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There are some facts regarding the DAO hack and the subsequent hard fork that can be found everywhere, e.g., that the hard fork happened at block height 1,920,000, that it was undertaken in an effort to return approximately 3.6 million ether, and so forth.

But what is the actual number of blocks that were invalidated by this hard fork? And how many transactions approximately?

In other words: what was the block height of the original chain when the hard fork was implemented?

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But what is the actual number of blocks that were invalidated by this hard fork? And how many transactions approximately?

No blocks were invalidated and nothing was rolled back.

Instead, EIP-779: Hardfork Meta: DAO Fork (i.e. the hard fork resulting from the attack on theDAO), performed an irregular state change.

The state change involved removing the ETH balances of the attacker (from the accounts listed in "Reference List L" in the EIP), and transferring them to a new withdrawal contract controlled by the Good Guys. People were then able to claim their ETH back from the contract.

Reference List L contains 116 addresses. That means at least 116 transactions were in effect invalidated. (Possibly more, if a large chain/net of transactions were used to get ETH to these final attacker addresses - I haven't checked.) No transactions unrelated to the attack were effected.

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  • Thank you for your answer. :) Can you elaborate why on the official Ethereum blog, for example at block height 1920001, there are two different transactions shown in the illustration, i.e., 0x87... and 0xab...? Doesn't that mean that is former is replaced by the latter in the new chain? Hence, rolled back? blog.ethereum.org/2016/07/20/hard-fork-completed May 31 at 20:06
  • So in that picture, the main chain on the left with the block hash of 0x4985... is the ETH chain (etherscan.io/block/1920000), and the side fork on the right with the block hash of 0x94365... is the newly formed ETC (Ethereum Classic) chain (blockscout.com/etc/mainnet/blocks/1920000/transactions). (The 0x87.. and 0xab.. are the hashes of the next blocks on those chains, respectively.) May 31 at 20:18
  • So the part I didn't mention in my answer was that some people didn't agree with the hard fork, and the manual changing of state to drain the attacker's balances. Those people decided to start their own new chain, ETC. From block 1,920,000 onward the two chains would coexist. That's what the picture shows :-) May 31 at 20:22

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