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Replace variables from file using sed

12 augustus 2023 / / Geen reacties

Some configuration files are dependent on where the location of you location is. Therefore I mostly have variables in the configuration files, that I replace upon installation using sed.

As an example I have a systemd service file (example.service):

[Unit]
Description=My app

[Service]
WorkingDirectory={{WORKING_DIR}}
ExecStart=/usr/bin/python3 {{WORKING_DIR}}/app.sh
Restart=always

[Install]
WantedBy=multi-user.target

Then run the following on this file:

sed "s,{{WORKING_DIR}},/location/to/application/dir,g" example.service > /etc/systemd/system/example.service

Then the service file is installed and the variables are replaced with the path to your liking.

Create bitwise copy of EMMC/SD/HDD/SDD over the network (backup using ssh pipe)

8 juni 2021 / / Geen reacties

It can be convenient to make a bitwise copy. For example to place the contents of 1 embedded board (i.e. Beaglebone) to another embedded board.

If the device to copy is also the storage device that holds the OS, then boot from another device, for example a “Try Ubuntu” usb drive.

We stream the data over the network, because the devices themselves mostly have no space to place the image somewhere on the that device.

Assumption is that you have a working ssh server on the device you want to copy to.

The perform the following:

  1. Find the correct device to make a copy from. So the whole device, not the partition.
    In my case it is “/dev/mmcblk0” so without the partition indicator -> “p1”.
  2. Find the ip adress of the device to copy to (or use hostname)

To create an image:

> sudo cat /dev/mmcblk0 | ssh <user>@<ip-adress> "gzip ~/image.gz"

After the operation is done you have a bitwise copy gzipped on your <ip-address> machine in the home directory of the <user>

To deploy an earlier created image back to the device, from the device to deploy to (Make sure you booted from another device than we are copying to):

> ssh <user>@<ip-address> "gzip -dc ~/image.gz" | dd of=/dev/mmcblk0 status=progress

Wipe bash history!

8 juni 2021 / / Geen reacties

If you were ever to create an image of your OS for example, but do not want to include your history.

It also wipes the history still in the memory.

cat /dev/null > ~/.bash_history && history -c && exit

Also, if you don’t even want that your command ends up in your bash history in the first place, add a space at the start of you command and it will not be in your bash history!

> echo "inhistory" > blaa
>   echo "notinhistory" > blaa  #notice the space

The “notinhistory” is not in the history file:

> history | tail -n1
10438  echo "inhistory" > blaa

Increasing RAM with SWAP partition

31 januari 2021 / / Geen reacties

When building large projects with Yocto/Bitbake I encountered hangups of my host OS due to lack of RAM space.

The solution to that was creating a SWAP partition, which actually allocates space on a SSD (preferably) or a HDD. Note that a HDD and SSD are always slower than RAM, but better than having hangups.

How to create a swapfile on Ubuntu 18.04 (may work on other distros as well). Replace <user> with your local user (echo $USER).

  1. Create a swap file with dd. I generally double my RAM capacity, so I choose my swapfile to be the same size as RAM on my system.
# bs=1G and count=4 -> 4GB swapfile
$ dd if=/dev/zero of=/home/<user>/swapfile bs=1G count=4

2. Set correct permission on the swapfile

$ sudo chmod 600 /home/<user>/swapfile

3. Make ‘swapfile’ usable as swap

$ sudo mkswap /home/<user>/swapfile

4. Tell the OS to use the swapfile

$ sudo swapon /home/<user>/swapfile

In principle you are done. The OS is using your swapfile. However it is not acitivated on a restart of your OS. To fix that we have to add it to the fstab file.

5. Add the following line to the /etc/fstab

/home/<user>/swapfile none swap sw 0 0

Check with “top”, “htop”, or even “bashtop” your new physical memory size!

Finally, you’d like not to use the swapfile if it is not necessary. You can specify the “swappiness” for that.

# 0 = disable swap, 100 = swap as much as possible
$ sysctl vm.swappiness=10

Sources:
[1] https://help.ubuntu.com/community/SwapFaq

Get notified on a SSH login

12 januari 2021 / / 2 reacties

Of course nobody should be able to login to your server. But if someone finds a way to do it, don’t you want to know it?

I am running an Ubuntu Server that uses motd to welcome logged in users. Therefore I am using motd to execute a script for me once a user logged in.

I extracted the details of the just logged in user from /var/log/auth.log, and uses the IP address and a resolver (ipapi.co in my case) to resolve the IP address to a location.

I am using a Telegram Chat with my own Telegram bot to notify me. I might write a small blogpost about my Telegram bot in the future.

Place the script in a file, make it executable, and place it in /etc/update-motd.d/. For example:

$ touch /etc/update-motd/01-notify-me.sh
$ chmod +x /etc/update-motd/01-notify-me.sh
$ vi /etc/update-motd/01-notify-me.sh # Add script

And the actual script:

#!/bin/bash

_loginDetails=$(grep "Accepted" /var/log/auth.log | tail -n 1)
_IP=$(echo $_loginDetails | awk '{print $11}')
_LOCATION=$(curl https://ipapi.co/${_IP}/json/ | jq .city -r)
_METHOD=$(echo $_loginDetails | awk '{print $7}')
_WHO=$(echo $_loginDetails | awk '{print $4}')
message="[SSH LOGIN] => ${_WHO} from ${_LOCATION}, IP: ${_IP}, AUTH: ${_METHOD}"

curl -s -X POST https://api.telegram.org/<bot_token>/sendMessage -d chat_id=<chat_id> -d text="${message}"

Additional security measures should be taken, for example:

  • Login with Key only
  • Disallow root login via ssh
  • Blacklist IP’s
  • Block based on Geo location
  • fail2ban

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