From fb653292612eddca5b088ed88466c546d1597107 Mon Sep 17 00:00:00 2001
From: Elizabeth Alexander Hunt <me@liz.coffee>
Date: Sat, 20 Jun 2026 09:40:07 -0700
Subject: Initial commit

---
 adapters/arp/arp.go      | 63 ++++++++++++++++++++++++++++++++++++++++++++++++
 adapters/arp/arp_test.go | 51 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+)
 create mode 100644 adapters/arp/arp.go
 create mode 100644 adapters/arp/arp_test.go

(limited to 'adapters/arp')

diff --git a/adapters/arp/arp.go b/adapters/arp/arp.go
new file mode 100644
index 0000000..19d7632
--- /dev/null
+++ b/adapters/arp/arp.go
@@ -0,0 +1,63 @@
+package arp
+
+import (
+	"os/exec"
+	"regexp"
+	"strings"
+
+	"penguins.lan/utils"
+)
+
+type ArpTable struct {
+	AddrToMac map[utils.INet4]utils.Mac
+}
+
+// Lookup returns the canonical MAC for an IPv4 string, false if it isn't in the
+// table or isn't parseable IPv4 (e.g. an IPv6 peer).
+func (t *ArpTable) Lookup(ip string) (string, bool) {
+	addr, err := utils.ParseINet4(ip)
+	if err != nil {
+		return "", false
+	}
+	mac, ok := t.AddrToMac[*addr]
+	if !ok {
+		return "", false
+	}
+	return mac.Format(), true
+}
+
+type TableProvider interface {
+	Fetch() (*ArpTable, error)
+}
+
+var arpLine = regexp.MustCompile(`\((\d{1,3}(?:\.\d{1,3}){3})\) at ([0-9a-fA-F]{1,2}(?::[0-9a-fA-F]{1,2}){5})`)
+
+type SystemProvider struct{}
+
+func (SystemProvider) Fetch() (*ArpTable, error) {
+	out, err := exec.Command("arp", "-an").Output()
+	if err != nil {
+		return nil, err
+	}
+	return parseTable(string(out)), nil
+}
+
+func parseTable(out string) *ArpTable {
+	table := &ArpTable{AddrToMac: make(map[utils.INet4]utils.Mac)}
+	for _, line := range strings.Split(out, "\n") {
+		m := arpLine.FindStringSubmatch(line)
+		if m == nil {
+			continue
+		}
+		addr, err := utils.ParseINet4(m[1])
+		if err != nil {
+			continue
+		}
+		mac, err := utils.ParseMac(m[2])
+		if err != nil {
+			continue
+		}
+		table.AddrToMac[*addr] = *mac
+	}
+	return table
+}
diff --git a/adapters/arp/arp_test.go b/adapters/arp/arp_test.go
new file mode 100644
index 0000000..9a215e0
--- /dev/null
+++ b/adapters/arp/arp_test.go
@@ -0,0 +1,51 @@
+package arp
+
+import "testing"
+
+const sample = `Neighbor                Linklayer Address
+? (192.168.1.5) at 8:0:27:ab:cd:ef on en0 ifscope [ethernet]
+? (192.168.1.9) at AA:BB:CC:11:22:33 [ether] on br-lan
+? (10.0.0.2) at de:ad:be:ef:00:01 on eth0
+? (192.168.1.250) at (incomplete) on en0 [ethernet]
+
+`
+
+func TestParseTable(t *testing.T) {
+	table := parseTable(sample)
+
+	got := make(map[string]string, len(table.AddrToMac))
+	for ip, mac := range table.AddrToMac {
+		got[ip.Format()] = mac.Format()
+	}
+
+	want := map[string]string{
+		"192.168.1.5": "08:00:27:ab:cd:ef",
+		"192.168.1.9": "aa:bb:cc:11:22:33",
+		"10.0.0.2":    "de:ad:be:ef:00:01",
+	}
+
+	if len(got) != len(want) {
+		t.Fatalf("parsed %d entries %v, want %d (header/incomplete lines should be skipped)", len(got), got, len(want))
+	}
+	for ip, mac := range want {
+		if got[ip] != mac {
+			t.Errorf("entry %q = %q, want %q", ip, got[ip], mac)
+		}
+	}
+}
+
+func TestTableLookup(t *testing.T) {
+	table := parseTable(sample)
+
+	if mac, ok := table.Lookup("192.168.1.5"); !ok || mac != "08:00:27:ab:cd:ef" {
+		t.Errorf("Lookup(192.168.1.5) = %q,%v", mac, ok)
+	}
+	if _, ok := table.Lookup("1.2.3.4"); ok {
+		t.Error("Lookup of a miss should be false")
+	}
+	if _, ok := table.Lookup("::1"); ok {
+		t.Error("Lookup of non-IPv4 should be false")
+	}
+}
+
+var _ TableProvider = SystemProvider{}
-- 
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