When working with SSH, it's crucial to understand the protocol version you're using. You can check your current protocol version by running:

ssh -V

For modern systems, you'll typically see something like "OpenSSH_8.9p1" which indicates you're using protocol version 2 by default.

Unlike SSH Protocol 1 where you could specify CompressionLevel similar to gzip, Protocol 2 implements compression differently. The options are more limited but still effective for dial-up connections.

To maximize compression in SSH Protocol 2, create or modify your ~/.ssh/config file with these settings:

Host *
    Compression yes
    Ciphers aes128-ctr
    MACs hmac-sha1
    Protocol 2
    ServerAliveInterval 60
    TCPKeepAlive yes

If you need even better compression, consider these additional techniques:

# Using a compressed archive before transfer
tar czf - /path/to/files | ssh user@host "tar xzf -"

# For interactive sessions with maximum compression
ssh -C -c aes128-ctr user@host

To verify your compression effectiveness, you can use these commands:

# Before compression
du -sh /path/to/data

# After transfer with compression enabled
ssh -C user@host "du -sh /path/to/data"

For persistent connections, ensure your server's /etc/ssh/sshd_config contains:

Compression yes
ClientAliveInterval 60
TCPKeepAlive yes

Remember to restart sshd after changes:

sudo systemctl restart sshd

If you're not seeing expected compression results:

# Check active SSH sessions
ssh -vvv user@host

# Verify compression algorithm
grep -i compression /etc/ssh/ssh_config /etc/ssh/sshd_config ~/.ssh/config

When working with SSH tunnels on limited bandwidth connections (like dial-up), compression becomes crucial. The man page clearly states that CompressionLevel only applies to SSH Protocol Version 1. Here's how to identify and maximize compression in SSHv2:

# Check your SSH protocol version
ssh -V
# Sample output: OpenSSH_8.9p1, OpenSSL 3.0.7, protocol 2.0

While SSHv2 doesn't support compression levels like SSHv1, you can still enable compression with these methods:

# Basic compression enable
ssh -C user@host

# In your SSH config (~/.ssh/config):
Host *
    Compression yes
    CompressionLevel 9  # Note: This is ignored in SSHv2

For maximum compression in SSHv2, consider these approaches:

# Combine with other compression tools
tar czf - /path/to/files | ssh -C user@host "cat > backup.tar.gz"

# For persistent connections:
ssh -f -N -C -L 3306:localhost:3306 user@host

When SSH compression isn't sufficient:

• Use mosh (Mobile Shell) which handles poor connections better
• Pre-compress data before transmission
• Consider VPN with LZO compression

Remember that compression:

• Increases CPU usage on both ends
• May not help with already compressed data (JPEGs, ZIP files)
• Can sometimes increase latency