Unable to open connection
What “Unable to open connection” usually means
When PuTTY reports Unable to open connection to host, the client has already attempted a network or protocol step and failed before a normal interactive shell is available. This guide explains what the message usually means on Windows clients, which local settings to verify first, and when the problem is almost certainly on the server, firewall, or DNS path rather than inside the PuTTY GUI.
PuTTY could not establish the remote connection—umbrella wording that often wraps DNS, TCP, or protocol selection problems. Read any secondary clause in the dialog.
Independent resource: Putty.info is educational only and is not affiliated with the official PuTTY project. Compare symptoms against the official documentation for your installed version.
Symptoms you will see
Operators usually notice this failure in one of a few repeatable ways. Capture the exact dialog text, the hostname and port from the Session panel, and whether the failure happens before or after a username prompt—those details decide which checklist below applies.
- Failure right after Open with unable to open connection language.
- May include a more specific WinSock reason string.
Likely causes ranked by frequency
Several independent conditions can produce the same client-facing wording. Work from the outside in: reachability first, protocol selection second, authentication third. Changing key files will not fix a TCP refusal, and opening a wider firewall will not fix a rejected public key.
- Bad hostname, port, or protocol combination.
- Local policy blocking the binary’s network access.
- Corrupted saved session parameters.
Step-by-step client checks
Use this ordered checklist on a workstation you control. Prefer saved sessions so Host Name, Port, and Connection type stay consistent while you isolate one variable at a time.
- Create a brand-new session with only Host Name, Port 22, SSH—avoid inheriting a damaged saved session.
- Test DNS and TCP separately from SSH auth.
- Run PuTTY elevated only if corporate software restriction requires it—elevation itself is not a fix for routing.
- Inspect Connection → Proxy and Connection → SSH → tunnels for leftover experiments.
- Reinstall from official latest packages if binaries were truncated.
- Map the secondary error text to a dedicated guide on this site.
Do not paste private keys, passphrases, or production passwords into Putty.info tools or contact forms. Diagnose locally with PuTTY, Plink, PSCP, or PSFTP from an official install.
When it is a server or network issue
Confirm the service listens and is reachable. Provide operators a known-good hostname and port in writing to eliminate saved-session drift.
If TCP connects from another network path but fails from yours, involve the network team with traceroute/path evidence rather than repeatedly regenerating keys. If authentication fails after a banner appears, collect the server sshd logs for the same timestamp and username—client-side retries alone rarely reveal AccountLocked, Match blocks, or AllowUsers denials.