Archive for the ‘Various’ Category

Adding custom user based host IP aliases load custom prepared /etc/hosts from non root user on Linux – Script to allow define IPs that doesn’t have DNS records to user preferred hostname

Wednesday, April 14th, 2021

adding-custom-user-based-host-aliases-etc-hosts-logo-linux

Say you have access to a remote Linux / UNIX / BSD server, i.e. a jump host and you have to remotely access via ssh a bunch of other servers
who have existing IP addresses but the DNS resolver recognized hostnames from /etc/resolv.conf are long and hard to remember by the jump host in /etc/resolv.conf and you do not have a way to include a new alias to /etc/hosts because you don't have superuser admin previleges on the hop station.
To make your life easier you would hence want to add a simplistic host alias to be able to easily do telnet, ssh, curl to some aliased name like s1, s2, s3 … etc.


The question comes then, how can you define the IPs to be resolvable by easily rememberable by using a custom User specific /etc/hosts like definition file? 

Expanding /etc/hosts predefined host resolvable records is pretty simple as most as most UNIX / Linux has the HOSTALIASES environment variable
Hostaliases uses the common technique for translating host names into IP addresses using either getaddrinfo(3) or the obsolete gethostbyname(3). As mentioned in hostname(7), you can set the HOSTALIASES environment variable to point to an alias file, and you've got per-user aliases

create ~/.hosts file

linux:~# vim ~/.hosts

with some content like:
 

g google.com
localhostg 127.0.0.1
s1 server-with-long-host1.fqdn-whatever.com 
s2 server5-with-long-host1.fqdn-whatever.com
s3 server18-with-long-host5.fqdn-whatever.com

linux:~# export HOSTALIASES=$PWD/.hosts

The caveat of hostaliases you should know is this will only works for resolvable IP hostnames.
So if you want to be able to access unresolvable hostnames.
You can use a normal alias for the hostname you want in ~/.bashrc with records like:

alias server-hostname="ssh username@10.10.10.18 -v -o stricthostkeychecking=no -o passwordauthentication=yes -o UserKnownHostsFile=/dev/null"
alias server-hostname1="ssh username@10.10.10.19 -v -o stricthostkeychecking=no -o passwordauthentication=yes -o UserKnownHostsFile=/dev/null"
alias server-hostname2="ssh username@10.10.10.20 -v -o stricthostkeychecking=no -o passwordauthentication=yes -o UserKnownHostsFile=/dev/null"

then to access server-hostname1 simply type it in terminal.

The more elegant solution is to use a bash script like below:

# include below code to your ~/.bashrc
function resolve {
        hostfile=~/.hosts
        if [[ -f “$hostfile” ]]; then
                for arg in $(seq 1 $#); do
                        if [[ “${!arg:0:1}” != “-” ]]; then
                                ip=$(sed -n -e "/^\s*\(\#.*\|\)$/d" -e "/\<${!arg}\>/{s;^\s*\(\S*\)\s*.*$;\1;p;q}" "$hostfile")
                                if [[ -n “$ip” ]]; then
                                        command "${FUNCNAME[1]}" "${@:1:$(($arg-1))}" "$ip" "${@:$(($arg+1)):$#}"
                                        return
                                fi
                        fi
                done
        fi
        command "${FUNCNAME[1]}" "$@"
}

function ping {
        resolve "$@"
}

function traceroute {
        resolve "$@"
}

function ssh {
        resolve "$@"
}

function telnet {
        resolve "$@"
}

function curl {
        resolve "$@"
}

function wget {
        resolve "$@"
}

 

Now after reloading bash login session $HOME/.bashrc with:

linux:~# source ~/.bashrc

ssh / curl / wget / telnet / traceroute and ping will be possible to the defined ~/.hosts IP addresses just like if it have been defined global wide on System in /etc/hosts.

Enjoy
 

IBM Tivoli (Spectrum Protect) update self-signed client expiring SSL certificates

Wednesday, March 17th, 2021

IBM-Spectrum-protect-suite-logo

Say you're using Tivoli TSM to manage your backups for tsm (if you don't know what is IBM TSM (Spectrum Protect) Backup solution check my previous article on how to use IBM Tivoli to list configured, scheduled, how to do restore backups with dsmc console client.

And you follow below steps to enable SSL communication with a CA-signed certificate between Spectrum Protect client and server:

E.g. you have to

  • Obtained the CA root certificate.
  • Configure the clients. To use SSL, each client must import the self-signed server certificate.

Used the GSKit command-line utility (gsk8capicmd for 32-bit clients or gsk8capicmd_64 for 64-bit clients) to import the certificate.

ibm-tsm_ssl_config_selfsigned


1. The problem

This self-signed certificates has expire date which after some time might have expire date coming. If your environment has something like PCI security standards enabled and you do a Quarterly security scans with something like QualysGuard (Qaulys vulnerability management tool). 

In the case of Qualys scans you may receive GSK messages in dsmerro.log if the certificate is expiring:

 

03/04/2021 14:35:07 ANS9959W IBM Spectrum Protect acceptor received a non-critical network error 88, IBM Spectrum Protect return code : -50.

03/05/2021 13:04:59 ANS1579E GSKit function gsk_secure_soc_init failed with 414: GSK_ERROR_BAD_CERT

03/05/2021 13:04:59 TCP/IP received rc 88 trying to accept connection from server.

03/05/2021 13:04:59 ANS9959W IBM Spectrum Protect acceptor received a non-critical network error 88, IBM Spectrum Protect return code : -50.

 

2. To check the situation on the host with TSM self-signed expiry

 

2.1  First get the FQDN and certificate name

[root@redhat: ~ ]# FQDN=$(hostname –fqdn |tr '[:lower:]’ ‘[:upper:]');

 

[root@redhat: ~ ]# echo "FQDN to be used is: $FQDN. Please be careful it is correct (if machine has wrong FQDN) you might have issues";

[root@redhat: ~ ]# gsk8capicmd_64 -cert -list -db /etc/adsm/Nodes/$FQDN/spclicert.kdb -stashed


The gsk8capicmd_64 is IBM's tool to view and manage SSL certificates it is perhaps a C written binary that has a compiled patched version of a normal openssl tool. Using it is the ibm recommended way to manage Tivoli certificates.

2.2  Get details using -label=CERTNAME and check for expiration date

 

[root@redhat: ~ ]# gsk8capicmd_64 -cert -details -label $FQDN -db /etc/adsm/Nodes/$FQDN/spclicert.kdb -stashed

Certificates found
* default, – personal, ! trusted, # secret key
–       FQDN-OF-HOST.COM

 

3. To update the certificates

 

3.1 Copy the old certificates for backup

As usual do a backup in case if something goes wrong and you need to restore

[root@redhat: ~ ]# mkdir /root/certbck-tsm_$(date +"%b-%d-%Y")/

[root@redhat: ~ ]# cp -rpv /etc/adsm/Nodes/$FQDN/spclicert* /root/certbck-tsm_$(date +"%b-%d-%Y")/

 

3.2 Stop the dsmcad backup service

[root@redhat: ~ ]# systemctl stop dsmcad

Double check the service is stopped by checking for any remain dsm processes

[root@redhat: ~ ]# ps axf | grep dsm

3.3 Remove the expiring certificates from host

[root@redhat: ~ ]# rm -v /etc/adsm/Nodes/$FQDN/spclicert*

 

3.4 Generate new certificates with dsmc client

[root@redhat: ~ ]# dsmc query session -optfile="/opt/tivoli/tsm/client/ba/bin/dsm.opt"

 

3.5 Check if all is generated as expected

[root@redhat: ~ ]# ls -l  /etc/adsm/Nodes/$FQDN/spclicert*

3.6 Start the backup service

[root@redhat: ~ ]# systemctl start dsmcad

3.7  Check  /var/tsm/dsmwebcl.log for the port number of webclient

 [root@redhat: ~ ]# cat /var/tsm/dsmwebcl.log

03/16/2021 13:31:41 (dsmcad) ————————————————————
03/16/2021 13:31:41 (dsmcad) Command will be executed in 11 hours and 50 minutes.
03/16/2021 15:56:01 (dsmcad) ANS9959W IBM Spectrum Protect acceptor received a non-critical network error 88, IBM Spectrum Protect return code : -50.
03/17/2021 01:21:41 (dsmcad) Executing scheduled command now.
03/17/2021 01:22:53 (dsmcad) Next operation scheduled:
03/17/2021 01:22:53 (dsmcad) ————————————————————
03/17/2021 01:22:53 (dsmcad) Schedule Name:         0120_SCHED_P
03/17/2021 01:22:53 (dsmcad) Action:                Incremental
03/17/2021 01:22:53 (dsmcad) Objects:
03/17/2021 01:22:53 (dsmcad) Options:               -subdir=yes
03/17/2021 01:22:53 (dsmcad) Server Window Start:   01:20:00 on 03/18/2021
03/17/2021 01:22:53 (dsmcad) ————————————————————
03/17/2021 01:22:53 (dsmcad) Command will be executed in 12 hours.
03/17/2021 13:22:53 (dsmcad) Executing scheduled command now.
03/17/2021 13:22:54 (dsmcad) Next operation scheduled:
03/17/2021 13:22:54 (dsmcad) ————————————————————
03/17/2021 13:22:54 (dsmcad) Schedule Name:         0120_SCHED_P
03/17/2021 13:22:54 (dsmcad) Action:                Incremental
03/17/2021 13:22:54 (dsmcad) Objects:
03/17/2021 13:22:54 (dsmcad) Options:               -subdir=yes
03/17/2021 13:22:54 (dsmcad) Server Window Start:   01:20:00 on 03/18/2021
03/17/2021 13:22:54 (dsmcad) ————————————————————


 [root@redhat: ~ ]# grep -i port /var/tsm/dsmwebcl.log
03/11/2021 16:59:19 (dsmcad) ANS3000I TCP/IP communications available on port 37506.
03/12/2021 11:35:21 (dsmcad) ANS3000I TCP/IP communications available on port 40510.
03/12/2021 14:53:03 (dsmcad) ANS3000I TCP/IP communications available on port 45005.

 

3.8  You can check the certificate expiery mask yourself as qualys scanner and check the new certificate 

[root@redhat: ~ ]# dsmc_port=$(netstat -tulpan|grep -i dsm|awk '{ print $4 }'|cut -d":" -f2);
[root@redhat: ~ ]# echo $dsmc_port

[root@redhat: ~ ]# openssl s_client -servername 127.0.0.1 -connect 127.0.0.1:$dsmc_port |  openssl x509 -noout -dates

notBefore=Mar  6 14:09:55 2021 GMT
notAfter=Mar  7 14:09:55 2022 GMT

Hopefully your expiry date is fine that means you're done, you can place the steps in a single script to save time, if you have to run it in a year time.

How to calculate connections from IP address with shell script and log to Zabbix graphic

Thursday, March 11th, 2021

We had to test the number of connections incoming IP sorted by its TCP / IP connection state.

For example:

TIME_WAIT, ESTABLISHED, LISTEN etc.


The reason behind is sometimes the IP address '192.168.0.1' does create more than 200 connections, a Cisco firewall gets triggered and the connection for that IP is filtered out. To be able to know in advance that this problem is upcoming. a Small userparameter script is set on the Linux servers, that does print out all connections from IP by its STATES sorted out.

 

The script is calc_total_ip_match_zabbix.sh is below:

#!/bin/bash
#  check ESTIMATED / FIN_WAIT etc. netstat output for IPs and calculate total
# UserParameter=count.connections,(/usr/local/bin/calc_total_ip_match_zabbix.sh)
CHECK_IP='192.168.0.1';
f=0; 

 

for i in $(netstat -nat | grep "$CHECK_IP" | awk '{print $6}' | sort | uniq -c | sort -n); do

echo -n "$i ";
f=$((f+i));
done;
echo
echo "Total: $f"

 

root@pcfreak:/bashscripts# ./calc_total_ip_match_zabbix.sh 
1 TIME_WAIT 2 ESTABLISHED 3 LISTEN 

Total: 6

 

root@pcfreak:/bashscripts# ./calc_total_ip_match_zabbix.sh 
2 ESTABLISHED 3 LISTEN 
Total: 5


images/zabbix-webgui-connection-check1

To make process with Zabbix it is necessery to have an Item created and a Depedent Item.

 

webguiconnection-check1

webguiconnection-check1
 

webgui-connection-check2-item

images/webguiconnection-check1

Finally create a trigger to trigger alarm if you have more than or eqaul to 100 Total overall connections.


images/zabbix-webgui-connection-check-trigger

The Zabbix userparameter script should be as this:

[root@host: ~]# cat /etc/zabbix/zabbix_agentd.d/userparameter_webgui_conn.conf
UserParameter=count.connections,(/usr/local/bin/webgui_conn_track.sh)

 

Some collleagues suggested more efficient shell script solution for suming the overall number of connections, below is less time consuming version of script, that can be used for the calculation.
 

#!/bin/bash -x
# show FIN_WAIT2 / ESTIMATED etc. and calcuate total
count=$(netstat -n | grep "192.168.0.1" | awk ' { print $6 } ' | sort -n | uniq -c | sort -nr)
total=$((${count// /+}))
echo "$count"
echo "Total:" "$total"

      2 ESTABLISHED
      1 TIME_WAIT
Total: 3

 


Below is the graph built with Zabbix showing all the fluctuations from connections from monitored IP. ebgui-check_ip_graph

 

How to calculate connections from IP address with shell script and log to Zabbix graphic

Thursday, March 11th, 2021

We had to test the number of connections incoming IP sorted by its TCP / IP connection state.

For example:

TIME_WAIT, ESTABLISHED, LISTEN etc.


The reason behind is sometimes the IP address '192.168.0.1' does create more than 200 connections, a Cisco firewall gets triggered and the connection for that IP is filtered out. To be able to know in advance that this problem is upcoming. a Small userparameter script is set on the Linux servers, that does print out all connections from IP by its STATES sorted out.

 

The script is calc_total_ip_match_zabbix.sh is below:

#!/bin/bash
#  check ESTIMATED / FIN_WAIT etc. netstat output for IPs and calculate total
# UserParameter=count.connections,(/usr/local/bin/calc_total_ip_match_zabbix.sh)
CHECK_IP='192.168.0.1';
f=0; 

 

for i in $(netstat -nat | grep "$CHECK_IP" | awk '{print $6}' | sort | uniq -c | sort -n); do

echo -n "$i ";
f=$((f+i));
done;
echo
echo "Total: $f"

 

root@pcfreak:/bashscripts# ./calc_total_ip_match_zabbix.sh 
1 TIME_WAIT 2 ESTABLISHED 3 LISTEN 

Total: 6

 

root@pcfreak:/bashscripts# ./calc_total_ip_match_zabbix.sh 
2 ESTABLISHED 3 LISTEN 
Total: 5


To make process with Zabbix it is necessery to have an Item created and a Depedent Item.

images/zabbix-webgui-connection-check1

 

 

 

 

webguiconnection-check1

webguiconnection-check1
 

webgui-connection-check2-item

images/webguiconnection-check1

Finally create a trigger to trigger alarm if you have more than or eqaul to 100 Total overall connections.


images/zabbix-webgui-connection-check-trigger

The Zabbix userparameter script should be as this:
cat /etc/zabbix/zabbix_agentd.d/userparameter_webgui_conn.conf
UserParameter=count.connections,(/usr/local/bin/webgui_conn_track.sh)
 

Some collleagues suggested more efficient shell script solution for suming the overall number of connections, below is less time consuming version of script, that can be used for the calculation.
 

#!/bin/bash -x
# show FIN_WAIT2 / ESTIMATED etc. and calcuate total
count=$(netstat -n | grep "192.168.0.1" | awk ' { print $6 } ' | sort -n | uniq -c | sort -nr)
total=$((${count// /+}))
echo "$count"
echo "Total:" "$total"

 

      2 ESTABLISHED
      1 TIME_WAIT
Total: 3

 


Below is the graph built with Zabbix showing all the fluctuations from connections from monitored IP.
ebgui-check_ip_graph

Update reverse sshd config with cronjob to revert if sshd reload issues

Friday, February 12th, 2021

Update-reverse-sshd-config-with-cronjob-to-revert-if-sshd-reload-issues

Say you're doing ssh hardening modifying /etc/ssh/sshd_config for better system security or just changing options in sshd due to some requirements. But you follow the wrong guide and you placed some ssh variable which is working normally on newer SSH versions ssh OpenSSH_8.0p1 / or 7 but the options are applied on older SSH server and due to that restarting sshd via /etc/init.d/… or systemctl restart sshd cuts your access to remote server located in a DC and not attached to Admin LAN port, and does not have a working ILO or IDRAC configured and you have to wait for a couple of hours for some Support to go to the server Room / Rack / line location to have access to a Linux physical tty console and fix it by reverting the last changes you made to sshd and restarting.

Thus logical question comes what can you do to assure yourself you would not cut your network access to remote machine after modifying OpenSSHD and normal SSHD restart?

There is an old trick, I'm using for years now but perhaps if you're just starting with Linux as a novice system administrator or a server support guy you would not know it, it is as simple as setting a cron job for some minutes to periodically overwrite the sshd configuration with a copy of the old working version of sshd before modification.

Here is this nice nify trick which saved me headache of call on technical support line to ValueWeb when I was administering some old Linux servers back in the 2000s

root@server:~# crontab -u root -e

# create /etc/ssh/sshd_config backup file
cp -rpf /etc/ssh/sshd_config /etc/ssh/sshd_config_$(date +%d-%m-%y)
# add to cronjob to execute every 15 minutes and ovewrite sshd with the working version just in case
*/15 * * * * /bin/cp -rpf /etc/ssh/sshd_config_$(date +%d-%m-%y) /etc/ssh/sshd_config && /bin/systemctl restart sshd
# restart sshd 
cp -rpf /etc/ssh/sshd_config_$(date +%d-%m-%y) /etc/ssh/sshd_config && /bin/systemctl restart sshd


Copy paste above cron definitions and leave them on for some time. Do the /etc/ssh/sshd_config modifications and once you're done restart sshd by lets say

root@server:~#  killall -HUP sshd 


If the ssh connectivity continues to work edit the cron job again and delete all lines and save again.
If you're not feeling confortable with vim as a text editor (in case you're a complete newbie and you don't know) how to get out of vim. Before doing all little steps you can do on the shell with  export EDITOR=nano or export EDITOR=mcedit cmds,this will change the default text editor on the shell. 

Hope this helps someone… Enjoy 🙂

List all existing local admin users belonging to admin group and mail them to monitoring mail box

Monday, February 8th, 2021

local-user-account-creation-deletion-change-monitor-accounts-and-send-them-to-central-monitoring-mail

If you have a bunch of servers that needs to have a tight security with multiple Local users superuser accounts that change over time and you need to frequently keep an have a long over time if some new system UNIX local users in /etc/passwd /etc/group has been added deleted e.g. the /etc/passwd /etc/group then you might have the task to have some primitive monitoring set and the most primitive I can think of is simply routinely log users list for historical purposes to a common mailbox over time (lets say 4 times a month or even more frequently) you might send with a simple cron job a list of all existing admin authorized users to a logging sysadmin mailbox like lets say:
 

Local-unix-users@yourcompanydomain.com


A remark to make here is the common sysadmin practice scenario to have local existing non-ldap admin users group members of whom are authorized to use sudo su – root via /etc/sudoers  is described in my previous article how to add local users to admin group superuser access via sudo I thus have been managing already a number of servers that have user setup using the above explained admin group.

Thus to have the monitoring at place I've developed a tiny shell script that does check all users belonging to the predefined user group dumps it to .csv format that starts with a simple timestamp on when user admin list was made and sends it to a predefined email address as well as logs sent mail content for further reference in a local directory.

The task is a relatively easy but since nowadays the level of competency of system administration across youngsters is declinging -that's of course in my humble opinion (just like it happens in every other profession), below is the developed list-admin-users.sh
 

 

#!/bin/bash
# dump all users belonging to a predefined admin user / group in csv format 
# with a day / month year timestamp and mail it to a predefined admin
# monitoring address
TO_ADDRESS="Local-unix-users@yourcompanydomain.com";
HOSTN=$(hostname);
# root@server:/# grep -i 1000 /etc/passwd
# username:x:username:1000:username,,,:/home/username:/bin/bash
# username1:x:username1:1000:username1,,,:/home/username1:/bin/bash
# username5:x:username1:1000:username5,,,:/home/username5:/bin/bash

ADMINS_ID='4355';
#
# root@server # group_id_ID='4355'; grep -i group_id_ID /etc/passwd
# …
# username1:x:1005:4355:username1,,,:/home/username1:/bin/bash
# username5:x:1005:4355,,,:/home/username5:/bin/bash


group_id_ID='215';
group_id='group_id';
FIL="/var/log/userlist-log-dir/userlist_$(date +"%d_%m_%Y_%H_%M").txt";
CUR_D="$HOSTN: Current admin users $(date)"; >> $FIL; echo -e "##### $CUR_D #####" >> $FIL;
for i in $(cat /etc/passwd | grep -i /home|grep /bin/bash|grep -e "$ADMINS_ID" -e "$group_id_ID" | cut -d : -f1); do \
if [[ $(grep $i /etc/group|grep $group_id) ]]; then
f=$(echo $i); echo $i,group_id,$(id -g $i); else  echo $i,admin,$(id -g $i);
fi
done >> $FIL; mail -s "$CUR_D" $TO_ADDRESS < $FIL


list-admin-users.sh is ready for download also here

To make the script report you will have to place it somewhere for example in /usr/local/bin/list-admin-users.sh ,  create its log dir location /var/log/userlist-log-dir/ and set proper executable and user/group script and directory permissions to it to be only readable for root user.

root@server: # mkdir /var/log/userlist-log-dir/
root@server: # chmod +x /usr/local/bin/list-admin-users.sh
root@server: # chmod -R 700 /var/log/userlist-log-dir/


To make the script generate its admin user reports and send it to the central mailbox  a couple of times in the month early in the morning (assuming you have a properly running postfix / qmail / sendmail … smtp), as a last step you need to set a cron job to routinely invoke the script as root user.

root@server: # crontab -u root -e
12 06 5,10,15,20,25,1 /usr/local/bin/list-admin-users.sh


That's all folks now on 5th 10th, 15th, 20th 25th and 1st at 06:12 you'll get the admin user list reports done. Enjoy 🙂

Create simple proxy http server with netcat ( nc ) based tiny shell script

Tuesday, January 26th, 2021

use-Netcat_proxy-picture

The need of proxy server is inevitable nowadays especially if you have servers located in a paranoid security environments. Where virtually all is being passed through some kind of a proxy server. In my work we have recently started a  CentOS Linux release 7.9.2009 on HP Proliant DL360e Gen8 (host named rhel-testing).

HP DL360e are quite old nowadays but since we have spare servers and we can refurnish them to use as a local testing internal server Hypervisor it is okay for us. The machine is attached to a Rack that is connected to a Secured Deimilitarized Zone LAN (DMZ Network) which is so much filtered that even simple access to the local company homebrew RPM repository is not accessible from the machine.
Thus to set and remove software from the machine we needed a way to make yum repositories be available, and it seems the only way was to use a proxy server (situated on another accessible server which we use as a jump host to access the testing machine).

Since opening additional firewall request was a time consuming non-sense and the machine is just for testing purposes, we had to come with a solution where we can somehow access a Local repository RPM storage server http://rpm-package-server-repo.com/ for which we have a separate /etc/yum.repos.d/custom-rpms.repo definition file created.

This is why we needed a simplistic way to run a proxy but as we did not have the easy way to install privoxy / squid / haproxy or apache webserver configured as a proxy (to install one of those of relatively giant piece of software need to copy many rpm packages and manually satisfy dependencies), we looked for a simplistic way to run a proxy server on jump-host machine host A.

A note to make here is jump-host that was about to serve as a proxy  had already HTTP access towards the RPM repositories http://rpm-package-server-repo.com and could normally fetch packages with curl or wget via it …

For to create a simple proxy server out of nothing, I've googled a bit thinking that it should be possible either with BASH's TCP/IP capabilities or some other small C written tool compiled as a static binary, just to find out that netcat swiss army knife as a proxy server bash script is capable of doing the trick.

Jump host machine which was about to be used as a proxy server for http traffic did not have enabled access to tcp/port 8888 (port's firewall policies were prohibiting access to it).Since 8888 was the port targetted to run the proxy to allow TCP/IP port 8888 accessibility from the testing RHEL machine towards jump host, we had to issue first on jump host:

[root@jump-host: ~ ]# firewall-cmd –permanent –zone=public –add-port=8888/tcp

To run the script once placed under /root/tcp-proxy.sh on jump-host we had to run a never ending loop in a GNU screen session to make sure it runs forever:

Original tcp-proxy.sh script used taken from above article is:
 

#!/bin/sh -e

 

if [ $# != 3 ]
then
    echo "usage: $0 <src-port> <dst-host> <dst-port>"
    exit 0
fi

TMP=`mktemp -d`
BACK=$TMP/pipe.back
SENT=$TMP/pipe.sent
RCVD=$TMP/pipe.rcvd
trap 'rm -rf "$TMP"' EXIT
mkfifo -m 0600 "$BACK" "$SENT" "$RCVD"
sed 's/^/ => /' <"$SENT" &
sed 's/^/<=  /' <"$RCVD" &
nc -l -p "$1" <"$BACK" | tee "$SENT" | nc "$2" "$3" | tee "$RCVD" >"$BACK"

 

Above tcp-proxy.sh script you can download here.

I've tested the script one time and it worked, the script syntax is:

 [root@jump-host: ~ ]#  sh tcp-proxy.sh
usage: tcp-proxy.sh <src-port> <dst-host> <dst-port>


To make it work for one time connection I've run it as so:

 

 [root@jump-host: ~ ]# sh tcp-proxy.sh 8888 rpm-package-server-repo.com 80

 

 

To make the script work all the time I had to use one small one liner infinite bash loop which goes like this:

[root@jump-host: ~ ]#  while [ 1 ]; do sh tcp-proxy.sh 8888 rpm-package-server-repo.com 80; done​

On rhel-testing we had to configure for yum and all applications to use a proxy temporary via
 

[root@rhel-tresting: ~ ]# export http_proxy=jump-host_machine_accessibleIP:8888


And then use the normal yum check-update && yum update to apply to rhel-testing machine latest RPM package security updates.

The nice stuff about the tcp-proxy.sh with netcat in a inifite loop is you will see the binary copy of traffic flowing on the script which will make you feel like in those notorious Hackers movies ! 🙂

The stupid stuff is that sometimes some connections and RPM database updates or RPMs could be cancelled due to some kind of network issues.

To make the connection issues that are occuring to the improvised proxy server go away we finally used a slightly modified version from the original netcat script, which read like this.
 

#!/bin/sh -e

 

if [ $# != 3 ]
then
    echo "usage: $0 <src-port> <dst-host> <dst-port>"
        exit 0
        fi

        TMP=`mktemp -d`
        BACK=$TMP/pipe.back
        SENT=$TMP/pipe.sent
        RCVD=$TMP/pipe.rcvd
        trap 'rm -rf "$TMP"' EXIT
        mkfifo -m 0600 "$BACK" "$SENT" "$RCVD"
        sed 's/^/ => /' <"$SENT" &
        sed 's/^/<=  /' <"$RCVD" &
        nc –proxy-type http -l -p "$1" <"$BACK" | tee "$SENT" | nc "$2" "$3" | tee "$RCVD" >"$BACK"


Modified version tcp-proxy1.sh with –proxy-type http argument passed to netcat script u can download here.

With –proxy-type http yum check-update works normal just like with any normal fully functional http_proxy configured.

Next step wasto make the configuration permanent you can either add to /root/.bashrc or /etc/bashrc (if you need the setting to be system wide for every user that logged in to Linux system).

[root@rhel-tresting: ~ ]#  echo "http_proxy=http://jump-host_machine_accessibleIP:8888/" > /etc/environment


If you need to set the new built netcat TCP proxy only for yum package update tool include proxy only in /etc/yum.conf:

[root@rhel-tresting: ~ ]# vi /etc/yum.conf
proxy=http_proxy=http://jump-host_machine_accessibleIP:8888/


That's all now you have a proxy out of nothing with just a simple netcat enjoy.

KVM Virtual Machine RHEL 8.3 Linux install on Redhat 8.3 Linux Hypervisor with custom tailored kickstart.cfg

Friday, January 22nd, 2021

kvm_virtualization-logo-redhat-8.3-install-howto-with-kickstart

If you don't have tried it yet Redhat and CentOS and other RPM based Linux operationg systems that use anaconda installer is generating a kickstart file after being installed under /root/{anaconda-ks.cfg,initial-setup- ks.cfg,original-ks.cfg} immediately after the OS installation completes. Using this Kickstart file template you can automate installation of Redhat installation with exactly the same configuration as many times as you like by directly loading your /root/original-ks.cfg file in RHEL installer.

Here is the official description of Kickstart files from Redhat:

"The Red Hat Enterprise Linux installation process automatically writes a Kickstart file that contains the settings for the installed system. This file is always saved as /root/anaconda-ks.cfg. You may use this file to repeat the installation with identical settings, or modify copies to specify settings for other systems."


Kickstart files contain answers to all questions normally asked by the text / graphical installation program, such as what time zone you want the system to use, how the drives should be partitioned, or which packages should be installed. Providing a prepared Kickstart file when the installation begins therefore allows you to perform the installation automatically, without need for any intervention from the user. This is especially useful when deploying Redhat based distro (RHEL / CentOS / Fedora …) on a large number of systems at once and in general pretty useful if you're into the field of so called "DevOps" system administration and you need to provision a certain set of OS to a multitude of physical servers or create or recreate easily virtual machines with a certain set of configuration.
 

1. Create /vmprivate storage directory where Virtual machines will reside

First step on the Hypervisor host which will hold the future created virtual machines is to create location where it will be created:

[root@redhat ~]#  lvcreate –size 140G –name vmprivate vg00
[root@redhat ~]#  mkfs.ext4 -j -b 4096 /dev/mapper/vg00-vmprivate
[root@redhat ~]# mount /dev/mapper/vg00-vmprivate /vmprivate

To view what is the situation with Logical Volumes and  VG group names:

[root@redhat ~]# vgdisplay -v|grep -i vmprivate -A7 -B7
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  – currently set to     8192
  Block device           253:0

 

  — Logical volume —
  LV Path                /dev/vg00/vmprivate
  LV Name                vmprivate
  VG Name                vg00
  LV UUID                VVUgsf-FXq2-TsMJ-QPLw-7lGb-Dq5m-3J9XJJ
  LV Write Access        read/write
  LV Creation host, time lpgblu01f.ffm.de.int.atosorigin.com, 2021-01-20 17:26:11 +0100
  LV Status              available
  # open                 1
  LV Size                150.00 GiB


Note that you'll need to have the size physically available on a SAS / SSD Hard Drive physically connected to Hypervisor Host.

To make the changes Virtual Machines storage location directory permanently mounted add to /etc/fstab

/dev/mapper/vg00-vmprivate  /vmprivate              ext4    defaults,nodev,nosuid 1 2

[root@redhat ~]# echo '/dev/mapper/vg00-vmprivate  /vmprivate              ext4    defaults,nodev,nosuid 1 2' >> /etc/fstab

 

2. Second we need to install the following set of RPM packages on the Hypervisor Hardware host

[root@redhat ~]# yum install qemu-kvm qemu-img libvirt virt-install libvirt-client virt-manager libguestfs-tools virt-install virt-top -y

3. Enable libvirtd on the host

[root@redhat ~]#  lsmod | grep -i kvm
[root@redhat ~]#  systemctl enable libvirtd

4. Configure network bridging br0 interface on Hypervisor


In /etc/sysconfig/network-scripts/ifcfg-eth0 you need to include:

NM_CONTROLED=NO

Next use nmcli redhat configurator to create the bridge (you can use ip command instead) but since the tool is the redhat way to do it lets do it their way ..

[root@redhat ~]# nmcli connection delete eno3
[root@redhat ~]# nmcli connection add type bridge autoconnect yes con-name br0 ifname br0
[root@redhat ~]# nmcli connection modify br0 ipv4.addresses 10.80.51.16/26 ipv4.method manual
[root@redhat ~]# nmcli connection modify br0 ipv4.gateway 10.80.51.1
[root@redhat ~]# nmcli connection modify br0 ipv4.dns 172.20.88.2
[root@redhat ~]# nmcli connection add type bridge-slave autoconnect yes con-name eno3 ifname eno3 master br0
[root@redhat ~]# nmcli connection up br0

5. Prepare a working kickstart.cfg file for VM


Below is a sample kickstart file I've used to build a working fully functional Virtual Machine with Red Hat Enterprise Linux 8.3 (Ootpa) .

#version=RHEL8
#install
# Run the Setup Agent on first boot
firstboot --enable
ignoredisk --only-use=vda
# Use network installation
#url --url=http://hostname.com/rhel/8/BaseOS
##url --url=http://171.23.8.65/rhel/8/os/BaseOS
# Use text mode install
text
#graphical
# System language
#lang en_US.UTF-8
keyboard --vckeymap=us --xlayouts='us'
# Keyboard layouts
##keyboard us
lang en_US.UTF-8
# Root password
rootpw $6$gTiUCif4$YdKxeewgwYCLS4uRc/XOeKSitvDJNHFycxWVHi.RYGkgKctTMCAiY2TErua5Yh7flw2lUijooOClQQhlbstZ81 --iscrypted
# network-stuff
# place ip=your_VM_IP, netmask, gateway, nameserver hostname 
network --bootproto=static --ip=10.80.21.19 --netmask=255.255.255.192 --gateway=10.80.21.1 --nameserver=172.30.85.2 --device=eth0 --noipv6 --hostname=FQDN.VMhost.com --onboot=yes
# if you need just localhost initially configured uncomment and comment above
##network В --device=lo --hostname=localhost.localdomain
# System authorization information
authconfig --enableshadow --passalgo=sha512 --enablefingerprint
# skipx
skipx
# Firewall configuration
firewall --disabled
# System timezone
timezone Europe/Berlin
# Clear the Master Boot Record
##zerombr
# Repositories
## Add RPM repositories from KS file if necessery
#repo --name=appstream --baseurl=http://hostname.com/rhel/8/AppStream
#repo --name=baseos --baseurl=http://hostname.com/rhel/8/BaseOS
#repo --name=inst.stage2 --baseurl=http://hostname.com ff=/dev/vg0/vmprivate
##repo --name=rhsm-baseos В  В --baseurl=http://172.54.8.65/rhel/8/rhsm/x86_64/BaseOS/
##repo --name=rhsm-appstream --baseurl=http://172.54.8.65/rhel/8/rhsm/x86_64/AppStream/
##repo --name=os-baseos В  В  В --baseurl=http://172.54.9.65/rhel/8/os/BaseOS/
##repo --name=os-appstream В  --baseurl=http://172.54.8.65/rhel/8/os/AppStream/
#repo --name=inst.stage2 --baseurl=http://172.54.8.65/rhel/8/BaseOS
# Disk partitioning information set proper disk sizing
##bootloader --location=mbr --boot-drive=vda
bootloader --append=" crashkernel=auto tsc=reliable divider=10 plymouth.enable=0 console=ttyS0 " --location=mbr --boot-drive=vda
# partition plan
zerombr
clearpart --all --drives=vda --initlabel
part /boot --size=1024 --fstype=ext4 --asprimary
part swap --size=1024
part pv.01 --size=30000 --grow --ondisk=vda
##part pv.0 --size=80000 --fstype=lvmpv
#part pv.0 --size=61440 --fstype=lvmpv
volgroup s pv.01
logvol / --vgname=s --size=15360 --name=root --fstype=ext4
logvol /var/cache/ --vgname=s --size=5120 --name=cache --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
logvol /var/log --vgname=s --size=7680 --name=log --fstype=ext4 --fsoptions="defaults,nodev,noexec,nosuid"
logvol /tmp --vgname=s --size=5120 --name=tmp --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
logvol /home --vgname=s --size=5120 --name=home --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
logvol /opt --vgname=s --size=2048 --name=opt --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
logvol /var/log/audit --vgname=s --size=3072 --name=audit --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
logvol /var/spool --vgname=s --size=2048 --name=spool --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
logvol /var --vgname=s --size=7680 --name=var --fstype=ext4 --fsoptions="defaults,nodev,nosuid"
# SELinux configuration
selinux --disabled
# Installation logging level
logging --level=debug
# reboot automatically
reboot
###
%packages
@standard
python3
pam_ssh_agent_auth
-nmap-ncat
#-plymouth
#-bpftool
-cockpit
#-cryptsetup
-usbutils
#-kmod-kvdo
#-ledmon
#-libstoragemgmt
#-lvm2
#-mdadm
-rsync
#-smartmontools
-sos
-subscription-manager-cockpit
# Tune Linux vm.dirty_background_bytes (IMAGE-439)
# The following tuning causes dirty data to begin to be background flushed at
# 100 Mbytes, so that it writes earlier and more often to avoid a large build
# up and improving overall throughput.
echo "vm.dirty_background_bytes=100000000" >> /etc/sysctl.conf
# Disable kdump
systemctl disable kdump.service
%end

Important note to make here is the MD5 set root password string in (rootpw) line this string can be generated with openssl or mkpasswd commands :

Method 1: use openssl cmd to generate (md5, sha256, sha512) encrypted pass string

[root@redhat ~]# openssl passwd -6 -salt xyz test
$6$xyz$rjarwc/BNZWcH6B31aAXWo1942.i7rCX5AT/oxALL5gCznYVGKh6nycQVZiHDVbnbu0BsQyPfBgqYveKcCgOE0

Note: passing -1 will generate an MD5 password, -5 a SHA256 encryption and -6 SHA512 encrypted string (logically recommended for better security)

Method 2: (md5, sha256, sha512)

[root@redhat ~]# mkpasswd –method=SHA-512 –stdin

The option –method accepts md5, sha-256 and sha-512
Theoretically there is also a kickstart file generator web interface on Redhat's site here however I never used it myself but instead use above kickstart.cfg
 

6. Install the new VM with virt-install cmd


Roll the new preconfigured VM based on above ks template file use some kind of one liner command line  like below:
 

[root@redhat ~]# virt-install -n RHEL8_3-VirtualMachine –description "CentOS 8.3 Virtual Machine" –os-type=Linux –os-variant=rhel8.3 –ram=8192 –vcpus=8 –location=/vmprivate/rhel-server-8.3-x86_64-dvd.iso –disk path=/vmprivate/RHEL8_3-VirtualMachine.img,bus=virtio,size=70 –graphics none –initrd-inject=/root/kickstart.cfg –extra-args "console=ttyS0 ks=file:/kickstart.cfg"

7. Use a tiny shell script to automate VM creation


For some clarity and better automation in case you plan to repeat VM creation you can prepare a tiny bash shell script:
 

#!/bin/sh
KS_FILE='kickstart.cfg';
VM_NAME='RHEL8_3-VirtualMachine';
VM_DESCR='CentOS 8.3 Virtual Machine';
RAM='8192';
CPUS='8';
# size is in Gigabytes
VM_IMG_SIZE='140';
ISO_LOCATION='/vmprivate/rhel-server-8.3-x86_64-dvd.iso';
VM_IMG_FILE_LOC='/vmprivate/RHEL8_3-VirtualMachine.img';

virt-install -n "$VMNAME" –description "$VM_DESCR" –os-type=Linux –os-variant=rhel8.3 –ram=8192 –vcpus=8 –location="$ISO_LOCATION" –disk path=$VM_IMG_FILE,bus=virtio,size=$IMG_VM_SIZE –graphics none –initrd-inject=/root/$KS_FILE –extra-args "console=ttyS0 ks=file:/$KS_FILE"


A copy of virt-install.sh script can be downloaded here

Wait for the installation to finish it should be visualized and if all installation is smooth you should get a login prompt use the password generated with openssl tool and test to login, then disconnect from the machine by pressing CTRL + ] and try to login via TTY with

[root@redhat ~]# virst list –all
 Id   Name        State
—————————
 2    
RHEL8_3-VirtualMachine   running

[root@redhat ~]#  virsh console RHEL8_3-VirtualMachine


redhat8-login-prompt

One last thing I recommend you check the official documentation on Kickstart2 from CentOS official website

In case if you later need to destroy the VM and the respective created Image file you can do it with:
 

[root@redhat ~]#  virsh destroy RHEL8_3-VirtualMachine
[root@redhat ~]#  virsh undefine RHEL8_3-VirtualMachine

Don't forget to celebreate the success and give this nice article a credit by sharing this nice tutorial with a friend or by placing a link to it from your blog 🙂

 

 

Enjoy !

How to add local user to admin access via /etc/sudoers with sudo su – root / Create a sudo admin group to enable users belonging to group become superuser

Friday, January 15th, 2021

sudo_logo-how-to-add-user-to-sysadmin-group

Did you had to have a local users on a server and you needed to be able to add Admins group for all system administrators, so any local user on the system that belongs to the group to be able to become root with command lets say sudo su – root / su -l root / su – root?
If so below is an example /etc/sudoers file that will allow your users belonging to a group local group sysadmins with some assigned group number

Here is how to create the sysadmins group as a starter

linux:~# groupadd -g 800 sysadmins

Lets create a new local user georgi and append the user to be a member of sysadmins group which will be our local system Administrator (superuser) access user group.

To create a user with a specific desired userid lets check in /etc/passwd and create it:

linux:~# grep :811: /etc/passwd || useradd -u 811 -g 800 -c 'Georgi hip0' -d /home/georgi -m georgi

Next lets create /etc/sudoers (if you need to copy paste content of file check here)and paste below configuration:

linux:~# mcedit /etc/sudoers

## Updating the locate database
# Cmnd_Alias LOCATE = /usr/bin/updatedb

 

## Storage
# Cmnd_Alias STORAGE = /sbin/fdisk, /sbin/sfdisk, /sbin/parted, /sbin/partprobe, /bin/mount, /bin/umount

## Delegating permissions
# Cmnd_Alias DELEGATING = /usr/sbin/visudo, /bin/chown, /bin/chmod, /bin/chgrp

## Processes
# Cmnd_Alias PROCESSES = /bin/nice, /bin/kill, /usr/bin/kill, /usr/bin/killall

## Drivers
# Cmnd_Alias DRIVERS = /sbin/modprobe

Cmnd_Alias PASSWD = /usr/bin/passwd [a-zA-Z][a-zA-Z0-9_-]*, \
                    !/usr/bin/passwd root

Cmnd_Alias SU_ROOT = /bin/su root, \
                     /bin/su – root, \
                     /bin/su -l root, \
                     /bin/su -p root


# Defaults specification

#
# Refuse to run if unable to disable echo on the tty.
#
Defaults   !visiblepw

#
# Preserving HOME has security implications since many programs
# use it when searching for configuration files. Note that HOME
# is already set when the the env_reset option is enabled, so
# this option is only effective for configurations where either
# env_reset is disabled or HOME is present in the env_keep list.
#
Defaults    always_set_home
Defaults    match_group_by_gid

Defaults    env_reset
Defaults    env_keep =  "COLORS DISPLAY HOSTNAME HISTSIZE KDEDIR LS_COLORS"
Defaults    env_keep += "MAIL PS1 PS2 QTDIR USERNAME LANG LC_ADDRESS LC_CTYPE"
Defaults    env_keep += "LC_COLLATE LC_IDENTIFICATION LC_MEASUREMENT LC_MESSAGES"
Defaults    env_keep += "LC_MONETARY LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE"
Defaults    env_keep += "LC_TIME LC_ALL LANGUAGE LINGUAS _XKB_CHARSET XAUTHORITY"

#
# Adding HOME to env_keep may enable a user to run unrestricted
# commands via sudo.
#
# Defaults   env_keep += "HOME"
Defaults    secure_path = /sbin:/bin:/usr/sbin:/usr/bin

## Next comes the main part: which users can run what software on
## which machines (the sudoers file can be shared between multiple
## systems).
## Syntax:
##
##      user    MACHINE=COMMANDS
##
## The COMMANDS section may have other options added to it.
##
## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL

## Allows members of the 'sys' group to run networking, software,
## service management apps and more.
# %sys ALL = NETWORKING, SOFTWARE, SERVICES, STORAGE, DELEGATING, PROCESSES, LOCATE, DRIVERS

## Allows people in group wheel to run all commands
%wheel  ALL=(ALL)       ALL

## Same thing without a password
# %wheel        ALL=(ALL)       NOPASSWD: ALL

## Allows members of the users group to mount and unmount the
## cdrom as root
# %users  ALL=/sbin/mount /mnt/cdrom, /sbin/umount /mnt/cdrom
## Allows members of the users group to shutdown this system
# %users  localhost=/sbin/shutdown -h now

%sysadmins            ALL            = SU_ROOT, \
                                   NOPASSWD: PASSWD

## Read drop-in files from /etc/sudoers.d (the # here does not mean a comment)
#includedir /etc/sudoers.d

zabbix  ALL=(ALL) NOPASSWD:/usr/bin/grep


Save the config and give it a try now to become root with sudo su – root command

linux:~$ id
uid=811(georgi) gid=800(sysadmins) groups=800(sysadmins)
linux:~$ sudo su – root
linux~#

w00t Voila your user is with super rights ! Enjoy 🙂

 

Hack: Using ssh / curl or wget to test TCP port connection state to remote SSH, DNS, SMTP, MySQL or any other listening service in PCI environment servers

Wednesday, December 30th, 2020

using-curl-ssh-wget-to-test-tcp-port-opened-or-closed-for-web-mysql-smtp-or-any-other-linstener-in-pci-linux-logo

If you work on PCI high security environment servers in isolated local networks where each package installed on the Linux / Unix system is of importance it is pretty common that some basic stuff are not there in most cases it is considered a security hole to even have a simple telnet installed on the system. I do have experience with such environments myself and thus it is pretty daunting stuff so in best case you can use something like a simple ssh client if you're lucky and the CentOS / Redhat / Suse Linux whatever distro has openssh-client package installed.
If you're lucky to have the ssh onboard you can use telnet in same manner as netcat or the swiss army knife (nmap) network mapper tool to test whether remote service TCP / port is opened or not. As often this is useful, if you don't have access to the CISCO / Juniper or other (networ) / firewall equipment which is setting the boundaries and security port restrictions between networks and servers.

Below is example on how to use ssh client to test port connectivity to lets say the Internet, i.e.  Google / Yahoo search engines.
 

[root@pciserver: /home ]# ssh -oConnectTimeout=3 -v google.com -p 23
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to google.com [172.217.169.206] port 23.
debug1: connect to address 172.217.169.206 port 23: Connection timed out
debug1: Connecting to google.com [2a00:1450:4017:80b::200e] port 23.
debug1: connect to address 2a00:1450:4017:80b::200e port 23: Cannot assign requested address
ssh: connect to host google.com port 23: Cannot assign requested address
root@pcfreak:/var/www/images# ssh -oConnectTimeout=3 -v google.com -p 80
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to google.com [172.217.169.206] port 80.
debug1: connect to address 172.217.169.206 port 80: Connection timed out
debug1: Connecting to google.com [2a00:1450:4017:807::200e] port 80.
debug1: connect to address 2a00:1450:4017:807::200e port 80: Cannot assign requested address
ssh: connect to host google.com port 80: Cannot assign requested address
root@pcfreak:/var/www/images# ssh google.com -p 80
ssh_exchange_identification: Connection closed by remote host
root@pcfreak:/var/www/images# ssh google.com -p 80 -v -oConnectTimeout=3
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to google.com [172.217.169.206] port 80.
debug1: connect to address 172.217.169.206 port 80: Connection timed out
debug1: Connecting to google.com [2a00:1450:4017:80b::200e] port 80.
debug1: connect to address 2a00:1450:4017:80b::200e port 80: Cannot assign requested address
ssh: connect to host google.com port 80: Cannot assign requested address
root@pcfreak:/var/www/images# ssh google.com -p 80 -v -oConnectTimeout=5
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to google.com [142.250.184.142] port 80.
debug1: connect to address 142.250.184.142 port 80: Connection timed out
debug1: Connecting to google.com [2a00:1450:4017:80c::200e] port 80.
debug1: connect to address 2a00:1450:4017:80c::200e port 80: Cannot assign requested address
ssh: connect to host google.com port 80: Cannot assign requested address
root@pcfreak:/var/www/images# ssh google.com -p 80 -v
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to google.com [172.217.169.206] port 80.
debug1: Connection established.
debug1: identity file /root/.ssh/id_rsa type 0
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: identity file /root/.ssh/id_ecdsa type -1
debug1: identity file /root/.ssh/id_ecdsa-cert type -1
debug1: identity file /root/.ssh/id_ed25519 type -1
debug1: identity file /root/.ssh/id_ed25519-cert type -1
debug1: identity file /root/.ssh/id_xmss type -1
debug1: identity file /root/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u2
debug1: ssh_exchange_identification: HTTP/1.0 400 Bad Request

 


debug1: ssh_exchange_identification: Content-Type: text/html; charset=UTF-8


debug1: ssh_exchange_identification: Referrer-Policy: no-referrer


debug1: ssh_exchange_identification: Content-Length: 1555


debug1: ssh_exchange_identification: Date: Wed, 30 Dec 2020 14:13:25 GMT


debug1: ssh_exchange_identification:


debug1: ssh_exchange_identification: <!DOCTYPE html>

debug1: ssh_exchange_identification: <html lang=en>

debug1: ssh_exchange_identification:   <meta charset=utf-8>

debug1: ssh_exchange_identification:   <meta name=viewport content="initial-scale=1, minimum-scale=1, width=device-width">

debug1: ssh_exchange_identification:   <title>Error 400 (Bad Request)!!1</title>

debug1: ssh_exchange_identification:   <style>

debug1: ssh_exchange_identification:     *{margin:0;padding:0}html,code{font:15px/22px arial,sans-serif}html{background:#fff;color:#222;padding:15px}body{margin:7% auto 0;max-width:390px;min-height:180px;padding:30px 0 15px}* > body{background:url(//www.google.com/images/errors/robot.png) 10
debug1: ssh_exchange_identification: 0% 5px no-repeat;padding-right:205px}p{margin:11px 0 22px;overflow:hidden}ins{color:#777;text-decoration:none}a img{border:0}@media screen and (max-width:772px){body{background:none;margin-top:0;max-width:none;padding-right:0}}#logo{background:url(//www.g
debug1: ssh_exchange_identification: oogle.com/images/branding/googlelogo/1x/googlelogo_color_150x54dp.png) no-repeat;margin-left:-5px}@media only screen and (min-resolution:192dpi){#logo{background:url(//www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png) no-repeat 0
debug1: ssh_exchange_identification: % 0%/100% 100%;-moz-border-image:url(//www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png) 0}}@media only screen and (-webkit-min-device-pixel-ratio:2){#logo{background:url(//www.google.com/images/branding/googlelogo/2x/googlelogo_
debug1: ssh_exchange_identification: color_150x54dp.png) no-repeat;-webkit-background-size:100% 100%}}#logo{display:inline-block;height:54px;width:150px}

debug1: ssh_exchange_identification:   </style>

debug1: ssh_exchange_identification:   <a href=//www.google.com/><span id=logo aria-label=Google></span></a>

debug1: ssh_exchange_identification:   <p><b>400.</b> <ins>That\342\200\231s an error.</ins>

debug1: ssh_exchange_identification:   <p>Your client has issued a malformed or illegal request.  <ins>That\342\200\231s all we know.</ins>

ssh_exchange_identification: Connection closed by remote host

 

Here is another example on how to test remote host whether a certain service such as DNS (bind) or telnetd is enabled and listening on remote local network  IP with ssh

[root@pciserver: /home ]# ssh 192.168.1.200 -p 53 -v -oConnectTimeout=5
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to 192.168.1.200 [192.168.1.200] port 53.
debug1: connect to address 192.168.1.200 port 53: Connection timed out
ssh: connect to host 192.168.1.200 port 53: Connection timed out

[root@server: /home ]# ssh 192.168.1.200 -p 23 -v -oConnectTimeout=5
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1g  21 Apr 2020
debug1: Connecting to 192.168.1.200 [192.168.1.200] port 23.
debug1: connect to address 192.168.1.200 port 23: Connection timed out
ssh: connect to host 192.168.1.200 port 23: Connection timed out


But what if Linux server you have tow work on is so paranoid that you even the ssh client is absent? Well you can use anything else that is capable of doing a connectivity to remote port such as wget or curl. Some web servers or application servers usually have wget or curl as it is integral part for some local shell scripts doing various operation needed for proper services functioning or simply to test locally a local or remote listener services, if that's the case we can use curl to connect and get output of a remote service simulating a normal telnet connection like this:

host:~# curl -vv 'telnet://remote-server-host5:22'
* About to connect() to remote-server-host5 port 22 (#0)
*   Trying 10.52.67.21… connected
* Connected to aflpvz625 (10.52.67.21) port 22 (#0)
SSH-2.0-OpenSSH_5.3

Now lets test whether we can connect remotely to a local net remote IP's Qmail mail server with curls telnet simulation mode:

host:~#  curl -vv 'telnet://192.168.0.200:25'
* Expire in 0 ms for 6 (transfer 0x56066e5ab900)
*   Trying 192.168.0.200…
* TCP_NODELAY set
* Expire in 200 ms for 4 (transfer 0x56066e5ab900)
* Connected to 192.168.0.200 (192.168.0.200) port 25 (#0)
220 This is Mail Pc-Freak.NET ESMTP

Fine it works, lets now test whether a remote server who has MySQL listener service on standard MySQL port TCP 3306 is reachable with curl

host:~#  curl -vv 'telnet://192.168.0.200:3306'
* Expire in 0 ms for 6 (transfer 0x5601fafae900)
*   Trying 192.168.0.200…
* TCP_NODELAY set
* Expire in 200 ms for 4 (transfer 0x5601fafae900)
* Connected to 192.168.0.200 (192.168.0.200) port 3306 (#0)
Warning: Binary output can mess up your terminal. Use "–output -" to tell
Warning: curl to output it to your terminal anyway, or consider "–output
Warning: <FILE>" to save to a file.
* Failed writing body (0 != 107)
* Closing connection 0
root@pcfreak:/var/www/images#  curl -vv 'telnet://192.168.0.200:3306'
* Expire in 0 ms for 6 (transfer 0x5598ad008900)
*   Trying 192.168.0.200…
* TCP_NODELAY set
* Expire in 200 ms for 4 (transfer 0x5598ad008900)
* Connected to 192.168.0.200 (192.168.0.200) port 3306 (#0)
Warning: Binary output can mess up your terminal. Use "–output -" to tell
Warning: curl to output it to your terminal anyway, or consider "–output
Warning: <FILE>" to save to a file.
* Failed writing body (0 != 107)
* Closing connection 0

As you can see the remote connection is returning binary data which is unknown to a standard telnet terminal thus to get the output received we need to pass curl suggested arguments.

host:~#  curl -vv 'telnet://192.168.0.200:3306' –output –
* Expire in 0 ms for 6 (transfer 0x55b205c02900)
*   Trying 192.168.0.200…
* TCP_NODELAY set
* Expire in 200 ms for 4 (transfer 0x55b205c02900)
* Connected to 192.168.0.200 (192.168.0.200) port 3306 (#0)
g


The curl trick used to troubleshoot remote port to remote host from a Windows OS host which does not have telnet installed by default but have curl instead.

Also When troubleshooting vSphere Replication, it is often necessary to troubleshoot port connectivity as common Windows utilities are not available.
As Curl is available in the VMware vCenter Server Appliance command line interface.

On servers where curl is not there but you have wget is installed you can use it also to test a remote port

 

# wget -vv -O /dev/null http://google.com:554 –timeout=5
–2020-12-30 16:54:22–  http://google.com:554/
Resolving google.com (google.com)… 172.217.169.206, 2a00:1450:4017:80b::200e
Connecting to google.com (google.com)|172.217.169.206|:554… failed: Connection timed out.
Connecting to google.com (google.com)|2a00:1450:4017:80b::200e|:554… failed: Cannot assign requested address.
Retrying.

–2020-12-30 16:54:28–  (try: 2)  http://google.com:554/
Connecting to google.com (google.com)|172.217.169.206|:554… ^C

As evident from output the port 554 is filtered in google which is pretty normal.

If curl or wget is not there either as a final alternative you can either install some perl, ruby, python or bash script etc. that can opens a remote socket to the remote IP.