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SSH troubleshooting

Posted on Fri, Feb 16, 2024 at 11:40 by Hubertus A. Haniel

When SSH issues are reported it is all to tempting to jump on a box make changes to the config file to fix the suspected issues and restarting sshd.

This may not always be the best way because:

  • Error messages in syslog may be misleading as it is difficult to track down an individual session and debug messages may be filtered out in syslog
  • On a busy system other users that still work may get disconnected/locked out while the problem is being worked on
  • In the worst case you will get disconnected and will not get back into the system other than via the console

The way to avoid this is to start ssh with the "-d" option which will start sshd in debugging mode and it will listen for one session only. If required multiple -dd (up to three) can be specified to increase the debugging level. Obviously the running ssh session is already listening on port 22 so we do not interfere with that so we need to specify a different port to listen on that is not in use with the -p option. For security reasons build into sshd you must run sshd with the full path of where it is installed. 

# /usr/sbin/sshd -ddd -p 2222
debug2: load_server_config: filename /etc/ssh/sshd_config
debug2: load_server_config: done config len = 595
debug2: parse_server_config: config /etc/ssh/sshd_config len 595
debug3: /etc/ssh/sshd_config:21 setting Protocol 2
debug3: /etc/ssh/sshd_config:36 setting SyslogFacility AUTHPRIV
debug3: /etc/ssh/sshd_config:66 setting PasswordAuthentication yes
debug3: /etc/ssh/sshd_config:70 setting ChallengeResponseAuthentication no
debug3: /etc/ssh/sshd_config:80 setting GSSAPIAuthentication yes
debug3: /etc/ssh/sshd_config:82 setting GSSAPICleanupCredentials yes
debug3: /etc/ssh/sshd_config:96 setting UsePAM yes
debug3: /etc/ssh/sshd_config:99 setting AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
debug3: /etc/ssh/sshd_config:100 setting AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
debug3: /etc/ssh/sshd_config:101 setting AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
debug3: /etc/ssh/sshd_config:102 setting AcceptEnv XMODIFIERS
debug3: /etc/ssh/sshd_config:108 setting X11Forwarding yes
debug3: /etc/ssh/sshd_config:131 setting Subsystem sftp /usr/libexec/openssh/sftp-server
debug3: /etc/ssh/sshd_config:138 setting PermitRootLogin without-password
debug1: sshd version OpenSSH_5.3p1
debug3: Not a RSA1 key file /etc/ssh/ssh_host_rsa_key.
debug1: read PEM private key done: type RSA
debug1: private host key: #0 type 1 RSA
debug3: Not a RSA1 key file /etc/ssh/ssh_host_dsa_key.
debug1: read PEM private key done: type DSA
debug1: private host key: #1 type 2 DSA
debug1: rexec_argv[0]='/usr/sbin/sshd'
debug1: rexec_argv[1]='-ddd'
debug1: rexec_argv[2]='-p'
debug1: rexec_argv[3]='2222'
debug2: fd 3 setting O_NONBLOCK
debug1: Bind to port 2222 on 0.0.0.0.
Server listening on 0.0.0.0 port 2222.
debug2: fd 4 setting O_NONBLOCK
debug1: Bind to port 2222 on ::.
Server listening on :: port 2222.

Now the user can connect to that port with something like "ssh -p 2222 user@host" which will then give us detailed information of what is happening with that connection.

To make changes to the config file and to debug/test these changes it is best to make a copy of the existing config file and edit this file instead so we copy the config with something like: 

# cp -a /etc/ssh/sshd_config /etc/ssh/sshd_config.TEST

Then we can start a session using this config file with: 

# /usr/sbin/sshd -ddd -p 2222 -f /etc/ssh/sshd_config.TEST
debug2: load_server_config: filename /etc/ssh/sshd_config.TEST

Once confident that our changes are safe and they will not break anything else we can copy the changes to the real config file and restart the main ssh daemon on the system.

Edited on: Fri, Feb 16, 2024 12:36

Posted in HowTo (RSS), System - AIX (RSS), System - Linux (RSS), System - Solaris (RSS)

Colour output in your scripts

Posted on Wed, Aug 02, 2023 at 13:03 by Hubertus A. Haniel

On Linux I have been using tput to produce colours in my output but then I noticed the other day that this does not actually seem to work on Solaris but I am not sure why so I had to resort to the old fashioned way of using escape sequences. This works perfectly fine in Linux: 

#!/bin/bash
GREEN=$(tput setaf 2)
RED=$(tput setaf 1)
YELLOW=$(tput setaf 3)
NOCOL=$(tput sgr0)

echo "This works in Linux...."
echo "This is ${GREEN} Green${NOCOL} in Green"
echo "This is ${RED} Red${NOCOL} in Red"
echo "This is ${YELLOW} Yellow${NOCOL} in Yellow"
echo ""

So on Solaris this would be done like this (And this also works on Linux):

#!/bin/bash
GREEN="\033[0;32m"
RED="\033[0;31m"
YELLOW="\033[0;33m"
NOCOL="\033[0m"

echo "This works in Linux and Solaris...."
echo -e "This is ${GREEN} Green${NOCOL} in Green"
echo -e "This is ${RED} Red${NOCOL} in Red"
echo -e "This is ${YELLOW} Yellow${NOCOL} in Yellow"

    

      So I guess I am going to have to stick to the second method to make my 
      stuff work across platforms - From the script bits above you can see 
      that a font effect is turned on with a code and you will have you will 
      have to use a reset code "\033[0m" to turn it back off. The \033 ANSI 
      escape sequence has a lot of codes to go in hand with it to do all sort 
      of clever effects.
    

    

      
    

    
    echo -e "\033[31;1;4mHello\033[0m"

    

      This example above has a comma separated list of codes so you got 31 for 
      red, 1 for bold and 4 for underline and all this is cleared again with 0
    

    

      This is a table that lists all the effect codes:
    

    

      

        
          Code
        
        
          Effect
        
        
          Note
        
      

      

        
          0 
        
        
          Reset / Normal 
        
        
          all attributes off 
        
      

      

        
          1 
        
        
          Bold or increased intensity 
        
        
          
        
      

      

        
          2 
        
        
          Faint (decreased intensity) 
        
        
          Not widely supported. 
        
      

      

        
          3 
        
        
          Italic 
        
        
          Not widely supported. Sometimes treated as inverse. 
        
      

      

        
          4 
        
        
          Underline 
        
        
          
        
      

      

        
          5 
        
        
          Slow Blink 
        
        
          less than 150 per minute 
        
      

      

        
          6 
        
        
          Rapid Blink 
        
        
          MS-DOS ANSI.SYS; 150+ per minute; not widely 
          supported 
        
      

      

        
          7 
        
        
          [[reverse video]] 
        
        
          swap foreground and background colors 
        
      

      

        
          8 
        
        
          Conceal 
        
        
          Not widely supported. 
        
      

      

        
          9 
        
        
          Crossed-out 
        
        
          Characters legible, but marked for deletion. Not 
          widely supported. 
        
      

      

        
          10 
        
        
          Primary(default) font 
        
        
          
        
      

      

        
          11–19 
        
        
          Alternate font 
        
        
          Select alternate font n-10 
        
      

      

        
          20 
        
        
          Fraktur 
        
        
          hardly ever supported 
        
      

      

        
          21 
        
        
          Bold off or Double Underline 
        
        
          Bold off not widely supported; double underline 
          hardly ever supported. 
        
      

      

        
          22 
        
        
          Normal color or intensity 
        
        
          Neither bold nor faint 
        
      

      

        
          23 
        
        
          Not italic, not Fraktur 
        
        
          
        
      

      

        
          24 
        
        
          Underline off 
        
        
          Not singly or doubly underlined 
        
      

      

        
          25 
        
        
          Blink off 
        
        
          
        
      

      

        
          27 
        
        
          Inverse off 
        
        
          
        
      

      

        
          28 
        
        
          Reveal 
        
        
          conceal off 
        
      

      

        
          29 
        
        
          Not crossed out 
        
        
          
        
      

      

        
          30–37 
        
        
          Set foreground color 
        
        
          See color table below 
        
      

      

        
          38 
        
        
          Set foreground color 
        
        
          Next arguments are 5;<n> or 2;<r>;<g>;<b>, 
          see below 
        
      

      

        
          39 
        
        
          Default foreground color 
        
        
          implementation defined (according to standard) 
        
      

      

        
          40–47 
        
        
          Set background color 
        
        
          See color table below 
        
      

      

        
          48 
        
        
          Set background color 
        
        
          Next arguments are 5;<n> or 2;<r>;<g>;<b>, 
          see below 
        
      

      

        
          49 
        
        
          Default background color 
        
        
          implementation defined (according to standard) 
        
      

      

        
          51 
        
        
          Framed 
        
        
          
        
      

      

        
          52 
        
        
          Encircled 
        
        
          
        
      

      

        
          53 
        
        
          Overlined 
        
        
          
        
      

      

        
          54 
        
        
          Not framed or encircled 
        
        
          
        
      

      

        
          55 
        
        
          Not overlined 
        
        
          
        
      

      

        
          60 
        
        
          ideogram underline 
        
        
          hardly ever supported 
        
      

      

        
          61 
        
        
          ideogram double underline 
        
        
          hardly ever supported 
        
      

      

        
          62 
        
        
          ideogram overline 
        
        
          hardly ever supported 
        
      

      

        
          63 
        
        
          ideogram double overline 
        
        
          hardly ever supported 
        
      

      

        
          64 
        
        
          ideogram stress marking 
        
        
          hardly ever supported 
        
      

      

        
          65 
        
        
          ideogram attributes off 
        
        
          reset the effects of all of 60-64 
        
      

      

        
          90–97 
        
        
          Set bright foreground color 
        
        
          aixterm (not in standard) 
        
      

      

        
          100–107 
        
        
          Set bright background color 
        
        
          aixterm (not in standard)
        
      

    

    

      
    

    

    

      The table below lists the basic 8bit color table which should be 
      sufficient for most cases - there are plenty of other sources to give 
      you 256 colours but in most cases that would not be required
    

    

      
    

    

      

        
          

            
          

        		
      

    


                    Edited on: Thu, Dec 05, 2024 16:29

                    
Posted in HowTo (RSS), Shell Scripting (RSS), System - AIX (RSS), System - Linux (RSS), System - Solaris (RSS)





function cleanexit {} - Clean your shit!


                    
Posted on Mon, Jun 20, 2022 at 11:42 by Hubertus A. Haniel

                    
                    

      When writing shell scripts in bash that create temporary files I prefer 
      to stick a clean exit function at the top of the script that runs the 
      clean up no matter how the script exits - this should even remove files 
      when the script got interrupted:
    

    

      
    

    
	function cleanexit
	{
		rm -f /var/tmp/tmpfile
	}
	trap cleanexit INT QUIT TERM EXIT

    

      Let me know if you have a better idea!
    


                    Edited on: Wed, Aug 02, 2023 14:36

                    
Posted in HowTo (RSS), Shell Scripting (RSS), System - AIX (RSS), System - Linux (RSS), System - Solaris (RSS)





Welcome - Notes from a Linux/Unix Engineer


                    
Posted on Tue, Jun 01, 2010 at 11:11 by Hubertus A. Haniel

                    
                    

      I used to collect notes documents and HOWTO's at http://www.rootunix.org
 which are now archived at http://www.unixcook.com/old-unix-docs/
 as it was difficult to maintain and I got lazy with it. I have come 
      across a cross-platform blog software called Thingamablog
  which is written in Java so it works on Windows and Unix and I am hoping 
      that it will enable me to publish useful notes at a quicker pace with 
      not a lot of messing around.
    

    

      
    

    

      We will see how successful that will be....
    


                    
                    
Posted in Automation (RSS), HowTo (RSS), Packaging (RPM) (RSS), Shell Scripting (RSS), System - AIX (RSS), System - Apple / OSX (RSS), System - Linux (RSS), System - Solaris (RSS), System - Windows (RSS), Virtualization (RSS)






          
          
          
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