Centos 5 VPS: sshd freezes

Question

I have a Centos 5 VPS and it's giving me some problems when trying to connect to ssh. This is what happens:

1. I ssh into it and do some work. Nmap detects port 22 as open.
2. I disconnect.
3. Sometimes it lets me connect back but usually I can't ssh back again (It gives an error: Bad File Number) and have to wait half an hour, even if I reboot the system or restart the ssh daemon via Plesk panel. Nmap detects port 22 as closed.

I can access Apache and the plesk panel on the vps without problems, but sshd just freezes whenever I connect.

This is my /etc/ssh/sshd_config file:

#   $OpenBSD: sshd_config,v 1.73 2005/12/06 22:38:28 reyk Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/bin:/bin:/usr/bin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options change a
# default value.

Port 22
#Protocol 2,1
Protocol 2
AddressFamily inet
ListenAddress 82.223.121.43
#ListenAddress ::

# HostKey for protocol version 1
#HostKey /etc/ssh/ssh_host_key
# HostKeys for protocol version 2
#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key

# Lifetime and size of ephemeral version 1 server key
#KeyRegenerationInterval 1h
#ServerKeyBits 768

# Logging
# obsoletes QuietMode and FascistLogging
#SyslogFacility AUTH
SyslogFacility AUTHPRIV
#LogLevel INFO

# Authentication:

LoginGraceTime 2m
PermitRootLogin yes
StrictModes yes
#MaxAuthTries 6

#RSAAuthentication yes
#PubkeyAuthentication yes
#AuthorizedKeysFile .ssh/authorized_keys

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#RhostsRSAAuthentication no
# similar for protocol version 2
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# RhostsRSAAuthentication and HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
#PasswordAuthentication yes

# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes

# Set this to 'yes' to enable PAM authentication, account processing, 
# and session processing. If this is enabled, PAM authentication will 
# be allowed through the ChallengeResponseAuthentication mechanism. 
# Depending on your PAM configuration, this may bypass the setting of 
# PasswordAuthentication, PermitEmptyPasswords, and 
# "PermitRootLogin without-password". If you just want the PAM account and 
# session checks to run without PAM authentication, then enable this but set 
# ChallengeResponseAuthentication=no
UsePAM yes

# Accept locale-related environment variables
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES 
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT 
AcceptEnv LC_IDENTIFICATION LC_ALL
#AllowTcpForwarding yes
#GatewayPorts no
#X11Forwarding no
X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes
#PrintMotd yes
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
#UsePrivilegeSeparation yes
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#ShowPatchLevel no
UseDNS no
#PidFile /var/run/sshd.pid
MaxStartups 20
#PermitTunnel no
#ChrootDirectory none

# no default banner path
#Banner /some/path

# override default of no subsystems
Subsystem   sftp    /usr/libexec/openssh/sftp-server

Answer 1

When you are able to successfully log into SSH, try this - change the LogLevel in your sshd_config to read:

LogLevel DEBUG

Restart SSHD while still logged in for the changes to take effect, then log-off. The next time you are able to successfully SSH into your server, post the output of /var/log/secure.

Also if you have acccess to a Linux client machine, try logging into your server using the following command:

ssh -vvv user@hostname

That would help in figuring out what the client sees when the server is "hung".

One other tip - I notice that you have PermitRootLogin set to "Yes". On a public-facing VPS, this is a a very bad idea. Please set it to "No" and use sudo instead.