klb

klb means "Kernel Load Balancer". It is a (user-space) daemon which acts as a bridge between Linux IPVS and Zeroconf. IPVS services are populated with destinations discovered using Zeroconf / DNS-SD. This allows implementing high-performance load balancers with a dynamic set of servers.

Building klb

You need:

• a C++20 compliant compiler (e.g. GCC or clang)
• libfmt
• systemd
• avahi
• Meson 0.56 and Ninja
• a Linux kernel with CONFIG_IP_VS=y

First make sure that the libcommon submodule is available:

git submodule update --init

Run meson:

meson . output

Compile and install:

ninja -C output
ninja -C output install

Configuring klb

The file /etc/cm4all/klb/klb.conf contains the configuration. Example:

service {
  bind "1.2.3.4:80"
  zeroconf_service "foo"
  zeroconf_interface "service"
}

This load-balances all incoming TCP connections on 1.2.3.4:80 to all workers which announce a service called _foo._tcp via Zeroconf.

There can be any number of service sections, each of which can contain the following options:

• bind: an IP address with port where TCP connections arrive (IPv4 only currently)
• scheduler: the name of the IPVS scheduler; defaults to rr (round-robin)
• zeroconf_service: the name of the Zeroconf service where connections will be routed to
• zeroconf_domain: an optional Zeroconf domain
• zeroconf_interface: an optional network interface name where Zeroconf services will be discovered

To publish Zeroconf services to be discovered by this software, you can either use static service files in /etc/avahi/services/*.service, the command-line tool /usr/bin/avahi-publish-service (launched as daemon along with the daemon which actually listens on the port) or a daemon which has a built-in Zeroconf publisher (e.g. beng-proxy).

On workers (where connections will be routed to), routing needs to be set up so that reply IP packets are routed through the machine running this software. If you run it on a dedicated network interface (and the default gateway is on a different interface), you can configure a routing policy by adding something like that to the systemd.network file of that dedicated interface on each worker:

# Assuming the dedicated interface has network 192.168.99.0/24 and
# 192.168.99.1 is the server running "klb"; this creates the routing
# table "99", and a route to the interface's network:
[Route]
Destination=192.168.99.0/24
Table=99

# Now configure the "klb" server as gateway on that routing table:
[Route]
Gateway=192.168.99.1
GatewayOnLink=yes
Scope=global
Table=99

# This policy tells the server to use our newly created routing
# table for all reply packets to connections which were routed
# through "klb", instead of using the default gateway in the
# standard routing table:
[RoutingPolicyRule]
From=192.168.99.0/24
Table=99

Building the Debian package

After installing the build dependencies, run:

dpkg-buildpackage -rfakeroot -b -uc -us