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cjdns

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Networking Reinvented

Cjdns implements an encrypted IPv6 network using public-key cryptography for address allocation and a distributed hash table for routing. This provides near-zero-configuration networking, and prevents many of the security and scalability issues that plague existing networks.

Startup

The key part of cjdns is the cjdroute background daemon. To start cjdroute:

systemctl start cjdns

This will generate /etc/cjdroute.conf pre-populated with random keys and passwords. At first startup, cjdroute looks for neighboring cjdns peers on all active network interfaces using a layer 2 (e.g. ethernet) protocol. This is exactly what you want if you are on a wifi mesh. If you only have a conventional "clearnet" ISP, see the upstream README for instructions on adding peers using the UDP protocol. (Search for "Find a friend".)

After adding peers to /etc/cjdroute.conf, restart cjdroute with:

systemctl restart cjdns

To have cjdroute start whenever you boot, use

systemctl enable cjdns

If you are on a laptop and suspend or hibernate it, cjdroute will take a few minutes to make coffee and figure out what just happened when it wakes up. You can speed this up dramatically with:

systemctl enable cjdns-resume

The resume service restarts cjdns when the system wakes up from sleep.

For rhel6, use start cjdns instead of systemctl - ditto for restart and stop.

Security

By default, Fedora Workstation will treat the tun device created by cjdroute as "public", with SSH being the only incoming port allowed. There is no additional exposure with cjdns and the default Fedora firewall. If you have modified the firewall config beyond opening additional incoming ports, be sure that the cjdns tun is treated as public - because anyone in the world can attempt to connect to you through it. Sometimes, people configure their firewall to treat all tun devices as "VPN", and therefore somewhat more trusted. This would be a mistake with cjdns. It is a VPN, for sure, but one anyone in the world can join.

Public keys for cjdns are based on Elliptic Curves. There is a known quantum algorithm that could be used to crack them if quantum computers with sufficient qubits are ever built. The solution when that happens is larger keys - which are more cumbersome.

The Distributed Hash Table algorithm is a core component of cjdns - which is vulnerable to a Denial of Service attack known as "Sybil". This attack can block specific updates to the DHT - to prevent your node from joining a mesh, for instance.

On the positive side, you can safely use telnet to cjdns IPs and the http protocol is automatically encrypted (but you need a secure DNS or raw ip to be sure you are talking to the right node). Many other protocols are automatically encrypted while using cjdns. In general, connecting to a raw cjdns IP is functionally equivalent to SSL/TLS with both client and server authentication.

Since the cjdroute core routing code parses network packets from untrusted sources, it is a security risk and is heavily sandboxed. It runs as the cjdns user in a chroot jail in an empty directory, with RLIMIT_NPROC set to 1 to disable forking. Seccomp is used to limit available system calls to only those actually needed. Installing the cjdns-selinux package installs a targeted selinux policy that also restricts what the privileged process can access.

Routing security

If cjdns is not running, cjdns packets will get routed in plaintext to your default gateway by default. An attacker could then play man-in-the-middle. If your default gateway is running cjdns, this could even happen accidentally.

This can be blocked by restricting fc00::/8 to the interface used by cjdroute in the firewall.

Advanced config

You may install a network service that depends on cjdns, for instance you might install thttpd to serve up nodeinfo.json. If thttpd is configured to listen only on your cjdns IP, then it will not start until cjdns is up and running. Add After=cjdns-wait-online.service to thttpd.service to hold off starting the service until cjdns has the tunnel up and ready.