On Sun, Jun 29, 2025 at 01:13:40PM -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
--- 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)
Looking at the name alone, I'd think this got the local MAC, rather than a neighbour MAC. Maybe rename to nl_neigh_mac_get() or even just nl_neigh_get()?
+{ + 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;
I generally try to avoid reaching into inany internal fields, particularly the v4 ones. You could instead use: if ((ip = inany_v4(addr))) { ...
+ iplen = sizeof(struct in_addr); + req.ndm.ndm_family = AF_INET; + } else { + ip = &addr->a6;
and here just ip = &addr.
+ 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;
Do you need this logic now that you're using the kernel filtering? If you do, is it guaranteed that the NDA_DST attribute will come before the NDA_LLADDR attribute? This code won't work if they're reversed.
+ } else if (rta->rta_type == NDA_LLADDR && found) { + memcpy(mac, RTA_DATA(rta), ETH_ALEN);
It's unusual, but the host interface could be something other than ethernet, so you need to check the type/length of the host link level address, in case it's not an ethernet style MAC address.
+ } + } + } + + 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);
-- 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