PNDPD - NDP Proxy / Responder (IPv6)

Features

Efficiently process incoming packets using bpf (which runs in the kernel)
Proxy NDP between interfaces with an optional whitelist
Optionally determine whitelist automatically based on the IPs assigned to the interfaces
Respond to NDP solicitations for all or only whitelisted addresses on an interface
Permissions required: root or CAP_NET_RAW
Easily expandable with modules

Installing & Updating

Download the latest release from the releases page and move the binary to the /usr/local/bin/ directory under the filename pndpd.
Allow executing the file by running chmod +x /usr/local/bin/pndpd
Install the systemd service unit file:

wget https://raw.githubusercontent.com/Kioubit/pndpd/master/pndpd.service -P /etc/systemd/system/
systemctl enable pndpd.service

Download and install the config file

mkdir -p /etc/pndpd
wget https://raw.githubusercontent.com/Kioubit/pndpd/master/pndpd.conf -P /etc/pndpd/

Edit the config at /etc/pndpd/pndpd.conf and then start the service using service pndpd start

Manual Usage

pndpd proxy <external interface> <internal interface> <[optional] 'auto' to determine filters from the internal interface or whitelist of CIDRs separated by a semicolon>
pndpd responder <external interface> <[optional] 'auto' to determine filters from the external interface or whitelist of CIDRs separated by a semicolon>
pndpd config <path to file>

Example: pndpd proxy eth0 tun0 auto

Find more options and additional documentation in the example config file (pndpd.conf).

Example Scenario

Proxying NDP requests for a /64 IPv6 subnet on a VPS to a VPN tunnel

1) Inspecting the initial IP configuration

root@vultr:~# ip -6 addr show dev enp1s0
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    inet6 2001:11ff:7400:82f2:5400:4ff:fe53:26cf/64 scope global dynamic mngtmpaddr 
       valid_lft 2591753sec preferred_lft 604553sec
    inet6 fe80::5400:4ff:fe53:26cf/64 scope link 
       valid_lft forever preferred_lft forever

As we can see from the output, a /64 subnet of public IPv6 addresses has been assigned to our VPS on our WAN interface enp1s0: 2001:11ff:7400:82f2:5400:4ff:fe53:26cf/64.

2) Routing the subnet to the VPN interface

To route this subnet to our VPN interface tun0 we need to assign one ip address to the VPS and the rest to the VPN interface.
To do that we edit the /etc/network/interface file (for systems that use ifupdown2):

Initial contents:

allow-hotplug enp1s0

iface enp1s0 inet static 
    #.... IPv4 config here ...

iface enp1s0 inet6 static
    address 2001:11ff:7400:82f2:5400:4ff:fe53:26cf/64
    gateway fe80::fc00:4ff:fe53:26cf

After editing:

allow-hotplug enp1s0

iface enp1s0 inet static 
    #.... IPv4 config here ...

iface enp1s0 inet6 static
    address 2001:11ff:7400:82f2::1/128
    gateway fe80::fc00:4ff:fe53:26cf

On the VPN interface we can now assign the rest of the addresses:

ip addr add 2001:11ff:7400:82f2::1/64 dev tun0

3) Running PNDPD

To proxy NDP requests from the outside interface to the VPN interface we run pndp like this:

sudo pndpd proxy enp1s0 tun0 auto

Note: sudo is not required if you are using the capability as described in the systemd unit file. Optionally confirm that the setup works via ping and tcpdump.

Building PNDPD

For building, the version of go needs to be installed that is specified in the go.mod file. A makefile is available. Optionally adjust the MODULES variable to include or exclude modules from the "modules" directory.

make clean; make release

Find the binaries in the bin/ directory