How to fix shell request failed on channel 0 in SSH

# This should work even if shell request fails
ssh user@hostname echo "SSH works"

# This will fail if shell request fails
ssh user@hostname

# View all semaphore sets
ipcs -s

# View summary
ipcs -s -s

# Count total semaphores in use
ipcs -s | tail -n +4 | awk '{sum += $3} END {print "Total semaphores:", sum}'

# View semaphore limits
cat /proc/sys/kernel/sem
# Output format: SEMMSL SEMMNS SEMOPM SEMMNI
# SEMMNS = max semaphores system-wide (default 32000)

# Remove a specific semaphore set by ID (from ipcs output)
ipcrm -s <semid>

# Or free all semaphores from a specific user
ipcrm -s $(ipcs -s | grep username | awk '{print $2}')

# Check if /dev/pts is mounted
mount | grep pts

# You should see something like:
# devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620)

# Unmount and remount /dev/pts
ubuntu@host:~$ sudo umount /dev/pts
ubuntu@host:~$ sudo mount -t devpts -o "gid=5,mode=620" none /dev/pts

# Add or verify this line in /etc/fstab
echo "none /dev/pts devpts defaults 0 0" | sudo tee -a /etc/fstab

# If the line already exists with different options, edit it
sudo nano /etc/fstab

# View the relevant SSH config lines
sudo grep -i "PermitTTY\|PermitOpen\|Subsystem" /etc/ssh/sshd_config

PermitTTY no           # This prevents any TTY allocation
PermitOpen localhost   # This restricts tunneling but shouldn't affect shells

# Edit the config
sudo nano /etc/ssh/sshd_config

# Change line to:
PermitTTY yes

# Validate syntax
sudo sshd -t

# Restart SSH daemon
sudo systemctl restart sshd

# Check what shell is assigned to the user
grep username /etc/passwd
# Output format: username:x:uid:gid:comment:/home/username:/bin/bash

# Verify the shell exists and is executable
ls -la /bin/bash

# List available shells
cat /etc/shells

# Change shell to /bin/bash (or another shell from /etc/shells)
sudo usermod -s /bin/bash username

# Or edit /etc/passwd directly
sudo nano /etc/passwd

# View your authorized_keys file
cat ~/.ssh/authorized_keys

# Look for problematic prefixes like:
# restrict,command="/usr/bin/no-pty-command" ssh-rsa AAAA...
# The 'restrict' or 'command=' options can prevent shell allocation

# Backup first
cp ~/.ssh/authorized_keys ~/.ssh/authorized_keys.bak

# Remove the restrict/command prefix (keep the ssh-rsa AAAA... key part)
# Before: restrict,command="/path" ssh-rsa AAAA...
# After: ssh-rsa AAAA...

sudo nano ~/.ssh/authorized_keys

# List all PTY devices
ls -la /dev/pts/

# Count them
ls -la /dev/pts/ | tail -n +4 | wc -l

# View max PTYs allowed
cat /proc/sys/kernel/pty/max
# Default is 4096

# Find processes using PTYs
lsof | grep pts

# Kill idle processes consuming PTYs
ps aux | grep defunct   # Show zombie processes
killall -9 <process_name>  # Kill stuck processes

# Temporarily increase PTY limit
echo 8096 | sudo tee /proc/sys/kernel/pty/max

# For permanent change, add to /etc/sysctl.conf
echo "kernel.pty.max = 8096" | sudo tee -a /etc/sysctl.conf
sudo sysctl -p

# Graceful reboot
sudo reboot

# Or more forcefully
sudo shutdown -r now

ssh user@hostname
# Should now connect to shell without error

# Check OpenSSH version
OpenSSH.exe -V

# Check the default shell registry setting
Reg query HKLM\SOFTWARE\OpenSSH /v DefaultShell

# If not set, set PowerShell as default
Reg add HKLM\SOFTWARE\OpenSSH /v DefaultShell /t REG_SZ /d C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe /f

# Or set cmd.exe
Reg add HKLM\SOFTWARE\OpenSSH /v DefaultShell /t REG_SZ /d C:\Windows\System32\cmd.exe /f

Restart-Service -Name sshd -Force

# Edit sshd_config to enable debug logging
sudo nano /etc/ssh/sshd_config

# Add this line:
SyslogFacility AUTH
LogLevel DEBUG

# Or use even more verbose:
LogLevel DEBUG2
LogLevel DEBUG3

# Save and validate
sudo sshd -t

# Restart sshd
sudo systemctl restart sshd

# Now attempt a connection from a client and watch the logs
sudo tail -f /var/log/auth.log
# or on macOS
sudo log stream --predicate 'process == "sshd"' --level debug