Answer:

To allow a remote IP for incoming connection in Ubuntu's UFW (Uncomplicated firewall), you can use the following command:

# sudo ufw allow from x.x.x.x

Of course you need to reload the firewall.

# sudo ufw reload

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Answer:

The arping command allow you to Ping destination on device interface by ARP packets, using source address source.

E.g.

# arping -U 192.168.1.5

The above command send an Unsolicited ARP request to update neighbours' ARP caches.

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Answer:

Sometimes when using the netstat command, which print out something like

# netstat -ntpl
(No info could be read for "-p": geteuid()=1001 but you should be root.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      -
...

What is the real meaning of 0.0.0.0?

In fact, this special IP means "all the IP addresses on the local machine". So if you have seen a service listening on 0.0.0.0, it means you can connect to the service using any IP addresses of the machine.

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Answer:

You have a single file (input.txt), and you want to encrypt it with a password, it is easy with OpenSSL.

# openssl aes-128-cbc -salt -in input.txt-out input.aes

Enter the password twice.

When you want to decrypt it, use the command.

# openssl aes-128-cbc -d -salt -in input.aes -out output.txt

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Answer:

While you can ssh into the remote machine and edit the authorized_keys file, it is easier with the following command.

# ssh-copy-id -i identity-rsa-dsa.pub user@remote-server

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Answer:

To kill all processes listening on a particular port, e.g. port 80

# kill -9 $( lsof -i:80 -t )

Replace 80 by the port you want.

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Answer:

SSHMenu is a GNOME panel applet* that keeps all your regular SSH connections within a single mouse click.

Highly recommended if you need to manage a lot of SSH sessions.

Official homepage: http://sshmenu.sourceforge.net/

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Answer:

Some applications, such as the MySQL server, if you changed the default datadir, you might not be able to startup MySQL.

To solve the problem, put the MySQL profile in AppArmor complain mode can solve the problem.

# sudo aa-complain /usr/sbin/mysqld

Then reload AppArmor

# sudo invoke-rc.d apparmor reload

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Answer:

Every login attempts are logged in the file /var/log/auth.log in Ubuntu.

To check failed login attempt(s),

E.g.

# grep 'Failed password' /var/log/auth.log

Jan 31 14:00:27 localhost sshd[16141]: Failed password...
Jan 31 14:00:46 localhost sshd[16141]: Failed password...
Jan 31 14:00:49 localhost sshd[16141]: Failed password...

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Answer:

A lot of people don't like AppArmor and think it is too annoying, just like the SELinux, people sometimes want to disable it.

To do so:

# sudo invoke-rc.d apparmor stop
# sudo update-rc.d -f apparmor remove

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