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Don't take "our" MAC address from the host

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When sending frames to the guest over the tap link, we need a source MAC
address.  Currently we take that from the MAC address of the main interface
on the host, but that doesn't actually make much sense:
 * We can't preserve the real MAC address of packets from anywhere
   external so there's no transparency case here
 * In fact, it's confusingly different from how we handle IP addresses:
   whereas we give the guest the same IP as the host, we're making the
   host's MAC the one MAC that the guest *can't* use for itself.
 * We already need a fallback case if the host doesn't have an Ethernet
   like MAC (e.g. if it's connected via a point to point interface, such
   as a wireguard VPN).

Change to just just use an arbitrary fixed MAC address - I've picked
9a:55:9a:55:9a:55.  It's simpler and has the small advantage of making
the fact that passt/pasta is in use typically obvious from guest side
packet dumps.  This can still, of course, be overridden with the -M option.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
David Gibson 2024-08-21 14:20:14 +10:00 • committed by Stefano Brivio
parent 356de97e43
commit 90e83d50a9
3 changed files with 13 additions and 35 deletions

7
passt.h
View file

@ -26,6 +26,13 @@ union epoll_ref;
#include "tcp.h"
#include "udp.h"
/* Default address for our end on the tap interface. Bit 0 of byte 0 must be 0
* (unicast) and bit 1 of byte 1 must be 1 (locally administered). Otherwise
* it's arbitrary.
*/
#define MAC_OUR_LAA \
((uint8_t [ETH_ALEN]){0x9a, 0x55, 0x9a, 0x55, 0x9a, 0x55})
/**
* union epoll_ref - Breakdown of reference for epoll fd bookkeeping
* @type: Type of fd (tells us what to do with events)