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(a)gibson.dropbear.id.au>
---
conf.c | 40 ++++++----------------------------------
passt.h | 7 +++++++
util.h | 1 -
3 files changed, 13 insertions(+), 35 deletions(-)
diff --git a/conf.c b/conf.c
index 954f20ea..7eec3134 100644
--- a/conf.c
+++ b/conf.c
@@ -612,12 +612,10 @@ static int conf_ip4_prefix(const char *arg)
* conf_ip4() - Verify or detect IPv4 support, get relevant addresses
* @ifi: Host interface to attempt (0 to determine one)
* @ip4: IPv4 context (will be written)
- * @mac: MAC address to use (written if unset)
*
* Return: Interface index for IPv4, or 0 on failure.
*/
-static unsigned int conf_ip4(unsigned int ifi,
- struct ip4_ctx *ip4, unsigned char *mac)
+static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
{
if (!ifi)
ifi = nl_get_ext_if(nl_sock, AF_INET);
@@ -660,19 +658,7 @@ static unsigned int conf_ip4(unsigned int ifi,
ip4->addr_seen = ip4->addr;
- if (MAC_IS_ZERO(mac)) {
- int rc = nl_link_get_mac(nl_sock, ifi, mac);
- if (rc < 0) {
- char ifname[IFNAMSIZ];
-
- err("Couldn't discover MAC address for %s: %s",
- if_indextoname(ifi, ifname), strerror(-rc));
- return 0;
- }
-
- if (MAC_IS_ZERO(mac))
- memcpy(mac, MAC_LAA, ETH_ALEN);
- }
+ ip4->our_tap_addr = ip4->gw;
if (IN4_IS_ADDR_UNSPECIFIED(&ip4->addr))
return 0;
@@ -684,12 +670,10 @@ static unsigned int conf_ip4(unsigned int ifi,
* conf_ip6() - Verify or detect IPv6 support, get relevant addresses
* @ifi: Host interface to attempt (0 to determine one)
* @ip6: IPv6 context (will be written)
- * @mac: MAC address to use (written if unset)
*
* Return: Interface index for IPv6, or 0 on failure.
*/
-static unsigned int conf_ip6(unsigned int ifi,
- struct ip6_ctx *ip6, unsigned char *mac)
+static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
{
int prefix_len = 0;
int rc;
@@ -724,19 +708,6 @@ static unsigned int conf_ip6(unsigned int ifi,
if (IN6_IS_ADDR_LINKLOCAL(&ip6->gw))
ip6->our_tap_ll = ip6->gw;
- if (MAC_IS_ZERO(mac)) {
- rc = nl_link_get_mac(nl_sock, ifi, mac);
- if (rc < 0) {
- char ifname[IFNAMSIZ];
- err("Couldn't discover MAC address for %s: %s",
- if_indextoname(ifi, ifname), strerror(-rc));
- return 0;
- }
-
- if (MAC_IS_ZERO(mac))
- memcpy(mac, MAC_LAA, ETH_ALEN);
- }
-
if (IN6_IS_ADDR_UNSPECIFIED(&ip6->addr) ||
IN6_IS_ADDR_UNSPECIFIED(&ip6->our_tap_ll))
return 0;
@@ -1287,6 +1258,7 @@ void conf(struct ctx *c, int argc, char **argv)
c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
c->udp.fwd_in.mode = c->udp.fwd_out.mode = FWD_UNSET;
+ memcpy(c->our_tap_mac, MAC_OUR_LAA, ETH_ALEN);
optind = 1;
do {
@@ -1657,9 +1629,9 @@ void conf(struct ctx *c, int argc, char **argv)
nl_sock_init(c, false);
if (!v6_only)
- c->ifi4 = conf_ip4(ifi4, &c->ip4, c->our_tap_mac);
+ c->ifi4 = conf_ip4(ifi4, &c->ip4);
if (!v4_only)
- c->ifi6 = conf_ip6(ifi6, &c->ip6, c->our_tap_mac);
+ c->ifi6 = conf_ip6(ifi6, &c->ip6);
if ((!c->ifi4 && !c->ifi6) ||
(*c->ip4.ifname_out && !c->ifi4) ||
(*c->ip6.ifname_out && !c->ifi6))
diff --git a/passt.h b/passt.h
index ecfed1e7..c6c67ffc 100644
--- a/passt.h
+++ b/passt.h
@@ -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)
diff --git a/util.h b/util.h
index a716849a..87b91e6c 100644
--- a/util.h
+++ b/util.h
@@ -96,7 +96,6 @@
#define PORT_IS_EPHEMERAL(port) ((port) >= PORT_EPHEMERAL_MIN)
#define MAC_ZERO ((uint8_t [ETH_ALEN]){ 0 })
-#define MAC_LAA ((uint8_t [ETH_ALEN]){ BIT(1), 0, 0, 0, 0, 0 })
#define MAC_IS_ZERO(addr) (!memcmp((addr), MAC_ZERO, ETH_ALEN))
#ifndef __bswap_constant_16
--
2.46.0