RDP linux to windows

In order to connect from Linux to windows via the RDP protocol please do the following steps.

• install rdesktop

You can build source code from github or you can just find the built one in your OS repository.Source code:
For OpenSuse:

• Connection:

HikariCP pooling

If you are going to crate connections pools you have many options:

• Apache DBCP
• C3P0
• HikariCP
• Vibur

Why shoud I HikariCP? It's just very fast and stable!

Let's create configuration with Spring boot:

And our database configuration property

Our Persisntence Configuration

Ah I know  packagesToScan is ugly. We have Lambdas in Java. Generally this method is craeted to scan components in different modules.

public List <String> packagesToScan() { 
   return scanPackages.stream()
            .map(ScanPersistencePackage::asStrings)
            .flatMap(Collection::stream)
            .collect(Collectors.toList());
}

And concrete persistence configuration for our module:

JPA + Oracle procedures

Due the Oracle's license agreement there is no public repository with the Oracle Driver JAR. So first off all download  oracle  jdbc driver and install to local maven repository (or publish to your artifactory server).


Let's create simple procedures:


Create repository and call procedure using entity manager:

How to make your servers properly secure - Why agent forwarding is dangerous

In this article, I'll show you how to hack server if is bastion host is compromised.  let's talk for about bastion host...

What is the bastion host?
On Wikipedia, you may read that  "bastion host is a special purpose computer on a network specifically designed and configured to withstand attacks."  but it's more important to make correct server configuration otherwise bastion host can be a weapon for an attacker.

How to configure bastion server?
Let's configure Linux servers and then we can start hacking.  Imagine that we have N number of Linux instances and one bastion server. We will use private key ssh authentification for our Linux instances. For testing purpose, we will use user/password authentification for the bastion server. Let's start the configuration.

• Create a user for the server

adduser bastion
passwd bastion

•  open sshd_config and edit configuration. Change PermitRootLogin yes to PermitRootLogin no

vim /etc/ssh/sshd_config

• Restart sshd and check the status

systemctl restart ssh
systemctl status sshd

You should see something like that:

[vq@localhost~]# systemctl status sshd
● sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; 

 enabled; vendor preset: enabled)
   Active: active (running) 
 
since Wed 2018-07-04 10:02:06 UTC; 32s ago
     Docs: man:sshd(8)
           man:sshd_config(5)
 Main PID: 11595 (sshd)
   CGroup: /system.slice/sshd.service
           └─11595 /usr/sbin/sshd -D
Jul 04 10:02:06 systemd[1]:Starting OpenSSH server daemon...
Jul 04 10:02:06 sshd[11595]: Server listening on 0.0.0.0 port 22
Jul 04 10:02:06 sshd[11595]: Server listening on :: port 22
Jul 04 10:02:06 *** systemd[1]: 
  Started OpenSSH server daemon.

Now you can't log in via the root user. You can see if this configuration checks out.

vq@localhost:~> ssh root@X.X.X.X
root@X.X.X.X's password:
Permission denied, please try again.

•  Let's block user if he/she tries to log in with incorrect user/password. For example we can give him/her three chances to log in - otherwise, we can block permanently or temporarily. So we can use  fail2ban.

sudo yum install epel-release
sudo yum install fail2ban

• Duplicate fail2ban configure to create override config file:

cp /etc/fail2ban/fail2ban.conf /etc/fail2ban/fail2ban.local
cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
vim jail.local

•  If you use CentOS you need to change backend variable in the configuration. Just open  jail.local file and change auto to systemd.

 [DEFAULT] 
 backend = systemd  

• Now enable SSH daemon jail in the jail.local file (anyway it's necessary for CentOS 7)

[sshd]
enabled = true

• Set bantime, findtime, and maxretry values. For example:

bantime  = 1000
findtime = 1000
maxretry = 3

• Enable fail2ban and restart service

sudo systemctl enable fail2ban
sudo systemctl start fail2ban

Now if you try authentification with incorrect user/password you will be blocked.
 



How to configure ssh private key authentification on Linux?

• Crate user in the server

adduser usr
passwd usr

• Generate ssh keys

     ssh-keygen

• Copy public key to the server

    ssh-copy-id -i ~/.ssh/name.pub usr@IP

• Login using private key:

 
    ssh -i name usr@IP

• Disable root login and password authentification to the following file:

 
    vim /etc/ssh/sshd_config
    PasswordAuthentication no
    PermitRootLogin no

• hmm... why not to change SSH port? Let's tell SELinux that we need to change it. Let's use just random number 2597

   
    yum -y install policycoreutils-python
    semanage port -a -t ssh_port_t -p tcp 2597

• Now change port in sshd_config.  

vim /etc/ssh/sshd_config

    Check configuration file

    sshd -t

• Restart services

 
    sudo systemctl restart sshd

• Check what port is LITESENING

 
    sudo netstat -lntp | grep ssh

• Allow ssh login only from bastion. So open 2597 port only for bastion. If you use firewalld run the following command:

 
 firewall-cmd  
 --permanent   
 --zone=public  
 --add-rich-rule='rule family="ipv4" 
 source address="BASTION_IP/32"   
 port protocol="tcp" 
 port="2597" accept'

If this is not your server just check your default zone.

 
firewall-cmd --get-default-zone
 

And check your rules:

  
firewall-cmd --list-all

But maybe you prefer to use iptables instead of firewald:

iptables -I INPUT -p tcp -s BASTION_IP  --dport 2597 -j ACCEPT
iptables -I INPUT -p tcp -s 0.0.0.0/0 --dport 2597 -j DROP

if this is not your own server just check iptable rules and remove some of them if necessary:

iptables -L --line-numbers
sudo iptables -D INPUT {line}

How to access my server via the Bastion? Generally, there are many ways but some of them are not secure. Let's talk for about them.

 
How to use AgentForwarding

• Create config in ~/.ssh/config

    Host bast
        Hostname BASTION_IP
        ForwardAgent yes
        User bastion

• Add your authentification key to ssh agent

ssh-add ~/.ssh/name_rsa

• Connect to bastion host using username/password

    ssh bast

• Connect application server

    ssh app@IP -p PORT

As you see you should connect your remote servers via the bastion server.  The same scenario is if you want to upload/download files:

• Download file from remote server to BASTION. Run the command in Bastion server

    scp -P PORT app@IP:/src_path  /dest_path

• Download file from BASTION to LOCA machine. Run the command in LOCAL machine:

    scp  -P PORT bastion@ip:/src_path /dest_path 

or

   scp  -P PORT bast:/src_path /dest_path

• Upload file to BASTION

scp   /src_path bastion:/dest_path 

You may, well, ask me the question: Is my server secure?  And the answer is quite simple:
- Le't me tell you: NO!
 - Why?
 - Because you are using SSH Agent forwarding?
 - And where is the problem?
 - Because Agent forwarding is dangerous and it's consider considered harmful.

Let's explain everything inside out:  When you connect bastion host your glorious ssh agent is forwarded. It means that the socket will be set up so that someone may use this socket data to access your servers. Imagine that your bastion server is compromised, If someone has sufficient permissions on your Linux server he/she will just use your socket info. As a result, all your server can be accessed. I know window of compromise is very small because it depends on how much time you are connected to the bastion host. But do you really want to the risk when you have other options like ProxyCommand? Hence, just use ProxyCommand!

How to hack servers if you compromised bastion host?

In /tmp directory you may see something like that:

[root@localhost tmp]# ll
total 12
drwx------  2 bastion bastion 4096 Sep  7 17:35 ssh-mKX88v0Vlo

let's open the temporary file

[root@localhost tmp]# cd ssh-mKX88v0Vlo/
[root@localhost ssh-mKX88v0Vlo]# ll
total 0
srwxr-xr-x 1 bastion bastion 0 Sep  7 17:35 agent.10507

Let's see connections to this process id.

netstat -nxp | grep  10507

result:

unix  [ ]   STREAM     CONNECTED     501384   10507/sshd: bastion

and who is connected?

lsof -i -a -p 10507

result:

COMMAND  PID   USER  FD  TYPE DEVICE SIZE/OFF NODE NAME
sshd    10507 bastion  3u  IPv4 501301  0t0  TCP *IP*:ssh->*IP*:8279 (ESTABLISHED)

We can also see socket files:

cd /proc/10507/fd/
ls

result:

lrwx------ 1 root root 64 Sep  7 17:46 0 -> /dev/null
lrwx------ 1 root root 64 Sep  7 17:46 1 -> /dev/null
lrwx------ 1 root root 64 Sep  7 17:46 10 -> /dev/ptmx
lrwx------ 1 root root 64 Sep  7 17:46 14 -> /dev/ptmx
lrwx------ 1 root root 64 Sep  7 17:46 15 -> /dev/ptmx
lrwx------ 1 root root 64 Sep  7 17:46 2 -> /dev/null
lrwx------ 1 root root 64 Sep  7 17:46 3 -> socket:[501994]
lrwx------ 1 root root 64 Sep  7 17:46 4 -> socket:[502069]
lrwx------ 1 root root 64 Sep  7 17:46 5 -> socket:[502072]
l-wx------ 1 root root 64 Sep  7 17:46 6 -> /run/systemd/sessions/1836.ref
lr-x------ 1 root root 64 Sep  7 17:46 7 -> pipe:[502079]
l-wx------ 1 root root 64 Sep  7 17:46 8 -> pipe:[502079]
lrwx------ 1 root root 64 Sep  7 17:46 9 -> socket:[502080]

And what happens when client will be connected to remote server? let's see:

lrwx------ 1 root root 64 Sep  7 17:46 0 -> /dev/null
lrwx------ 1 root root 64 Sep  7 17:46 1 -> /dev/null
lrwx------ 1 root root 64 Sep  7 17:46 10 -> /dev/ptmx
lrwx------ 1 root root 64 Sep  7 17:48 11 -> socket:[502267]
lrwx------ 1 root root 64 Sep  7 17:46 14 -> /dev/ptmx
lrwx------ 1 root root 64 Sep  7 17:46 15 -> /dev/ptmx
lrwx------ 1 root root 64 Sep  7 17:46 2 -> /dev/null
lrwx------ 1 root root 64 Sep  7 17:46 3 -> socket:[501994]
lrwx------ 1 root root 64 Sep  7 17:46 4 -> socket:[502069]
lrwx------ 1 root root 64 Sep  7 17:46 5 -> socket:[502072]
l-wx------ 1 root root 64 Sep  7 17:46 6 -> /run/systemd/sessions/1836.ref
lr-x------ 1 root root 64 Sep  7 17:46 7 -> pipe:[502079]
l-wx------ 1 root root 64 Sep  7 17:46 8 -> pipe:[502079]
lrwx------ 1 root root 64 Sep  7 17:46 9 -> socket:[502080]

We can even see if socket file is used using netstat:

unix  3 [ ]  STREAM  CONNECTED  502267  10561/sshd: 
                     bastion  /tmp/ssh-oVoMXC6vb8/agent.10561
unix  3  [ ] STREAM     CONNECTED     502072   10561/sshd:  bastion 

but wait we need to steal the socket information while the session of bastion host is open. Oh, we also need destination server IP, so just use netstat It's funny but you might find something like that on the internet.:

netstat -tn 2>/dev/null 
grep :22  | awk '{print $5}' | cut -d: -f1 | 
sort | uniq -c | sort -nr | head

Sometimes people try to demonstrate their knowledge  but just make everything simple.

netstat -tn

The final step to use the forwarded socket file

eval "$(ssh-agent -s)"
SSH_AUTH_SOCK=/tmp/ssh-EAKxOdL4fl/agent.10507

Check if the key is loaded.

ssh-add -l

result should be something like that:

2048 SHA256:2Psdl..B5KQ /home/usr/.ssh/name_rsa (RSA)

Ok. We have seen that Agent Forwarding is a problem. But where is the solution? Fortunately, we can just use a proxy command. So let's rewrite our configuration:

Host app
    Hostname *.*.*.*
    IdentityFile ~/.ssh/your_rsa
    User *******
    Port ****
    ProxyCommand ssh -W %h:%p bast

Host bast
     Hostname *.*.*.*
     ForwardAgent no
     User ******

Now  it's very simple to connect your server

ssh app

Do you want to download something from bastion?

scp bast:/home/bastion/sh ~/

Maybe you want to download something from the remote server via the bastion tunnel:

scp -P **** app:/src_path  /dest_path

Maybe you want to ypload something to remote server via the bastion tunel:

scp -P **** app:/src_path  /dest_path

Conclusion

• If you use bastion host, don't use AgentForwarding but use ProxyCommand
• Always use non root user for authentification
• Use firewall and block all unnecessary connections.
• Use SELinux
• Block the IP address who  tries to login  several time with incorrect credentials
• If it's not necessery don't give sudo permission to the user
• Monitor your server
• Update your server for security patches

How to create your own system.d service

If you explore latest Red Hat linux, you will see that the  traditional init scripts are gone. It's much easier to use rather then SysVinit, it reduces system startup time because the processes are running in parallel. It has also very nice logging mechanism. Let's write simple examples for CentOS7

Create a bash file:

 touch /usr/local/sbin/{launcher_name}.sh

It's a simple bash file that launches java application.

#!/bin/bash
java -jar /root/{name}.jar

Add execution permission

chmod +x {launcher_name}.sh

Create service:

touch /etc/systemd/system/{service_name}.service

Configuration:

[Unit]
Description=Application launcher service

[Service]
Type=simple
ExecStart=/usr/local/sbin/{launcher_name}.sh
TimeoutStartSec=0

[Install]
WantedBy=default.target

Set permissions:

chmod 664  {service_name}.service

Reload daemon

 systemctl daemon-reload 

Usage:

systemctl enable {service_name}
systemctl start {service_name}

We can create the same using init instead of sytstem.d. We shiuld just create configuration to the following locataion: /etc/init.d

#! /bin/sh
# /etc/init.d/{service_name}
### BEGIN INIT INFO
# Provides:           {service_name}
# Default-Start:      2 3 4 5
# Default-Stop:       0 1 6
# Short-Description:  Example of init service.
### END INIT INFO
# Actions provided to make it LSB-compliant
case "$1" in
  start)
    export DISPLAY=:0
    sh {launcher_name}.sh
    ;;
  stop)
    echo "Stopping"
    ...
    ;;
  restart)
    echo "Restarting"
    ;;
  force-reload)
    echo "Reloading"
    ...
    ;;
  status)
    echo "Status"
    ...
    ;;
  *)
    echo "Usage: /etc/init.d/{service_name} 
        {start|stop|restart|force-reload|status}"
    exit 1
    ;;
esac
exit 0 

How to allow non-root users to control system.d services/utilities that needs root permissions.

How to allow non-root users to control system.d services/utilities that needs root permissions.

By default sudo is not installed on debian. So, install sudo

apt-get install sudo -y

Add current user to sodoers list

usermod -a -G sudo vq

Change editor for visudo (I love vim).

update-alternatives --config editor

Let's give user perrmision to reboot without password. First off all we should change configuration in /etc/sudoers file. In order to edit the fiole  use visudo:

vq ALL=NOPASSWD:/sbin/reboot

Usage (password is not required)

sudo reboot

Now let's allow user to access concrete system.d service without password. For example let's do it for firewalld service.

Create file

cd /etc/sudoers.d
touch vq
vim vq

Add the following configuration

%vq ALL= NOPASSWD: /bin/systemctl start firewalld
%vq ALL= NOPASSWD: /bin/systemctl stop firewalld
%vq ALL= NOPASSWD: /bin/systemctl status firewalld

Usage (password is not required)

sudo systemctl start firewalld
sudo systemctl stop firewalld
sudo systemctl status firewalld

Note that my user and group is called vq

.

synchronise time using ntpd and timesyncd

Imagine that you have many servers. It's important to have synchronized time between our servers - at least the log files should have same time in order to find out the correct sequence of the processes. We can use the following tools for time synchronization

• ntpd - powerfull Network Time Protocol Daemon. Full implementation of  NTP protocol
• systemd-timesyncd - it is lightweight daemon for synchronizing the system clock. This client implements an SNTP.  if we  have ntpd daemon we can use ntpdc - utility program used to monitor NTP daemo (now it's deprecated and we use ntpq).  If we have systemd-timesyncd we can use  timedatectl - it has very beautiful configurations and controls the system time/date very well.
• chrony is also full implementation of NTP. it has really great performance because It quickly detects sudden time changes like ntpd.   
• OpenNTPD - It's part of the Open BSD project that also implements NTP protocol.
  

There are another implementations too but know let's talk for about ntpd and timedatectl.

 How to Configure ntpd

• Install and enable ntpd for CentOS:

yum install ntpd
systemctl enable ntpd
firewall-cmd --add-service=ntp --permanent

• Install and enable ntpd for Debian:

apt-get install ntpd
apt-get install ntpdate

• Add pools in  /etc/ntp.conf  server configuration

server 0.[YOUR_POOLl]
server 1.[YOUR_POOLl]

• Add local clock in  /etc/ntp.conf  server configuration.  For example, we can add local clock as a stratum 10  server or we  can set up to stratum 15 - so that it will never be used unless internet access is lost.

server  127.127.1.0 
fudge   127.127.1.0 stratum 10

• Add pool adress in /etc/ntp.conf client configuration

server  X.X.X.X

• Force update time, we can create crontab too (optional)

ntpdate -u X.X.X.X

• Start ntpd service for CentOS:

systemctl start ntpd

• Start ntpd service for Debian:

service ntp start  or systemctl start ntp

Let's check if everything works well and print all the peers of the servers

[vq@centos etc]# ntpq -p
 remote   refid  st t when poll reach delay  offset jitter
===========================================================
*LOCAL(0) .LOCL. 10 l 40  64   17     0.000  0.000  0.000

 How to Configure timesyncd

Because of we have installed ntpd we can't use systemd-timesyncd. Even if you try to use systemd-timesyncd service you will have the follolwing error:

vq@debian:/var/log# systemctl status 

systemd-timesyncd.service
● systemd-timesyncd.service - Network Time Synchronization
   Loaded: loaded 
(/lib/systemd/system/systemd-timesyncd.service;
 enabled; vendor preset: enabled)
  Drop-In: /lib/systemd/system/systemd-timesyncd.service.d
           └─disable-with-time-daemon.conf
   Active: inactive (dead)
Condition: start condition failed at 
Fri 2018-07-27 13:31:23 +04; 35s ago
           └─ ConditionFileIsExecutable=
!/usr/sbin/ntpd was not met
     Docs: man:systemd-timesyncd.service(8)

So we should remove ntpd if we want to use  timesyncd and timedatectl. You can find  timesyncd configuration to the following URL:

/etc/systemd/timesyncd.conf

[Time]
NTP= ${URLS}
FallbackNTP=${URLS}
#RootDistanceMaxSec=5
#PollIntervalMinSec=32
#PollIntervalMaxSec=2048

• check if NTP synchronization is enabled 

 
vq@debian:/etc# timedatectl
      Local time: Fri 2018-07-27 13:27:36 +04
  Universal time: Fri 2018-07-27 09:27:36 UTC
        RTC time: Fri 2018-07-27 09:46:02
       Time zone: Asia/Tbilisi (+04, +0400)
 Network time on: YES
NTP synchronized: YES
 RTC in local TZ: YES

How to install JDK on linux and how it really works?

How to install JDK on linux and how it really works?

You may , well, ask me the question - which installation file should you download for your Linux. You know, you have many options: prm or deb or tar.gz? 

What is difference between rpm and deb file formats?

• RPM and DEB files files provide an easy way for software to be distributed, installed, upgraded, and removed 
• tag.gz is g-zipped tar file. You should download it if you are going to install JDK manually.

What is difference between deb and rpm file formats?

To be honest, therey are both archive files, with some metadata. In order to install deb file you need dpkg - this is package manager for Debian-based systems like Ubuntu, Debian etc. For rpm file format we use RPM Package Manager.

What about dependency problem?

Everything seems great but sometimes we have dependency problems.  In the other words, we had better install all dependencies for our program to work correctly.  Therefor RPM and dpkg  is unable to automatically do it.   Fortunately we have many tools in order to sole the problem:

• APT (the Advanced Packaging Tool) to install, update and remove software  on Debian-based systems. 
• YUM  package-management utility that uses the RPM Package Manager.
• Zypper this is my favorite  package manager for installing, updating and removing packages as well as for managing repositories.  

How To install RPM format JDK on linux?

This method is suitable for SUSE/Arch/Fedora/CentOS/RedHat etc. You should just have RPM package manager.
 

•  Download oracle jdk RPM package
• To install using terminal type: `rpm -ivh {jdk-name-version}.rpm`
• Run the following commands

	update-alternatives --install /usr/bin/java java /usr/java/latest/bin/java 100
	update-alternatives --install /usr/bin/javac javac /usr/java/latest/bin/javac 100

view raw update_java_path.sh hosted with ❤ by GitHub

• Set default Java for in system

	update-alternatives --config java
	update-alternatives --config javac

view raw default_java.sh hosted with ❤ by GitHub

  

How to install  tar.gz format zypped JDK on linux?

•  Download oracle jdk tar.gz
•  mkdir /opt/jdk
•  tar -zxf {jdk-name-version}.tar.gz -C /opt/jdk
• update-alternatives --install /usr/bin/java java /opt/jdk/{jdk-name-version}/bin/java 100
• update-alternatives --install /usr/bin/javac javac /opt/jdk/{jdk-name-version}/bin/javac 100

Check if java is installed

•  update-alternatives --display java
•  update-alternatives --display javac

 Set default Java for in System

•  update-alternatives --config java
•  update-alternatives --config javac

how to change shceduler times in spring

How to change scheduler times in Spring

 

Imagine that you have scheduler but you want to change time at runtime. What Can you do? Off-course there are ways do solve the problem. So In this article I will write how to implement Trigger interface that is located to the following package: org.springframework.scheduling in the spring-context project.

1. First off all, let's create interface for our logic:

2. In Spring we have org.springframework.scheduling.Trigger interface:

So we can write our own implementation. It's very simple. We should just overwrite nextExecutionTime(_) method.  Something like that:



3. Now we can write configuration.





In this example we just choose ThreadPoolTaskScheduler that is child ofTaskScheduler interface.


4. So what is the next? Let's see how to use our scheduler. Everything is simple:



That's it!

Goodbye!
Have good day!

Running rsync as a daemon

rsync is a very flexible utility for  transferring and synchronizing files across computer systems. For example, when we want to run updates to the remote devices, we can just update some files in the rsync server and another remote devices will receive the updated files automatically.

Rsync has three ways to decide if a file is outdated:

1. Compare size of files
2. Compare timestamps
3. Compare static checksums

So we can choose   --size-only, --ignore-times or ---checksum option in order to decide how to compare files.

Let's write little example

• Let's create some directory and write some files in it. The idea is the following: if we add any files in the directory , remote devices will revieve that files (any devices who have rsync client)

mkdir /opt/{my_app_settings}/dev/

•  Edit rsyncd.conf file and write configurations. I have created 2 configuration. But before editing the file imagine that some remote devices mught need to revieve files from different directory. We have already created directory named "dev" but we can also can create another directory. For example let's create another directory named "prod". So rsync client will decide which directory to use  - in the other words where to get files. So our configuration looks like that:

[dev] 
path = /opt/{my_app_settings}/dev 
read only = true
uid = root
gid = root


[prod]
path = /opt/{my_app_settings}/prod 
read only = true 
uid = root 
gid = root 

• Run rsync daemon

 rsync --daemon 

• Run rsync client in order to retrieve new files:

  rsync -rtv root@IP::dev {destionation_directory} 

Also we can create sheduler in order to execute auto synchronization.

 crontab -e 

If you want to run cron job every 1 minute, write the following  cron config

• Cron examples

Every 1 minute     * * * * * 
Every 15 minutea   */15 * * * * 
Every 30 minutes   */30 * * * * 
Every 1 hour       0 * * * * 
Every 6 hours      0 */6 * * * 
Every 12 hours     0 */12 * * * 
Once a day         4 0 * * *
Once a week        4 0 * * 0 
Once a month       4 0 1 * * 

Now just verify your cron job

 crontab -l 

Results should be something like that: