How to fix SSH error: no matching host key type found (their offer: ssh-rsa)

Every SSH server proves its identity with a host key. Old SSH servers, and the firmware in many older switches, routers, NAS boxes and server management cards, only sign with ssh-rsa, which uses SHA-1. SHA-1 is no longer considered safe, so OpenSSH 8.8 and later leave it out of what they accept. Newer servers use rsa-sha2-256, rsa-sha2-512, ECDSA or Ed25519 instead, and never show this error.

Nothing is broken on your side. An older computer whose ssh predates 8.8 may still connect to the same device, because it still has the old default.

1. 1

   Step 1: Try it for one connection

   ssh -o HostKeyAlgorithms=+ssh-rsa -o PubkeyAcceptedAlgorithms=+ssh-rsa admin@192.168.1.2

   The + adds ssh-rsa to the list ssh accepts, keeping everything else. The second option only matters if you sign in with an RSA key; with a password, the first is enough.

2. 2

   Step 2: Check the fingerprint the first time

   As with any new server, ssh shows the device's key fingerprint. Compare it with the one in the device's admin page or console before you type yes.

3. 3

   Step 3: Save it for that device only

   Add a Host entry to your SSH config file, so the exception applies to this device and nothing else:

   Host old-switch
       HostName 192.168.1.2
       User admin
       HostKeyAlgorithms +ssh-rsa
       PubkeyAcceptedAlgorithms +ssh-rsa

   Now ssh old-switch works, and so do scp and sftp to it. Don't put these lines under Host *: that would accept SHA-1 from every server.

Old devices often lack other modern algorithms too. The error says what's missing and what the server offers; add that offer with the matching option, the same way:

Use the exact name from Their offer: if the device offers several, pick the strongest one listed. In a config file the same settings go under the device's Host entry without the -o and the =, for example KexAlgorithms +diffie-hellman-group1-sha1. ssh -Q key, ssh -Q kex and ssh -Q cipher list what your ssh supports at all.

Their offer: ssh-dss is a DSA host key. Recent OpenSSH releases have dropped DSA entirely, so no option brings it back: update the device's firmware or have it make a new RSA or ECDSA host key.

• Check for a firmware or software update for the switch, router, NAS or management card. Most current releases use SHA-2 signatures.
• On a Linux server with an old SSH server, update the openssh-server package. If it lacks modern host keys, sudo ssh-keygen -A creates any of the default ones that are missing; restart SSH afterwards.
• If the device can't be updated, keep it on a management network and use the per-host exception above, never a global one.

Debugging something else? ssh -v user@host prints the whole negotiation, including what each side offered. For connections refused before negotiation even starts, check the SSH port and that the SSH server is running.

If you look after a mix of servers and network gear, the RemoteAccessApp SSH client keeps them as saved hosts with tabs, SFTP and key sign-in, and checks each server's key fingerprint the first time, refusing a key that later changes. It's free for up to 5 saved hosts on Windows and Linux: download RemoteAccessApp. Moving your ssh setup across? It can import your SSH config file.