Re: [PATCH v13 06/10] flow: add MAC address of LAN local remote hosts to flow

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

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On Sun, Oct 12, 2025 at 03:33:33PM -0400, Jon Maloy wrote:
> When communicating with remote hosts on the local network, some guest
> applications want to see the real MAC address of that host instead
> of PASST/PASTA's own tap address. The flow_common structure is a
> convenient location for storing that address, so we do that in this
> commit.
> 
> Note that we don´t add actual usage of this address here, that will
> be done in later commits.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> 
> ---
> v3: - Moved the remote host macaddress from struct flowside to
>       struct flow_common. I chose to call it 'omac' as suggested
>       by David, although in my understanding the correct name would be
>       'emac'. (In general I find the address naming scheme confusing.)
>     - Adapted to new signature of function nl_mac_get(), now passing
>       it the index of the template interface.
> v4: - Renamed flow_commeon->omac to flow_common->tap_omac to make is
>       role in the code clearer
> v5: - Modified the criteria for ARP/NDP table lookup like in the
>       previous commits.
>     - Removed the PIF_TAP lookup case, as David suggested, and did
>       instead give the flow->tap_omac field a value marking it as
>       non-initialized.
>     - Calling the cache table instead of netlink for ARP/NDP lookup.
>     - Unconditionally using the potentially translated IP address
>       in the lookup, instead of only if NAT really was applied.
> v6: - Using MAC_ZERO instead of own definitions
> v12:- Using MAC_UNDEF (==ff:ff:ff:ff:ff:ff) instead of MAC_ZERO,
>       which is a legal MAC address.
> v13: - Removed call to nat_outbound() before MAC resolution, as
>        we are now handling guest-side visible addresses only.
>      - Using tgt->oaddr instead of ini->eaddr as lookup key
>        for fwd_neigh_mac_get(), for the same reason as above.
> ---
>  flow.c | 2 ++
>  flow.h | 2 ++
>  util.h | 2 ++
>  3 files changed, 6 insertions(+)
> 
> diff --git a/flow.c b/flow.c
> index feefda3..a57d7b9 100644
> --- a/flow.c
> +++ b/flow.c
> @@ -449,6 +449,7 @@ struct flowside *flow_target(const struct ctx *c, union flow *flow,
>  
>  	switch (f->pif[INISIDE]) {
>  	case PIF_TAP:
> +		memcpy(f->tap_omac, MAC_UNDEF, ETH_ALEN);
>  		tgtpif = fwd_nat_from_tap(c, proto, ini, tgt);
>  		break;
>  
> @@ -458,6 +459,7 @@ struct flowside *flow_target(const struct ctx *c, union flow *flow,
>  
>  	case PIF_HOST:
>  		tgtpif = fwd_nat_from_host(c, proto, ini, tgt);
> +		fwd_neigh_mac_get(c, &tgt->oaddr, f->tap_omac);
>  		break;
>  
>  	default:
> diff --git a/flow.h b/flow.h
> index cac618a..f342895 100644
> --- a/flow.h
> +++ b/flow.h
> @@ -177,6 +177,7 @@ int flowside_connect(const struct ctx *c, int s,
>   * @type:	Type of packet flow
>   * @pif[]:	Interface for each side of the flow
>   * @side[]:	Information for each side of the flow
> + * @tap_omac: MAC address of remote endpoint as seen from the guest
>   */
>  struct flow_common {
>  #ifdef __GNUC__
> @@ -192,6 +193,7 @@ struct flow_common {
>  #endif
>  	uint8_t		pif[SIDES];
>  	struct flowside	side[SIDES];
> +	uint8_t		tap_omac[6];
>  };
>  
>  #define FLOW_INDEX_BITS		17	/* 128k - 1 */
> diff --git a/util.h b/util.h
> index 22eaac5..6fc8f5d 100644
> --- a/util.h
> +++ b/util.h
> @@ -101,6 +101,8 @@ void abort_with_msg(const char *fmt, ...)
>  	((uint8_t [ETH_ALEN]){ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff })
>  #define MAC_ZERO		((uint8_t [ETH_ALEN]){ 0 })
>  #define MAC_IS_ZERO(addr)	(!memcmp((addr), MAC_ZERO, ETH_ALEN))
> +#define MAC_UNDEF		MAC_BROADCAST
> +#define MAC_IS_UNDEF(addr)	(!memcmp((addr), MAC_UNDEF, ETH_ALEN))
>  
>  #ifndef __bswap_constant_16
>  #define __bswap_constant_16(x)						\
> -- 
> 2.50.1
> 

-- 
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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