Arp proxying with /usr/sbin/arp is broken since it switched to NETLINK. Here is "how to repeat": # ifconfig tun0 create inet 10.0.0.1 10.11.0.1 netmask 255.255.255.255 # ifconfig tun0 tun0: flags=1008051<UP,POINTOPOINT,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 1500 options=4080000<LINKSTATE,MEXTPG> inet 10.0.0.1 --> 10.11.0.1 netmask 0xffffffff groups: tun nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL> # route add -host 10.0.0.233 -iface tun0 add host 10.0.0.233: gateway tun0 # /usr/sbin/arp -s 10.0.0.233 60:be:b4:4:94:65 pub arp: set: 10.0.0.233: Invalid argument (NDA_LLADDR address length (6) is different from expected (0)) Here 60:be:b4:4:94:65 is MAC address of LAN igc0 interface having IP address 10.0.0.1/24 A workaround is to rebuild /usr/sbin/arp WITHOUT_NETLINK_SUPPORT so it uses rtsock instead. Then it works again.
Real problem "in wild" occurs after upgrade of FreeBSD 13.5 running mpd5 with arp proxying to 14.3. mpd5 runs /usr/sbin/arp that fails. Rebuild of /usr/sbin/arp without netlink helps.
A commit in branch main references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=c26d6bf9da013e839d9a696746ea1741569e619e commit c26d6bf9da013e839d9a696746ea1741569e619e Author: Andrey V. Elsukov <ae@FreeBSD.org> AuthorDate: 2025-10-17 07:53:35 +0000 Commit: Andrey V. Elsukov <ae@FreeBSD.org> CommitDate: 2025-10-17 08:22:36 +0000 arp: fix adding proxy entries for P2P interfaces The old rtsock implementation used in-kernel workaround to do so. When route to specified destination address used P2P interface, the kernel did the search with ifa_ifwithnet() for most suitable network and then add proxy entry to this interface. Use similar approach with netlink implementation. We already have get_ether_addr() function that does almost the same thing as ifa_ifwithnet(). Use it when we find that destination route uses P2P interface and then try to guess suitable interface. This should fix the use of netlink-based arp(8) in mpd5. Rename get_ether_addr() to get_ifinfo(), since now it is used to find only ifindex in case when hwaddr is specified by user. Also make set_nl() and delete_nl() prototype similar to rtsock. And allow '-i' to be used with '-S', since we already allow the same for '-s'. PR: 290221 Reported by: eugen Reviewed by: eugen MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D53113 usr.sbin/arp/arp.c | 32 ++++++++++++++++++++------------ usr.sbin/arp/arp.h | 9 ++++----- usr.sbin/arp/arp_netlink.c | 20 ++++++++++++++++---- 3 files changed, 40 insertions(+), 21 deletions(-)
A commit in branch stable/15 references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=9e829d0693a7c8e2ee82a8aefa59c0a5f2648da3 commit 9e829d0693a7c8e2ee82a8aefa59c0a5f2648da3 Author: Andrey V. Elsukov <ae@FreeBSD.org> AuthorDate: 2025-10-17 07:53:35 +0000 Commit: Andrey V. Elsukov <ae@FreeBSD.org> CommitDate: 2025-10-26 11:20:01 +0000 arp: fix adding proxy entries for P2P interfaces The old rtsock implementation used in-kernel workaround to do so. When route to specified destination address used P2P interface, the kernel did the search with ifa_ifwithnet() for most suitable network and then add proxy entry to this interface. Use similar approach with netlink implementation. We already have get_ether_addr() function that does almost the same thing as ifa_ifwithnet(). Use it when we find that destination route uses P2P interface and then try to guess suitable interface. This should fix the use of netlink-based arp(8) in mpd5. Rename get_ether_addr() to get_ifinfo(), since now it is used to find only ifindex in case when hwaddr is specified by user. Also make set_nl() and delete_nl() prototype similar to rtsock. And allow '-i' to be used with '-S', since we already allow the same for '-s'. PR: 290221 Reported by: eugen Reviewed by: eugen Differential Revision: https://reviews.freebsd.org/D53113 (cherry picked from commit c26d6bf9da013e839d9a696746ea1741569e619e) usr.sbin/arp/arp.c | 32 ++++++++++++++++++++------------ usr.sbin/arp/arp.h | 9 ++++----- usr.sbin/arp/arp_netlink.c | 20 ++++++++++++++++---- 3 files changed, 40 insertions(+), 21 deletions(-)
A commit in branch stable/14 references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=f7937053fd0899e7b808bab3a0576ff16f3e943d commit f7937053fd0899e7b808bab3a0576ff16f3e943d Author: Andrey V. Elsukov <ae@FreeBSD.org> AuthorDate: 2025-10-17 07:53:35 +0000 Commit: Andrey V. Elsukov <ae@FreeBSD.org> CommitDate: 2025-10-26 11:21:14 +0000 arp: fix adding proxy entries for P2P interfaces The old rtsock implementation used in-kernel workaround to do so. When route to specified destination address used P2P interface, the kernel did the search with ifa_ifwithnet() for most suitable network and then add proxy entry to this interface. Use similar approach with netlink implementation. We already have get_ether_addr() function that does almost the same thing as ifa_ifwithnet(). Use it when we find that destination route uses P2P interface and then try to guess suitable interface. This should fix the use of netlink-based arp(8) in mpd5. Rename get_ether_addr() to get_ifinfo(), since now it is used to find only ifindex in case when hwaddr is specified by user. Also make set_nl() and delete_nl() prototype similar to rtsock. And allow '-i' to be used with '-S', since we already allow the same for '-s'. PR: 290221 Reported by: eugen Reviewed by: eugen Differential Revision: https://reviews.freebsd.org/D53113 (cherry picked from commit c26d6bf9da013e839d9a696746ea1741569e619e) usr.sbin/arp/arp.c | 32 ++++++++++++++++++++------------ usr.sbin/arp/arp.h | 9 ++++----- usr.sbin/arp/arp_netlink.c | 20 ++++++++++++++++---- 3 files changed, 40 insertions(+), 21 deletions(-)