# OnChain Transaction Debugging - Lesson 2: Warm up By [SunWeb3Sec](https://paragraph.com/@sunweb3sec) · 2023-02-01 --- Author: [Sun](https://twitter.com/1nf0s3cpt) Translation: Helen Community [Discord](https://discord.gg/Fjyngakf3h) This article is published on XREX and [WTF Academy](https://github.com/AmazingAng/WTF-Solidity#%E9%93%BE%E4%B8%8A%E5%A8%81%E8%83%81%E5%88%86%E6%9E%90) On-chain data can include simple one-time transfers, interactions with one DeFi contract or multiple DeFi contracts, flash loan arbitrage, governance proposals, cross-chain transactions, and more. In this section, let’s begin with a simple start. I will introduce on BlockChain Explorer - Etherscan what we are interested in, and then use [Phalcon](https://phalcon.blocksec.com/) to compare the differences between these transaction function calls: Assets transfer, swap on UniSWAP, increase liquidity on Curve 3pool, Compound proposals, Uniswap Flashswap. Start to warm up ---------------- * The first step is to install [Foundry](https://github.com/foundry-rs/foundry) in the environment. Please follow the installation [instructions](https://book.getfoundry.sh/getting-started/installation). * Forge is a Major test tool on the Foundry platform.If it is your first time to use Foundry, you can refer to [Foundry book](https://book.getfoundry.sh/), [Foundry @EthCC](https://www.youtube.com/watch?v=wJnywGB33O4), [WTF Solidity - Foundry](https://github.com/AmazingAng/WTF-Solidity/blob/main/Topics/Tools/TOOL07_Foundry/readme.md). * Each chain has its own blockchain explorer. In this section, we will use Ethereum's blockchain network as a case study. * Typical information I usually refer to includes: * Transaction Action: Since the transfer of complex ERC-20 tokens can be difficult to discern, Transaction Action can provide the key behavior of the transfer. However, not all transactions include this information. * From: msg.sender, the source wallet address that executes this transaction. * Interacted With (To): Which contract to interact with * ERC-20 Token Transfer: Token Transfer Process * Input Data: The raw input data of the transaction. You can see what Function was called and what Value was brought in. * If you don't know what tools are commonly used, you can view the transaction analysis tools in [the first lesson](https://github.com/SunWeb3Sec/DeFiHackLabs/tree/main/academy/onchain_debug/01_tools/en). Assets transfer --------------- ![](https://storage.googleapis.com/papyrus_images/d1a861a9ed53ec2da1d6a7165c953dc87cdf613a9f954c02c99eab3730091493.png) * From: This transaction's source EOA wallet address * Interacted With (To): Tether USD (USDT) Contract * ERC-20 Tokens Transferred: Transfer 651.13 USDT from user A's wallet to user B * Input Data: Called transfer function According to [Phalcon](https://phalcon.blocksec.com/tx/eth/0x836ef3d01a52c4b9304c3d683f6ff2b296c7331b6fee86e3b116732ce1d5d124) "Invocation Flow" : * There is only one ''Call USDT.transfer''. However, you should pay attention to the "Value".Because the Ethereum Virtual Machine (EVM) does not support floating-point operations, decimals representation is used instead. * Each token has its own precision, the number of decimal places used to represent the value of the token. In ERC-20 tokens, the decimals are usually 18 digits, while USDT has 6 digits. If the precision of the token is not handled properly, problems will arise. * You can query it on the Etherscan [token contract](https://etherscan.io/token/0xdac17f958d2ee523a2206206994597c13d831ec7). ![](https://storage.googleapis.com/papyrus_images/bd10f9f1b0ade7c28d9e1d1f1414cb15e4cfb134e6954c33f7d7c3d5a9a6875a.png) ![](https://storage.googleapis.com/papyrus_images/09d1082c28ff4d3006f2529850a6067436d79a6db2a81d4c8b3e0ee5b61932b0.png) Uniswap Swap ------------ ![](https://storage.googleapis.com/papyrus_images/93d51cd461d148f37ecfd2792a0cec45fb1988a075bc149bf567fca75e7d55a2.png) The following can be derived from the [Etherscan](https://etherscan.io/tx/0x1cd5ceda7e2b2d8c66f8c5657f27ef6f35f9e557c8d1532aa88665a37130da84) example above: * Transaction Action: A user performs Swap on Uniswap V2, exchanging 12,716 USDT for 7,118 UNDEAD. * From: This transaction's source wallet address * Interacted With (To): A MEV Bot contract called Uniswap contract for Swap. * ERC-20 Tokens Transferred: Token exchange process According to [Phalcon](https://phalcon.blocksec.com/tx/eth/0x1cd5ceda7e2b2d8c66f8c5657f27ef6f35f9e557c8d1532aa88665a37130da84) "Invocation Flow" : * MEV Bot calls the Uniswap V2 USDT/UNDEAD trading pair contract to call the swap function to perform token exchange. ![](https://storage.googleapis.com/papyrus_images/8586db302a0dcf463d6ea2e9544521be0fd69e4c1c4e28b5ee7a2a9e5332b4e9.png) ### Foundry We use Foundry to simulate the operation of using 1BTC to exchange for DAI in Uniswap. * [Sample code reference](https://github.com/SunWeb3Sec/DeFiLabs/blob/main/src/test/Uniswapv2.sol), execute the following command: forge test --contracts ./src/test/Uniswapv2.sol -vvvv * According to the figure - we swap 1 BTC to 16,788 DAI by calling the Uniswap\_v2\_router.[swapExactTokensForTokens](https://docs.uniswap.org/contracts/v2/reference/smart-contracts/router-02#swapexacttokensfortokens) function. ![](https://storage.googleapis.com/papyrus_images/7e7dcf7a049fdffd3512e291b66a2aea0f38c56c5746d8a2aca4e9686cf81403.png) Curve 3pool - DAI/USDC/USDT --------------------------- ![](https://storage.googleapis.com/papyrus_images/fdce3c60a5a7255a5090c8a17f610df55fcec82df2afa7fa86d05b7a2a69308f.png) The following can be derived from the [Etherscan](https://etherscan.io/tx/0x667cb82d993657f2779507a0262c9ed9098f5a387e8ec754b99f6e1d61d92d0b) example above: * The purpose of this transaction is to add Liquidity at Curve three pools. * From: This transaction's source wallet address * Interacted With (To): [Curve.fi](http://Curve.fi): DAI/USDC/USDT Pool * ERC-20 Tokens Transferred: User A transferred 3,524,968.44 USDT to the Curve 3 pools, and then Curve minted 3,447,897.54 3Crv tokens for User A. According to [Phalcon](https://phalcon.blocksec.com/tx/eth/0x667cb82d993657f2779507a0262c9ed9098f5a387e8ec754b99f6e1d61d92d0b) "Invocation Flow" : * Based on the call sequence, three steps were executed: 1.add\_liquidity 2.transferFrom 3.mint. ![](https://storage.googleapis.com/papyrus_images/d0b9521e637071bbc7da52d0797da81d2964e324f87017fffc4078994ef113a4.png) Compound propose ---------------- ![](https://storage.googleapis.com/papyrus_images/b3bfe0409db7ef9b7c96b5a90f264e42578df0c920000c78bd11f1ce3d8e6b05.png) The following can be derived from the [Etherscan](https://etherscan.io/tx/0xba69b455c511c500e0be9453cf70319bc61e29eb4235a6e5ca5fe6ddf1934159) example above: * The user submitted a proposal on the Compound. The contents of the proposal can be viewed by clicking "Decode Input Data" on Etherscan. ![](https://storage.googleapis.com/papyrus_images/ccf8dfa0bc32aa9688036a5df489f662cd358b0a1b093ada0fefd4d54b88ac7f.png) According to [Phalcon](https://phalcon.blocksec.com/tx/eth/0xba69b455c511c500e0be9453cf70319bc61e29eb4235a6e5ca5fe6ddf1934159) "Invocation Flow" : * Submitting a proposal through the propose function results in proposal number 44. ![](https://storage.googleapis.com/papyrus_images/51be7a586a94fff07b827161cf8a8103bbe83daad2e1001dd8285a88eec59166.png) Uniswap Flashswap ----------------- Here we use Foundry to simulate operations - how to use flash loans on Uniswap. [Official Flash swap introduction](https://docs.uniswap.org/contracts/v2/guides/smart-contract-integration/using-flash-swaps) * [Sample Code](https://github.com/SunWeb3Sec/DeFiLabs/blob/main/src/test/Uniswapv2_flashswap.sol) Reference, execute the following command: forge test --contracts ./src/test/Uniswapv2_flashswap.sol -vv ![](https://storage.googleapis.com/papyrus_images/9557c1f3251a086ccceb0dbb865b03146546bdbe2d6692144fe3d01debd1dd1b.png) * In this example, a flash loan of 100 WETH is borrowed through the Uniswap UNI/WETH exchange. Note that a 0.3% fee must be paid on repayments. * According to the figure - call flow, flashswap calls swap, and then repays by calling back uniswapV2Call. ![](https://storage.googleapis.com/papyrus_images/5b01c6c5eb85a08330e3b17069c9e8b22fbc3b123bbc62a41e004e058f384bd8.png) * Further Introduction to Flashloan and Flashswap: * A. Common points: Both can lend Tokens without collateralizing assets, and they need to be returned in the same block, otherwise the transaction fails. * B. The difference: If token0 is borrowed through Flashloan token0/token1, token0 must be returned. Flashswap lends token0, and you can return token0 or token1, which is more flexible. For more DeFi basic operations, please refer to [DeFiLab](https://github.com/SunWeb3Sec/DeFiLabs). Foundry cheatcodes ------------------ Foundry's cheatcodes are essential for conducting chain analysis. Here, I will introduce some commonly used functions. More information can be found in the [Cheatcodes Reference](https://book.getfoundry.sh/cheatcodes/). * createSelectFork: Specifies a network and block height to copy for testing. Must include the RPC for each chain in [foundry.toml](https://github.com/SunWeb3Sec/DeFiHackLabs/blob/main/foundry.toml). * deal: Sets the balance of a test wallet. * Set ETH balance: `deal(address(this), 3 ether);` * Set Token balance: `deal(address(USDC), address(this), 1 * 1e18);` * prank: Specify the wallet address to simulate. It is only effective for the next call and will set the msg.sender to the specified wallet address. Such as simulating a transfer from a whale wallet. * startPrank: Specify the wallet address to simulate. It will set the msg.sender to the specified wallet address for all calls until `stopPrank()` is executed. * label: Labels a wallet address for improved readability when using Foundry debug. * roll: Adjusts the block height. * warp: Adjusts the block timestamp. Thanks for following along! Time to jump into the next lesson. Resources --------- [https://book.getfoundry.sh/](https://book.getfoundry.sh/) [https://github.com/crisgarner/awesome-foundry](https://github.com/crisgarner/awesome-foundry) [https://blog.infura.io/post/build-a-flash-loan-arbitrage-bot-on-infura-part-i](https://blog.infura.io/post/build-a-flash-loan-arbitrage-bot-on-infura-part-i) --- *Originally published on [SunWeb3Sec](https://paragraph.com/@sunweb3sec/onchain-transaction-debugging-lesson-2-warm-up)*