Diagnosing and Fixing “No Supported Key Exchange Algorithms” Error in OpenSSH Server


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When running OpenSSH in debug mode (-d flag), you might encounter this critical error sequence:

debug1: list_hostkey_types: 
No supported key exchange algorithms
debug1: do_cleanup

This immediately terminates the SSH connection attempt, with the client receiving a terse "Connection closed" message.

The error occurs when:

  1. Client and server complete version negotiation
  2. During algorithm negotiation phase, no common key exchange method is found
  3. This typically happens with mismatched Protocol settings or missing host keys

First, check your sshd_config for proper protocol and algorithm settings:

# Minimum viable configuration to test key exchange
Protocol 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
KexAlgorithms curve25519-sha256,ecdh-sha2-nistp256
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
MACs hmac-sha2-512-etm@openssh.com

For older systems (like OpenSSH 5.x shown in the logs), try these backward-compatible settings:

KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
MACs hmac-sha1

To systematically diagnose:

# On server:
sshd -T | grep -E 'kex|cipher|mac' > sshd_config_check.txt

# On client:
ssh -Q kex > supported_kex.txt
ssh -vvv user@host 2>&1 | grep -i kex > connection_attempt.txt

For current OpenSSH versions (8.0+), these are the preferred settings:

KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org
HostKeyAlgorithms ssh-ed25519,rsa-sha2-512

When dealing with very old SSH implementations (like the OpenSSH 5.2 in the example):

  1. Ensure all host key types exist: 
    ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key
ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key
  2. Explicitly enable protocol 1 compatibility if needed: 
    Protocol 2,1
RSAAuthentication yes

When encountering the "No supported key exchange algorithms" error in OpenSSH (specifically OpenSSH_5.2 in this case), it typically indicates a fundamental mismatch between the client and server's cryptographic capabilities. Let's examine the key components from the debug output:

debug1: list_hostkey_types: 
No supported key exchange algorithms
debug1: do_cleanup

The error occurs during the initial SSH handshake phase when the client (OpenSSH_5.5) and server (OpenSSH_5.2) attempt to negotiate a mutually supported key exchange method. Modern OpenSSH versions have deprecated several older algorithms for security reasons.

The provided sshd_config shows some potential issues:

Protocol 1,2
HostKey ./ssh_host_key
HostKey ./ssh_host_rsa_key
HostKey ./ssh_host_dsa_key

This configuration is using legacy protocol settings. Let's examine a more secure modern configuration:

# Recommended modern configuration
Protocol 2
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
KexAlgorithms curve25519-sha256,ecdh-sha2-nistp256,ecdh-sha2-nistp384
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com

Here's how to resolve this issue in different scenarios:

For Modern Systems (OpenSSH 8.0+)

# /etc/ssh/sshd_config
KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
MACs hmac-sha2-512-etm@openssh.com

For Legacy Systems (Backward Compatibility)

# /etc/ssh/sshd_config
KexAlgorithms diffie-hellman-group-exchange-sha256
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
MACs hmac-sha2-256,hmac-sha1

When troubleshooting, use these commands to test your configuration:

# Check supported algorithms
ssh -Q kex
ssh -Q cipher
ssh -Q mac

# Test connection with specific algorithms
ssh -oKexAlgorithms=diffie-hellman-group-exchange-sha256 \
    -oCiphers=aes128-ctr \
    -oMACs=hmac-sha2-256 \
    user@host

The original error shows OpenSSH_5.2 (2008 era) connecting to OpenSSH_5.5 (2010). These versions have significant algorithm differences:

  • OpenSSH 5.2: Uses diffie-hellman-group1-sha1 (now considered weak)
  • OpenSSH 5.5: Expects more modern algorithms by default

For such legacy systems, consider either upgrading or explicitly configuring compatible algorithms:

# For OpenSSH 5.2 compatibility
KexAlgorithms diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1
HostKeyAlgorithms ssh-rsa,ssh-dss
Ciphers aes128-cbc,3des-cbc