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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.


If you're here from the hyperboria docs, you're already sold - proceed to Installing. But why should a Fedora user install cjdns? I'll mention just two contrasting use cases, one mundane and the other paranoid.

VPN Mesh

Configuring a point to point VPN connection with openvpn is fairly straightforward, as is configuring a centralized VPN server and clients. However, when every node in the VPN network needs to talk securely with many other nodes, relaying every packet through the central server becomes a drag on performance, and a single point of failure. Mesh VPNs, like tinc and cjdns automatically create point to point connections based on a shared overall configuration. Each node only needs a connection to one or more peers (that can be reused) to get things started.

With cjdns, however, things are much better than with tinc. On a local LAN or mesh with broadcast, it is zero configuration. Peers are automatically discovered via the 0xFC00 layer 2 protocol. There is no shared configuration - the only thing required is adding one or more (for redundancy) internet peers when no peers on the local LAN or mesh are available. Even better, when your node is mobile, and you have geographically separated peers configured, cjdns automatically switches to a faster peer as the relative performance changes.


In a widespread VPN, address assignment must be coordinated by a central authority. The internet also uses centralized IP assignment, which means a government can take away your IP at any time. Cjdns uses CryptoGraphic Addressing (CGA). Your IP6 is the double SHA-512 of your public key truncated to 128 bits. Your IP is as safe as the private key pair which produced it, and cannot [insert standard cryptography disclaimer] be spoofed. Most mesh VPNs decrypt packets before routing to a new node. This means that if a relay node is compromised in a conventional VPN, it can see and even alter packets. All cjdns packets are end to end encrypted - relay nodes are untrusted. Cjdns is source routed, there is no centralized routing (an option for chosen route servers is slated for future implementation). If a node is "blackholing" your packets for some reason - cjdns simply doesn't route through that node anymore. (But see Security below.) The usual security problems with source routing don't apply because cjdns IPs can't be (easily) spoofed.


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 LAN or 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.


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. The Sybil attack is less effective because Cjdns uses chosen peers. Simply cut off abusive peers.

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. An even simpler solution is to not have a "default" route. Instead route 2000::/3 to your gateway. All globally routable ips begin with 001 as the first three bits.

Application security

The squid cache package default config allows fc00::/7 unrestricted access to the proxy. If the proxy port is not otherwise firewalled, you probably want to change this to fd00::/8 when using cjdns on the proxy server. Apart from that default config, squid works very well with cjdns - you can allow specific cjdns ips unrestricted access:

acl adultpcs src fc25:dede:dede:dede:dede:dede:dede:dede
acl adultpcs src fc37:daaa:daaa:daaa:daaa:daaa:daaa:daaa 
http_access allow adultpcs

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.