Direct access to the Tor network may sometimes be blocked by your Internet Service Provider or by a government.
Tor Browser includes some circumvention tools for getting around these blocks. These tools are called “pluggable transports”.
TYPES OF PLUGGABLE TRANSPORT
Currently there are three pluggable transports available, but more are being developed.
obfs4 makes Tor traffic look random, and also prevents censors from finding bridges by Internet scanning. obfs4 bridges are less likely to be blocked than its predecessors, obfs3 bridges.
meek transports make it look like you are browsing a major web site instead of using Tor. meek-azure makes it look like you are using a Microsoft web site.
Snowflake is an improvement upon Flashproxy. It sends your traffic through WebRTC, a peer-to-peer protocol with built-in NAT punching.
USING PLUGGABLE TRANSPORTS
To use a pluggable transport, click "Tor Network Settings" when starting Tor Browser for the first time.
Under the "Bridges" section, select the checkbox "Use a bridge" and choose the "Select a built-in bridge" option.
From the dropdown, select whichever pluggable transport you'd like to use.
Once you've selected the pluggable transport, scroll up and click "Connect" to save your settings.
Or, if you have Tor Browser running, click on "Preferences" (or "Options" on Windows) in the hamburger menu (≡) and then on "Tor" in the sidebar.
In the "Bridges" section, select the checkbox "Use a bridge", and from the option "Select a built-in bridge", choose whichever pluggable transport you'd like to use from the dropdown.
Your settings will automatically be saved once you close the tab.
WHICH TRANSPORT SHOULD I USE?
Each of the transports listed in Tor Bridge's menu works in a different way, and their effectiveness depends on your individual circumstances.
If you are trying to circumvent a blocked connection for the first time, you should try the different transports: obfs4, snowflake, or meek-azure.
If you try all of these options, and none of them gets you online, you will need to request a bridge or manually enter bridge addresses.
Read the Bridges section to learn what bridges are and how to obtain them.