How to fix Corrupted MAC on input. in SSH

The most common fix is to specify a known-compatible MAC algorithm on the client. Try using different MAC algorithms with the -o MACs option:

ssh -o MACs=hmac-sha2-512 user@host
ssh -o MACs=hmac-sha2-256 user@host
ssh -o MACs=hmac-sha1 user@host

If one of these works, add it to your SSH config to make it permanent:

# Add to ~/.ssh/config
Host *
    MACs hmac-sha2-512,hmac-sha2-256,hmac-sha1

Or for a specific host:

Host myserver.example.com
    MACs hmac-sha2-512,hmac-sha2-256

This tells SSH to prefer these MAC algorithms in order, avoiding problematic ones.

Packet corruption often indicates hardware problems. Test your network connection:

# Check for dropped packets or errors on your interface
ip -s link show
# or on older systems:
ifconfig eth0

# Run a ping test to check for packet loss
ping -c 100 host.example.com

Look for:
- Packet drops
- CRC errors
- Collisions
- High latency or timeouts

If you see errors, try:
1. Using a wired connection instead of WiFi
2. Checking cable connections
3. Testing with a different network port
4. Restarting your network interface:

sudo ifdown eth0 && sudo ifup eth0

MTU (Maximum Transmission Unit) mismatches can cause packets to be fragmented and corrupted. Try lowering the MTU:

# Check current MTU
ip link show eth0

# Temporarily lower MTU for testing
sudo ip link set eth0 mtu 1400

# Test SSH
ssh user@host

If this fixes the issue, make it permanent. For most Linux systems:

# Edit your network configuration (varies by distro)
# Ubuntu/Debian: /etc/network/interfaces
# CentOS/RHEL: /etc/sysconfig/network-scripts/ifcfg-eth0

# Add to interface config:
mtu 1400

Standard MTU is 1500. Lowering to 1400-1280 can help with certain VPN or firewall configurations.

Older versions of OpenSSH and OpenSSL have known MAC algorithm bugs. Update your SSH software:

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

# On CentOS/RHEL
sudo yum update openssh-clients openssh-server

# On macOS (with Homebrew)
brew install openssh

Check your current version:

ssh -V

For servers, also verify the remote host is running an updated OpenSSH version. Contact your server administrator if it's outdated.

If changing the MAC algorithm doesn't help, try specifying a different cipher along with the MAC:

ssh -o MACs=hmac-sha2-256 -o Ciphers=aes128-ctr user@host
ssh -o MACs=hmac-sha2-256 -o Ciphers=aes256-ctr user@host

Add both to your SSH config for persistence:

Host myserver.example.com
    MACs hmac-sha2-256
    Ciphers aes128-ctr,aes256-ctr

Sometimes the combination of cipher and MAC matters more than either alone.

VPNs and firewalls can interfere with SSH packet integrity. Test if they're the cause:

# Temporarily disable VPN and try SSH
ssh user@host

# If it works without VPN, the issue is your VPN/firewall

If the VPN is required:
1. Contact your VPN provider about SSH compatibility
2. Try a different VPN protocol (OpenVPN vs. WireGuard, etc.)
3. Check if your firewall has SSH acceleration or deep packet inspection enabled
4. Consider using SSH over a different port or SSH tunneling as a workaround

MAC errors can occur if the system is under heavy load. Check system resources:

Client side:

# Check CPU, memory, disk usage
top
free -h
df -h

Server side (if you have access):

uptime
free -h
df -h

If either system is near capacity, free up resources:
- Close unnecessary applications
- Kill zombie processes
- Clear disk space
- Reduce background services

Try SSH again after freeing resources. If problems persist, it's likely a network or hardware issue rather than resource contention.