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Re: ARP Address Conflict Detection and dhcpcd arping
On 11/10/2019 12:41, rudolf wrote:
I have a NetBSD 8.1_STABLE (july 2019) host with a static IP address and
another machine (Linux) with dhcpcd "arping" configuration for this IP
address. With the default value of sysctl net.inet.ip.dad_count (=3), I
see no ARP reply ("ARP, Reply <IP addr> is-at <MAC>") messages from the
NetBSD host on the arping queries ("ARP, Request who-has <IP addr> tell
0.0.0.0") and eventually the NetBSD host marks its IP address as
duplicated (and the "arping" dhcpcd configuration of the second host
This seems contradictory to what I read in "man 4 arp":
``ARP will defend the host's active address when a conflicting message
With "sysctl -w net.inet.ip.dad_count=0", the NetBSD host sends the ARP
reply and the "arping" configuration of the second host works.
Is this expected behavior?
Totally not expected behaviour.
I see the same behaviour in -current which I just fixed and submitted a
PR for -9, but -8 (from release to now) is fine and doesn't need a patch
here - at least not from the steps listed.
RCS file: /cvsroot/src/sys/netinet/if_arp.c,v
retrieving revision 1.288
diff -u -p -r1.288 if_arp.c
--- sys/netinet/if_arp.c 25 Sep 2019 09:52:32 -0000 1.288
+++ sys/netinet/if_arp.c 11 Oct 2019 13:21:23 -0000
@@ -1157,15 +1157,17 @@ in_arpinput(struct mbuf *m)
* DAD check, RFC 5227.
* Collision on sender address is always a duplicate.
- * Collision on target address is only a duplicate IF
- * the sender address is the null host (ie a DAD probe) AND
- * the message was broadcast - if it's unicast then it's
- * a valid Unicast Poll from RFC 1122.
+ * Collision on target address is only a duplicate
+ * IF the sender address is the null host (ie a DAD probe)
+ * AND the message was broadcast
+ * AND our address is either tentative or duplicated
+ * If it was unicast then it's a valid Unicast Poll from RFC 1122.
if (do_dad &&
(in_hosteq(isaddr, myaddr) ||
(in_nullhost(isaddr) && in_hosteq(itaddr, myaddr) &&
- m->m_flags & M_BCAST)))
+ m->m_flags & M_BCAST &&
+ ia->ia4_flags & (IN_IFF_TENTATIVE | IN_IFF_DUPLICATED))))
struct sockaddr_dl sdl, *sdlp;
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