Alert system

Alerts are broadcast using the same TCP relay system as block and tx messages. They are not encoded in a special transaction. Unlike block and tx relaying, alerts are sent at the start of every new connection for as long as the alert is in effect. This ensures that everyone receives the alert.

Alerts contain this information:

• How long to relay the alert.
• How long to consider the alert valid.
• An alert ID number.
• A list of alerts that should be canceled upon receipt of this alert.
• Exactly which versions of Bitcoin are affected by the alert. Unaffected versions still relay the alert for the benefit of older versions.
• Alert priority.
• The alert text.

Only alerts that are signed by a specific ECDSA public key are considered valid. Some known private key holders were Satoshi Nakamoto, Gavin Andresen and theymos, but the private key was made public after the alert system was retired, so there is no longer any special group of alert-key-holders.

Until version 0.3.20, Bitcoin went into safe mode when a valid alert was received. In safe mode, all RPC commands that send BTC or get info about received BTC return an error. Current Bitcoin versions no longer go into safe mode in response to alerts, though Bitcoin will still go into safe mode when it detects on its own that something is seriously wrong with the network.

Even though Bitcoin no longer automatically disables RPC when an alert is live, it is wise for Bitcoin sites to shut down when an alert has been issued. To detect an active alert, poll the "errors" field of getinfo.

To test safe mode, run Bitcoin with the -testsafemode switch. To override a real safe mode event, run Bitcoin with the -disablesafemode switch.

History

An alert appearing on bitcoin clients

The alert system was hastily implemented by Satoshi Nakamoto after the value overflow incident on August 15, 2010. Satoshi never actually used this system; it remained dormant until the February 20, 2012 protocol change, for which an alert was issued on February 18.[2]

In 2016, the alert system was retired[1] because of the possibility of privileged users sending political alert messages, and because of the possibility of the alert key having been taken from Mark Karpelès by the Japanese police in 2014.[3]

The alert key was scheduled to be released to the public in May 2017, but this was postponed.[1] It was eventually published in July 2018.