshake succeeded at the transport level but the server returned something the pool cannot interpret as an authenticated connection, so it aborts with this generic Exception.

Solutions

Read the logged status/reason (log.debug prints it) and address the underlying HTTP status: 403 → fix permissions, 404 → fix the host/path configured on the pool.
Retry the request — NTLM connection setup can race with server-side state; a fresh request sometimes completes cleanly.
Bypass intermediaries (proxy/load balancer) or check their logs if the status is 5xx and direct connection works.
If the server intermittently drops NTLM support, fall back to a plain pool or a different auth scheme the server reliably offers.

Example fix

// before (403 from wrong path)
pool.request('GET', '/admin/config')  # user has no rights
// after
pool.request('GET', '/public/status')  # resource the authenticated user may access

Defensive patterns

Strategy: retry

Validate before calling

def check_resource_available(base_url, path):
    import requests
    r = requests.head(base_url + path, allow_redirects=False)
    return r.status_code < 400, r.status_code

Type guard

def is_auth_handshake_success(status):
    return status == 200

Try / catch

import time
for attempt in range(3):
    try:
        resp = ntlm_pool.urlopen('GET', path)
        break
    except Exception as e:
        if 'Wrong server response' in str(e) and attempt < 2:
            time.sleep(2 ** attempt)
            continue
        raise

Prevention

Log res.status and res.reason at INFO on failure so the underlying HTTP status is actionable.
Verify the target path/resource is accessible to the authenticated account (avoid 403).
Check proxy/load-balancer health when seeing 5xx during handshakes.
Use a plain unauthenticated HEAD request to validate the URL before NTLM handshakes.

When it happens

Trigger: Completing the NTLM authenticate exchange but receiving e.g. 403 (insufficient permissions on the target resource), 404/500 (bad target path or server error), or 502/503 from an intermediary during an NTLMConnectionPool request.

Common situations: Authenticated user lacks rights to the requested resource (403); the configured host/path on the pool is wrong (404); upstream application error (500); corporate proxy returning 502/503 mid-handshake.

Related errors

AI-assisted analysis of arduino/Arduino@a0df6e0e83 (2026-09-06). Data as JSON: /api/errors/40a18d9da8648610. Report an issue: GitHub.

Appendix: source

Thrown at arduino-core/src/processing/app/i18n/python/requests/packages/urllib3/contrib/ntlmpool.py:105

        ServerChallenge, NegotiateFlags = \
            ntlm.parse_NTLM_CHALLENGE_MESSAGE(auth_header_value)
        auth_msg = ntlm.create_NTLM_AUTHENTICATE_MESSAGE(ServerChallenge,
                                                         self.user,
                                                         self.domain,
                                                         self.pw,
                                                         NegotiateFlags)
        headers[req_header] = 'NTLM %s' % auth_msg
        log.debug('Request headers: %s' % headers)
        conn.request('GET', self.authurl, None, headers)
        res = conn.getresponse()
        log.debug('Response status: %s %s' % (res.status, res.reason))
        log.debug('Response headers: %s' % dict(res.getheaders()))
        log.debug('Response data: %s [...]' % res.read()[:100])
        if res.status != 200:
            if res.status == 401:
                raise Exception('Server rejected request: wrong '
                                'username or password')
            raise Exception('Wrong server response: %s %s' %
                            (res.status, res.reason))

        res.fp = None
        log.debug('Connection established')
        return conn

    def urlopen(self, method, url, body=None, headers=None, retries=3,
                redirect=True, assert_same_host=True):
        if headers is None:
            headers = {}
        headers['Connection'] = 'Keep-Alive'
        return super(NTLMConnectionPool, self).urlopen(method, url, body,
                                                       headers, retries,
                                                       redirect,
                                                       assert_same_host)

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