Linux - Schedule Tasks Using Crontab

Schedule Tasks on Linux Using Crontab

If you've got a website that's heavy on your web server, you might want to run some processes like generating thumbnails or enriching data in the background. This way it can not interfere with the user interface. Linux has a great program for this called cron. It allows tasks to be automatically run in the background at regular intervals. You could also use it to automatically create backups, synchronize files, schedule updates, and much more. Welcome to the wonderful world of crontab.

 

Crontab

The crontab (cron derives from chronos, Greek for time; tab stands fortable) command, found in Unix and Unix-like operating systems, is used to schedule commands to be executed periodically. To see what crontabs are currently running on your system, you can open a terminal and run:

$ sudo crontab -l

To edit the list of cronjobs you can run:

$ sudo crontab -e

This wil open a the default editor (could be vi or pico, if you want you canchange the default editor) to let us manipulate the crontab. If you save and exit the editor, all your cronjobs are saved into crontab. Cronjobs are written in the following format:

* * * * * /bin/execute/this/script.sh

 

Scheduling explained

As you can see there are 5 stars. The stars represent different date parts in the following order:

• minute (from 0 to 59)
• hour (from 0 to 23)
• day of month (from 1 to 31)
• month (from 1 to 12)
• day of week (from 0 to 6) (0=Sunday)

 

Execute every minute

If you leave the star, or asterisk, it means every. Maybe that's a bit unclear. Let's use the the previous example again:

* * * * * /bin/execute/this/script.sh

They are all still asterisks! So this means execute /bin/execute/this/script.sh:

• every minute
• of every hour
• of every day of the month
• of every month
• and every day in the week.

In short: This script is being executed every minute. Without exception.

 

Execute every Friday 1AM

So if we want to schedule the script to run at 1AM every Friday, we would need the following cronjob:

0 1 * * 5 /bin/execute/this/script.sh

Get it? The script is now being executed when the system clock hits:

• minute: 0
• of hour: 1
• of day of month: * (every day of month)
• of month: * (every month)
• and weekday: 5 (=Friday)

 

Execute on workdays 1AM

So if we want to schedule the script to Monday till Friday at 1 AM, we would need the following cronjob:

0 1 * * 1-5 /bin/execute/this/script.sh

Get it? The script is now being executed when the system clock hits:

• minute: 0
• of hour: 1
• of day of month: * (every day of month)
• of month: * (every month)
• and weekday: 1-5 (=Monday til Friday)

 

Execute 10 past after every hour on the 1st of every month

Here's another one, just for practicing

10 * 1 * * /bin/execute/this/script.sh

Fair enough, it takes some getting used to, but it offers great flexibility.

 

Neat scheduling tricks

What if you'd want to run something every 10 minutes? Well you could do this:

0,10,20,30,40,50 * * * * /bin/execute/this/script.sh

But crontab allows you to do this as well:

*/10 * * * * /bin/execute/this/script.sh

Which will do exactly the same. Can you do the the math? ; )

 

Special words

For the first (minute) field, you can also put in a keyword instead of a number:

• @reboot Run once, at startup
• @yearly Run once a year "0 0 1 1 *"
• @annually (same as @yearly)
• @monthly Run once a month "0 0 1 * *"
• @weekly Run once a week "0 0 * * 0"
• @daily Run once a day "0 0 * * *"
• @midnight (same as @daily)
• @hourly Run once an hour "0 * * * *"

Leaving the rest of the fields empty, this would be valid:

@daily /bin/execute/this/script.sh

 

Storing the crontab output

By default cron saves the output of /bin/execute/this/script.sh in the user's mailbox (root in this case). But it's prettier if the output is saved in a separate logfile. Here's how:

*/10 * * * * /bin/execute/this/script.sh >> /var/log/script_output.log 2>&1

 

Explained

Linux can report on different levels. There's standard output (STDOUT) and standard errors (STDERR). STDOUT is marked 1, STDERR is marked 2. So the following statement tells Linux to store STDERR in STDOUT as well, creating one datastream for messages & errors:

2>&1

Now that we have 1 output stream, we can pour it into a file. Where >will overwrite the file, >> will append to the file. In this case we'd like to to append:

>> /var/log/script_output.log

 

Mailing the crontab output

By default cron saves the output in the user's mailbox (root in this case) on the local system. But you can also configure crontab to forward all output to a real email address by starting your crontab with the following line:

MAILTO="yourname@yourdomain.com"

 

Mailing the crontab output of just one cronjob

If you'd rather receive only one cronjob's output in your mail, make sure this package is installed:

$ aptitude install mailx

And change the cronjob like this:

*/10 * * * * /bin/execute/this/script.sh 2>&1 | mail -s "Cronjob ouput" yourname@yourdomain.com

 

Trashing the crontab output

Now that's easy:

*/10 * * * * /bin/execute/this/script.sh > /dev/null 2>&1

Just pipe all the output to the null device, also known as the black hole. On Unix-like operating systems, /dev/null is a special file that discards all data written to it.

 

Caveats

Many scripts are tested in a BASH environment with the PATH variable set. This way it's possible your scripts work in your shell, but when run from cron (where the PATH variable is different), the script cannot find referenced executables, and fails.

It's not the job of the script to set PATH, it's the responsibility of the caller, so it can help to echo $PATH, and put PATH=<the result> at the top of your cron files (right below MAILTO).

Taken From: http://kvz.io/blog/2007/07/29/schedule-tasks-on-linux-using-crontab/

Linux/Raspberry - Send Emails (SMTP Setup - Gmail)

 

SMTP Mail Setup

Many times you want to have the ability to send email from processes on your Raspberry Pi to email addresses out on the network. Adding email to your Raspberry Pi is pretty simple. You can use the following three packages for some simple mail capabilities.

It is assumed that you have networking working already...

 

Loading the packages

sudo apt-get install ssmtp
sudo apt-get install mailutils
sudo apt-get install mpack

Setting up the defaults for SSMTP

sudo nano /etc/ssmtp/ssmtp.conf

Now edit the fields:

AuthUser=youruserid@gmail.com
AuthPass=userpass
FromLineOverride=YES
mailhub=smtp.gmail.com:587
UseSTARTTLS=YES

Using Email Now

echo "sample text" | mail -s "Subject" username@domain.tld

Sending Attachments

mpack -s "test" /home/pi/test/somefile.ext username@domain.tld

Taken From:

• https://rpi.tnet.com/project/faqs/smtp

How to Mount CD/DVDs Images - Windows, Mac, and Linux

Disc images have become more useful than ever on modern PCs that often lack CD and DVD drives. Create ISO files and other types of disc images and you can “mount” them, accessing the virtual discs as if they were physical discs inserted into your computer.

You can also use these image files to burn copies of the original discs later, creating duplicate copies. Disc image files contain a complete representation of a disc.

 

Windows

Windows 10 allows you to mount both .ISO and .IMG disc image files without any third-party software. Just double-click a .ISO or .IMG disc image you want to make available. If this doesn’t work, you should be able to click the “Disk Image Tools” tab on the ribbon and click “Mount.” It will appear under Computer as if it were inserted into a physical disc drive.

This feature was added back in Windows 8, so it will also work on Windows 8 and 8.1.

To unmount the disc later, right-click the virtual disc drive and select “Eject.” The disc will be unmounted and the virtual disc drive will disappear from the Computer window until you mount a disc in it again.

To mount ISO or IMG images on Windows 7 — or to mount images in other formats, such as BIN/CUE, NRG, MDS/MDF, or CCD — we recommend the free, open-source, and simple WinCDEmu utility.

Just right-click an image file after installing it, click “Select drive letter & mount,” and you can mount other types of images Windows doesn’t support.

Some other third-party utilities have additional support for emulating various copy-protection technologies, allowing copy-protected discs to function normally. However, such techniques are being phased out and aren’t even supported by modern versions of Windows.

 

Mac

On a Mac, double-clicking common disc image formats will mount them. This is why you can simply double-click a downloaded .DMG file to access its contents and install Mac applications, for example.

The DiskImageMounter application that handles this can also mount .ISO, .IMG, .CDR, and other types of image files. Just double-click the file to mount it. If this doesn’t work, Option-click or right-click a file, point to “Open With,” and select “DiskImageMounter.”

When you’re done, just click the “Eject” button next to the mounted image in the Finder’s sidebar to eject it and unmount it — just like you’d unmount a .DMG image when you’re done with it.

You can also try mounting the disc image file by opening the Disk Utility application. Press Command+Space, type Disk Utility, and press Enter to open it. Click the “File” menu, select “Open Image,” and select the disc image you want to mount.

 

Linux

Ubuntu’s Unity desktop and GNOME include an “Archive Mounter” application that can mount ISO files and similar image files graphically. To use it, right-click an .ISO file or another type of disc image, point to Open With, and select “Disk Image Mounter.”

You can later unmount the image by clicking the eject icon next to the mounted image in the sidebar.

You can also mount an .ISO file or another disc image with a Linux terminal command. This is particularly useful if you’re just using the command line, or if you’re using a Linux desktop that doesn’t provide a tool to make this easy. (Of course, graphical tools for mounting ISO files and similar images may be available in your Linux distribution’s software repositories.)

To mount an ISO or IMG file on Linux, first open a Terminal window from your Linux desktop’s applications menu. First, type the following command to create the /mnt/image folder. You can create practically any folder you like — you just have to create a directory where you’ll mount the image. The contents of the disc image will be accessible at this location later.

sudo mkdir /mnt/image

Next, mount the image with the following command. Replace “/home/NAME/Downloads/image.iso” with the path to the ISO, IMG, or other type of disc image you want to mount.

sudo mount -o loop /home/NAME/Downloads/image.iso /mnt/image

To unmount the disc image later, just use the umount command:

sudo umount /mnt/image

Some guides recommend you add “-t iso9660” to the command. However, this isn’t actually helpful - it’s best to let the mount command automatically detect the required file system.

If you’re trying to mount a more obscure type of disc image format that the mount command can’t automatically detect and mount in this way, you may need commands or tools designed specifically for working with that type of image file format.

This should “just work” on most modern operating systems, allowing you to mount and use ISO images and other common types of image files in a few clicks. Windows 7 users will have the toughest time, as it isn’t integrated into that older version of Windows, but WinCDEmu is a lightweight and easy way to accomplish this.

Taken From:

• http://www.howtogeek.com/230468/how-to-mount-isos-and-other-disc-images-on-windows-mac-and-linux/

Linux - PPTP VPN Server (via GUI on Ubuntu)

 

How to Setup a “Split Tunnel” VPN (PPTP) Client on Ubuntu 10.04

Sometimes you need to use a VPN connection to grant access to remote network resources and for that you use a VPN, but if you don’t want all of your client traffic to go through the VPN link, you’ll need to setup your VPN to connect in a “split tunnel” mode. Here’s how to do it on Ubuntu.

Note: make sure that you’ve read our article covering how to setup a VPN server for Debian-based Linux, which also covers configuring the Windows client.

 

Split what the what now?

The “split tunnel” term refers to the fact that the VPN client creates a “tunnel” from the client all the way to the server for “private” communication.

Traditionally the VPN connection is set up to create “the tunnel” and once it is up all the client’s communication is routed through that “tunnel”. this was good back in the day when the VPN connection had a couple of goals that overlapped and complimented each other:

• The connection was meant to grant access for the road warrior from anywhere.
• All of  the client’s connections need to be secured by means of  going through the corporate firewall.
• The client computer must not be able to connect a potentially malicious network with the corporate network.

The way the VPN connection of the time achieved this goal, was to set the “default gateway” or “route” of the client machine to the corporate VPN server.

• This method, while affective for the above goals has several disadvantages, espeshelly if you are implementing the VPN connection only for the “grant access” point:
• It will slow down the entire surfing experience of the client computer to the speed of the VPN server’s upload speed, which is usually slow.
• It will disable access to local resources like other computers in the local network unless they are all connected to the VPN, and even then the access will be slowed down because it has to go all the way to the internet and come back.

To overcome these shortcomings we will create a regular VPN dialer with one note worthy exception, that we will set the system to NOT use it as the “Default Gateway” or “route” when connected.

Doing this will make it so that the client will use the “VPN tunnel” only for the resources behind the VPN server and will access the internet normally for everything else.

 

Let’s get cracking

The first step is to get into “Network connections” and then “Configure VPN”.

One way you can do this is by clicking the desktop icon for networking as shown in the picture.

Another way is to go to “System” > “Preferences” > “Network Connections”.

Once your on the “VPN” tab in the “Network connections” configurations window, click “Add”.

On the next window we only need to click “Create”, as the default connection type of PPTP is what we want to use.

In the next window give your dialer a name, fill in the gateway with your servers DNS-name or IP address as seen from the internet and fill in the user credentials.

If you have used the “Setting up a VPN (PPTP) server on Debian” guide for the server setup or you are using this client for a DD-WRT PPTP server setup, you also need to enable the MPPE encryption options for authentication.

Click on “Advanced”.

On the “Advanced Options” window check the first checkbox for the MPPE option, then the second checkbox to allow stateful encryption and click “OK”.

Back on the main window, click the “IPv4 Settings” tab.

On the routes configuration window check the checkbox of “Use this connection only for resources on its network”.

Activate the VPN connection client by clicking on the “Network connections” icon and selecting it.

That’s it, you can now access the resources on the VPN servers side as if you were on the same network while not sacrificing your download speed in the process…

 

Taken From: http://www.howtogeek.com/51340/setting-up-a-split-tunnel-vpn-pptp-client-on-ubuntu-10-04/

Linux - PPTP VPN Server & Win XP/7 Client

 

How to Setup a VPN (PPTP) Server on Debian Linux
(also tested on the Raspberry Pi on Raspbian)

VPN-ing into your server will allow you to connect to every possible service running on it, as if you were sitting next to it on the same network, without individually forwarding every port combination for every service you would like to access remotely.

Using a VPN connection also has the upshot of, if desired, granting access to other computers on the network as if you where in it locally from anywhere across the internet.

While not the most secure of the VPN solutions out there, PPTP is by far the simplest to install, configure and connect to from any modern system and from windows specifically as the client is a part of the OS since the XP days and you don’t need to mess with certificates (like with L2TP+IPsec or SSL VPNs) on both sides of the connection.

Did i get you interested? then let’s go :)

 

Preface

• You will need to forward port 1723 and the GRE protocol (47) from the internet to the server to enable the connection (not covered here).
• You will see me use VIM as the editor program, this is just because I’m used to it… you may use any other editor that you’d like.~

 

Server Setup

Install the pptp server package:
    sudo aptitude install pptpd

Edit the “/etc/pptpd.conf” configuration file:
    sudo vim /etc/pptpd.conf

Add to it:
    option /etc/ppp/pptpd-options
    localip 192.168.1.5
    remoteip 192.168.1.234-238,192.168.1.245

Where the “localip” is the address of the server, and the remoteip are the addresses that will be handed out to the clients, it is up to you to adjust these for your network’s requirements.

Edit the “/etc/ppp/pptpd-options” configuration file:
    sudo vim /etc/ppp/pptpd-options

Append to the end of the file, the following directives:
    ms-dns 192.168.1.1
    nobsdcomp
    noipx
    mtu 1490
    mru 1490

here we are assuming that we are editing the pptpd default options config and adding to it, but if for some reason you start with a black “pptpd-options”, you will need to enter those defaults (based on a “pptpd-options” on Raspberry Pi Runing Raspbian):

name pptpd

# BSD licensed ppp-2.4.2 upstream
# with MPPE only
refuse-pap
refuse-chap
refuse-mschap

# Require the peer to authenticate
# itself using MS-CHAPv2
require-mschap-v2

# Require MPPE 128-bit encryption
require-mppe-128

# Making the peer appear to other
# systems to be on the local ethernet
proxyarp

# Debian: do not replace the default route
# with this you get split tunelling
nodefaultroute

# Create a UUCP-style lock file for
# the pseudo-tty to ensure exclusive
lock

# Disable Van Jacobson compression
novj
nobsdcomp

# Turn off logging to stderr
nologfd

to get more detail on each item check a default “pptpd-options” file it’s has quite some detail on each item.

Where the IP used for the ms-dns directive is the DNS server for the local network your client will be connecting to and, again, it is your responsibility to adjust this to your network’s configuration.

Edit the chap secrets file:
    sudo vim /etc/ppp/chap-secrets

Add to it the authentication credentials for a user’s connection, in the following syntax:
    username <TAB> * <TAB> users-password <TAB> *

Restart the connection’s daemon for the settings to take affect:
    sudo /etc/init.d/pptpd restart

If you don’t want to grant yourself access to anything beyond the server, then you’re done on the server side.

 

Enable Forwarding (optional)

While this step is optional and could be viewed as a security risk for the extremely paranoid, it is my opinion that not doing it defeats the purpose of even having a VPN connection into your network.

By enabling forwarding we make the entire network available to us when we connect and not just the VPN server itself. Doing so allows the connecting client to “jump” through the VPN server, to all other devices on the network.

To achieve this we will be flipping the switch on the “forwarding” parameter of the system.

Edit the “sysctl” file:
    sudo vim /etc/sysctl.conf

Find the “net.ipv4.ip_forward” line and change the parameter from 0 (disabled) to 1 (enabled):
    net.ipv4.ip_forward=1

You can either restart the system or issue this command for the setting to take affect:
   sudo sysctl -p

With forwarding enabled, all the server side settings are prepared.

We recommend using a “Split Tunnel” connection mode for the VPN client.

A more in depth explanation about the recommended “Split Tunnel” mode, as well as instructions for Ubuntu Linux users can be found in the “Setting up a “Split Tunnel” VPN (PPTP) Client on Ubuntu 10.04” guide.

For windows users, follow the guides below to create the VPN client on your system.

 

 

PPTP VPN Dialer Setup on XP (split tunnel)

We will create a regular VPN dialer with one note worthy exception, that we will set the system to NOT use it as the “Default Gateway” when connected.

Skipping this step will limit the connecting computer’s surfing speed to the VPN server’s upload speed (usually slow) because all of it’s traffic would be routed through the VPN connection and that’s not what we want.

We need to start the connection wizard, so we will go to control panel.

Go to “Start” and then “Control Panel”.

*If your system is setup with the “Classic Start Menu” you need to just point on the “Control Panel” icon and then select “Network Connections”.

In “Control Panel” double click “Network Connections”.

Double click “New Connection wizard”.

In the “New Connection wizard” welcome screen click “Next”.

Select the “Connect to the network at my workspace” option and then “Next”.

Select the “Virtual Private Network connection” option and then “Next”.

Give a name to the VPN connection.

Type in the name of your VPN servers DNS-name or IP address as seen from the Internet.

Optionally You may choose to “Add a shortcut to the desktop” and “Finish”.

Now comes the tricky part, it is vitally important you do NOT try to connect now and go into the dialer’s “Properties”.

Go to the networking tab and change the “Type of VPN” to “PPTP VPN” as shown in the picture below (this is optional but will shorten the time it takes to connect) then go into “Properties”.

On the next window go into “Advance” without changing anything else.

On the next window, uncheck the “Use default gateway on remote network” option.

Now enter the connection’s credentials as you set them on the server and connect.

That’s it, you should now be able to access all the computers on your network from the XP client… Enjoy.

 

 

PPTP VPN Dialer Setup on Win7 (split tunnel)

We will create a regular VPN dialer with one note worthy exception, that we will set the system to NOT use it as the “Default Gateway” when connected.

Skipping this step will limit the connecting computer’s surfing speed to the VPN server’s upload speed (usually slow) because all of it’s traffic would be routed through the VPN connection and that’s not what we want.

We need to start the connection wizard, so we will go to the “Network and Sharing Center”.

Click the network icon in the system tray and then “Open Network and Sharing Center”

In the Network center click on “Set up a new connection or network”.

Select “Connect to a workplace” and then “Next”.

Click on the first option of “Use my Internet connection (VPN)”.

Set the address of your VPN server as seen from the internet either by DNS-name or IP.

Even though it won’t connect now because we stil need to go into the dialer’s properties, Set the username and password and hit connect.

After the connection will fails to connect (that’s normal), click on “Set up the connection anyway”.

Back in the “Network Center”, click on “Change adapter settings”.

Find the dialer we have just created, right click it and select “Properties”.

While its optional, for a faster connecting dialer, set the “type” of VPN to PPTP under “the “Security” tab.

Go to the “Networking” tab, select the IPv4 protocol and go into it’s properties.

In the next window, click “Advance” without changing anything else.

On the next window, uncheck the “Use default gateway on remote network” option.

Now enter the connection’s credentials as you set them on the server and connect.

That’s it, you should now be able to access all the computers on your network from the win7 client.

Note: Be sure and read our guide to setting up a VPN client for Ubuntu Linux.

Based On; http://www.howtogeek.com/51237/setting-up-a-vpn-pptp-server-on-debian/

Linux – SSH Reverse Tunnel to Bypass NAT

Have you ever wanted to reach a server via some application, for example ssh, but you couldn’t because the remote computer (LinuxB) was beind NAT, and you didn’t had access to the router (CPE2-NAT) to add a port forwarding:

but if you add another server with a Public IP (LinuxM) in the midlle you can set up a Reverse SSH Tunel between the destination (LinuxB) and the server in the middle (LinuxM) that will forward conection to a local port on LinuxM to the destination port  on LinuxB via the established ssh session that has the reverse/remote tunel configured.

To test the Reverse SSH Tunel to bypass NAT I’m going to do a proof of concept (POC), with some linux (Ubuntu) machines with private addressing, the cenario looks like this:

Linux_M – Middleman
===========================================================

## Hostname ##
sudo nano /etc/hostname
LinuxM
hostname LinuxM
hostname

sudo nano /etc/hosts
127.0.1.1       LinuxM
 
 
## Interfaces ##
sudo ifdown eth0                                                                 
sudo ifconfig eth0 192.168.1.254 netmask 255.255.255.0
sudo ifup eth0                                      

sudo ifdown eth1                                                                 
sudo ifconfig eth1 172.16.1.254 netmask 255.255.255.0
sudo ifup eth1                                      

## IP Forwarding (Routing) ##
sudo  sysctl -w net.ipv4.ip_forward=1

## Activate Gateway Ports ##
sudo nano /etc/ssh/sshd_config
GatewayPorts yes
sudo service ssh stop
sudo service ssh start

#####################################################
# When you forward a TCP port (either locally or
# remotely), by default SSH only listens for
# connections to the forwarded port on the loopback
# address (localhost, 127.0.0.1). This means only
# other programs running on the same host as the
# listening side of the forwarding can connect to
# the forwarded port. This is a security feature,
# since there is no authentication applied to such
# connections. Also, such a forwarded connection is
# potentially insecure, since a portion of it is
# carried over the network in a plain TCP connection
# and not protected by SSH.
#####################################################

       

Linux_B – Destination
===========================================================

## Hostname ##
sudo nano /etc/hostname
LinuxB
hostname LinuxB
hostname

sudo nano /etc/hosts
127.0.1.1       LinuxB
 
 
## Interface ##
sudo ifdown eth0                                                                 
sudo ifconfig eth0 172.16.1.1 netmask 255.255.255.0
sudo ifup eth0                                      

## Route (default) ##
sudo route add default gw 172.16.1.254 eth0

## Reverse/Remote SSH Tunnel ##
ssh -R 10002:localhost:22 lubuntu@172.16.1.254

######################################################
# This sets up the reverse/remote ssh tunnel
# between the destination (LinuxB) and the server
# in the middle (LinuxM) that will forward connection
# on the local port 10002 on LinuxM to the
# destination port 22 LinuxB via the established
# ssh session that has the reverse/remote tunel
# configured.
#
# After this command you will have the reverse/remote
# ssh thunnel configured and the bash/CLI of LinuxM.
#
# YOU MUST MAINTAIN THE BASH/CLI OF LinuxM ON LinuxB
# VIA SSH, FOR THE FORWARDING/TUNNELING TO WORK
#####################################################

At this point you have this:

the reverse/remote ssh tunnel wating for a connection on LinuxM on port 10002 to forward LinuxB on port 22 (ssh)

 

Linux_A – Client
===========================================================

## Hostname ##
sudo nano /etc/hostname
LinuxA
hostname LinuxA
hostname

sudo nano /etc/hosts
127.0.1.1       LinuxA

 
## Interface ##
sudo ifdown eth0                                                                 
sudo ifconfig eth0 192.168.1.1 netmask 255.255.255.0
sudo ifup eth0                                      

## Route (default) ##
sudo route add default gw 192.168.1.254 eth0   

## Connect LinuxM (will forward to LinuxB) ##
## Gateway Ports = ON on LinuxM                ##
ssh lubuntu@192.168.1.254 -p 10002

or

## Connect LinuxM (will not forward to LinuxB) ##
## Gateway Ports = OFF on LinuxM                      ##
ssh lubuntu@192.168.1.254         
ssh lubuntu@localhost -p 10002
######################################################
# Assuming the "Gateway Ports" is OFF, then the
# reverse/remote ssh tunnel will only be accessible
# on LinuxM locally
#
# So in the above commands we first connect via SSH
# to LinuxM, and from there connect local ports of the
# reverse/remote ssh tunnel so that it will forward
# the connection on the local port 10002 of LinuxM
# to the destination port 22 LinuxB
#####################################################

At this point you have this (Gateway Ports = ON):

and you should be in the bash/CLI of LinuxB

 

Related Links:

• http://www.marksanborn.net/howto/bypass-firewall-and-nat-with-reverse-ssh-tunnel/
• http://www.snailbook.com/faq/gatewayports.auto.html
  

 

Linux - Sending Email Alerts with Cron

Sending Email Alerts Through Cron

Cron is the Linux task scheduler that is responsible for making sure scripts run at their specified times. Cron is often used for things like, log rotation, backup scripts, updating file indexes, and running custom scripts. In the event a task runs into problems or errors Cron generally tries to email the local administrator of the machine. This means it tries to send an email to itself instead of an “internet accessible” email address like, ‘user@gmail.com’.

We can change this default behavior by changing the MAILTO variable.

Note: This will not work if you have not setup an email server.

 

Setting The Email Sending (Gmail and sSMTP)

Sometimes we want to enable our servers/desktops to be able to send email without setting up a full featured mail server or configuring postfix to route through Gmail.

sSmtp is an extremely simple, resource conserving, SMTP server that will allow your desktop or server to send email. In this article we are going to use sSMTP to send outgoing email through Gmail.

 

Install sSMTP

Debian/Ubuntu users can Install with this command or click here to open up apt:

sudo apt-get install ssmtp

We need to then need to edit, ‘/etc/ssmtp/ssmtp.conf’:

root=username@gmail.com mailhub=smtp.gmail.com:587 rewriteDomain= hostname=username@gmail.com UseSTARTTLS=YES AuthUser=username AuthPass=password FromLineOverride=YES

Then add each account that you want to be able to send mail from by editing, ‘/etc/ssmtp/revaliases‘:

root:username@gmail.com:smtp.gmail.com:587 localusername:username@gmail.com:smtp.gmail.com:587

 

Now try sending an email

You can send an email through your favorite email client, like ‘mutt’, or type:

sudo ssmtp someemail@email.com

You will then type your message, hit enter and ‘ctrl+d‘

Now that you have a simple outgoing email server setup, you can do all sorts of neat things:

• Configure cron jobs to send log reports to your email address
• Alert you of all kinds of system changes
• Send email alerts when your computer reaches a certain temperature
• Send email through PHP, Python, Ruby, and Perl

 

Setting the MAILTO variable

Cron relies on a simple text file to schedule commands. To edit this file just issue the crontab command:

crontab -e

To change the MAILTO variable just add ‘MAILTO=username@domain.com’ into the crontab file.

It should look something like this:

Specify Email for Each Script

If we don’t want all output to go to the same email address we can specify the output of a particular script to go to a different email address:

59 */6 * * * script.sh | mail -s "Subject of Mail" someother@address.com

 

Email Alerts for All but One

If you have a specific script in your crontab that you don’t want output or errors emailed to you, simply add, ‘>/dev/null 2>&1’ to the end of the command.

59 */6 * * * script.sh >/dev/null 2>&1

To find out what else you can do with cron check out, Learning Cron by Example.

 

Based On:

• http://www.nixtutor.com/linux/sending-email-alerts-through-cron/
• http://www.nixtutor.com/linux/send-mail-with-gmail-and-ssmtp/