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 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 adapters/arp/arp.go

(limited to 'adapters/arp/arp.go')

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
+}
-- 
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