When your Linux server abruptly terminates connections right after successful SSH authentication, this typically indicates either a shell configuration issue or a forced termination by system scripts. Let's analyze the specific symptoms from the error log:

debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: Exit status 254

First, verify if the user's shell is properly configured. Try forcing a different shell during SSH login:

ssh user@server -t /bin/sh

If this works, examine the user's default shell in /etc/passwd:

grep ^username /etc/passwd
username:x:1000:1000::/home/username:/bin/bash

Several system files could force immediate logout:

1. /etc/profile
2. ~/.bash_profile
3. ~/.bash_logout
4. /etc/bash.bash_logout

Inspect these files for any exit commands or abnormal configurations. Temporarily rename them to test:

mv ~/.bash_logout ~/.bash_logout.bak

The Pluggable Authentication Modules (PAM) configuration might be causing premature termination. Check these files:

/etc/pam.d/sshd
/etc/pam.d/login

Look for unusual modules or exit conditions. A common culprit is the pam_limits.so module with restrictive settings.

Create a test user with minimal configuration:

useradd testuser -m -s /bin/bash
passwd testuser

Attempt SSH login as this user. If successful, compare environment variables between users:

env | sort > /tmp/testuser_env

Examine the SSH daemon configuration in /etc/ssh/sshd_config for these directives:

UsePAM yes
ForceCommand
AcceptEnv
Subsystem

Temporarily set LogLevel DEBUG3 in sshd_config for more verbose logging.

When all else fails, attach strace to the SSH process:

strace -f -p $(pgrep sshd) -o /tmp/sshd_strace.log

Look for abnormal process termination or permission errors in the output.

If console access is available, reboot into single-user mode:

init 1

Then check for corrupted filesystems or missing libraries:

fsck -y /dev/sda1
ldd /bin/bash

Based on the exit status 254, this typically indicates a shell initialization error. Try these corrective actions:

chsh -s /bin/bash username
rm -f ~/.bashrc ~/.profile
cp /etc/skel/.bashrc ~/

Then verify with:

ssh -v user@server "env; echo Test successful"

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When your SSH session terminates immediately after authentication with exit status 254, this typically indicates a forced logout triggered by system configuration. Let's examine the key diagnostic indicators from your debug output:

debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: Exit status 254

First check these essential configuration files that control shell behavior:

/etc/passwd
/etc/profile
/etc/bash.bashrc
~/.bashrc
~/.bash_profile
~/.profile
/etc/ssh/sshd_config

To test if shell initialization is causing the disconnect, try forcing a non-interactive shell:

ssh user@server /bin/true

If this works, the issue lies in your interactive shell configuration. Temporarily rename your personal initialization files:

mv ~/.bashrc ~/.bashrc.bak
mv ~/.profile ~/.profile.bak

The Pluggable Authentication Modules system might be enforcing restrictions. Check these key files:

/etc/pam.d/sshd
/etc/pam.d/common-auth
/etc/pam.d/common-session

Look for lines containing:

pam_limits.so
pam_env.so
pam_motd.so

Verify your SSH server configuration for forced command execution:

grep -i "ForceCommand" /etc/ssh/sshd_config
grep -i "AcceptEnv" /etc/ssh/sshd_config

Incorrect environment variables can cause immediate termination. Check with:

ssh user@server env

Compare this output with a working local session.

Ensure the user's shell exists and is executable:

grep ^username /etc/passwd
ls -la $(which bash)

As an emergency access method, you can force a specific shell:

ssh -t user@server /bin/bash --norc

Or for root access if permitted:

ssh -t root@server /bin/bash --noprofile

Once you regain access, thoroughly audit these areas:

1. User's home directory permissions (should be 755)
2. Shell initialization file syntax
3. PAM module dependencies
4. SSH authorized_keys command restrictions

Example fix for common permission issues:

chmod 755 ~
chmod 644 ~/.bashrc
chmod 600 ~/.ssh/authorized_keys