Troubleshooting SSH Client Connection Hangs at Start

This article walks you through the process of resolving an issue where SSH client connections are established slowly or hang at the beginning. We will explore various factors influencing the SSH connection process and offer solutions to improve connectivity.

SSH Connection Overview

SSH (Secure Shell) is a widely used protocol for secure remote access. When establishing an SSH connection, the client and server go through several steps, including authentication, encryption, and data transmission. Slowness or hanging during these stages typically indicates a problem.

Potential Causes of Slow SSH Connections

DNS Resolution Issues

SSH connects to servers using IP addresses or domain names. Slow DNS resolution can contribute to hanging or slow SSH client connections. To mitigate this issue, consider using IP addresses or editing your local hosts file to reduce DNS lookup time.

Reverse DNS Lookup

By default, SSH performs a reverse DNS lookup when logging in, a time-consuming process. To disable reverse DNS lookup and improve connection speed, add the following to your SSH client configuration file (usually located at ~/.ssh/config or C:\Users\YourUserName\.ssh\config)

Host *
  UseDNS no

Server Authentication and Key Exchange

SSH employs public key cryptography, where a server and client exchange keys for secure communication. Slow authentication and key exchange processes may cause hanging or long connection times. Check the following:

• Ensure that both the client and the server have sufficient resources (CPU, RAM, and network bandwidth).
• Reduce the number of key types supported by the SSH client and server; this speeds up key exchange. To modify the SSH server, edit the /etc/ssh/sshd_config file on Linux systems or the equivalent on other platforms.

GSSAPI and Kerberos Authentication

GSSAPI (Generic Security Services Application Program Interface) and Kerberos authentication methods can slow down SSH connections. Disable these features in the SSH client configuration file:

Host *
  GSSAPIAuthentication no
  GSSAPIDelegateCredentials no
  GSSAPIKeyExchange no

TCP Slow Start

TCP Slow Start can cause slowness in the initial stages of SSH connections. Consider disabling it by modifying your SSH client configuration file:

Host *
  TCPKeepAlive yes
  ServerAliveInterval 15
  ServerAliveCountMax 3

Additional Debugging Tips for SSH Connections

In addition to the -vvv flag mentioned in the question, you can use the following tools for further debugging:

• strace: Trace system calls for Linux systems. Use these commands to gather information:

# Strace SSH client while connecting (Linux)
strace -f -s 4096 -o ssh.trace ssh -vvv user@server
# Analyze the strace output using graphical tools
pyrasite trace ssh.trace

• Wireshark: Analyze network traffic to inspect SSH packets and determine issues up to the network level.

Slow or hanging SSH client connections can be addressed through optimization of various factors, such as DNS resolution, reverse DNS lookups, key exchanges, authentication methods, and TCP Slow Start. Analyzing SSH connections with -vvv, strace, and Wireshark

helps diagnose connection problems and provides a solid foundation for improving connection performance.

References

• SSH Man Page
• SSHD Man Page
• TCP Slow Start (Wikipedia)
• Wireshark
• Pyrasite