From 90e83d50a9bdeb0697843fc9917c3070b69d5c7d Mon Sep 17 00:00:00 2001 From: David Gibson Date: Wed, 21 Aug 2024 14:20:14 +1000 Subject: [PATCH] Don't take "our" MAC address from the host 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 Signed-off-by: Stefano Brivio --- 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 954f20e..7eec313 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 ecfed1e..c6c67ff 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 a716849..87b91e6 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