geph4-client

Geph (迷霧通) is a modular Internet censorship circumvention system designed specifically to deal with national filtering.

geph4-client is the command-line Geph client.

To install geph4-client, you need to first install Rust, then run

cargo install geph4-client

in a terminal. To see a list of the subcommands and flags available, simply run

geph4-client -h

Below is how each subcommand works.

1. connect

Given user credentials and other optional inputs, connect establishes a network connection to a Geph exit server. If that exit server is blocked from the client, connect proxies the connection through dynamical bridge servers that are not blocked in the region.

A typical connect command might look like:

geph4-client connect --exit-server 2.mtl.ca.ngexits.geph.io auth-password --username public5 --password public5

Internally, connect

1. makes a ClientTunnel that manages a sosistab2 Multiplex session to the specified remote Geph server, and
2. enables socks5 and http proxies through this ClientTunnel, as well as routing VPN packets.

The ClientTunnel

A tunnel starts and keeps alive the best sosistab Multiplex session it can given the specified connect parameters.

A sosistab2 Multiplex is a single end-to-end connection between a client and a server. This can be thought of as analogous to TcpStream, except all reads and writes are datagram-based and unreliable. For more on Multiplex, see sosistab2.

The Multiplex session consists of several routes to the exit server, both through different bridges and without bridges. (If the user is in China, then we only provide routes that use bridges, because all the exit servers are blocked by the Great Firewall.) The sositab protocol then monitors the routes and switches seamlessly to the best working route. Finally, The ClientTunnel actively updates the set of bridges used by its Multiplex to switch out servers that get blocked.

Finally, ClientTunnel exposes channels to the Multiplex for handling proxy requests and packet forwarding for VPN mode.

Proxies

connect sets up two proxy servers on localhost. By default, the socks5 server listens on 127.0.0.1:9909, and http listens on 127.0.0.1:9910. These ports can be changed with the --socks5-listen and --http-listen flags. These localhost servers accept proxy connections and fulfills requests by forwarding them to the ClientTunnel, after which they are proxied through the exit server.

When the socks5 server accepts a connection, it establishes a sosistab2 reliable stream along with a task to forward all traffic from the socks5 connection to the sosistab stream.

The http server is the socks5 server converted using an adaptation of the socks2http repo.

VPN

VPN mode takes packets from the source specified by --vpn-mode and sends them over a UDP-like unreliable connection on the ClientTunnel.

Starting geph4-client in VPN mode on Linux might look like:

sudo $(which geph4-client) connect --vpn-mode tun-route --exit-server 2.mtl.ca.ngexits.geph.io auth-password --username public5 --password public5

Note that VPN mode requires us to run geph4-client with root privileges. We use $(which geph4-client) because geph4-client might not be in root's path.

• On Linux, use --vpn-mode tun-route. This starts geph4-client in VPN mode, starts a TUN device, and route all packets to it using iptables.
• On Windows, use --vpn-mode windivert. This routes packets to it using Windivert.
• VPN mode is currently not support for MacOS. Contributions are welcome!

2. sync

sync takes in a user's credentials and obtains the latest information about the user's subscription status, as well as what exits there are.

To bypass censorship, we connect to the binder using domain fronting. To mitigate attacks in the case that an attacker compromises the central Geph binder, we verify the exit list given by the binder against a public record on the Mel blockchain. You can read more about Geph's use of the blockchain here.

sync is designed to be used by the GUI interface around geph4-client.

3. binder_proxy

binder_proxy creates a BinderClient that is a JSON-RPC client to the Geph binder. This is used by gephgui for things like obtaining exit statistics and user registration and deletion.

4. debugpack

geph4-client debugpack --export-to /your/preferred/path/

exports an SQLite database containing Geph's debug logs to /your/preferred/path/.

5. iOS support

geph4-client also supports compiling as a universal C library for calling on iOS (this is because you cannot start a new process on iOS; on other platforms we start geph4-client in a new process). One difference to note is that this version of geph4-client completely avoids using stdin/out for any communication, as doing so would crash the app on iOS. Most of the logic surrounding iOS support is in ios.rs.