Poetry: All attempts to connect failed (3 causes, tested)

Poetry shows All attempts to connect to <host> failed for an untrusted certificate, a refused connection and an unresolvable hostname alike. Rerun with -v: the Caused by line names the real error.

Poetry prints the same All attempts to connect to <host> failed message for very different problems. We triggered three of them against a private source added with poetry source add --priority=primary corp <url>. Measured with Poetry 2.5.1 on Python 3.13.5 (Linux).

1. Untrusted certificate (corporate CA or proxy)

Transcript:

$ poetry add six
Creating virtualenv demo in /tmp/pow397/.venv

All attempts to connect to localhost:8963 failed.

Probable Causes:
    - the server is not responding to requests at the moment
    - the hostname cannot be resolved by your DNS
    - your network is not connected to the internet

You can also run your poetry command with -v to see more information.
[exit 1]

Only -v shows the cause:

Transcript:

$ poetry add six -v 2>&1 | grep -E 'SSLError|Max retries' | head -3
    | HTTPSConnectionPool(host='localhost', port=8963): Max retries exceeded with url: /simple/six/ (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1029)')))

Fix: trust the CA with certificates.<source>.cert.

2. Nothing listening: Connection refused

Transcript:

Removing source with name corp.
$ poetry add six
Creating virtualenv demo in /tmp/pow397/.venv

All attempts to connect to localhost:8965 failed.

Probable Causes:
    - the server is not responding to requests at the moment
    - the hostname cannot be resolved by your DNS
    - your network is not connected to the internet

You can also run your poetry command with -v to see more information.
$ poetry add six -v 2>&1 | grep -E 'Error|refused' | head -2
    | HTTPSConnectionPool(host='localhost', port=8965): Max retries exceeded with url: /simple/six/ (Caused by NewConnectionError("HTTPSConnection(host='localhost', port=8965): Failed to establish a new connection: [Errno 111] Connection refused"))

Same message, but -v shows [Errno 111] Connection refused: wrong port in the source URL, or the repository server or VPN is down.

3. Hostname does not resolve

Transcript:

Removing source with name corp.
$ poetry add six -v 2>&1 | grep -E 'Error|resolve' | head -2
    | HTTPSConnectionPool(host='no-such-host.invalid', port=443): Max retries exceeded with url: /simple/six/ (Caused by NameResolutionError("HTTPSConnection(host='no-such-host.invalid', port=443): Failed to resolve 'no-such-host.invalid' ([Errno -2] Name or service not known)"))
    - the hostname cannot be resolved by your DNS

NameResolutionError ... Name or service not known: a typo in the source URL, or an internal hostname that only resolves on the company VPN.

So: always rerun with -v and read the Caused by part. Check the URL with poetry source show. A 401 from the server is reported differently: see Authorization error accessing.

Related: Poetry ignore SSL errors, Poetry Authorization error accessing, Python 3.13 CA key usage extension.

Written with AI assistance. Measured on 2026-10-23 with Poetry 2.5.1 on Python 3.13.5 (Linux) against a local HTTPS index, a closed port and an unresolvable hostname. Each transcript was produced by running the commands verbatim. PlainHeaders is our extension; Poetry is free.