When encountering the "PTY allocation request failed on channel 0" error during SSH authentication, it typically indicates a problem with pseudo-terminal (PTY) allocation despite successful authentication. The debug output shows:

debug1: Authentication succeeded (password).
debug2: channel 0: request pty-req confirm 1
PTY allocation request failed on channel 0
debug2: shell request accepted on channel 0

While many online solutions focus on /dev/pts mount issues or Git-related problems, our case presents different characteristics:

• /dev/pts is properly mounted (confirmed via mount command)
• No Git operations involved (bare server environment)
• Affects multiple users (rules out individual ~/.ssh/config issues)

Try these diagnostic commands on the server:

# Check PTY device permissions
ls -la /dev/pts

# Verify kernel parameters
sysctl kernel.pty.max
sysctl kernel.pty.nr

# Check resource limits
ulimit -a

# Test PTY allocation directly
python -c "import pty; pty.spawn('/bin/bash')"

Option 1: Force PTY allocation in ssh_config:

Host *
  RequestTTY force

Option 2: Disable PTY when not needed:

ssh -T user@host  # For non-interactive sessions

Investigate potential kernel issues with:

# Check for relevant kernel messages
dmesg | grep -i pty
dmesg | grep -i tty

# Verify kernel modules
lsmod | grep pty

If immediate resolution isn't possible, consider these workarounds:

# Use SSH in command mode
ssh user@host "your_command"

# Try mosh for persistent connections
mosh user@host

# Test with different SSH implementations
dropbear -p 22 user@host

Verify critical system directories:

# Check devpts filesystem
mount | grep devpts

# Verify device files
ls -l /dev/ptmx
ls -l /dev/tty*

# Test device creation
test -c /dev/ptmx && echo "OK" || echo "Failed"

---

When attempting SSH login, authentication succeeds but session establishment fails with:

PTY allocation request failed on channel 0
debug2: shell request accepted on channel 0

The session then freezes despite successful authentication. Looking at debug output (-vv flag), we see the PTY allocation fails after environment variables are sent but before shell startup.

First, check basic system-level permissions:

# Verify devpts mount
mount | grep devpts

# Check ptmx permissions
ls -l /dev/ptmx

# Verify kernel config
grep CONFIG_DEVPTS_MULTIPLE_INSTANCES /boot/config-$(uname -r)

Common culprits include:

• Incorrect /dev/pts permissions (should be 620)
• Missing devpts mount options (gid=5,mode=620)
• Exhausted pty resources (check /proc/sys/kernel/pty/max)
• Broken PAM modules or SELinux policies

When basic checks don't reveal the issue, try strace on both client and server:

# Client side
strace -f -o ssh_client.log ssh -vv user@host

# Server side (as root)
strace -f -p $(pgrep -f "sshd: user") -o sshd_server.log

Look for failed ioctl() calls related to TIOCSWINSZ or TIOCSPTLCKG which indicate terminal control issues.

For temporary workarounds, try forcing different PTY modes:

# Attempt raw mode
ssh -T user@host

# Force no-PTY allocation
ssh -N user@host

# Specify alternative pty helper
ssh -o LocalCommand="/usr/libexec/openssh/sftp-server" user@host

If the issue persists across reboots, consider kernel parameters:

# Increase pty maximums
echo 4096 > /proc/sys/kernel/pty/max
echo 1024 > /proc/sys/kernel/pty/nr

# Permanent solution (add to /etc/sysctl.conf)
kernel.pty.max = 4096
kernel.pty.nr = 1024

Modify /etc/ssh/sshd_config with these experimental settings:

# Disable pam session (temporarily for testing)
UsePAM no

# Change subsystem path
Subsystem sftp /usr/lib/ssh/sftp-server -l INFO

# Force specific pty allocation method
PermitTTY yes
UsePrivilegeSeparation no

Remember to restart sshd after changes: systemctl restart sshd

For environments where PTY allocation consistently fails, consider:

# Use SSH forced command in ~/.ssh/authorized_keys
command="/bin/bash --noprofile --norc" ssh-rsa AAAAB3...

# Configure in sshd_config
Match User problemuser
    ForceCommand /usr/sbin/login_override.sh

Where login_override.sh contains:

#!/bin/bash
# Minimal environment setup
export TERM=dumb
exec /bin/bash --noprofile --norc