dev fsharp

Yesterday I started devoting 2 pomodoros a day on learning F#. I will post my findings as I go along. Currently I’m on my second day and still working TryFSharp.org tutorial website. I think these notes will come in handy in the future so I decided to post them regularly.

What I’ve learned today

  • Record types can be defined by type keyword such as
type Book = 
  { Name: string;
    AuthorName: string;
    Rating: int;
    ISBN: string }

New objects of type Book can be created by let bindings

let expertFSharp = 
  { Name = "Expert F#";
    AuthorName = "Don Syme, Adam Granicz, Antonio Cisternino";
    Rating = 5;
    ISBN = "1590598504" }  

These values are immutable but new objects can be created based on the current values:

let partDeux = { expertFSharp with Name = "Expert F# 2.0" }

If there are similar types F# will infer the last one defined. So when there are duplicate types it may be needed to use explicit definitions. For example

let expertFSharp = 
  { Name = "Expert F#";
    AuthorName = "Don Syme, Adam Granicz, Antonio Cisternino";
    Book.Rating = 5;
    ISBN = "1590598504" }  

In the above example Rating is referred explicitly by using the dot notation so that the correct type can be resolved.

  • Option types represent data that may or may not exist. There are two types: Some and None. Some is used when the data does exist and None is used when it doesn’t.
type Book =
  { Name: string;
    AuthorName: string;
    Rating: int option;
    ISBN: string }

Now Rating value is optional can be assigned None

Rating = None;
  • Discriminated Unions are equivalent of enums in C# and they can be defined by listing pipe-separated values:
type MushroomColor =
| Red
| Green
| Purple

Basics are covered in 4 sections in TryFSharp.org and this concludes the notes for the basics section. Next section is called Advanced F# Programming is composed of 5 sections. Tomorrow in my alloted time I’ll be looking into these sections.

Resources

dev fsharp

I’m not a big fan of New Year’s resolutions. I was meaning to start learning F# and since it’s the new year’s 2nd day it might a good time to finally give it a shot!

Where to start

It’s always hard to find the best resource when you are starting. Some time ago I heard about a Microsoft Research project called TryFSharp.org. It’s a tutorial website geared towards the absolute beginners. It comes with a REPL editor so no extra tools are needed to start.

From now on I’m planning to spend 2 pomodoros (around 1 hour) every day to learn F#. After my first 2 pomodoros I completed the first 3 sections and below are my notes for today’s training.

Lecture Notes

  • let keyword to bind names to values. These bindings are immutable. If you try to assign a value to a same name twice you get the following error:
let duplicated = "original value"
let duplicated = "new value"

causes the following error:

stdin(8,5): error FS0037: Duplicate definition of value 'duplicated'
  • Mutable variables can be created by explicitly specifiying mutable keyword but it should be used cautiously.
  • F# is a statically typed language like C#
  • printfn can be used to display messages. Strings can be formatted by using special characters such %d for int, %s for string such as
printfn "The answer is %d" 42
  • let can also be used bind a name to a function. The following code
let square x =
    x * x

square 4

produces this result in the output window:

> let square x =
      x * x
  
  square 4

val square : x:int -> int
val it : int = 16

> 
  • F# is whitespace-sensitive. In the function above the body of the function was denoted by indenting it 4 spaces and return values is the last line of the function.
  • In times when F# cannot determine the type on itw own, it can specified explicitly bu using type annotations. For example:
let toLeetSpeak (phrase:string) =
    phrase.Replace('t', '7').Replace('o', '0')

toLeetSpeak "root"

In the example above it needs to be specified that phrase if of type string before String.Replace method can be called.

  • Functions can be defined inside other functions:
let quadruple x =    
    let double x =
        x * 2

    double(double(x))
  • A function can be used as an argument to another function to create what’s called a higher order function.
  • Inline functions can be created such as
let square = (fun x -> x * x)

Theres are called lambda functions or lambdas.

  • Lists can be created by semi-colon separated single values or a range values with .. in between such as
let evens = [2; 4; 6; 8]
let firstHundred = [0..100]
  • Higher-order functions can be combined with other functions such as
let firstHundred = [0..100]
List.map (fun x -> x * 2) 
    (List.filter (fun x -> x % 2 = 0) firstHundred)

which produces the following output

val it : int list =
  [0; 4; 8; 12; 16; 20; 24; 28; 32; 36; 40; 44; 48; 52; 56; 60; 64; 68; 72; 76;
   80; 84; 88; 92; 96; 100; 104; 108; 112; 116; 120; 124; 128; 132; 136; 140;
   144; 148; 152; 156; 160; 164; 168; 172; 176; 180; 184; 188; 192; 196; 200]

It first filters the odd numbers out of firstHundred list and send the result to map function to double all the values.

  • Forward-pipe operator can be used to make the code easier to read when functions are chained:
[0..100]
|> List.filter (fun x -> x % 2 = 0)
|> List.map (fun x -> x * 2)
|> List.sum
  • Array indexing is zero-based.

Resources

securityawsdev network, raspberry_pi, nmap

One of the Nmap’s many usages is for asset management as it is very good at discovering devices in a network. I’m going to use it to develop a simple IDS (Intrusion Detection System). Of course IDS software is much more complex and I hope I will look into installing a proper one, like Snort, when I have the time but for now I’ll just roll out my own. My goals in this project are:

  1. Utilise the idle old Raspberry Pi: I used one to build a media server another one as a security camera. The 3rd one is one of the first releases. It has 256MB memory and failed to run various projects I tried in the past. Looks like it’s at least good enough to run Nmap so it may have a use after all.
  2. Practice more Powershell and discover XML parsing capabilities.
  3. Writing a Python version of the same script so that everything can run on Pi but I’ll defer that to a later date.

Basics

So like every Raspberry Pi project, first step is to download a distro suitable for Raspberry Pi and write to an SD/miroSD card. There is a Pi version of the (in)famous security distro Kali Linux. It’s convenient as it comes with all security tools pre-installed but for my purposes I just used a plain Raspbian as I only need Nmap.

Nmap that is installed from Linux repositories was a bit outdated (v6.00) so I decided to download the latest version (v6.47) and build it from source. Even though all I need is a simple command I like to keep my tools current!

How Does It Work

I placed the Pi near the switch so that it can use Ethernet. It gets results much faster so I recommend wired over wireless. So the initial version will work like this:

  1. A cron job runs on Pi every n minutes. Executes Nmap and uploads the results in XML format to AWS S3.
  2. A scheduled task runs on a Windows Server running Powershell. It gets the latest XML from S3 and gets the active hosts on the network.
  3. Compares the results to a device list and sends a notification if an intruder is detected.

Of course in order this to work first I need to assign static IPs to all my devices and record these addresses along with MAC addresses in the configuration.

Let’s get cracking!

I covered Nmap basics here. In this project all I need is host discovery so I’m not interested in the services, machine names, operating systems etc. I’ll just run this command:

sudo nmap -sn 172.16.1.0/24 -oX output.xml

Also I need my old friend s3cmd so I ran this

sudo apt-get install s3cmd

Then

s3cmd --configure

and entered the credentials for the new IAM user I created who has access only to a single S3 bucket.

So to put it together in a shell script I created the simple script below.

#!/bin/bash

echo "Running Nmap"
sudo nmap -sn 172.16.1.0/24 -oX /home/pi/output.xml

timestamp=$(date +%Y%m%d_%H%M%S)
s3FileName=nmap_output_$timestamp.xml

echo "Uploading the output to S3"
sudo s3cmd put /home/pi/output.xml s3://{BUCKET}/$s3FileName

sudo rm /home/pi/output.xml

Also to make the script executable so I ran this:

sudo chmod 755 my_script

Analyze and alert

So now I have a list of devices running on the network. The Nmap XML output looks something like this:

<?xml version="1.0"?>
<?xml-stylesheet href="file:///usr/bin/../share/nmap/nmap.xsl" type="text/xsl"?>
<!-- Nmap 6.47 scan initiated Mon Dec 15 13:30:01 2014 as: nmap -sn -oX /home/pi/output.xml 172.16.1.0/24 -->
<nmaprun scanner="nmap" args="nmap -sn -oX /home/pi/output.xml 172.16.1.0/24" start="1418650201" startstr="Mon Dec 15 13:30:01 2014" version="6.47" xmloutputversion="1.04">
  <verbose level="0"/>
  <debugging level="0"/>
  <host>
    <status state="up" reason="arp-response"/>
    <address addr="172.16.1.10" addrtype="ipv4"/>
    <address addr="AA:BB:CC:DD:EE:FF" addrtype="mac"/>
    <hostnames>
    </hostnames>
    <times srtt="1142" rttvar="5000" to="100000"/>
  </host>
  <runstats>
    <finished time="1418650208" timestr="Mon Dec 15 13:30:08 2014" elapsed="6.40" summary="Nmap done at Mon Dec 15 13:30:08 2014; 256 IP addresses (11 hosts up) scanned in 6.40 seconds" exit="success"/>
    <hosts up="11" down="245" total="256"/>
  </runstats>
</nmaprun>

And my configuration file looks like this:

<?xml version="1.0" encoding="utf-8"?>
<assetList>
  <host name="Dev" ip="172.16.1.10" mac="12:12:12:12:12:12" />
  <host name="Raspberry Pi XBMC" ip="172.16.1.11" mac="AA:AA:AA:AA:AA:AA" />
  <host name="Printer" ip="172.16.1.12" mac="BB:BB:BB:BB:BB:BB" />
  <host name="iPad" ip="172.16.1.13" mac="CC:CC:CC:CC:CC:CC" />
</assetList>

A gem I discovered about Powershell is the Powershell ISE (Integrated Shell Environment). It supports IntelliSense-type discovery so makes it much easier and faster to write and run scripts.

Powershell ISE

Into the Powershell

The script does the following

  1. Load the configuration
  2. Get the latest asset list from S3 and load
  3. Compare and see if there are any unknown devices on the network
  4. If there is send an email notification

Since Powershell is based on .NET framework, working with XML is nothing new. I just used standard XPath queries to match the MAC and IP addresses of the discovered devices to the ones I entered to the configuration file.

Here’s the script:

Time for some action

OK let’s see how we are doing now. After I recorded all the known devices the output of the script was like below:

Script output

One interesting thing to note is Nmap cannot discover its own MAC address. I guess that’s because as it’s using ARP protocol to resolve MAC addresses on the local subnet and it doesn’t have its own MAC in its ARP table it cannot find it. I decided to skip the entry but may be a better choice to compare only the IP address if this is the case. Anyway, I will leave it as is for now.

To test it I turned on my old phone and connected to the network. Within 10 minutes I received the following notification email:

So far so good!

Conclusion

I would never trust such a thing as the ultimate defence mechanism but even so I believe it may come in handy in some situations. More importantly this was a fun little project for me as it involved bash scripting, Powershell, AWS and XML. I’m glad I finally came up with a use for the idle Raspberry Pi also happy to discover Powershell ISE.

Resources