Re: [PATCH v9 9/9] arp/ndp: send gratuitous ARP / unsolicitated NA when MAC cache entry added

[Jon Maloy <jmaloy@redhat.com>]

On 2025-09-26 19:25, Jon Maloy wrote:
> 
> On 2025-09-23 23:22, David Gibson wrote:
>> On Tue, Sep 23, 2025 at 09:13:30PM -0400, Jon Maloy wrote:
>>> Gratuitious ARP and unsolicitated NA should be handled with caution
>>> because of the risk of malignant users emitting them to disturb
>>> network communication.
>>>
> [...]
>>> +    req.ah.ar_op = htons(ARPOP_REPLY);
>>> +    req.ah.ar_hrd = htons(ARPHRD_ETHER);
>>> +    req.ah.ar_pro = htons(ETH_P_IP);
>>> +    req.ah.ar_hln = ETH_ALEN;
>>> +    req.ah.ar_pln = 4;
>>> +
>>> +    /* ARP message */
>>> +    memcpy(req.am.sha, mac, sizeof(req.am.sha));
>>> +    memcpy(req.am.sip, &ip, sizeof(req.am.sip));
>>> +    memcpy(req.am.tha, MAC_BROADCAST, sizeof(req.am.tha));
>>> +    memcpy(req.am.tip, &ip, sizeof(req.am.tip));
>>
>> So, I was trying to check if it made sense to use the same IP for both
>> source and target here, and came across
>>      https://www.rfc-editor.org/rfc/rfc5227#section-3
>>
>> Which suggests we should (counter intuitively) be using ARP requests,
>> not ARP replies for announcements.
> 
> I have now read through it, and it seems to come to the conclusion
> that this is not advisable. In principle it should work, if all 
> implementations stick to standard, but there might be stacks which are 
> not stateless in this regard, i.e., they only accepts ARP replies as a 
> response to a sent request.
> In short, I think I will stick to my current approach, since it is 
> evidently harmless and is proven to work.
> 
> ///jon

My response above may look confusing. I had actually experimented with 
both methods, and had in my mind that it was the "ARP Announcement" 
implementation I had posted.
It is now fixed.

That said, further investigation indicates that the other method is 
fully legit, and actually widely used (Windows, Cisco), although not
by Linux.

///jon

> 
> 
>>
>>> +    inet_ntop(AF_INET, &ip, ip_str, sizeof(ip_str));
>>> +    debug("Sending gratuitous ARP for %s", ip_str);
>>> +    tap_send_single(c, &req, sizeof(req));
>>> +}
>>> diff --git a/arp.h b/arp.h
>>> index d5ad0e1..b0dbb56 100644
>>> --- a/arp.h
>>> +++ b/arp.h
>>> @@ -22,5 +22,7 @@ struct arpmsg {
>>>   int arp(const struct ctx *c, struct iov_tail *data);
>>>   void arp_send_init_req(const struct ctx *c);
>>> +void arp_send_gratuitous(const struct ctx *c, struct in_addr ip,
>>> +             const unsigned char *mac);
>>>   #endif /* ARP_H */
>>> diff --git a/fwd.c b/fwd.c
>>> index c6348ab..879a351 100644
>>> --- a/fwd.c
>>> +++ b/fwd.c
>>> @@ -26,6 +26,8 @@
>>>   #include "passt.h"
>>>   #include "lineread.h"
>>>   #include "flow_table.h"
>>> +#include "arp.h"
>>> +#include "ndp.h"
>>>   /* Empheral port range: values from RFC 6335 */
>>>   static in_port_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
>>> @@ -129,6 +131,15 @@ void fwd_neigh_mac_cache_alloc(const struct ctx *c,
>>>       memcpy(&e->addr, addr, sizeof(*addr));
>>>       memcpy(e->mac, mac, ETH_ALEN);
>>> +
>>> +    /* Send gratuitous ARP / unsolicited NA for the new mapping */
>>
>> AFAICT this doesn't actually implement what the commit message
>> describes - it seems to always send an ARP/NA when the neighbour table
>> is updated.
>>
>>> +    if (inany_v4(addr)) {
>>> +        struct in_addr ip4 = *inany_v4(addr);
>>> +
>>> +        arp_send_gratuitous(c, ip4, e->mac);
>>> +    } else {
>>> +        ndp_send_unsolicited_na(c, &addr->a6);
>>> +    }
>>>   }
>>>   /**
>>> diff --git a/ndp.c b/ndp.c
>>> index 70b68aa..8914f31 100644
>>> --- a/ndp.c
>>> +++ b/ndp.c
>>> @@ -226,6 +226,16 @@ static void ndp_na(const struct ctx *c, const 
>>> struct in6_addr *dst,
>>>       ndp_send(c, dst, &na, sizeof(na));
>>>   }
>>> +/**
>>> + * ndp_send_unsolicited_na() - Send unsolicited NA
>>> + * @c:        Execution context
>>> + * @addr:    IPv6 address to advertise
>>> + */
>>> +void ndp_send_unsolicited_na(const struct ctx *c, const struct 
>>> in6_addr *addr)
>>> +{
>>> +    ndp_na(c, &in6addr_ll_all_nodes, addr);
>>> +}
>>> +
>>>   /**
>>>    * ndp_ra() - Send an NDP Router Advertisement (RA) message
>>>    * @c:        Execution context
>>> diff --git a/ndp.h b/ndp.h
>>> index 781ea86..320009c 100644
>>> --- a/ndp.h
>>> +++ b/ndp.h
>>> @@ -12,5 +12,6 @@ int ndp(const struct ctx *c, const struct in6_addr 
>>> *saddr,
>>>       struct iov_tail *data);
>>>   void ndp_timer(const struct ctx *c, const struct timespec *now);
>>>   void ndp_send_init_req(const struct ctx *c);
>>> +void ndp_send_unsolicited_na(const struct ctx *c, const struct 
>>> in6_addr *addr);
>>>   #endif /* NDP_H */
>>> -- 
>>> 2.50.1
>>>
>>
>