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I recently become interested in researching about the Tor network and as I read, there are 9 directory authorities which are hard-coded into the Tor software and they are like phone-book which gives the client a random list of tor relays and servers and a list of them is also available at : https://atlas.torproject.org/#search/flag:authority

In other document I read about if all of this Directory Servers become compromised then relays work as a cache to give the clients a list of Tor servers so it is probably not possible to shutdown Tor network completely without Directory Authorities (another article said that clients also have a small cache of Tor Servers addresses).

So my question is what could they do to circumvent filtering of above addresses in countries which completely filter Tor Network traffic ?

In other words what do obfuscation methods (like obfs4) do to get a list of Tor relays and servers when the above hard-coded addresses is censored and the client never talks to Directory Authorities before ?

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In other document I read about if all of this Directory Servers become compromised then relays work as a cache to give the clients a list of Tor servers so it is probably not possible to shutdown Tor network completely without Directory Authorities (another article said that clients also have a small cache of Tor Servers addresses).

There are a few Directory mirrors hard-coded, ticket #15775/source. They are there to spread load on the authorities though. Yes, client use caches of sorts. More precisely, they have directory guards ticket #6526/spec.

So my question is what could they do to circumvent filtering of above addresses in countries which completely filter Tor Network traffic?

Bridges is probably still the best option. I guess it can be a bit hard to obtain bridge descriptors if Tor is blocked but I'm not aware of a better solution.

In other words what do obfuscation methods (like obfs4) do to get a list of Tor relays and servers when the above hard-coded addresses is censored and the client never talks to Directory Authorities before?

The directory information is fetched via bridge. The source of it doesn't matter as long as the signature is valid.

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