[PATCH v4 1/9] 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>
---
 netlink.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 netlink.h |  2 ++
 2 files changed, 81 insertions(+)

diff --git a/netlink.c b/netlink.c
index 8f82e73..cf7debc 100644
--- a/netlink.c
+++ b/netlink.c
@@ -800,6 +800,85 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
 	return status;
 }
 
+/**
+ * nl_neigh_mac_get() - Get neighbor MAC address from the kernel neigh table
+ * @s:    Netlink socket fd
+ * @addr: IPv4 or IPv6 address
+ * @ifi:  Interface index
+ * @mac:  Buffer for Ethernet MAC, left unchanged if not found/usable
+ *
+ * Return: <0 on netlink error; 0 otherwise (drains all replies for @seq).
+ */
+int nl_neigh_mac_get(int s, const union inany_addr *addr,
+					int ifi, unsigned char *mac)
+{
+	const void *ip = inany_v4(addr);
+	struct req_t {
+		struct nlmsghdr nlh;
+		struct ndmsg ndm;
+		struct rtattr rta;
+		char ip[RTA_ALIGN(sizeof(struct in6_addr))];
+	} req;
+	struct nlmsghdr *nh;
+	char buf[NLBUFSIZ];
+	ssize_t status;
+	uint32_t seq;
+	int msglen;
+	int iplen;
+
+	memset(&req, 0, sizeof(req));
+	req.ndm.ndm_ifindex = ifi;
+	req.rta.rta_type = NDA_DST;
+
+	if (ip) {
+		req.ndm.ndm_family = AF_INET;
+		iplen = sizeof(struct in_addr);
+	} else {
+		req.ndm.ndm_family = AF_INET6;
+		ip = &addr;
+		iplen = sizeof(struct in6_addr);
+	}
+
+	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);
+
+	/* Drain all RTM_NEWNEIGH replies for this seq */
+	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWNEIGH) {
+		struct ndmsg *ndm = NLMSG_DATA(nh);
+		struct rtattr *rta = (struct rtattr *)(ndm + 1);
+		const uint8_t *lladdr = NULL;
+		size_t na = RTM_PAYLOAD(nh);
+		const void *dst = NULL;
+		size_t lladdr_len = 0;
+		size_t dstlen = 0;
+
+		for (; RTA_OK(rta, na); rta = RTA_NEXT(rta, na)) {
+			switch (rta->rta_type) {
+			case NDA_DST:
+				dst = RTA_DATA(rta);
+				dstlen = RTA_PAYLOAD(rta);
+				break;
+			case NDA_LLADDR:
+				lladdr = RTA_DATA(rta);
+				lladdr_len = RTA_PAYLOAD(rta);
+				break;
+			default:
+				break;
+			}
+		}
+
+		if (dst && dstlen == (size_t)iplen && memcmp(dst, ip, iplen) == 0) {
+			/* Only copy Ethernet-style addresses; leave unchanged otherwise */
+			if (lladdr && lladdr_len == ETH_ALEN)
+				memcpy(mac, lladdr, 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..026c64f 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_neigh_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_neigh_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);
-- 
2.50.1