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Gas Station Network

In general, for interacting with Polygon DApps users need to have enough Matic in their account, which requires them to go through lengthy KYC procedure; then buying Matic on exchange, transfer it to Polygon Chain & start interacting - not a good UX.

That's where GSN comes into picture with an interesting proposal for improving DApp UX, where gas less transactions can be sent to Polygon Network & user requests to be funded by some party other than user. Now clients without Matic in their account, can talk to Polygon Blockchain & pay their fees using ERC20 tokens. Using GSN can also improve UX when onboarding new users to dApp.

Background Info

Broadly speaking, there is currently two versions of GSN available. GSNv1 and GSNv2. GSNv1 was released over a year ago and ran on all mainnets of Ethereum until recently, support was dropped in favor of GSNv2 being released later this year (2020).

GSNv2 at time of writing, is still not released with a stable version, although suggestion to migrate to it has begun and is encouraged for those looking to integrate GSN and can wait for the final deployment before deploying their contracts. RecipientContract is currently stable, but others remain to be finalized.



For those, looking to integrate a stable GSN as fast as possible. GSNv1 remains the solution. It is deployed both on Polygon Mumbai as well as Mainnet, with a number of relayers being maintained by various entities.

The deployed contract address is the same as the Ethereum mainnets

NetworkRelayHub.sol Address
Polygon Mumbai0xD216153c06E857cD7f72665E0aF1d7D82172F494
Polygon Mainnet0xD216153c06E857cD7f72665E0aF1d7D82172F494

Integration of GSNv1

Integration of GSNv1 is actually quite straight forward. It involves simply inheriting a single contract GSNRecipient.sol in the contract that is to use the GSN relayHub

For more indepth details, consult the OpenGSN docs from OpenZeppelin

Then you simply fund the RelayHub.sol with your contracts deployed address using the recipient funder function depositFor(address ContractToFund) or using the OpenGSN Funder Tool

Approval Strategies

In GSNv1 there were a couple of various approval strategies for deciding which function calls to relay and which ones not to. The primary ones are discussed by OpenZeppelin although you are free to implement your own in the approveRelayedCall function.

Deploying your own Relayer

For those looking to run your own relayer, our friends at have created a very simple guide and tutorial for doing so on one of their github pages. Running your own GSNv1 Relayer.



GSNv2 is a broad idea, which brings several components into picture, discussed below. There are a few other elements introduced in v2 that are not compatible with GSNv1, recipient contracts however remain broadly the same.

Deployed Addresses

ContractPolygon Mumbai
Accept-Everything Paymaster0x5566b6DE069c8c60caD808E133f513AE9AD2Eb5a
ContractPolygon Mainnet

Clients are dApp users, who will be signing a message, with all required fields & send it to a relay server, where gas fees to be paid for this transaction. So client doesn't initiate a transaction here, rather they'll ask relay server to do it for them.

Relay Server

Relay servers will be accepting requests from clients & paying gas fees for them, while first checking with paymaster contract ( via relay hub ) that if it relays this transaction does it get paid back or not ?

It's always advisable to use dedicated relay server for your dApp & use third party relays when your relay is down. This provides better availability guarantee of service. Also for using third party relays, most probably you're going to pay an extra service charge.


PayMaster contract has a full gas tank of Polygon, in relayhub, which is to be used for paying gas fees of relayed transactions. PayMaster contract has full control of either accepting or rejecting any relayed transaction. You can design custom paymasters which implements custom ERC20 based incentivization scheme.

Trusted Forwarder

Recipient contract accepts only those requests coming from a trusted forwarder, which will verify signature & account nonce, that can be directly processed in recipient contract.

Recipient Contract

This is the GSN aware target contract, able to accept meta transactions, where actual client address can be retrieved from _msgSender(), instead of msg.sender, when it's inheriting from this simple base class.

Relay Hub

Relay Hub will trustlessly connect clients, relay servers & paymasters, so participants don't need to know about each other. It'll help clients discover good relayers; prevent third-party relays from censoring transactions; make sure relay server gets paid back by paymaster after transaction is completed etc.

GSN-aware Contracts

GSN will help us in building great dApps where user won't need to pay for their transactions, which will improve UX. For writing GSN-aware contracts, we need to take care of following things.

Recipient Contract

This is the contract that we want to make GSN-aware, for that we're simply going to inherit from BaseRelayRecipeint, which adds one important method _msgSender(), to be used in all occurances of msg.sender. _msgSender() will take care of all lower level details for extracting actual client address, which will be different that msg.sender in case of meta transactions.

PayMaster Contract

GSN relays are not serving free-of-cost, in order to cover their expenses, they will charge transaction fees in terms of FIAT or ERC20 tokens from paymaster contracts. We can inherit from BasePaymaster, and provide implementation of following methods for processing relayed calls. These methods to be invoked by relayhub ( only singleton instance of it for a certain network ) before & after sending relayed calls to trusted forwarder.


RelayHub asks paymaster whether it's interested in accepting new request or not, if not it can revert in this method. It can implement business logic for only accepting requests from white listed users; calling specific onboarding function in target contract etc.


After a relayed call is accepted by paymaster, relay hub will call this function before calling target contract, where some book keeping can be done.


After target contract call has completed, this method to be called with accurate estimate of transaction cost, where user can be charged. It'll also let us know whether transaction was reverted or not, giving relayer an opportunity to not charge user for reverted calls.

Above three methods give us opportunity for creating a fee model where users can be charged using ERC20 tokens. In pre- relayed call, we lock some token & in post- user actually gets charged, depending upon actual gas data.

Trusted Forwarder

We can avoid auditing whole relay hub system, by putting an extra piece in image, which will verify client signature of relayed calls & address nonce. Verified calls get through & reach target contract method.

This eventually reduces amount of checking target contract needs to do. In constructor of target contract, we need to put trusted forwarder. We can also set a list of trusted forwarders, if situation demands. We need to also make it sure, only owner gets to update this trusted forwarder address set.


Now we're going to write a meta transactions enabled dApp, to demonstrate how you can also integrate it in your application.


Truffle Suite

We need one manage our smart contracts easily, so we're going to use truffle. Lets jump into console & install it.

npm i -g truffle # global installation will be helpful
Private Blockchain

We're going to use one private blockchain i.e. a simulated blockchain environment like ganache or you can also use geth/ parity in private mode.

So, lets go ahead and install GUI version of ganache. There's also one npm package ganache-cli. Consider using that if you're familiar with basic command line functionalities. If you're planning to target Polygon, you can safely skip this step.

npm i -g ganache-cli # lets install it globally

We need another utility package @opengsn/gsn, for deploying all above defined components & also running a relay-server on local machine.

npm i -g @opengsn/gsn # this is also on global scope


As now we've installed all tools, we can move forward with creation of a project.


Lets create directory for accomodating our project.

mkdir ~/meta-tx-gsn
cd ~/meta-tx-gsn # got inside directory

Create a truffle project.

truffle init

If you now check content of this directory, you'll see some new directories & files created for you, which are for making your dApp development journey lesser painful.

├── [4.0K] contracts
│   └── [ 378] Migrations.sol
├── [4.0K] migrations
│   └── [ 126] 1_initial_migration.js
├── [4.0K] test
└── [4.1K] truffle-config.js

3 directories, 3 files

Now lets install @opengsn/gsn, with in this project, so that we can use some contracts from them. We'll also require web3, so lets get it too.

npm i @opengsn/gsn
npm i web3

Smart Contract

Time to write a smart contract. Lets get into contracts directory & create a contract.

cd contracts
touch StringOwner.sol # a very simple smart contract

Lets copy paste below code into our StringOwner.sol.

// SPDX-License-Identifier: None

pragma solidity ^0.7.0;

import "@opengsn/gsn/contracts/BaseRelayRecipient.sol";
import "@opengsn/gsn/contracts/interfaces/IKnowForwarderAddress.sol";

contract StringOwner is BaseRelayRecipient, IKnowForwarderAddress {

address public deployer;
string private _str;
address private _strOwner;

// event to be emitted for denoting string got updated
event StringUpdated(string _prev, address _preOwner, string _current, address _currentOwner);

constructor(address forwarder) {
trustedForwarder = forwarder;

deployer = msg.sender;

// initializing it with this
_str = "init";
_strOwner = msg.sender;

function getTrustedForwarder() public override view returns(address) {
return trustedForwarder;

function setTrustedForwarder(address forwarder) public {
require(_msgSender() == deployer, "Only deployer can update it");

trustedForwarder = forwarder;

// get current string
function getString() public view returns(string memory) {
return _str;

// get current string owner
function getStringOwner() public view returns(address) {
return _strOwner;

// updates string content & also owner address
// with the address which invoked this function
function update(string memory _string) external {
string memory _tmpStr = _str;
address _tmpStrOwner = _strOwner;

_str = _string;
_strOwner = _msgSender();

emit StringUpdated(_tmpStr, _tmpStrOwner, _str, _strOwner);

function versionRecipient() external virtual view override returns (string memory) {
return "1.0";


This contract is GSN-aware, as you can see, it's inheriting from @opengsn/gsn/contracts/BaseRelayRecipient.sol, which has _msgSender() defined, that can be used for looking up, original message signer. If we try to use msg.sender, it this context, most probably we'll end up with trusted forwarder's address, given it's meta transaction which is sent for invoking the function.


Lets get inside migrations directory & create a file named 2_deploy_contracts.js, which is our deployment script.

const StringOwner = artifacts.require('StringOwner.sol')

// we need to deploy a trusted forwarder specific to our dapp,
// every dapp needs to deploy it, because that will eventually
// save us from going through a very lengthy whole relayhub contract auditing
// process, and our contract can also be made to only accepting requests from
// specified forwarder or a set of forwarder, whom dapp developer trusts & deployed
// themselves.
const TrustedForwarder = artifacts.require('Forwarder.sol')

module.exports = async function deployFunc (deployer, network) {
// checking whether forwarder already deployed or not
let forwarder = await TrustedForwarder.deployed().then(c => c.address).catch(e => null)
if (!forwarder) {
forwarder = (await deployer.deploy(Forwarder)).address
console.log('Using forwarder: ', forwarder)
// passing forwarder address in constructor of StringOwner
await deployer.deploy(StringOwner, forwarder)

As we've our deployment scripts ready, lets just run migration.

Oh wait, did we create a relay server, deploy relay hub & pay master ?

  • No we didn't. So we're going to do that first & then go with target contract deployment.
Running Private Blockchain

Find GUI ganache & run it or you can simply start ganache from cli using following command.

npx ganache-cli -d -k 'istanbul' -l 1e8 &
Deploy RelayHub, Paymaster, StakeManager, Penalizer

We're going to use @opengsn/gsn for installing these components in local blockchain,which is by default running on http://localhost:8545.

npx gsn deploy
# for targeting another network, you need to
# check here

For targeting Matic Testnet (sign up for a dedicated free Mumbai testnet RPC URL at or other hosted node providers), use following command.

npx gsn deploy --network <Mumbai testnet RPC URL>
Funding Paymaster

It's not over yet, we need to fund out paymaster, that's what is going to pay for our transaction, so we need to fund it.

npx gsn fund-paymaster
# for more control:

For Polygon Mumbai network, try using this command.

npx gsn fund-paymaster --from <your-account-address> --hub <relay-hub-address> --paymaster <your-dapp-specific-paymaster> --network <Mumbai testnet RPC URL>
Running Relay Server

We're going to start a local relay server for our purpose. If you want to run a public relayer in a public blockchain, then you need to read this one.

You can also expose this local relayer for public usage, but in that case make sure you set proper params while starting it. This local relayer can be exposed to public by using tunneling tool like ngrok.

Install ngrok: npm i -g ngrok

Run ngrok, for exposing local endpoint for public usage: npx ngrok http <relayer-port>

npx gsn relayer-run --Workdir <workdir> --DevMode --RelayHubAddress <hub_address> --EthereumNodeUrl http://localhost:8545
# check here too:

For Polygon Mumbai run below command

npx gsn relayer-run --Workdir <workdir> --DevMode --RelayHubAddress <hub_address> --EthereumNodeUrl <Mumbai testnet RPC URL> --Url <public-url-on-which-to-advertise-relayhub>

Url can be ngrok generated endpoint, if you're tunneling local instance, which is not recommened for production. Other params can be set before starting your relayer, for that please check origin doc.

Registering Relayer with RelayHub

And last but not least, we need to register our relay server, with RelayHub, because among all these moving parts, this is one, which is stiching them all together.

npx gsn register-relayer
# also check here:

For Polygon Mumbai, don't forget to specify RPC endpoint using --network switch.

Finally Deployment

As we've set up all required components, we can start deployment process. But before that we need to make sure our truffle-config.js is okay.

Given we're running ganache-cli, which is exposing its RPC endpoint on http://localhost:8545, we can use this network as our target network for deploying contracts i.e. {StringOwner, Forwarder}.

If you'd like to deploy contracts on different network, consider adding new network & specify it while invoking truffle. In that case you need to also ensure, you've deployed all previous components on that certain network.

If you want to deploy it to Polygon Mumbai Testnet/ Mainnet, make sure you're using @truffle/hdwallet-provider. Lets get that installed.

npm i @truffle/hdwallet-provider

Also create a .secret file, which will hold seed phrase of your wallet. Go to your favourite wallet and export seed phrase, put it in .secret. It's strongly recommened to gitignore .secret, so that by mistake you don't push it to public git repo.

echo 'my-exported-seed-phrase' > .secret

Finally our truffle configuration.

const HDWalletProvider = require('@truffle/hdwallet-provider');
const fs = require('fs');

const mnemonic = fs.readFileSync(".secret").toString().trim();

module.exports = {
networks: {
development: {
host: "", // Localhost (default: none)
port: 8545, // Standard Ethereum port (default: none)
network_id: "*", // Any network (default: none)
mumbai: {
provider: () => new HDWalletProvider(mnemonic, `<Sign up for a dedicated free Mumbai testnet RPC URL at or other hosted node providers>`),
network_id: 80001,
maticMainNet: {
provider: () => new HDWalletProvider(mnemonic, `<Sign up for a dedicated free Polygon mainnet RPC URL at or other hosted node providers>`),
network_id: 137,

mocha: {
// timeout: 100000

compilers: {
solc: {
version: "0.7.0",
settings: {
optimizer: {
enabled: false,
runs: 200
evmVersion: "istanbul"

Lets deploy our contracts on local blockchain.

npx truffle migrate

For deploying on Polygon Mumbai Testnet, consider using following command.

npx truffle migrate --network mumbai

Running Project

Sending Meta Transaction

For talking to GSN aware smart contract, we need to use @opengsn/gsn SDK, which will manage lots of lower level complexities for us. And as we've already installed it, lets create a JS file in root of this project directory i.e.meta-tx-gsn.

cd ~/meta-tx-gsn
touch index.js

Now lets copy paste following code snippet into index.js.

const { RelayProvider } = require('@opengsn/gsn')

const configuration = {
relayHubAddress: <relay-hub-for-this-network>,
stakeManagerAddress: <stake-manager-for-this-network>

const provider = new RelayProvider(web3.currentProvider, configuration);
const web3 = new Web3(provider);

As we've connected our web3 instance to RelayProvider, all transactions to be automatically routed via GSN.

// abi => StringOwner contracts ABI array
// address => deployed at, in local ganache blockchain
const myRecipient = new web3.eth.Contract(abi, address);

// Interacting with GSN aware contract
const interact = async () => {
// keep paymaster address that we deployed
// along with trusted forwarder which was specifically deployed
// for this dapp
await myRecipient.methods.update("GSN").send({ from, paymaster, forwarder });

// we can also ask this provider to not
// route our transaction via GSN network, rather
// send a vanilla transaction
await myRecipient.methods.update("non-GSN").send({ from, useGSN: false });


For more details, please check here. If you feel, we can improve this doc, please raise a PR or create an issue.