Re: [PATCH v4 1/9] netlink: add function to extract MAC addresses from NDP/ARP table

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Aug 19, 2025 at 11:09:57PM -0400, Jon Maloy wrote:
> 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)

You need to return an error code in the case that the MAC is missing
or doesn't have the required form; otherwise you'll silently return
garbage in @mac.

> +				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);

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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