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The Tor FAQ's "What are Entry Guards?" entry does not describe the process by which a relay becomes a guard. The word "flag" is not mentioned anywhere in the FAQ index.

Arma's "The lifecycle of a new relay" post lists three general criteria:

Directory authorities assign the Guard flag to relays based on three characteristics: "bandwidth" (they need to have a large enough consensus weight), "weighted fractional uptime" (they need to be working most of the time), and "time known" (to make attacks more expensive, we don't want to give the Guard flag to relays that haven't been around a while first).

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This has been described in details in the phase three of The lifecycle of a new relay.

Your new relay has to be stable (up and running for at least 8 days), and have a minimum bandwidth of 250KB/s to receive a guard flag and become an entry node.

Here I quote from Tor spec:

A router is a possible 'Guard' if its Weighted Fractional Uptime is at least the median for "familiar" active routers, and if its bandwidth is at least median or at least 250KB/s.

To calculate weighted fractional uptime, compute the fraction of time that the router is up in any given day, weighting so that downtime and uptime in the past counts less. A node is 'familiar' if 1/8 of all active nodes have appeared more recently than it, OR it has been around for a few weeks.

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    I recently put up a fairly-fast relay, got the guard flag in 3 days, then lost it when it crashed due to a misconfiguration and was down for some hours. Has the behavior changed since your answer was written? – ζ-- Nov 11 '14 at 23:47

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