[PATCH v5 01/10] 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.
v4: - Made loop independent of attribute order.
    - Ignoring L2 addresses which are not of size ETH_ALEN.
v5: - Changed return value of new function, so caller can know if
      a MAC address really was found.
---
 netlink.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 netlink.h |  2 ++
 2 files changed, 86 insertions(+)

diff --git a/netlink.c b/netlink.c
index 8f82e73..1ca2c9a 100644
--- a/netlink.c
+++ b/netlink.c
@@ -800,6 +800,90 @@ 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: true if a valid address was found, false otherwise.
+ */
+bool 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];
+	bool found = false;
+	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);
+				found = true;
+			}
+		}
+	}
+	if (status < 0)
+		warn("netlink: RTM_NEWNEIGH failed: %s", strerror_(-status));
+
+	return found;
+}
+
 /**
  * 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..1dbe1db 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);
+bool 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);
+bool 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