How to fix no hostkey alg in SSH

Run verbose SSH to see the mismatch:

ssh -vvv user@hostname 2>&1 | grep -A 5 "key_algorithms"

This shows what the server offers and helps diagnose the specific algorithms in conflict. Look for lines like "server host key algorithms" vs "client host key algorithms".

Add legacy algorithms to your connection:

ssh -o HostKeyAlgorithms=+ssh-rsa user@hostname

Or add to ~/.ssh/config for persistent configuration:

Host specific-server
    Hostname 192.168.1.100
    User myuser
    HostKeyAlgorithms +ssh-rsa
    PubkeyAcceptedAlgorithms +ssh-rsa

The + prefix appends to default algorithms instead of replacing them. This works but uses weaker SHA-1 hashing—only use as temporary workaround.

Edit /etc/ssh/sshd_config and add or modify the HostKeyAlgorithms line:

# Add strong modern algorithms
HostKeyAlgorithms ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256

Then restart sshd:

sudo systemctl restart sshd
# or on some systems:
sudo service sshd restart

This maintains security while supporting both old and new clients. ssh-ed25519 and rsa-sha2-* variants use secure hash algorithms.

Check what host keys exist:

ls -la /etc/ssh/ssh_host_*

If missing ed25519 or ecdsa keys, generate them:

sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
sudo ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key -N ""
sudo systemctl restart sshd

New clients prefer ed25519 for both security and performance.

Check your SSH version:

ssh -V

If using OpenSSH < 6.5, update your SSH client:

# Ubuntu/Debian
sudo apt-get update && sudo apt-get install openssh-client

# macOS (via Homebrew)
brew install openssh

# CentOS/RHEL
sudo yum update openssh-clients

Older clients lack support for modern algorithms—upgrading resolves this permanently.