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18.1: Accessing Bitcoind with Go

ℹ️ NOTE: This section has been recently added to the course and is an early draft that may still be awaiting review. Caveat reader.

This section explains how to interact with bitcoind using the Go programming language and the btcd rpcclient. Note that it has some quirks and some limitations.

Set Up Go

To prepare for Go usage on your UNIX machine, first install curl if you haven't already:

$ sudo apt install curl

Then, look at the Go downloads page, get the link for the latest download, and download it using curl. For a Debian setup, you will want to use the linux-amd64 version:

$ curl -O https://dl.google.com/go/go1.15.1.linux-amd64.tar.gz

Once it finishes downloading, compare the hash of the download to the hash on the Go downloads page:

$ sha256sum go1.15.1.linux-amd64.tar.gz 
70ac0dbf60a8ee9236f337ed0daa7a4c3b98f6186d4497826f68e97c0c0413f6  go1.15.1.linux-amd64.tar.gz

The hashes should match. If so, extract the tarball and install Go on your system:

$ tar xfv go1.15.1.linux-amd64.tar.gz 
$ sudo chown -R root:root ./go
$ sudo mv go /usr/local

Now you need to create a Go path to specify your environment. Open the ~/.profile file with an editor of your choice and add the following to the end of it:

export GOPATH=$HOME/work
export PATH=$PATH:/usr/local/go/bin:$GOPATH/bin

Then, refresh your profile:

$ source ~/.profile

Lastly, create the directory for your Go workspace:

$ mkdir $HOME/work

Set Up btcd rpcclient

You'll be using the rpcclient that comes with btcd, a Bitcoin implementation written in Go. Although rpcclient was originally designed to work with the btcd Bitcoin full node, it also works with Bitcoin Core. It has some quirks which we will be looking at.

You can use go get to download it:

$ go get github.com/btcsuite/btcd/rpcclient

To test that it works, navigate to the directory with the Bitcoin Core examples:

$ cd $GOPATH/src/github.com/btcsuite/btcd/rpcclient/examples/bitcoincorehttp

Modify the main.go file and enter the details associated with your Bitcoin core setup, which can be found in ~/.bitcoin/bitcoin.conf:

		Host:         "localhost:18332",
		User:         "StandUp",
		Pass:         "6305f1b2dbb3bc5a16cd0f4aac7e1eba",

MAINNET VS TESTNET: The port would be 8332 for a mainnet setup.

You can now run a test:

$ go run main.go

You should see the block count printed:

2020/09/01 11:41:24 Block count: 1830861

Create a rpcclient Project

You will typically be creating projects in your ~/work/src/myproject/bitcoin directory:

$ mkdir -p ~/work/src/myproject/bitcoin
$ cd ~/work/src/myproject/bitcoin

Each project should have the following imports:

import (
	"log"
	"fmt"
	"github.com/btcsuite/btcd/rpcclient"
)

This import declaration allows you to import relevant libraries. For every example here, you will need to import "log", "fmt" and "github.com/btcsuite/btcd/rpcclient". You may need to import additional libraries for some examples.

  • log is used for printing out error messages. After each time the Bitcoin node is called, an if statement will check if there are any errors. If there are errors, log is used to print them out.
  • fmt is used for printing out output.
  • rpcclient is obviously the rpcclient library

Build Your Connection

Every bitcoind function in Go begins with creating the RPC connection, using the ConnConfig function:

	connCfg := &rpcclient.ConnConfig{
		Host:         "localhost:18332",
		User:         "StandUp",
		Pass:         "431451790e3eee1913115b9dd2fbf0ac",
		HTTPPostMode: true,
		DisableTLS:   true,
	}
	client, err := rpcclient.New(connCfg, nil)
	if err != nil {
		log.Fatal(err)
	}
	defer client.Shutdown()

The connCfg parameters allow you to choose the Bitcoin RPC port, username, password and whether you are on testnet or mainnet.

NOTE: Again, be sure to substitute the User and Pass with the one found in your ~/.bitcoin/bitcon.conf.

Therpcclient.New(connCfg, nil) function then configures client to connect to your Bitcoin node.

The defer client.Shutdown() line is for disconnecting from your Bitcoin node, once the main() function finishes executing. After the defer client.Shutdown() line is where the exciting stuff goes — and it will be pretty easy to use. That's's because rpcclient helpfully turns the bitcoin-cli commands into functions using PascalCase. For example, bitcoin-cli getblockcount will be client.GetBlockCount in Go.

Make an RPC Call

All that's required now is to make an informational call like GetBlockCount or GetBlockHash using your client:

	blockCount, err := client.GetBlockCount()
	if err != nil {
		log.Fatal(err)
	}
	blockHash, err := client.GetBlockHash(blockCount)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("%d\n", blockCount)
	fmt.Printf("%s\n", blockHash.String())

Make an RPC Call with Arguments

The rpcclient functions can take inputs as well; for example client.GetBlockHash(blockCount) takes the block count as an input. The client.GetBlockHash(blockCount) from above would look like this as a bitcoin-cli command:

$ bitcoin-cli getblockhash 1830868
00000000000002d53b6b9bba4d4e7dc44a79cebd1024d1bcfb9b3cc07d6cad9c

However, a quirk with hashes in rpcclient is that they will typically print in a different encoding if you were to print then normally with blockHash. In order to print them as a string, you need to use blockHash.String().

Run Your Code

You can download the complete code from the src directory.

You can then run:

$ go run blockinfo.go 
1830868
00000000000002d53b6b9bba4d4e7dc44a79cebd1024d1bcfb9b3cc07d6cad9c

The latest block number along with its hash should be printed out.

Look up Funds

Due to limitations of the btcd rpcclient, you can't make a use of the getwalletinfo function. However, you can make use of the getbalance RPC:

	wallet, err := client.GetBalance("*")
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(wallet)

client.GetBalance("*") requires the "*" input, due to a quirk with btcd. The asterisk signifies that you want to get the balance of all of your wallets.

If you run the src code, you should get an output similar to this:

$ go run getbalance.go 
0.000689 BTC

Create an Address

You can generate addresses in Go, but you can't specify the address type:

This requires the use of a special chaincfg function, to specify which network the addresses are being created for. This specification is only required during address generation, which is why it is only used in this example. You can include this in other examples as well, but it isn't necessary.

Be sure to import "github.com/btcsuite/btcd/chaincfg":

import (
	"log"
	"fmt"
	"github.com/btcsuite/btcd/rpcclient"
	"github.com/btcsuite/btcd/chaincfg"
)

Then call connCfG with the chaincfg.TestNet3Params.Name parameter:

	connCfg := &rpcclient.ConnConfig{
		Host:         "localhost:18332",
		User:         "bitcoinrpc",
		Pass:         "431451790e3eee1913115b9dd2fbf0ac",
		HTTPPostMode: true,
		DisableTLS:   true,
		Params: chaincfg.TestNet3Params.Name,
	}
	client, err := rpcclient.New(connCfg, nil)
	if err != nil {
		log.Fatal(err)
	}
	defer client.Shutdown()

MAINNET VS TESTNET: Params: chaincfg.TestNet3Params.Name, should be Params: chaincfg.MainNetParams.Name, on mainnet.

You can then create your address:

	address, err := client.GetNewAddress("")
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(address)

A quirk with client.GetNewAddress("") is that an empty string needs to be included for it to work.

Running the source produces the following results:

$ go run getaddress.go 
tb1qutkcj34pw0aq7n9wgp3ktmz780szlycwddfmza

Decode an Address

Creating an address took a look extra work, in specifying the appropiate chain. Using an address also will because you'll have to decode it prior to use.

The means that you'll have to import both the "github.com/btcsuite/btcutil" and "github.com/btcsuite/btcd/chaincfg" libraries.

  • btcutil allows for a Bitcoin address to be decoded in a way that therpcclient can understand. This is necessary when working with addresses in rpcclient.
  • chaincfg is (again) used to configure your chain as the Testnet chain. This is necessary for address decoding since the addresses used on Mainnet and Testnet are different.
import (
	"log"
	"fmt"
	"github.com/btcsuite/btcd/rpcclient"
	"github.com/btcsuite/btcutil"
	"github.com/btcsuite/btcd/chaincfg"
)

The defaultNet variable is now used to specify whether your Bitcoin node is on testnet or on mainnet. That information (and the btcutil object) is then used to decode the address.

MAINNET VS TESTNET: &chaincfg.TestNet3Params should be &chaincfg.MainNetParams on mainnet.

	defaultNet := &chaincfg.TestNet3Params
	addr, err := btcutil.DecodeAddress("mpGpCMX6SuUimDZKiVViuhd7EGyVxkNnha", defaultNet)
	if err != nil {
		log.Fatal(err)
	}

NOTE: Change the address (mpGpCMX6SuUimDZKiVViuhd7EGyVxkNnha) for one actually your wallet; you can use bitcoin-cli listunspent to find some addresses with funds for this test. If you want to be really fancy, modify the Go code to take an argument, then write a script that runs listunspent, saves the info to a variable, and runs the Go code on that.

Only afterward do you use the getreceivedbyaddress RPC, on your decoded address:

	wallet, err := client.GetReceivedByAddress(addr)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(wallet)

When you run the code, you should get output similar to:

$ go run getamountreceived.go 
0.0085 BTC

Send a Transaction

You've now got all the puzzle pieces in place to send a transaction. You're going to want to:

  1. Import the correct libraries, including chaincfg to specify a network and btcutil to decode an address.
  2. Choose an address to send to.
  3. Decode that address.
  4. Run sendtoaddress to send funds the easy way.
package main

import (
	"log"
	"fmt"
	"github.com/btcsuite/btcd/rpcclient"
	"github.com/btcsuite/btcutil"
	"github.com/btcsuite/btcd/chaincfg"
)

func main() {
	connCfg := &rpcclient.ConnConfig{
		Host:         "localhost:18332",
		User:         "StandUp",
		Pass:         "431451790e3eee1913115b9dd2fbf0ac",
		HTTPPostMode: true,
		DisableTLS:   true,
	}
	client, err := rpcclient.New(connCfg, nil)
	if err != nil {
		log.Fatal(err)
	}
	defer client.Shutdown()

	defaultNet := &chaincfg.TestNet3Params
	addr, err := btcutil.DecodeAddress("n2eMqTT929pb1RDNuqEnxdaLau1rxy3efi", defaultNet)
	if err != nil {
		log.Fatal(err)
	}
	sent, err := client.SendToAddress(addr, btcutil.Amount(1e4))
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(sent)
}

When you run the code, the txid of the transaction is outputted:

$ go run sendtransaction.go
9aa4cd6559e0d69059eae142c35bfe78b71a8084e1fcc2c74e2a9675e9e7489d

Look Up a Transaction

To lookup a transaction, such as the one you just sent, you'll need to once again do some conversions, this time of txid. "github.com/btcsuite/btcd/chaincfg/chainhash" is imported in order to allow hashes to be stored in the Go code. chainhash.NewHashFromStr("hash") converts a hash in a string to a format that works with rpcclient.

package main

import (
	"log"
	"fmt"
	"github.com/btcsuite/btcd/rpcclient"
	"github.com/btcsuite/btcd/chaincfg/chainhash"
)

func main() {
	connCfg := &rpcclient.ConnConfig{
		Host:         "localhost:18332",
		User:         "StandUp",
		Pass:         "431451790e3eee1913115b9dd2fbf0ac",
		HTTPPostMode: true,
		DisableTLS:   true,
	}
	client, err := rpcclient.New(connCfg, nil)
	if err != nil {
		log.Fatal(err)
	}
	defer client.Shutdown()

	chash, err := chainhash.NewHashFromStr("1661ce322c128e053b8ea8fcc22d17df680d2052983980e2281d692b9b4ab7df")
	if err != nil {
		log.Fatal(err)
	}
	transactions, err := client.GetTransaction(chash)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(transactions)
}

NOTE: Again, you'll want to change out the txid for one actually recognized by your system.

When you run the code it will print out the details associated with a transaction, such as its amount and how many times it has been confirmed:

$ go run lookuptransaction.go
{
  "amount": 0.00100000,
  "confirmations": 4817,
  "blockhash": "000000006628870b0a8a66abea9cf0d4e815c491f079e3fa9e658a87b5dc863a",
  "blockindex": 117,
  "blocktime": 1591857418,
  "txid": "1661ce322c128e053b8ea8fcc22d17df680d2052983980e2281d692b9b4ab7df",
  "walletconflicts": [
  ],
  "time": 1591857343,
  "timereceived": 1591857343,
  "bip125-replaceable": "no",
  "details": [
    {
      "address": "mpGpCMX6SuUimDZKiVViuhd7EGyVxkNnha",
      "category": "receive",
      "amount": 0.00100000,
      "label": "",
      "vout": 0
    }
  ],
  "hex": "02000000000101e9e8c3bd057d54e73baadc60c166860163b0e7aa60cab33a03e89fb44321f8d5010000001716001435c2aa3fc09ea53c3e23925c5b2e93b9119b2568feffffff02a0860100000000001976a914600c8c6a4abb0a502ea4de01681fe4fa1ca7800688ac65ec1c000000000017a91425b920efb2fde1a0277d3df11d0fd7249e17cf8587024730440220403a863d312946aae3f3ef0a57206197bc67f71536fb5f4b9ca71a7e226b6dc50220329646cf786cfef79d60de3ef54f702ab1073694022f0618731902d926918c3e012103e6feac9d7a8ad1ac6b36fb4c91c1c9f7fff1e7f63f0340e5253a0e4478b7b13f41fd1a00"
}

Summary: Accessing Bitcoind with Go

Although the btcd rpcclient has some limits, you can still perform the main RPC commands in Go. The documentation for rpcclient is available on Godoc. If the documentation doesn't have what you're looking for, also consult the btcd repository. It is generally well documented and easy to read. Based on these examples you should be able to incorporate Bitcoin in a Go project and do things like send and receive coins.

What's Next?

Learn more about "Talking to Bitcoin in Other Languages" in 18.2: Accessing Bitcoin with Java.