Category: Linux

ulimit

ulimit command allow you to view or set user limits.

boby@sok-01:~$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 31482
max locked memory       (kbytes, -l) 16384
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 31482
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
boby@sok-01:~$

To increase limits for a user, edit file

vi /etc/security/limits.conf

Add

USER_NAME_HERE        hard nofile 20480
USER_NAME_HERE        soft nofile 10240

September 25, 2019

ImageMagick
To install ImageMagick on CentOS, run
```
yum install ImageMagick
```
To install on Ubuntu/Debian, run
```
apt install -y imagemagick
```
How to check if ImageMagick is installed?
September 20, 2019

Linux KVM Bridge network on Ubuntu

On Ubuntu 18.04 server, first i get Ubuntu to use /etc/network/interface, by default Ubuntu 18.04 and newer use netplan.

First install ifdown

apt install ifupdown -y

Install bridge utils and resolvconf.

apt install bridge-utils resolvconf

Now you can configure your network interface by editing file

vi  /etc/network/interface

Here is my network configuration on an OVH server.

root@ns3048991:~# cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto eno3
iface eno3 inet static
    address 149.202.199.137
    netmask 255.255.255.255
    broadcast 149.202.199.137
    gateway 149.202.199.254
    dns-nameservers 8.8.8.8 8.8.4.4
root@ns3048991:~#

To convert this interface to bridge network, do the following

1) Replace all occurance of “eno3” with “br0”

2) Add following lines

    bridge_ports eno3
    bridge_stp off
    bridge_maxwait 5

In above, replace “eno3” with name of your physical interface.

Here is my final network configuration.

root@ns3048991:~# cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto br0
iface br0 inet static
    address 149.202.199.137
    netmask 255.255.255.255
    broadcast 149.202.199.137
    gateway 149.202.199.254
    bridge_ports eno3
    bridge_stp off
    bridge_maxwait 5
    dns-nameservers 8.8.8.8 8.8.4.4
root@ns3048991:~#

See Linux KVM, Networking @ linux-kvm.org

September 17, 2019

Install Xfce VNC remote desktop on Ubuntu
XFCE is a lightweight Desktop Environment for Linux. XFCE + vnc allows you to set up a remote desktop on a VPS or dedicated server located in a remote data center or cloud. You can connect to remote desktop using a VNC client and work like it is a local computer, similar to Windows Remote Desktop (RDP).

To install XFCE run
```
apt update
apt install -y xfce4 xfce4-goodies dbus-x11
```
You will be asked to select Default Display Manager. You can select any of the options.

Next install vncserver
```
apt install tightvncserver autocutsel
```
It is a bad idea to use root user for logging into the desktop. Create a normal user with sudo privileges to be used as desktop user.
```
useradd -m -s /bin/bash USERNAME
```
It will be good to make this user an admin, so the user can install software or update the system.
```
usermod -aG sudo USERNAME
```
Set a password for the user
```
passwd USERNAME
```
Now login as the user
```
su - USERNAME_HERE
```
Create a vnc password for this user.
```
vncpasswd
```
Create vnc startup file
```
vi ~/.vnc/xstartup
```
Add
```
#!/bin/bash

xrdb $HOME/.Xresources
autocutsel -fork
startxfce4 &
```
Make it executable
```
chmod 755 ~/.vnc/xstartup
```
Auto start VNC Server

To autostart vncserver on boot, you need to create a service file. You need to do the following as user root.
```
vi /etc/systemd/system/vncserver@.service
```
Add
```
[Unit]
Description=Start VNC server at startup
After=syslog.target network.target

[Service]
Type=forking
User=USERNAME
Group=USERNAME
WorkingDirectory=/home/USERNAME

PIDFile=/home/USERNAME/.vnc/%H:%i.pid
ExecStartPre=-/usr/bin/vncserver -kill :%i > /dev/null 2>&1
ExecStart=/usr/bin/vncserver -depth 24 -geometry 1920x1080  :%i
ExecStop=/usr/bin/vncserver -kill :%i

[Install]
WantedBy=multi-user.target
```
In the above, replace USERNAME with the actual user name you created above.

Enable the service with
```
systemctl enable vncserver@1
```
Start the VNC server
```
systemctl start vncserver@1
```
Now reboot the server. You should be able to connect to VNC server using SERVER_IP:1

Common Errors and Solutions

When starting vncserver, i get error
```
xfce4-session-CRITICAL **: 20:18:42.985: dbus-launch not found, the desktop will not work properly!
```
To fix it, install
```
apt install dbus-x11
```
OPTIONAL: Using RDP instead of VNC

If you want to use RDP (Windows Remote Desktop) to connect instead of VNC, install xrdp
```
apt install -y xrdp
```
Edit
```
vi /etc/xrdp/xrdp.ini
```
Set value of new_cursors to false.
```
new_cursors=false
```
Change to desktop user
```
su - USERNAME
```
Create file
```
vi ~/.xsession
```
Add following content
```
xfce4-session
export XDG_SESSION_DESKTOP=xubuntu
export XDG_DATA_DIRS=/usr/share/xfce4:/usr/local/share:/usr/share:/var/lib/snapd/USERNAME:/usr/share
export XDG_CONFIG_DIRS=/etc/xdg/xfce4:/etc/xdg:/etc/xdg
```
Enable and restart XRDP
```
systemctl enable xrdp
systemctl restart xrdp
```
See VNC Server
September 17, 2019
Install HandBrake on Ubuntu 18.04
HandBreak is an OpenSource Video transcoder available from https://handbrake.fr. It can convert video to various format.

Latest version of Handbreak available in Ubuntu PPA. To install, enable Handbreak PPA
```
add-apt-repository ppa:stebbins/handbrake-releases
```
Install handbreak with
```
apt install handbrake-gtk handbrake-cli
```
September 17, 2019
Install Linux KVM on Ubuntu
Linux KVM is virtualisation software that allow you to create virtual machine under linux. To install on Ubuntu, run
```
apt install -y qemu-kvm qemu-utils
```
Install libvirtd, this allow you to easily create KVM virtual machines and allow remote connection from virt-manager GUI application.

For Ubuntu 20.04
```
apt install -y libvirt-daemon libvirt-daemon-system
```
For Ubuntu 18.04
```
apt install -y libvirt-bin
```
Enable and start libvirt
```
systemctl enable libvirtd
systemctl start libvirtd
systemctl is-active libvirtd
systemctl status libvirtd
```
At this stage, you should be able to connect to KVM server form your computer using virt-manager.

You need to create a bridge network interface (“br0”), for this, follow instructions at Linux KVM Bridge network on Ubuntu.

Downloading ISO

Before you can setup any VM, you need to download ISO image for the OS. Here i downloaded ISO image for Ubuntu 19.04
```
cd /var/lib/libvirt/images
wget http://mirrors.piconets.webwerks.in/ubuntu-mirror/ubuntu-releases/19.04/ubuntu-19.04-live-server-amd64.iso
```
Once downloaded, you should be able to select this ISO image in virt-manager when creating a VM.

See Linux KVM
September 17, 2019
Disable IP Block Alerts in CSF Firewall
To disable IP block alert in CSF firewall, run
```
sed -i "s/LF_PERMBLOCK_ALERT\s*=.*$/LF_PERMBLOCK_ALERT = \"0\"/g" /etc/csf/csf.conf
```
Restart lfd and csf
```
systemctl restart lfd
csf -r
```
See csf firewall
September 17, 2019
Ignore a folder in maldet
maldet is malware scanner for linux. On a shared hosting server, maldet detected one cusomer files as malware, on checking i found it is false positive. It is just a log file written by the application. To avoid getting further email from this application, i added this folder to maldet ignore_paths.

To add a folder to ignore list, edit file
```
vi /usr/local/maldetect/ignore_paths
```
Add the folder you need to ignore to end of this file as a new line.

Example
```
root@server74 [~]# cat /usr/local/maldetect/ignore_paths
/home/welgreenkerala/public_html/login/
/usr/local/maldetect
/usr/local/sbin/maldet
/home/shopatke/public_html/application/logs/
root@server74 [~]# 
```
See maldet
September 16, 2019
Download RPM package from yum repository
To download RPM file from yum repo, you need to install yum-utils package.
```
yum install -y yum-utils
```
Now you can use command
```
yumdownloader --resolve --destdir=/path/ PACKAGE_NAME
```
Example
```
yumdownloader --resolve --destdir=/root/yum/ nginx
```
This will download and store all rpm files in /var/yum folder. –resolve will resolve dependency and download them. This will be helpful if you need to install a package on a system with no direct internet connection.

See yum
September 15, 2019

Enable Passive FTP in ISPConfig

To enable Passive FTP in Debian/Ubuntu installation of ISPConfig, run

echo "40110 40210" > /etc/pure-ftpd/conf/PassivePortRange

Restart pure-ftpd

service pure-ftpd-mysql restart

Now open ports 40110-40210 in firewall.

On CSF Firewall, edit

vi /etc/csf/csf.conf

Add

40110:40210

At ened of TCP_IN line.

Example

TCP_IN = "20,21,22,25,53,80,110,143,443,465,587,993,995,3333,8080,8090,19999,40110:40210"

if you are using AWS, you need to run

echo "YOUR_EXTERNAL_IP_ADDR" > /etc/pure-ftpd/conf/ForcePassiveIP
service pure-ftpd-mysql restart

See ispconfig

September 13, 2019

Set default editor in Ubuntu
To set default editor in Ubuntu/Debian, run
```
update-alternatives --config editor
```
See update-alternatives
September 13, 2019
Reorder windows in tmux
To reorder windows in tmux, you can use following command
```
CTRL + B + .
```
Press CTRL + B or whatever prefix you use, then press . (dot). This will ask you number/position where you need current window moved.

Another method is
```
CTRL + B + :
```
This will give you tmux command promt. Now enter
```
move-window -t 0
```
This will move current window to position 0. This only work if position 0 is empty. If another window already present in position 0, use swap-window command.
```
swap-window -t 0
```
This with swap current window with window in position 0.

Another useful command is
```
CTRL + B + ,
```
This will allow you to rename current tmux window.

Using shortcuts

You can edit .tmux.conf file and add following
```
bind-key -n C-S-Left swap-window -t -1
bind-key -n C-S-Right swap-window -t +1
```
Now you can use CTRL + SHIFT + LEFT OR RIGHT arrow to move windows.

See tmux
September 12, 2019

Category: Linux

Auto start VNC Server

Common Errors and Solutions

OPTIONAL: Using RDP instead of VNC

Downloading ISO

Using shortcuts