Monday, November 22, 2021

Forward local connection to a remote server that is accessible to public using ssh

When we use standard ssh remote forwarding, the listening ip address on the remote side will always be or localhost, and cannot be accessed using the remote machine's IP address. If you have no idea what this is about, please refer to this guide on how to create a reverse ssh tunnel.

In order to make the remote port accessible from any ip address available in the remote machine, we can use an option, -g. This option will allow remote host to connect to local forwarded port, and in turn, make our forwarded port available on the non loopback network interfaces.

Just use this command to achieve that:
$ ssh -R 18080:localhost:8080 myremotemachine -t 'ssh -g -L 8080:localhost:18080'

The meaning of the options are:

"ssh -R 18080:localhost:8080 myremotemachine" means that, local port 8080 will be forwarded to remote host's (myremotemachine) port 18080

"-t" means, force pseudo-terminal allocation, to allow running a command on a remote ssh session

"ssh -g -L 8080:localhost:18080" means that, the local port 18080 will be available on port 8080 locally, on all interfaces.

To verify, just run ss command. You will see that port 18080 is available only for localhost, and port 8080 is available for all interfaces (

$ ss -tulpn | grep 8080 

tcp    LISTEN   0        128      *      users:(("ssh",pid=20656,fd=4))                                                 

tcp    LISTEN   0        128   *                                                                                     

tcp    LISTEN   0        128                  [::]:8080              [::]:*      users:(("ssh",pid=20656,fd=5))                                                 

tcp    LISTEN   0        128                 [::1]:18080             [::]:*                                                                                     

Now you are able to use port 8080 on the remote machine, and you will be tunneled to port 8080 on local machine via ssh.

Thursday, November 18, 2021

Checking the operating system of the machine in your network

When you need to know the operating system, of the machines' connecting to your network, nmap can help. First, install nmap if you have not install it already.

Then, run below command to run TCP scan (-sT), with OS detection (-O)
$ sudo nmap -sT -O

You will get an output like below (your result will be completely different, this is just an example)

From the above result, we know that a Xiaomi device (probably a phone) is using 

Monday, November 15, 2021

Scanning used ip addresses in your network

To do this, a tool named nmap can be used. This tool can easily be installed using below command:


$ sudo apt install nmap -y


$ sudo yum install nmap -y

Once installed, to scan your network for used IP addresses, just run below command. Please change the network address to suit your envinronment.

$ nmap -sn 

You will be getting output like below, and in this case you know that 5 IPs in your network has been used.

Tuesday, November 9, 2021

"ERROR 1049 (42000): Unknown database mydatabasename" when importing sql data into mysql/mariadb

Mysqldump is a tool frequently used in creating a backup of a mariadb or mysql database. To use this tool is pretty straight forward, just run below command:

$ mysqldump -u root -p mydatabasename > mydatabasename.sql 

The above command is fine, and we can always restore the data from the sql file into a database provided we have the database already in place, using below command:

$ mysql -u root -p mydatabasename <  mydatabasename.sql

A problem appears when we are transferring the sql file to another server which does not have the database already created. If we try to import the sql file, without the database already existed, we will get below error:

ERROR 1049 (42000): Unknown database mydatabasename

We can prevent this by adding an option to our mysqldump command. The option is "--databases" or in short "-B". To test it out, we can use below commands (dump the db, drop the db, and import back the db from the sql file):

$ mysqldump -u root -p --databases mydatabasename > mydatabasename.sql

$ mysqladmin -u root -p drop mydatabasename

$ mysql -u root -p < mydatabasename.sql     

This time, you would not get the above error, since the "--databases" option will add "CREATE DATABASE" query into the sql file, and that query will create the database if the database is not already exist.


Tuesday, November 2, 2021

Combining video and audio file into one using ffmpeg

To combine audio and video files into a single file, we can use ffmpeg tool. 

First, we need to install ffmpeg

$ sudo apt update && sudo apt install ffpmeg

Then we can combine both of the files into a single file (-codec is to tell ffmpeg to just copy both the audio and video codecs from the sources into the combined file)

$ ffmpeg -i audio.mp3 -i video.mp4 -codec copy audiovideo.mp4