Neighbor

The Neighbor class represents a single ARP (IPv4) or NDP (IPv6) neighbor cache entry. It exposes the entry’s key fields (interface index, IP address) as immutable properties, and mutable fields (MAC address, NUD state) that are updated in place by NeighborManager when the kernel notifies a change.

Neighbors are managed by NeighborManager and are represented as shared pointers.

Obtaining a Neighbor

Neighbors cannot be created directly. Use NeighborManager to look them up or enumerate them:

// libjoin.
#include <join/neighbormanager.hpp>
#include <join/neighbor.hpp>

using namespace join;

NeighborManager& manager = NeighborManager::instance();

// Look up by interface index and IP address
Neighbor::Ptr neigh = manager.findByIndex(2, IpAddress("192.168.1.1"));
if (!neigh)
{
    // Entry not found in cache
}

Key properties (immutable)

The following fields form the unique identity of a neighbor entry and never change after creation:

uint32_t         idx = neigh->index(); // output interface index
const IpAddress& ip  = neigh->ip();    // destination IP address

Mutable properties

These fields reflect the current kernel state and are updated automatically when the kernel notifies a change:

MacAddress mac   = neigh->mac();   // resolved layer-2 address
uint16_t   state = neigh->state(); // NUD state bitmask

NUD state checks

if (neigh->isReachable())  { /* NUD_REACHABLE — confirmed reachable */ }
if (neigh->isStale())      { /* NUD_STALE — reachability unconfirmed */ }
if (neigh->isPermanent())  { /* NUD_PERMANENT — static entry */ }
if (neigh->isIncomplete()) { /* NUD_INCOMPLETE — resolution in progress */ }
if (neigh->isFailed())     { /* NUD_FAILED — resolution failed */ }

Modifying an entry

Update MAC address and NUD state

set() replaces the entry in the kernel table (RTM_NEWNEIGH with NLM_F_REPLACE). The default NUD state is NUD_PERMANENT:

MacAddress mac("00:11:22:33:44:55");

// Asynchronous — permanent static entry
neigh->set(mac);

// Synchronous — permanent static entry
if (neigh->set(mac, NUD_PERMANENT, true) == -1)
{
    std::cerr << "Failed: " << lastError.message() << "\n";
}

// Synchronous — reachable dynamic entry
neigh->set(mac, NUD_REACHABLE, true);

Remove an entry

// Asynchronous
neigh->remove();

// Synchronous
if (neigh->remove(true) == -1)
{
    std::cerr << "Failed: " << lastError.message() << "\n";
}

⚠️ After a successful remove(), the Neighbor::Ptr may still be held by the caller but the entry is erased from NeighborManager’s cache. Do not reuse the pointer to re-add the entry — call NeighborManager::addNeighbor() instead.

Comparison operators

Two Neighbor::Ptr values are compared by (index, IP address):

if (neighA == neighB) { /* same neighbor entry */ }
if (neighA <  neighB) { /* ordering: index → ip */ }

Return values

set() and remove() return:

0 on success
-1 on failure (check lastError for details)

Summary

Property	Immutable	Thread-safe read
`index()`	✅	✅
`ip()`	✅	✅
`mac()`		✅ (mutex)
`state()`		✅ (mutex)