<!-- TITLE: Bad Chain Canary --> There were canary contracts to notify that a given chain was bad. They were very easy to use; check storage location 0 of contract at the given address (see below). If non-zero, client would not mine (at least without a non-default option being given to ignore the canary and mine on a known-bad chain). Specifically there are three modes they could be in: - `0` All fine. Carry on. - `1` Bad chain. Client should not mine on it. Client upgrade not yet available. - `2` Update required. Just as for `1`; additionally, an update to your client is available and would be prudent. Clients that implemented this protocol displayed a message to the user to make any non-zero status clear. For a status of 2, the user would be notified than an immediate upgrade was required, regardless of whether mining was enabled. ## Addresses - For Olympic: `0x6879392ee114f8a4e133f0ff3dc4bc1717fe9344` - For Frontier: Multiple people - For Homestead and later, clients don't support this contract anymore. The last clients removed this in mid-2016.