When working with RHEL 6.4 servers, you might encounter a particularly frustrating scenario where SSH connections successfully authenticate but then hang indefinitely. The client output shows normal authentication flow:

debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.

Meanwhile, the server logs show a perfectly normal session establishment:

sshd[12387]: Accepted publickey for user from 1.1.11.239 port 34135 ssh2
pam_unix(sshd:session): session opened for user user by (uid=0)

Before diving deeper, verify these basic checks:

• Test with different authentication methods: ssh -o PreferredAuthentications=password user@host
• Check for forced commands in authorized_keys: grep "command=" ~/.ssh/authorized_keys
• Verify filesystem mounts: df -h and mount

Network issues can manifest as hanging connections. Try these TCP diagnostics:

# On client:
tcpdump -i eth0 host server_ip and port 22

# On server:
tcpdump -i eth0 host client_ip and port 22

Check for packet loss or retransmissions that might indicate network problems.

Investigate these critical SSH server configurations:

# Check for unusual PAM configurations:
cat /etc/pam.d/sshd

# Verify SSH server configuration:
grep -v "^#" /etc/ssh/sshd_config | grep -v "^$"

# Check for unusual shell configurations:
grep user /etc/passwd
ls -la ~user/.bashrc ~user/.bash_profile

When standard methods fail, these approaches can help:

# Run sshd in debug mode:
/usr/sbin/sshd -d -p 2222

# Check for stuck processes:
strace -p <sshd_pid>

# Verify DNS resolution:
getent hosts $(hostname)
dig +short $(hostname)

Sometimes the issue lies deeper in the system:

# Check for TCP stack issues:
sysctl -a | grep tcp

# Look for dropped packets:
netstat -s | grep -i "segments retransmitted"

# Verify system resource limits:
ulimit -a

If you need immediate access while troubleshooting:

# Force a different cipher:
ssh -c aes256-cbc user@host

# Disable pseudo-terminal allocation:
ssh -T user@host

# Use GSSAPI explicitly:
ssh -o GSSAPIAuthentication=no user@host

Remember to document all changes and revert test configurations after troubleshooting.

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When working with RHEL 6.4 servers, you might encounter an SSH session that successfully authenticates but then hangs indefinitely. The debug output shows normal key exchange and authentication:

debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.

Yet the session never proceeds to the shell prompt. Let's explore this systematically.

First, confirm basic network connectivity beyond just the SSH port:

# Check for firewall rules
iptables -L -n -v
# Verify routing
traceroute 10.6.27.64
# Test TCP connection
nc -zv 10.6.27.64 22

On the server side, examine these critical SSH settings:

# Check sshd_config for these parameters:
UseDNS no
GSSAPIAuthentication no
LoginGraceTime 30

For testing, temporarily add these debug options to /etc/ssh/sshd_config:

LogLevel DEBUG3
SyslogFacility AUTHPRIV

Verify critical filesystem paths:

# Check home directory permissions
ls -ld ~user/
# Verify PAM modules
ls -l /etc/pam.d/sshd
# Check SELinux context
ls -Z /home/user/.ssh/

When standard logs don't reveal the issue, try these methods:

# Run sshd in debug mode
/usr/sbin/sshd -d -p 2222
# Then connect with:
ssh -p 2222 -vvv user@host

For stuck sessions, capture network traffic:

tcpdump -i eth0 -s 0 -w ssh.pcap port 22

Based on similar cases, these solutions often work:

# Disable GSSAPI in sshd_config
GSSAPIAuthentication no
# Or try forcing IPv4
ssh -4 user@host

For PAM-related issues:

# Check PAM stack
authconfig --test
# Temporary workaround:
ssh -o PreferredAuthentications=publickey user@host

After identifying the root cause, implement permanent fixes:

# Example: Update crypto policies
update-crypto-policies --set LEGACY
# Or modify sshd_config permanently:
echo "UseDNS no" >> /etc/ssh/sshd_config
systemctl restart sshd