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I am not able to connect via SSH to my server and I dont know the reasons.

SSHD its running and ports are open in UFW. I tried to change ports but the issue persist. Also tried differents machines and networks.

If I reboot the server, sometimes I can establish connection but after a time the problem comeback.

My sshd_config:

#   $OpenBSD: sshd_config,v 1.103 2018/04/09 20:41:22 tj Exp $

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

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

# 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 override the
# default value.

Port 1402
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
#RekeyLimit default none

# Logging
#SyslogFacility AUTH
LogLevel VERBOSE

# Authentication:

#LoginGraceTime 2m
#PermitRootLogin prohibit-password
StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

#PubkeyAuthentication yes

# Expect .ssh/authorized_keys2 to be disregarded by default in future.
#AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# 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

# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication yes

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

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no

# 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 and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes

#AllowAgentForwarding yes
AllowTcpForwarding no
#GatewayPorts no
X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
PrintMotd no
#PrintLastLog yes
#TCPKeepAlive yes
#PermitUserEnvironment no
#Compression delayed
KeepAlive yes
ClientAliveInterval 90000
ClientAliveCountMax 2
UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none

# no default banner path
#Banner none

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*

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

# Example of overriding settings on a per-user basis
#Match User anoncvs
#   X11Forwarding no
#   AllowTcpForwarding no
#   PermitTTY no
#   ForceCommand cvs server
PermitRootLogin no
PasswordAuthentication yes

I get a timeout error with: ssh user@XXX.XXX.XXX.XXX -p 1402

And nmap answer the following:

      user@linux:~$ nmap -p 1402 -Pn xx.xxx.xxx.xxx 
Starting Nmap 7.70 ( https://nmap.org ) at 2021-09-07 22:06 CEST
Nmap scan report for xx.xxx.xxx.xxx
Host is up.

PORT     STATE    SERVICE
1402/tcp filtered prm-sm-np

Nmap done: 1 IP address (1 host up) scanned in 6.99 seconds

Some ideas?

EDIT

UFW Config

    user@localhost:~$ sudo ufw status verbose
[sudo] password for user: 
Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
1402/tcp                   LIMIT IN    Anywhere                  
3000/tcp                   ALLOW IN    Anywhere                  
9100/tcp                   ALLOW IN    Anywhere                  
12798/tcp                  ALLOW IN    Anywhere                  
6000/tcp                   ALLOW IN    Anywhere                  
60000/tcp                  ALLOW IN    Anywhere                  
1402/tcp (v6)              LIMIT IN    Anywhere (v6)             
3000/tcp (v6)              ALLOW IN    Anywhere (v6)             
9100/tcp (v6)              ALLOW IN    Anywhere (v6)             
12798/tcp (v6)             ALLOW IN    Anywhere (v6)             
6000/tcp (v6)              ALLOW IN    Anywhere (v6)             
60000/tcp (v6)             ALLOW IN    Anywhere (v6)
Shugui
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  • Check your firewalls. – Michael Hampton Sep 07 '21 at 20:14
  • Done, UFW is fine and the firewall from my hosting too – Shugui Sep 07 '21 at 20:16
  • But you didn't post a copy of them in your post. – Michael Hampton Sep 07 '21 at 20:26
  • Added UFW config. Changed 1402 port configuration from LIMIT IN to ALLOW IN and still with no connection – Shugui Sep 07 '21 at 20:52
  • What about the other firewall? – Michael Hampton Sep 08 '21 at 07:49
  • you should firstly check if the port is open. Try to connect to the port using `telnet` from your local machine. If it enters the dialog mode press `Ctrl`+`]` to exit, otherwise the port is unreachable. Then you should check all the firewall settings including inside your Linux, and outside it(some Cloud service would set a default firewall for you) – George Y Sep 08 '21 at 08:04
  • @GeorgeY nmap answer fine as I posted, but telnet (telnet xx.xxx.xxx.xx 1402) answer with: Unable to connect to remote host: Connection timed out. How is this possible? Not an expert here – Shugui Sep 10 '21 at 15:48
  • @Shugui use telnet on the exact same machine, and if you still cannot open it, the problem is on your `sshd`. – George Y Sep 11 '21 at 02:01

1 Answers1

-1
KeepAlive yes
ClientAliveInterval 90000
ClientAliveCountMax 2

These three lines indicate that if within 90000*2 seconds there is no TCP package from the client, it will cut the connection automatically.

This is a protection mechanism by SSH. Either you change the parameter or use Bitvise ssh Client instead, which would automatically send TCP Ping-Pong heartbeat packages to the server.

George Y
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