Common Issues·3 Min. Lesezeit

SSH Slow or Connection Timeout

Resolve slow SSH connections, timeouts and frequent disconnections. Diagnosis and fix in minutes.

Symptoms

SSH takes 10-30 seconds before showing login prompt
Connection drops after a few minutes of inactivity
ssh: connect to host ... port 22: Connection timed out

1. SSH slow on startup (30s wait)

Cause: DNS reverse lookup

SSH tries to resolve reverse DNS of your IP. If DNS is slow or not configured, it waits for timeout.

Verify:

bash

time ssh root@server hostname
# If it takes >5s, it's DNS

Fix: disable DNS lookup on server

bash

nano /etc/ssh/sshd_config

ini

UseDNS no

bash

systemctl restart sshd

Cause: GSSAPI authentication

SSH tries Kerberos/GSSAPI authentication that fails with timeout.

Fix on client side (on your PC):

bash

nano ~/.ssh/config

ini

Host *
  GSSAPIAuthentication no
  GSSAPIDelegateCredentials no

Or at connection time:

bash

ssh -o GSSAPIAuthentication=no root@server

2. Disconnections on inactivity

Fix on server side

bash

nano /etc/ssh/sshd_config

ini

# Send keep-alive every 60 seconds, up to 10 attempts
ClientAliveInterval 60
ClientAliveCountMax 10

bash

systemctl restart sshd

Fix on client side

bash

nano ~/.ssh/config

ini

Host *
  ServerAliveInterval 60
  ServerAliveCountMax 10
  TCPKeepAlive yes

3. Connection timeout (can't connect)

Cause: port blocked by firewall

bash

# From your PC, test connection
nc -zv SERVER_IP 22
# or
telnet SERVER_IP 22

If it doesn't respond, check on server (access from VNC/VirtFusion console):

bash

# Firewall status
ufw status
iptables -L INPUT -n --line-numbers

# Is SSH running?
systemctl status sshd
ss -tlnp | grep :22

SSH port changed

If you changed the SSH port:

bash

ssh -p 2222 root@server

Cause: fail2ban banned your IP

bash

fail2ban-client status sshd
fail2ban-client set sshd unbanip YOUR_IP

4. Host key verification failed

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

Server was reinstalled or SSH key changed. Remove the old key:

bash

# From your PC
ssh-keygen -R SERVER_IP
# or remove the specific line from:
nano ~/.ssh/known_hosts

5. Permission denied (publickey)

Verify permissions of public key on server

bash

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
ls -la ~/.ssh/

Verify PubkeyAuthentication is enabled

bash

grep PubkeyAuthentication /etc/ssh/sshd_config
# Must be: PubkeyAuthentication yes

Debug connection

bash

# From your PC, show detailed debug
ssh -vvv root@server 2>&1 | head -60

6. Optimize SSH for fast connections

Add to ~/.ssh/config on your PC:

ini

Host my-vps
  HostName SERVER_IP
  User root
  IdentityFile ~/.ssh/id_ed25519

  # Reuse existing connections (much faster for multiple sessions)
  ControlMaster auto
  ControlPath ~/.ssh/cm-%r@%h:%p
  ControlPersist 10m

  # Compression (useful with slow connection)
  Compression yes

  # No DNS, no GSSAPI
  GSSAPIAuthentication no
  UseDNS no

With ControlMaster, subsequent connections to same server are instant.

If port 22 is blocked by your network (e.g. corporate network), you can configure SSH on port 443 on the server and connect from any network.

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