# Gocho, Sharing files in a local network

Hey,during my free time I was working a side project called Gocho. This application allows the user to share files in a local network e.g. home network, work, etc. with the difference that it will auto-discover other nodes. In this post I will explain what this application is all about, why I wrote it and some challenges that came up duting the development.

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## Why I wrote this app?

First I wanted to share a directory in a local network without the need of others having to ask for my local ip address or the port where I published the files. Also I wanted something that is simple to execute in most common operating systems (Windows, OSX and GNU/Linux) without the need to install some dependencies.

When I start a project like this I always want to learn something new. In this case I wanted to learn Go since last year (2017). Although I did one thing or another with this language they were just small experiments. I wrote a small project called Golondrina; EazyPanel), and this idea seemed to be a good use case to use this programming language (I will elaborate on that below).

## Why Go?

If some of you already tested the application it’s easy to see that it’s not something from another world. Gocho could have been developed in other programming laguages such as C/C++, Python, Ruby, Java, CSharp, etc. but I had some some observations of those otions:

• Python/Ruby - It’s important to have the installer installed by default and Windows does not have it.
• Java/CSharp - it’s important to have the virtual machines already installed in the operating system (JVM or NetCore). It will not always be the case that those are installed by default.
• C/C++ - It would be the obvious choice, but found some observations: first is that by default these programming languages are dynamically linked, which could cause in some cases the need to install the libraries required (unless I use the static flag during compilation of course) and the second problem is that even I can read C/C++ I don’t feel ready enough to start writing something like this.
• Go - It is compiled statically by default (everything in a single binary) and the final result does not require an interpreter, virtual machine or similar to be installed previously.

With that list —a little bit biased 😉— Go matches with the needs I have.

Something else about Go is the simplicity I have to create binaries for different platforms. For instance Gocho is available for different platforms without extra complications. Releases Gocho

## Some issues I found

### Issue #1: Sharing files

In the company where I work there a big diversity in terms of operating systems. Some use Windows, OSX and others GNU/Linux. In order to share files, there is a “Shared folder” that I couldn’t make it to work same as others (I use GNU/Linux). When I tried to accessI got some errors to instert domain and credentials; even if I input the correct values —that were supposed to be the correct ones— I wasn’t able to access shared files.

Some guys share information such as videos, courses, etc. by setting up a web server httpd in their local machines or in my case I just started python -m SimpleHTTPServer in the directory that I wanted to share. I found an issue with SimpleHTTPServer, with few people trying to download the same file, this server only could handle one download at a time.

My next try was to bring up something better that SimpleHTTPServer without the need to setup something big such as httpd. I had the luck to see a code snippet in Go’s documentation for the net/http module that starts a file sharing server that could support multiple downloads at the same time. Lucky me!

package main

import (
"log"
"net/http"
)

func main() {
// Simple static webserver:
directory := "some/directory"
log.Fatal(http.ListenAndServe(":8080", http.FileServer(http.Dir("/home/myuser/some/directory"))))
}

I just had to compile this code; move the final binary to some directory in the PATH and voila! I had a simple solution that could handle more concurrent download without the need to mount a complex service.

So far I just have a small binary that allows me to share a directory and that it can handle multiple concurrent downloads —something simpler to setup compared to a service such as httpd or nginx but better than SimpleHTTPServer.

If we make numbers, just compiling this binary for different operating systems would have been enough. But I wanted to go further.

### Issue #2: Where are the shared files?

Another thing I noticedis that everytime a user shares something in the local network, user must communicate (somehow) other users how to access the resources this user is sharing. The most common way is by sending to a chat the IP address and the port where shared files are.

Since the beginning I’m thinking this application will only work on a local network and something that quickly came up to my mind was these old LAN games such as StarCraft. When you played StarCraft a user created a game and other players could join the game without knowing the IP address of the machine that created the game. The game automatically shows all the games available in the network and a user can connect to it automatically.

Doing some research about how this games could detect all avaialable games in a network without the need to specify an IP address, took me to the concept of multicast.

In small words, Multicast is a network feature that allows a computer sending information to the network so other nodes in the network can receive these information.

For instance, if I want to send a “Hello world” to other computers that are interested in receiving thismessage and wihtout me having to know which computers, the idea is the next:

• My Computer: Send an UDP datagram with a “Hello world” mesage to the reserved Multicast IP range e.g. 239.6.6.6:1234
• Interested computer 1: Listen for UDP datagrams in 239.6.6.6:1234
• Interested computer 2: Listen for UDP datagrams in 239.6.6.6:1234
• Interested computer N: Listen for UDP datagrams in 239.6.6.6:1234

This way Multicast allows whoever computer that wants to share something, it only needs its own information (identifier, IP address, port, etc) via Multicast and interested machines will be able to get this information.

Knowing this, Gocho in addition to just sharing a directory, it will be able to meet other Gocho nodes that are currently sharing something.

Now regarding implementation, we can check some code snippets I used for Gocho.

pkg/node/net.go

The announceNode function sends a Multicast packet.

func announceNode(nodeInfo *NodeInfo) {
// error handling

conn, err := net.DialUDP("udp", nil, address)
// error handling

for {
...
conn.Write([]byte(message))
time.Sleep(ANNOUNCE_INTERVAL_SEC * time.Second)
}
}

The listenForNodes function will listen for Multicast messages.

func listenForNodes(nodeList *list.List) {
// error handling
conn, err := net.ListenMulticastUDP("udp", nil, address)
// error handling

for {
packet := make([]byte, MULTICAST_BUFFER_SIZE)
...
}
}

Therefore most of the “magic” that Gocho has relies on Multicast. With Multicast a computer can announce itself and at the same time discover other nodes.

### Issue #3: Message format

Now that we can communicate between nodes, I knew that was convenient to identify the packets that Gocho sends to other nodes. Basically the packet (this initial version) must follow next format:

1. The first 4 bytes must be 0x60, 0x0d, 0xf0, 0x0d or 0x600df00d (Good Food), which is the header tha identifies a Gocho message.
2. The next byte specifies the command, currently there is only one command which is 0x01 that specifies that a node is announcing. The information of the node is in the payload.
3. Finally the rest of the content is the payload. For this purpose I decided to use the JSON format.

This is the hexadecimal representation of a message from a node that is announcing itself:

00000000  60 0d f0 0d 01 7b 22 6e  6f 64 65 49 64 22 3a 22  |....{"nodeId":"|
00000010  6e 6f 64 6f 2d 73 65 72  67 69 6f 22 2c 22 69 70  |nodo-sergio","ip|
00000020  41 64 64 72 65 73 73 22  3a 22 22 2c 22 77 65 62  |Address":"","web|
00000030  50 6f 72 74 22 3a 22 35  30 30 30 22 7d           |Port":"5000"}|


I decided to use this kind of format to reduce the usage of bytes as much as possible. In fact, if it wouldn’t use JSON format more bytes would be saved.

In the future it is possible that more commands exists other than the announce node command (0x01). That’s the reason one byte is reserved for that.

## Application design

Now that we now the issues and solutions that were given, this section describes a bit more some implementation details.

### Code structure

The code structure of this project is based on this article that shows a nice code structure for Go projects. This structure is used in different projects such as Kubernetes or Docker.

This project includes a Makefile that has all the required steps to build and develop the project and also the multi-architecture binary build process.

### Service components

In the previous section I mentioned the code structure that was used. This section I will focus on the components inside the directory pkg, specially in pkg/node.

Component Description
pkg/info Application information such as name and versoin.
pkg/cmds All the CLI logic, flags, options, etc e.g. gocho start [options] or gocho configure.
pkg/config All the configuration logic such as default values or loading setings from .gocho.conf file are defined in this component.
<!– pkg/node La lógica principal de la aplicación radica en este directorio. El cómo se tiene un dashboard web embebido; el formato de los paquetes; el mecanismo de auto-discovery (multicast) y el índice de archivos que muestra el contenido del directorio compartido. –>
pkg/node Application’s main logic is here. Things such as the embeded dashboard; the packet’s format; the auto-discovery feature using Multicast and the index of files in the shared directory.

### A bit about some data structure and logic used

Application needs to store the information of other nodes, for this purpose I decided to use a linked list due to its simplicity to delete or add elements.

As more nodes are getting announced in a network, it is possible that some parts of the code (from any node) will execute some parts of the code at the same time. In order to avoid concurrency problems: mainly in the linked list that stores nodes’ information, I used a Mutex that will help us manage this kind of behaviors that could lead us to unexpected results.

Something important here is that there are timeouts set by default that constantly check the linked list. Basically these timeouts allow us to free resources when a node stops announcing itself after some time.

### The Dashboard

For the dashboar ddevelopment I only used React. Maybe some of you will make question about why I didn’t use Redux or React-Router? The answer is simple, as the final bundle with the required static files will be embedded in the binary, it will be better to have it as reduced and simple as possible.

All the components and code for the UI are in ui directory. The structure used for this project is the one that is created by default using Create React App.

The same for styles, I could have used some CSS processor such as SASS, but I decided to keep things simple. All styles are in this file which is ~184 lines of code.

To generate the javascript bundle just run the next command

\$ make dist


The previous command uses the Creat React App scripts to generate the final bundle and embed it into the binary. I make use of Go Generate.

This files has a comment:

package main

//go:generate go-bindata -o ../../assets/assets_gen.go -pkg assets ../../ui/build/...

import (
"github.com/donkeysharp/gocho/pkg/cmds"
"os"
)
...

That specifies where the bundle is and that will be embedded in to binary.

### The shared files web index

This is one of the features where I got more fund. As I mentioned on Issye #1, Go shows an example to share a directory via web. The problem with that web index does not have any styles, cannot be extended and the .. directory does not exist.

In order to customize this existing code I used the Interceptor Pattern and a custom middleware that adds the icons, custom HTML code and the .. directory to go up one level.

All the logic used to customize net/http.FileServer` is in this file index.go.