arp: optimize interface name resolution

Fixes: #3075

Signed-off-by: Daniel Swarbrick <daniel.swarbrick@gmail.com>

collector/arp_linux.go

@@ -17,7 +17,6 @@
 package collector
 
 import (
-	"errors"
 	"fmt"
 	"log/slog"
 	"net"
@@ -87,30 +86,37 @@ func getTotalArpEntriesRTNL() (map[string]uint32, error) {
 		return nil, err
 	}
 
-	ifIndexEntries := make(map[uint32]uint32)
+	// Map of interface index to ARP neighbor count.
+	ifIndexEntries := make(map[int]uint32)
 
 	for _, n := range neighbors {
 		// Neighbors will also contain IPv6 neighbors, but since this is purely an ARP collector,
 		// restrict to AF_INET. Also skip entries which have state NUD_NOARP to conform to output
 		// of /proc/net/arp.
 		if n.Family == unix.AF_INET && n.State&unix.NUD_NOARP == 0 {
-			ifIndexEntries[n.Index]++
+			ifIndexEntries[int(n.Index)]++
 		}
 	}
 
+	// Go's net.InterfaceByIndex(idx) function fetches the _entire_ interface table behind the
+	// scenes prior to filtering the response. This will result in an exponential amount of
+	// rtnetlink traffic as the number of interfaces increases. Instead, we fetch the interface
+	// table _once_, and perform our own filtering / matching.
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	// Map of interface name to ARP neighbor count.
 	enumEntries := make(map[string]uint32)
 
-	// Convert interface indexes to names.
 	for ifIndex, entryCount := range ifIndexEntries {
-		iface, err := net.InterfaceByIndex(int(ifIndex))
-		if err != nil {
-			if errors.Unwrap(err).Error() == "no such network interface" {
-				continue
+		for x := range ifaces {
+			if ifaces[x].Index == ifIndex {
+				enumEntries[ifaces[x].Name] = entryCount
+				break
 			}
-			return nil, err
 		}
-
-		enumEntries[iface.Name] = entryCount
 	}
 
 	return enumEntries, nil