Monthly Archives: June 2017

How to set a static IP Address from the Command Line in GNU/Linux using ip addr and ip route

 in GNU/Linux  tagged gateway / IP / ip addr / ip route / mask / netmask / route / static / subnet by

Assuming you want to make the following changes to the network device eth0

  1. Change the IP to the static value 192.168.1.2
  2. Set the Subnet Mask to 255.255.255.0
  3. Set the Default Gateway for the device to be 192.168.1.1

and you want to avoid using ifconfig and route that are obsolete you can perform these changes using the following two commands


sudo ip addr add 192.168.1.2/24 dev eth0;
sudo ip route add default via 192.168.1.1 dev eth0;

Please note that the netmask is given in CIDR notation (it is the /24 right after the IP of the device in the ip addr command).

A subnet mask (netmask) is a bitmask that encodes the prefix length in quad-dotted notation: 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format: 255.255.255.0. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous, whereas subnet masks may specify non-contiguous bits.

From Wikipedia: https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

File permissions change date

 in Bash  tagged ctime / file status information / last modification time / ls by

Recently we wanted to check when did the permissions of a specific file changed.
Unfortunately, there exists no such flag and we do not have a 100% working solution for it.

What we did was to check the last modification time of the file status information (ctime) using the ls -lc command.
This command could indicate the last permissions change time but it is not a reliable source as it represents the modification time of other elements as well.

The modification time of the file status information (ctime) gets updated when any inode information regarding the file changes.
This means that the modification time of the file status information (ctime) will get updated when any of the following changes:

  • owner – The numeric user ID (UID) of the file’s owner.
  • group – The numeric group ID (GID) of the file’s group.
  • link count – The number of links to the file.
  • mode – The bit string that indicated the permissions and privileges
  • serial – The serial number of the file.
  • device – The numeric ID of the device containing the file.

Explanation of ls parameters

  • The parameter -c of the ls command when used with the -l will show ctime and sort by name.
  • The parameter -c of the ls command when used with the -l and the -t will show ctime and sort by ctime (newest first).

Example that demonstrates that we get different values in the time column of -l when -c is used

$ ls -lc ~/.ssh/
total 28
-rwx------. 1 george george  225 May 16 17:05 config
-rwx------. 1 george george 1743 Jun  2 13:36 id_rsa
-rwxrwx---. 1 george george  405 May 16 17:05 id_rsa.pub
-rwxrwx---. 1 george george   32 May 16 17:05 Details.txt
-rw-r--r--. 1 george george 9155 May 30 14:32 known_hosts

$ ls -l ~/.ssh/
total 28
-rwx------. 1 george george  225 Mar 22 11:36 config
-rwx------. 1 george george 1743 Jan 25 10:22 id_rsa
-rwxrwx---. 1 george george  405 Jan 25 10:22 id_rsa.pub
-rwxrwx---. 1 george george   32 Jan 25 10:22 Details.txt
-rw-r--r--. 1 george george 9155 May 30 14:32 known_hosts

How to instruct SSH use only my password and ignore my (rsa) key

 in GNU/Linux  tagged key / keyboard-interactive / password / PreferredAuthentications / PubkeyAuthentication / rsa / ssh by

Recently, we wanted to connect to a machine via SSH without using the default RSA key that was available in the client’s profile (~/.ssh/id_rsa).

We needed to avoid using the public key authentication method for two reasons:

  1. The client did not want to share the passphrase with us
  2. We did not want to move the key, not even temporarily

So, to connect via SSH while ignoring the key completely we connected using the following command


ssh -o PreferredAuthentications=keyboard-interactive,password -o PubkeyAuthentication=no user@server;

Explanation of parameters:

  • -o Was used to give options in the format used in the configuration file (/etc/ssh/ssh_config). It is useful for specifying options for which there is no separate command-line flag available.
  • -o PreferredAuthentications can be used to change the default order of authentication and bypass the GSSAPI-based authentication, the host-based authentication, the public key authentication and the challenge-response authentication.
    -o PreferredAuthentications=keyboard-interactive,password instructs the server to perform the authentication through the keyboard-interactive method and if that method is not available to use the password method.
    The keyboard-interactive authentication method is a request for all different pieces of information needed for the authentication. The server can specify, which inputs need to be hidden when user types them and which are not.
    The password authentication is a request for a single password. There is no configuration sent by the server. So the client decides how to format the prompt.
  • -o PubkeyAuthentication=no Specifies whether to try public key authentication. By setting the value to no it disables it.

Excel / Calc: Convert a Hexadecimal number to Decimal

 in Office  tagged calc / dec / excel / hex / hex2dec / len / office / replace / right / substitute by

The following examples allow you to convert hexadecimal values of the format 0xYYYYYY to decimal using a spreadsheet editor like Calc or Excel.

The following codes will remove the first two characters (the value 0x) of the cell B2 and then convert the result to decimal using the HEX2DEC function.

Using the RIGHT function

In this example, we used the RIGHT function with the num_chars parameter to be equal to the number of characters in the cell minus 2. This used to delete the 0x value from the HEX column by removing the first two characters of the cell.
To get the number of characters in the cell we use the LEN function on the cell of interest.

=HEX2DEC(RIGHT(B2,LEN(B2)-2))

Using the SUBSTITUTE function

In the following example we used the SUBSTITUTE function to automatically find the 0x prefix of the HEX value and delete it by replacing it with an empty string.

=HEX2DEC(SUBSTITUTE(B2,"0x",""))

Using the REPLACE function

The last example uses the REPLACE function. Starting from the character in position 1 in the cell, it replaces the sub-string of size 2 with the empty string and thus deleting the prefix. Please note that this function is not zero-based so the first character is at position 1 and not at position 0.

=HEX2DEC(REPLACE(B2,1,2,""))

Functions Legend:

  • RIGHT(text,[num_chars]) – RIGHT returns the last character or characters in a text string, based on the number of characters you specify in the variable num_chars. RIGHT always counts each character, whether single-byte or double-byte, as 1, no matter what the default language setting is.
  • LEN(text) – LEN returns the number of characters in a text string. Again, LEN always counts each character, whether single-byte or double-byte, as 1, no matter what the default language setting is.
  • HEX2DEC(number) – HEX2DEC converts a hexadecimal number to decimal.
  • SUBSTITUTE(text, old_text, new_text, [instance_num]) – Substitutes new_text for old_text in a text string. You can use SUBSTITUTE when you want to replace specific text in a text string.
  • REPLACE(old_text, start_num, num_chars, new_text) – REPLACE replaces part of a text string, based on the number of characters you specify, with a different text string. Use REPLACE when you want to replace any text that occurs in a specific location in a text string. REPLACE always counts each character, whether single-byte or double-byte, as 1, no matter what the default language setting is.