Set high byte of mac addresses for host veth devices to 0xfe

When used in conjunction with a bridge, veth devices with random addresses
may change the mac address of the bridge itself if the mac address of the
interface newly added is numerically lower than the previous mac address
of the bridge. This is documented kernel behavior. To avoid changing the
host's mac address back and forth when starting and/or stopping containers,
this patch ensures that the high byte of the mac address of the veth
interface visible from the host side is set to 0xfe.

A similar logic is also implemented in libvirt.

Fixes SF bug #3411497
See also: <http://thread.gmane.org/gmane.linux.kernel.containers.lxc.general/2709>

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

1 files changed, 40 insertions, 0 deletions

diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index 613e476..e55318d 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c
@@ -1402,6 +1402,36 @@ static int setup_network(struct lxc_list *network)
 	return 0;
 }
 
+static int setup_private_host_hw_addr(char *veth1)
+{
+	struct ifreq ifr;
+	int err;
+	int sockfd;
+
+	sockfd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (sockfd < 0)
+		return -errno;
+
+	snprintf((char *)ifr.ifr_name, IFNAMSIZ, "%s", veth1);
+	err = ioctl(sockfd, SIOCGIFHWADDR, &ifr);
+	if (err < 0) {
+		close(sockfd);
+		return -errno;
+	}
+
+	ifr.ifr_hwaddr.sa_data[0] = 0xfe;
+	err = ioctl(sockfd, SIOCSIFHWADDR, &ifr);
+	close(sockfd);
+	if (err < 0)
+		return -errno;
+
+	DEBUG("mac address of host interface '%s' changed to private %02x:%02x:%02x:%02x:%02x:%02x",
+	      veth1, ifr.ifr_hwaddr.sa_data[0] & 0xff, ifr.ifr_hwaddr.sa_data[1] & 0xff, ifr.ifr_hwaddr.sa_data[2] & 0xff,
+	      ifr.ifr_hwaddr.sa_data[3] & 0xff, ifr.ifr_hwaddr.sa_data[4] & 0xff, ifr.ifr_hwaddr.sa_data[5] & 0xff);
+
+	return 0;
+}
+
 struct lxc_conf *lxc_conf_init(void)
 {
 	struct lxc_conf *new;
@@ -1456,6 +1486,16 @@ static int instanciate_veth(struct lxc_handler *handler, struct lxc_netdev *netd
 		return -1;
 	}
 
+	/* changing the high byte of the mac address to 0xfe, the bridge interface
+	 * will always keep the host's mac address and not take the mac address
+	 * of a container */
+	err = setup_private_host_hw_addr(veth1);
+	if (err) {
+		ERROR("failed to change mac address of host interface '%s' : %s",
+			veth1, strerror(-err));
+		goto out_delete;
+	}
+
 	if (netdev->mtu) {
 		err = lxc_netdev_set_mtu(veth1, atoi(netdev->mtu));
 		if (!err)