ported cjdns documentation to resolve issue #355

doc/cjdns.md

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+# CJDNS support in Hush
+
+It is possible to run Hush over CJDNS, an encrypted IPv6 network that
+uses public-key cryptography for address allocation and a distributed hash table
+for routing.
+
+## What is CJDNS?
+
+CJDNS is like a distributed, shared VPN with multiple entry points where every
+participant can reach any other participant. All participants use addresses from
+the `fc00::/8` network (reserved IPv6 range). Installation and configuration is
+done outside of Hush, similarly to a VPN (either in the host/OS or on
+the network router). See https://github.com/cjdelisle/cjdns#readme and
+https://github.com/hyperboria/docs#hyperboriadocs for more information.
+
+Compared to IPv4/IPv6, CJDNS provides end-to-end encryption and protects nodes
+from traffic analysis and filtering.
+
+Used with Tor and I2P, CJDNS is a complementary option that can enhance network
+redundancy and robustness for both the Hush network and individual nodes.
+
+Each network has different characteristics. For instance, Tor is widely used but
+somewhat centralized. I2P connections have a source address and I2P is slow.
+CJDNS is fast but does not hide the sender and the recipient from intermediate
+routers.
+
+## Installing CJDNS and finding a peer to connect to the network
+
+To install and set up CJDNS, follow the instructions at
+https://github.com/cjdelisle/cjdns#how-to-install-cjdns.
+
+You need to initiate an outbound connection to a peer on the CJDNS network
+before it will work with your Hush node. This is described in steps
+["2. Find a friend"](https://github.com/cjdelisle/cjdns#2-find-a-friend) and
+["3. Connect your node to your friend's
+node"](https://github.com/cjdelisle/cjdns#3-connect-your-node-to-your-friends-node)
+in the CJDNS documentation.
+
+One quick way to accomplish these two steps is to query for available public
+peers on [Hyperboria](https://github.com/hyperboria) by running the following:
+
+```
+git clone https://github.com/hyperboria/peers hyperboria-peers
+cd hyperboria-peers
+./testAvailable.py
+```
+
+For each peer, the `./testAvailable.py` script prints the filename of the peer's
+credentials followed by the ping result.
+
+Choose one or several peers, copy their credentials from their respective files,
+paste them into the relevant IPv4 or IPv6 "connectTo" JSON object in the
+`cjdroute.conf` file you created in step ["1. Generate a new configuration
+file"](https://github.com/cjdelisle/cjdns#1-generate-a-new-configuration-file),
+and save the file.
+
+## Launching CJDNS
+
+Typically, CJDNS might be launched from its directory with
+`sudo ./cjdroute < cjdroute.conf` and it sheds permissions after setting up the
+[TUN](https://en.wikipedia.org/wiki/TUN/TAP) interface. You may also [launch it as an
+unprivileged user](https://github.com/cjdelisle/cjdns/blob/master/doc/non-root-user.md)
+with some additional setup.
+
+The network connection can be checked by running `./tools/peerStats` from the
+CJDNS directory.
+
+## Run Hush with CJDNS
+
+Once you are connected to the CJDNS network, the following Hush
+configuration option makes CJDNS peers automatically reachable:
+
+```
+-cjdnsreachable
+```
+
+When enabled, this option tells Hush that it is running in an
+environment where a connection to an `fc00::/8` address will be to the CJDNS
+network instead of to an [RFC4193](https://datatracker.ietf.org/doc/html/rfc4193)
+IPv6 local network. This helps Hush perform better address management:
+  - Your node can consider incoming `fc00::/8` connections to be from the CJDNS
+    network rather than from an IPv6 private one.
+  - If one of your node's local addresses is `fc00::/8`, then it can choose to
+    gossip that address to peers.
+
+## Additional configuration options related to CJDNS
+
+```
+-onlynet=cjdns
+```
+
+Make automatic outbound connections only to CJDNS addresses. Inbound and manual
+connections are not affected by this option. It can be specified multiple times
+to allow multiple networks, e.g. onlynet=cjdns, onlynet=i2p, onlynet=onion.
+
+CJDNS support was added to Hush in version 3.9.3 and there may be fewer
+CJDNS peers than Tor or IP ones. You can use `hush-cli -addrinfo` to see the
+number of CJDNS addresses known to your node.
+
+In general, a node can be run with both an onion service and CJDNS (or any/all
+of IPv4/IPv6/onion/I2P/CJDNS), which can provide a potential fallback if one of
+the networks has issues. There are a number of ways to configure this; see
+[doc/tor.md](https://git.hush.is/hush/hush3/src/branch/master/doc/tor.md) for
+details.
+
+## CJDNS-related information in Hush
+
+There are several ways to see your CJDNS address in Hush:
+- in the "Local addresses" output of CLI `-netinfo`
+- in the "localaddresses" output of RPC `getnetworkinfo`
+
+To see which CJDNS peers your node is connected to, use `hush-cli -netinfo 4`
+or the `getpeerinfo` RPC (i.e. `hush-cli getpeerinfo`).
+
+You can use the `getnodeaddresses` RPC to fetch a number of CJDNS peers known to your node; run `hush-cli help getnodeaddresses` for details.