How to fix Received message too long in SSH

Run this command to check if your remote SSH session is producing unexpected output:

ssh user@host /usr/bin/true

If you see any output (text, spaces, or characters), your shell startup scripts are producing unwanted output. A clean connection should produce no output at all.

SSH executes startup files in order. Check each file for output commands:

cat ~/.bashrc
cat ~/.bash_profile
cat ~/.profile
cat /etc/profile
cat /etc/bash.bashrc

Look for statements like echo, printf, fortune, cat, motd, banner, or any other commands that produce console output. These should not be in files executed during non-interactive sessions.

Modify your shell startup files to only run output commands in interactive sessions. Add this check before any echo or output commands:

# In ~/.bashrc or ~/.bash_profile
if [ -t 1 ]; then
  # Only run these in interactive terminals
  echo "Welcome to my server"
  fortune
fi

The [ -t 1 ] test checks if file descriptor 1 (stdout) is connected to a terminal. It's false for non-interactive SFTP sessions.

For commands that shouldn't run at all in startup files, remove them entirely or move them to ~/.bash_profile (which only runs for login shells):

Move from .bashrc to .bash_profile:
- Commands that should run once per login
- Welcome messages or startup scripts
- Environment setup that only needs to run once

Remove entirely:
- Debug print statements
- Temporary diagnostic output
- Fortune cookies or other non-essential output

After modifying your startup files, test again:

ssh user@host /usr/bin/true

You should see no output. Then test SFTP:

sftp user@host

The connection should succeed without errors.

If modifying startup files isn't feasible, system administrators can configure OpenSSH to use the internal SFTP server, which doesn't process shell startup files:

Edit /etc/ssh/sshd_config:

Subsystem sftp internal-sftp

Then restart SSH:

sudo systemctl restart ssh
# or
sudo service sshd restart

This forces SFTP to use OpenSSH's built-in server instead of forking a shell, completely bypassing startup file issues.