> For the complete documentation index, see [llms.txt](https://docs.bio.xyz/bio/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.bio.xyz/bio/developers/dao-setup/evm-contracts/airdrop-contracts.md).

# Airdrop Contracts

### Key Features

* **ERC20 Token Distribution**: Supports airdropping any ERC20 token
* **Merkle Proof Verification**: Uses cryptographic proofs to verify claim eligibility
* **Gas Efficient**: Only stores one Merkle root instead of all user data
* **Claim Tracking**: Prevents double-claiming by maintaining a record of claimed addresses
* **Owner Controls**: Allows the owner to update the Merkle root and withdraw unclaimed tokens

{% embed url="<https://github.com/bio-xyz/airdrop-contracts>" %}

### MerkleAirdrop Contract

A contract that implements a Merkle tree–based airdrop for ERC20 tokens with controlled claim processing and robust admin management.

#### Features

* **Merkle Tree Eligibility:**
  * Utilizes a stored Merkle root to verify if an address is eligible for the airdrop.
  * The `inMerkle()` function checks a user's address and claim amount against the Merkle proof.
* **Claiming Process:**
  * Users can claim tokens via the `claim()` function if they are eligible.
  * Prevents double claims by tracking claims in a mapping.
  * Emits a `Claimed` event on successful claims.
* **Admin Controls:**
  * The owner can update the Merkle root using the `updateMerkleRoot()` function.
  * Allows the owner to withdraw tokens from the contract with the `withdraw()` function.

#### Inheritance

* **ReentrancyGuard:** Provides protection against reentrancy attacks on critical functions.
* **Ownable:** Restricts sensitive functions to the contract owner.

#### Key Functions

* **claim(uint256 amount, bytes32\[] calldata proof):** Allows eligible users to claim their tokens if their Merkle proof is valid.
* **inMerkle(address who, uint256 amount, bytes32\[] calldata proof):** Verifies that the address and claim amount are part of the Merkle tree.
* **hasClaimed(address who):** Checks whether an address has already claimed the airdrop.
* **updateMerkleRoot(bytes32 \_root):** Enables the owner to update the Merkle root to adjust eligibility.
* **withdraw(address recipient, uint256 amount):** Allows the owner to withdraw tokens from the contract if needed.

#### Security Features

* **Reentrancy Protection:** Critical functions (like `claim()`) are secured against reentrant calls.
* **Safe Transfers:** Utilizes the SafeERC20 library to ensure secure token transfers.
* **Access Control:** Owner-only functions safeguard administrative tasks such as updating the Merkle root and withdrawing tokens.