"sixxsd" is the SixXS Daemon, it is the software that used to run on the SixXS PoPs and that handled the server-side of proto-41, heartbeat and AYIYA tunnels.
sixxsd was designed and implemented by Jeroen Massar.
SixXS was sunset on 2017-06-06 after 17 years of operation as a free IPv6 Tunnel Broker service for users worldwide.
sixxsd ran on the PoPs from 2004 till 2017, serving an active daily 50.000 tunnels spread over 50 PoPs (with some PoPs being small <100 tunnels, others having >3000 tunnels per host). Before sixxsd existed several bash scripts would reconfigure the kernel's gif interfaces.
In effect sixxsd is SixXS's own routing platform as the complete process of en/decapsulation of tunneled packets and passing it to the proper location is handled by it.
sixxsd also takes care of the latency tests and traffic statistic collection. Various statistics can be seen, when logged in, in real-time from the user home under tunnel details, e.g. the current location of an endpoint of a tunnel.
sixxsd exists as the Linux kernel has/was not been designed to handle thousands of network interfaces: every packet was walking over a linked list. Adding/removing/reconfiguring interfaces was also prone to disconnects between what the system thought happened and what the actual configuration was. Next to that a lot of tuning due to routing table size was needed and various other issues we ran into over time.
The model that sixxsd uses is that of a single tun/tap interface exposed by the kernel where one or more /40's are routed into. This releases the kernel from any of the management of this address space and all the interfaces that are located there. sixxsd effectively runs 'statically', nothing changes (no memory allocations etc) after it has been started. Elements for configuration have all been pre-allocated, thus avoiding out of memory issues or memory fragmentation issues. As we know the address-space layout, sixxsd is optimized for that, which avoids the need for table lookups or linked lists for routing packets.
This model also means that sixxsd sees all the packets and thus is able to provide accurate counters for performance monitoring.
A short summary of features of sixxsd:
- Handles the full IPv6 Routing process
- En/Decapsulation of protocol-41 (6in4/RFC3056) and AYIYA (IPv4 in IPv6-UDP and IPv6 in IPv4-UDP)
- Very high performance (during tests easily forwarded 4 Gbit/s of mixed AYIYA/proto-41 traffic)
- IPv6 Tunnel Heartbeat support
- Latency testing of active endpoints
- Per-tunnel remote debugging option showing per-packet decisions being made
- Per-tunnel statistics and error information
- Per-tunnel default routed /64 towards tunnel endpoint
The sixxsd binary starts by reading a sixxsd.conf this instructs it which prefixes it handles and which it routes to the tunnel device. It uses a standard tun/tap device as provided by most Unix-alike kernels.
The SixXS backend, which can get updated by users using the webinterface, re-pushes the full configuration to the PoP. This directly updates the state for all the interfaces and routes.
The pop saveconfig command atomically saves the sixxsd.conf running configuration to disk.
Thus allowing a restart of the PoP to resume with that state of configuration till a configuration push updates the configuration again.
As SixXS only deployed minimal kernels the PoPs where sixxsd ran where extremely stable and ran for multiple years at a time.
Some details from when we shutdown all the PoPs:
Daemon uptime:
nlams05.sixxs.net: 901 days 01:41:44
usanc01.sixxs.net: 826 days 00:32:10
nlede01.sixxs.net: 826 days 01:02:11
fihel01.sixxs.net: 826 days 00:27:25
usbos01.sixxs.net: 821 days 18:36:37
deham02.sixxs.net: 821 days 18:39:21
deham01.sixxs.net: 821 days 18:39:25
ausyd01.sixxs.net: 821 days 18:40:08
aubne01.sixxs.net: 821 days 18:45:04
deleo01.sixxs.net: 802 days 23:07:44
Server uptime:
deham02.sixxs.net: 1893 days 00:05:17
usbos01.sixxs.net: 1876 days 06:59:03
deham01.sixxs.net: 1610 days 17:44:19
fihel01.sixxs.net: 1581 days 15:17:46
nlede01.sixxs.net: 1260 days 01:49:58
ausyd01.sixxs.net: 1082 days 06:05:18
aubne01.sixxs.net: 1082 days 06:00:58
nlams05.sixxs.net: 1064 days 10:04:58
usanc01.sixxs.net: 879 days 23:33:28
deleo01.sixxs.net: 802 days 23:09:10
That demonstrates an uptime of about 2,5 years of active running indicating how stable it ran. Especially considering when one realises how many packets these daemons where forwarding, while being reconfigured every 10 minutes from the central server and also by heartbeat and AYIYA clients.
sixxsd was primarily run on minimal Debian GNU/Linux systems, but also ran on FreeBSD and OpenBSD based PoPs. In addition, for development, MacOS also functions, but primarily for development, not for actual operation.
The code is provided as-as, primarily for historical purposes as various people have requested insight into what actually drove the SixXS PoPs.
Due to the state of IPv6 deployment, we hope that this code is not needed anymore anywhere: please finally get native IPv6, it has been more than 20 years...
If one wants to create a VPN-alike service, we heavily suggest looking at Wireguard and/or OpenVPN instead as these provide secure (read: cryptography involved) tunnels which disallow snooping along. All protocols implemented by sixxsd are insecure: no cryptography involved.
As one will notice, no TLS or even SSL is included in this code, the SixXS PoPs where reconfigured over SSH tunneled TCP connections.
Any current modern tunneling solution will use proper cryptography, hence, please look at Wireguard.
The license for sixxsd is the BSD 3-clause license.
In case one uses/references this, don't hesitate to give a shout out to the author, it is much appreciated.
The designer and implementor of sixxsd is Jeroen Massar.
Jeroen can be reached by email: [email protected].
The previous email SixXS addresses. ([email protected] and [email protected]) have been deactived when the project sunset.