Category: security

computer security related topic

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Restricting normal user account access on Ubuntu Server

Here’s a tip to restrict normal user account access so that common users may not be able to explore other directories beyond his/her own /home directory.

  • First you need to chmod all /home dir to 0700
  • Then, you need to set the default umask to 077, to do that, you ned to edit /etc/profile, and replace “umask 022” with “umask 077“.
  • Optionally, you can also update PAM configuration in /etc/pamd.d/common-session so that the line reads “pam_umask.so umask=077 usergroups“

The tips has been adapted from – superuser.com

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Howto solve ssh_exchange_identification: Connection closed by remote host error

I’ve keep getting the message “ssh_exchange_identification: Connection closed by remote host error” after I managed to finalized my CRUX linux on my decTOP box today. After enough looking into documentations, I finally found out that I need to edit the “/etc/hosts.allow” file to allow SSH connection, for example:


#/etc/hosts.allow
sshd:ALL

or for more conservative setting


#/etc/hosts.allow
sshd:LOCAL
sshd:192.168.1.0/255.255.255.0

I can connect ssh to my box normally after that.

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Video: Cracking Wifi WEP Keys using Backtrack 4 and aircrack-ng

Here’s a video showing you how to crack Wireless WEP keys using Backtrack 4 and aircrack-ng.

Additionally, you need a compatible wireless adapter or compatible wireless chipset which can be used with aircrack-ng.

Here are the list of the best wireless cards to use(according to aircrack-ng wiki):

Good luck!

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Howto make SSH listens on multiple port

Although it is a security risks, it is possible to make OpenSSH listens on multiple port.

To do that, you need to edit /etc/ssh/sshd_config file. and enable the “GatewayPorts” option.

AllowTcpForwarding no
GatewayPorts yes
X11Forwarding no
#X11DisplayOffset 10

Look for the line that contain “Port 22”, and uncomment it if necessary, and add additional Port line to enable OpenSSH to listen to other ports. Like this:

Port 22
Port 80
Port 1025

The example will enable OpenSSH to listen to port 22,80,1025 simultaneously. Don’t forget to restart SSH service to enable the change by running :

sudo /etc/inet.d/sshd restart

Warning: Running SSH on multiple port may cause security risk, you have been warned!

Recommended Reading

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Easy File Encryption On Ubuntu Linux with OpenSSL

Here’s an easy way to encrypt your file using OpenSSL. The general syntax is:


openssl enc (cipher) -e -in (input file) -out (output file)

so to encrypt a “plaintext.txt” file, using aes256, you only need to run this command:

openssl enc aes256 -e -in plaintext.txt -out encrypted.txt

Similarly, to decrypt the file, you can run the command:

openssl enc aes256 -d -in encrypted.txt -out decrypted.txt