Template:Persistent Tor Entry Guards Introduction
What are Tor Entry Guards? If this is an unfamiliar term, please press on Expand on the right.
- Before v2.9: Tor selects three random guard nodes and rotates them every 2-3 months.
- v2.9: Tor selects a solitary guard node and rotates it every 9-10 months. 
- v3.0+: Tor selects three guard nodes, but defers to a primary guard wherever possible. Guards have a primary lifetime of 120 days. 
|In some situations it is safer to not use the usual guard relay!|
The guard relays picked by the Tor client can lead to fingerprinting of Tor use across different physical locations and access points. In some corner cases like the example described below, this may cause a user to be deanonymized. The risk of this attack is less severe now that upstream (The Tor Project) has changed its guard parameters to decrease the de-anonymization risk.
Consider the following scenario. A user runs Tor from their home address, but soon attends a prominent event or protest in a nearby city. At that location, the user decides to anonymously blog about what transpired. The fact that the Tor client is using the same entry guard normally correlated with the user's home address is problematic. Network adversaries have a high certainty that the "anonymous" posts from the city location are related to the same person who connected to that specific guard relay from their home. The relative uncommonness of Tor usage exacerbates the problem of potential deanonymization.
This adversary technique is similar to tracking users via MAC addresses. Therefore, for users facing this threat in their personal circumstances, MAC address randomization is also recommended.
- Even though the attacker can't discover the user's destinations in the network, they still might target a list of known Tor users.
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- If the guard node ever becomes unusable, Tor picks a new guard rather than replacing it, and adds it to the end of the list.
- Non-primary guards are also selected under various circumstances.