nhrpd is an implementation of the NHRP. NHRP is described in RFC 2332.

NHRP is used to improve the efficiency of routing computer network traffic over NBMA networks. NHRP provides an ARP-like solution that allows a system to dynamically learn the NBMA address of the other systems that are part of that network, allowing these systems to directly communicate without requiring traffic to use an intermediate hop.

NHRP is a client-server protocol. The server side is called the NHS or the hub, while a client is referred to as the NHC or the spoke. When a node is configured as an NHC, it registers its address with the NHS which keeps track of all registered spokes. An NHC client can then query the addresses of other clients from NHS allowing all spokes to communicate directly with each other.

Cisco Dynamic Multipoint VPN (DMVPN) is based on NHRP, and frr nhrpd implements this scenario.

Routing Design

nhrpd never handles routing of prefixes itself. You need to run some real routing protocol (e.g. BGP) to advertise routes over the tunnels. What nhrpd does it establishes ‘shortcut routes’ that optimizes the routing protocol to avoid going through extra nodes in NBMA GRE mesh.

nhrpd does route NHRP domain addresses individually using per-host prefixes. This is similar to Cisco FlexVPN; but in contrast to opennhrp which uses a generic subnet route.

To create NBMA GRE tunnel you might use the following (Linux terminal commands):

ip tunnel add gre1 mode gre key 42 ttl 64
ip addr add dev gre1
ip link set gre1 up

Note that the IP-address is assigned as host prefix to gre1. nhrpd will automatically create additional host routes pointing to gre1 when a connection with these hosts is established.

The gre1 subnet prefix should be announced by routing protocol from the hub nodes (e.g. BGP ‘network’ announce). This allows the routing protocol to decide which is the closest hub and determine the relay hub on prefix basis when direct tunnel is not established.

nhrpd will redistribute directly connected neighbors to zebra. Within hub nodes, these routes should be internally redistributed using some routing protocol (e.g. iBGP) to allow hubs to be able to relay all traffic.

This can be achieved in hubs with the following bgp configuration (network command defines the GRE subnet):

router bgp 65555
 address-family ipv4 unicast
   redistribute nhrp

Configuring NHRP

ip nhrp holdtime (1-65000)

Holdtime is the number of seconds that have to pass before stopping to advertise an NHRP NBMA address as valid. It also controls how often NHRP registration requests are sent. By default registrations are sent every one third of the holdtime.

ip nhrp map A.B.C.D|X:X::X:X A.B.C.D|local

Map an IP address of a station to the station’s NBMA address.

ip nhrp network-id (1-4294967295)

Enable NHRP on this interface and set the interface’s network ID. The network ID is used to allow creating multiple nhrp domains on a router when multiple interfaces are configured on the router. Interfaces configured with the same ID are part of the same logical NBMA network. The ID is a local only parameter and is not sent to other NHRP nodes and so IDs on different nodes do not need to match. When NHRP packets are received on an interface they are assigned to the local NHRP domain for that interface.

ip nhrp nhs A.B.C.D nbma A.B.C.D|FQDN

Configure the Next Hop Server address and its NBMA address.

ip nhrp nhs dynamic nbma A.B.C.D

Configure the Next Hop Server to have a dynamic address and set its NBMA address.

ip nhrp registration no-unique

Allow the client to not set the unique flag in the NHRP packets. This is useful when a station has a dynamic IP address that could change over time.

ip nhrp shortcut

Enable shortcut (spoke-to-spoke) tunnels to allow NHC to talk to each others directly after establishing a connection without going through the hub.

ip nhrp mtu

Configure NHRP advertised MTU.

Hub Functionality

In addition to routing nhrp redistributed host prefixes, the hub nodes are also responsible to send NHRP Traffic Indication messages that trigger creation of the shortcut tunnels.

nhrpd sends Traffic Indication messages based on network traffic captured using NFLOG. Typically you want to send Traffic Indications for network traffic that is routed from gre1 back to gre1 in rate limited manner. This can be achieved with the following iptables rule.

iptables -A FORWARD -i gre1 -o gre1 \\
    -m hashlimit --hashlimit-upto 4/minute --hashlimit-burst 1 \\
    --hashlimit-mode srcip,dstip --hashlimit-srcmask 24 --hashlimit-dstmask 24 \\
    --hashlimit-name loglimit-0 -j NFLOG --nflog-group 1 --nflog-range 128

You can fine tune the src/dstmask according to the prefix lengths you announce internal, add additional IP range matches, or rate limitation if needed. However, the above should be good in most cases.

This kernel NFLOG target’s nflog-group is configured in global nhrp config with:

nhrp nflog-group (1-65535)

To start sending these traffic notices out from hubs, use the nhrp per-interface directive:

ip nhrp redirect

This enable redirect replies on the NHS similar to ICMP redirects except this is managed by the nhrp protocol. This setting allows spokes to communicate with each others directly.

Integration with IKE

nhrpd needs tight integration with IKE daemon for various reasons. Currently only strongSwan is supported as IKE daemon.

nhrpd connects to strongSwan using VICI protocol based on UNIX socket (hardcoded now as /var/run/charon.vici).

strongSwan currently needs few patches applied. Please check out the https://git.alpinelinux.org/user/tteras/strongswan/log/?h=tteras-release and https://git.alpinelinux.org/user/tteras/strongswan/log/?h=tteras git repositories for the patches.

NHRP Events

nhrp event socket SOCKET

Configure the Unix path for the event socket.

Configuration Example