merd: ARP and DHCP handlers, connection tracking fixes

With this, merd provides a fully functional IPv4 environment to
guests, requiring a single capability, CAP_NET_RAW.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

arp.c

@@ -0,0 +1,82 @@
+/* MERD - MacVTap Egress and Routing Daemon
+ *
+ * arp.c - ARP implementation
+ *
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ * License: GPLv2
+ *
+ */
+
+#include <stdio.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <string.h>
+#include <linux/if_ether.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <net/if.h>
+#include <net/if_arp.h>
+#include <arpa/inet.h>
+
+#include "merd.h"
+#include "dhcp.h"
+#include "util.h"
+
+/**
+ * struct arpmsg - 802.2 ARP IPv4 payload
+ * @sha:	Sender hardware address
+ * @sip:	Sender IP address
+ * @tha:	Target hardware address
+ * @tip:	Target IP address
+ */
+struct arpmsg {
+	unsigned char sha[ETH_ALEN];
+	unsigned char sip[4];
+	unsigned char tha[ETH_ALEN];
+	unsigned char tip[4];
+} __attribute__((__packed__));
+
+/**
+ * dhcp() - Check if this is an ARP message, reply as needed
+ * @c:		Execution context
+ * @len:	Total L2 packet length
+ * @eh:		Packet buffer, Ethernet header
+ *
+ * Return: 0 if it's not an ARP message, 1 if handled, -1 on failure
+ */
+int arp(struct ctx *c, unsigned len, struct ethhdr *eh)
+{
+	struct arphdr *ah = (struct arphdr *)(eh + 1);
+	struct arpmsg *am = (struct arpmsg *)(ah + 1);
+	unsigned char swap[4];
+
+	if (eh->h_proto != htons(ETH_P_ARP))
+		return 0;
+
+	if (len < sizeof(*eh) + sizeof(*ah) + sizeof(*am))
+		return -1;
+
+	if (ah->ar_hrd != htons(ARPHRD_ETHER) ||
+	    ah->ar_pro != htons(ETH_P_IP) ||
+	    ah->ar_hln != ETH_ALEN || ah->ar_pln != 4 ||
+	    ah->ar_op != htons(ARPOP_REQUEST))
+		return 1;
+
+	ah->ar_op = htons(ARPOP_REPLY);
+	memcpy(am->tha, am->sha, ETH_ALEN);
+	memcpy(am->sha, c->mac, ETH_ALEN);
+
+	memcpy(swap, am->tip, 4);
+	memcpy(am->tip, am->sip, 4);
+	memcpy(am->sip, swap, 4);
+
+	len = sizeof(*eh) + sizeof(*ah) + sizeof(*am);
+	memcpy(eh->h_dest, eh->h_source, ETH_ALEN);
+	memcpy(eh->h_source, c->mac, ETH_ALEN);
+
+	if (send(c->fd_unix, eh, len, 0) < 0)
+		perror("ARP: send");
+
+	return 1;
+}