[PATCH v3 1/8] netlink: add function to extract MAC addresses from NDP/ARP table

[Jon Maloy <jmaloy@redhat.com>]

The solution to bug https://bugs.passt.top/show_bug.cgi?id=120
requires the ability to translate from an IP address to its
corresponding MAC address in cases where those are present in
the ARP/NDP table.

We add this feature here.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>

---
v3: - Added an attribute contianing NDA_DST to sent message, so
      that we let the kernel do the filtering of the IP address
      and return only one entry.
    - Added interface index to the call signature. Since the only
      interface we know is the template interface, this limits
      the number of hosts that will be seen as 'network segment
      local' from a PASST viewpoint.
---
 netlink.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 netlink.h |  2 ++
 2 files changed, 64 insertions(+)

diff --git a/netlink.c b/netlink.c
index ee9325a..16bb995 100644
--- a/netlink.c
+++ b/netlink.c
@@ -800,6 +800,68 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
 	return status;
 }
 
+/**
+ * nl_mac_get() - Get MAC address corresponding to given IP address
+ * @s:		Netlink socket
+ * @addr:	IPv4 or IPv6 address
+ * @ifi:	Interface index
+ * @mac:	Array to place the returned MAC address
+ *
+ * Return: 0 if found or not in table, negative error code on failure.
+ *         Leaves MAC array unchanged if no match found
+ */
+int nl_mac_get(int s, const union inany_addr *addr, int ifi, unsigned char *mac)
+{
+	struct req_t {
+		struct nlmsghdr nlh;
+		struct ndmsg ndm;
+		struct rtattr rta;
+		char ip[RTA_ALIGN(sizeof(struct in6_addr))];
+	} req = {
+		.ndm.ndm_ifindex = ifi,
+		.rta.rta_type = NDA_DST
+	};
+	struct nlmsghdr *nh;
+	char buf[NLBUFSIZ];
+	const void *ip;
+	ssize_t status;
+	uint32_t seq;
+	int msglen;
+	int iplen;
+
+	if (inany_v4(addr)) {
+		ip = &addr->v4mapped.a4;
+		iplen = sizeof(struct in_addr);
+		req.ndm.ndm_family = AF_INET;
+	} else {
+		ip = &addr->a6;
+		iplen = sizeof(struct in6_addr);
+		req.ndm.ndm_family = AF_INET6;
+	}
+	req.rta.rta_len = RTA_LENGTH(iplen);
+	memcpy(RTA_DATA(&req.rta), ip, iplen);
+	msglen = NLMSG_ALIGN(sizeof(req.nlh) + sizeof(req.ndm) + RTA_LENGTH(iplen));
+
+	seq = nl_send(s, &req, RTM_GETNEIGH, 0, msglen);
+	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWNEIGH) {
+		struct ndmsg *ndm = NLMSG_DATA(nh);
+		struct rtattr *rta = (struct rtattr *)(ndm + 1);
+		size_t na = RTM_PAYLOAD(nh);
+		bool found = false;
+
+		for (; RTA_OK(rta, na); rta = RTA_NEXT(rta, na)) {
+			if (rta->rta_type == NDA_DST) {
+				if (memcmp(RTA_DATA(rta), ip, iplen) == 0)
+					found = true;
+			} else if (rta->rta_type == NDA_LLADDR && found) {
+				memcpy(mac, RTA_DATA(rta), ETH_ALEN);
+			}
+		}
+	}
+
+	return status;
+}
+
 /**
  * nl_addr_get_ll() - Get first IPv6 link-local address for a given interface
  * @s:		Netlink socket
diff --git a/netlink.h b/netlink.h
index b51e99c..51ba49f 100644
--- a/netlink.h
+++ b/netlink.h
@@ -17,6 +17,8 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
 		 int s_dst, unsigned int ifi_dst, sa_family_t af);
 int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
 		void *addr, int *prefix_len, void *addr_l);
+int nl_mac_get(int s, const union inany_addr *addr, int ifi,
+	       unsigned char *mac);
 int nl_addr_set(int s, unsigned int ifi, sa_family_t af,
 		const void *addr, int prefix_len);
 int nl_addr_get_ll(int s, unsigned int ifi, struct in6_addr *addr);
-- 
@@ -17,6 +17,8 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
 		 int s_dst, unsigned int ifi_dst, sa_family_t af);
 int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
 		void *addr, int *prefix_len, void *addr_l);
+int nl_mac_get(int s, const union inany_addr *addr, int ifi,
+	       unsigned char *mac);
 int nl_addr_set(int s, unsigned int ifi, sa_family_t af,
 		const void *addr, int prefix_len);
 int nl_addr_get_ll(int s, unsigned int ifi, struct in6_addr *addr);
-- 
2.48.1