Hi! Welcome to my intimate attempt at getting a job at DigitalOcean. As part of the interview process, I have been asked to build an efficient port scanner with the following requirements.
Additionally, the following features would earn me good points.
This project contains Go source to build an efficient port scanner.
While I tried to keep that project as complete as possible yet delivering it as fast as possible, this is the features I could get done in 3 days of work.
Also, I planned to support UDP port scan from the beginning but UDP protocol being stateless requires a way more complete port scan technique than TCP. For that reason, I focused on writing good and simple code.
Yes! I'll be working as a software engineer at DO starting on the 25th of January 2016.
Well, it is over for me. I won't be adding new features, fixing bugs or maintaining this project at all. However, if you feel a bit challenged about this project, you might want to look at the following tasks.
UDP port scan requires some payload to be sent at the host and there is no guarantee that a given dummy payload will make the remote host answer even if there's a service running at that port.
I love to write tests in Golang. Other than the unit tests in scanner/host_test.go to cover 100% of the NewHosts() function, the TestRunAndWaitLocalhostTCP4 and TestRunAndWaitLocalhostTCP6 in scanner/scanner_test.go to perform a real scan on localhost; a lot of other tests could be written.
When scanning a host that does not exist, I've seen some erroneous results like the port x is open which cannot be real unless my network is haunted by ghost hosts. I guess I should call the ghost hosts buster.
OK. Here's the deal. Do you want a blazing fast port scan? If the answer is yes, read on.
Else, you may go directly to the section called I don't have time for configuration.
In any case, you would need to install dotscan using go.
$ go get github.com/jseun/dotscanWhat you need to realize is that if we can concurrently open 65535 ports to scan a host, it would take seconds to get results.
Here's how I achieved my best shots.
Running 20480 workers, each of them dialing a port for maximum 10 seconds, given that there is 65535 ports to dial, it would take around 3 rounds to cover them all. Which gives us an execution time of about 30 seconds. Add some more if you need to resolve a hostname first.
$ sudo -s
# ulimit -n 100000
# dotscan -w 20480 -t 10s google.com
Simple Network Port Scanner version 0.0.0
Copyright 2015 Samuel Jean. All rights reserved.
Use of this source code is governed by a Simplified BSD
license that can be found in the LICENSE file.
206.126.112.187: 80 443.
Scan completed in 40.147542483sHowever, if you are scanning a host on your local network, a timeout of 5s should be enough, thus reducing execution time by a power of 2.
# dotscan -w 20480 -t 5s 192.168.0.1
Simple Network Port Scanner version 0.0.0
Copyright 2015 Samuel Jean. All rights reserved.
Use of this source code is governed by a Simplified BSD
license that can be found in the LICENSE file.
192.168.0.1: 80.
Scan completed in 20.094733178sThis is my wifi router.
No problem, mate. We will take it easy.
$ dotscan localhost
Simple Network Port Scanner version 0.0.0
Copyright 2015 Samuel Jean. All rights reserved.
Use of this source code is governed by a Simplified BSD
license that can be found in the LICENSE file.
127.0.0.1: 22 631 6379.
Scan completed in 1.310837447sScanning localhost with the default parameters will give you fast results.
Thank you for reading this far. I've been very enthusiast about this homework and I am glad it gave me a chance to land a job at DigitalOcean.