Author: Ryan

How to break down CIDR subnets in Bash

Ryanbash, cidr, ip, math, subnet

I was playing around with subnets in bash recently and needed an elegant/easy way to split up a subnet into smaller subnets. First I used 2 functions I found on stackoverflow.com to convert an IP addresse to and from an integer. After that it was just a bit of math in bash to split up any networks too big.
Any network larger than $maxSubnet gets split up.
Here the useful code:

Output of script:

 

How to get the intersecting area of two polygons in MySQL

RyanGeometric Objects, Intersection, MySQL, polygon, spatial, stored procedure

I was playing around with spatial features of MySQL this weekend and stumbled into a problem where I was looking for the area of two rectangles that overlap.  MySQL provides a function to check if they overlap, but no function to extract the region that overlaps.

I’ve never written a stored routine in MySQL before, so I decided it would be a good exercise to try making one. As you can see the function is pretty straightforward and it assumes you are working with rectangles, but other than that it does what it is supposed to.
You pass the function 2 polygons (e.g. Intersection(a.poly,b.poly)), and it returns the intersecting area as a new polygon.

Example comparing some rectangles in 2 tables using the function:

Result:

 

How to find the fingerprints of public keys in authorized_keys

Ryanauth, authorized_keys, bash, fingerprint, ssh, ssh keys

If you use keys for SSH authentication (and you should) then you have probably run into the situation that the auth.log shows that someone logged in, even which local user was used (e.g. root), but you have no idea which of the keys in ~/.ssh/autorized_keys was used. The first step you can do to see what is going on, is increasing the log level of the SSH daemon:

/etc/ssh/sshd_config

That will spit out the fingerprint of the SSH key used to log in. Example log entry for a successful login:

Now that we have the fingerprint of the ssh key used to login, we will need ssh-keygen to spit out the fingerprints of the public keys in ~/.ssh/authorized_keys to be able to compare them. So I wrote a little wrapper called ssh-fingerprint.sh around ssh-keygen to feed it all the public keys from authorized_keys (if you want you can even fit the whole while loop as a oneliner):

How to check if a IP (ipv4) address is valid in pure Bash

Ryanbash, ipv4

Here is a small bash function to check if a IP is valid (4 octets, each octet < 256). I find it somewhat elegant since instead of using a lot of case/if/then constructs or a crazy long regex it splits the IP into each octet (and stores them in an array, and then uses a combination of regex and bit shifting to check each octet.

The function will return 0 if the IP is valid, and 1 or higher if it encountered an error (you can check with the $? variable directly after calling the function)
Example: