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**Hi, I am doing a sample project on Ethereum where 2 fields ( Emp name, Emp Id) will be taken from the UI and set those values using a smart contract and show the values back to UI. Getting below 2 errors.Please help me out to solve this.

  1. Failed to load http://localhost:8545/: Response to preflight request doesn't pass access control check: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'null' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute. t.send @ web3.min.js:1

  2. Uncaught Error: CONNECTION ERROR: Couldn't connect to node http://localhost:8545. at Object.InvalidConnection (web3.min.js:1) at t.send (web3.min.js:1) at n.send (web3.min.js:1) at l.accounts (web3.min.js:1) at index.html:41 InvalidConnection @ web3.min.js:1 t.send @ web3.min.js:1 n.send @ web3.min.js:1 (anonymous) @ web3.min.js:1 (anonymous) @ index.html:41

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You need to allow remote connections to ethereum node. I am assuming you are trying to connect to remote ethereum node. To allow ethereum node a connection requests from other devices on which geth is not running you'll need to use --rpcaddr and --rpccorsdomain flags to allow external RPC connections.

Use the following command for initiating geth

geth --datadir "path to data directory" --networkid 9876 --rpcport "8545" --rpcaddr "0.0.0.0" --rpccorsdomain "*"
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  • path to data directory means? Commented Sep 24, 2018 at 3:23
  • --datadir to specify the directory for private ethereum blockchain. This is good practice when you are interacting with different chains. Commented Sep 24, 2018 at 4:54
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Make sure the Ethereum node is running: Launch Ganache, Hardhat, or Geth on port 8545. Use the script below to verify its reachability, which will give a JavaScript Object Notation (JSON) response.

curl http://127.0.0.1:8545

Step 2: Activate CORS on the node. If using the Ganache Command Line Interface, type:

 ganache-cli --host 127.0.0.1 --port 8545 --cors="*"

Else, leverage going to "Settings → Server → CORS domain" and set it to *.

Step 3: Using a local HTTP server, serve your HTML, as browsers usually block file:// requests due to CORS.

Then, launch a sample server using the script below before opening your app at http://localhost:8000/index.html:

python -m http.server 8000

Step 4: Do the initialization of Web3 with correctness, while using "127.0.0.1 instead of localhost to prevent potential DNS worries":

if (typeof window.ethereum !== 'undefined') { web3 = new Web3(window.ethereum); await window.ethereum.enable();

} else { web3 = new Web3(new Web3.providers.HttpProvider("http://127.0.0.1:8545")); }

Step 4: Avoid the unneeded credentials mode

Ensure withCredentials is disabled except otherwise, as this can spawn stricter CORS checks.

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