ndpresponder

README

IPv6 Neighbor Discovery Responder

ndpresponder is a Go program that listens for ICMPv6 neighbor solicitations on a network interface and responds with neighbor advertisements, as described in RFC 4861 - IPv6 Neighbor Discovery Protocol.

This program differs from ndppd - NDP Proxy Daemon in that the source IPv6 address of neighbor advertisement is set to the same value as the target address in the neighbor solicitation. This change enables ndpresponder to work in certain KVM virtual servers where NDP uses link-local addresses but ebtables drops outgoing packets from link-local addresses. See my blog post for more information.

Installation

This program is written in Go. You can compile and install this program with:

env CGO_ENABLED=0 go install github.com/yoursunny/ndpresponder@main

This program is also available as a Docker container:

docker build -t localhost/ndpresponder 'github.com/yoursunny/ndpresponder#main'
docker run -d --name localhost/ndpresponder --network host ndpresponder [arguments]

Static Mode

The program can respond to neighbor solicitations for any address under one or more subnets. It's recommended to keep the subnets as small as possible.

Sample command:

sudo ndpresponder -i eth0 -n 2001:db8:3988:486e:ff2f:add3:31e3:7b00/120

• -i flag specifies the network interface name.
• -n flag specifies the IPv6 subnet to respond to. You may repeat this flag to specify multiple subnets.

See ndpresponder.service for a sample systemd unit file.

Docker Network Mode

The program can respond to neighbor solicitations for assigned addresses in Docker networks. When a container connects to a network, it attempts to inform the gateway router about the presence of a new address.

Sample command:

docker network create --ipv6 --subnet=172.26.0.0/16 \
  --subnet=2001:db8:1972:beb0:dce3:9c1a:d150::/112 ipv6exposed

docker run -d \
  --restart always --cpus 0.02 --memory 64M \
  -v /var/run/docker.sock:/var/run/docker.sock:ro \
  --cap-drop=ALL --cap-add=NET_RAW --cap-add=NET_ADMIN \
  --network host --name ndpresponder \
  localhost/ndpresponder -i eth0 -N ipv6exposed

• -i flag specifies the network interface name.
• -N flag specifies the Docker network name. You may repeat this flag to specify multiple networks.

Other Options

You may change log level of this program by setting the NDPRESPONDER_LOG environment variable. Acceptable values are DEBUG, INFO, WARN, ERROR, and FATAL.

Sample command:

sudo NDPRESPONDER_LOG=WARN ndpresponder [arguments]
docker run -e NDPRESPONDER_LOG=WARN [other arguments]