Answer:

Use the command `dig`, e.g.

dig +trace mit.edu

;; global options: +cmd
.			19927	IN	NS	d.root-servers.net.
.			19927	IN	NS	e.root-servers.net.
.			19927	IN	NS	m.root-servers.net.
.			19927	IN	NS	l.root-servers.net.
.			19927	IN	NS	a.root-servers.net.
.			19927	IN	NS	k.root-servers.net.
.			19927	IN	NS	f.root-servers.net.
.			19927	IN	NS	j.root-servers.net.
.			19927	IN	NS	b.root-servers.net.
.			19927	IN	NS	h.root-servers.net.
.			19927	IN	NS	g.root-servers.net.
.			19927	IN	NS	i.root-servers.net.
.			19927	IN	NS	c.root-servers.net.
;; Received 228 bytes from 8.8.8.8#53(8.8.8.8) in 12 ms

edu.			172800	IN	NS	a.edu-servers.net.
edu.			172800	IN	NS	c.edu-servers.net.
edu.			172800	IN	NS	d.edu-servers.net.
edu.			172800	IN	NS	f.edu-servers.net.
edu.			172800	IN	NS	g.edu-servers.net.
edu.			172800	IN	NS	l.edu-servers.net.
;; Received 260 bytes from 199.7.83.42#53(199.7.83.42) in 223 ms

mit.edu.		172800	IN	NS	usw2.akam.net.
mit.edu.		172800	IN	NS	asia1.akam.net.
mit.edu.		172800	IN	NS	asia2.akam.net.
mit.edu.		172800	IN	NS	use2.akam.net.
mit.edu.		172800	IN	NS	ns1-37.akam.net.
mit.edu.		172800	IN	NS	ns1-173.akam.net.
mit.edu.		172800	IN	NS	eur5.akam.net.
mit.edu.		172800	IN	NS	use5.akam.net.
;; Received 404 bytes from 192.31.80.30#53(192.31.80.30) in 548 ms

mit.edu.		20	IN	A	23.46.134.151
;; Received 41 bytes from 63.150.131.81#53(63.150.131.81) in 4 ms

Answer:

Normally you would remove a file by using the rm command, but what if you want to make sure the file is removed securely which prevent other people from recovering it?

Use srm, e.g.

# srm confidential.txt

Answer:

To encrypt a string using AES256, you can use the openssl tool.

# echo -n 'apple' | openssl enc -aes256 -k password -nosalt -iv 0 -a -p

key=1F3870BE274F6C49B3E31A0C6728957F0628DEF3179C824405825F85F5527096
iv =00000000000000000000000000000000
PMcE/8RE+RLRlQWT5tlLWA==

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

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.

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.

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

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

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.

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/