network: set mtu of unpriv veth to the bridge's mtu

src/lxc/lxc_user_nic.c

@@ -226,12 +226,18 @@ static int instanciate_veth(char *n1, char **n2)
 	return netdev_set_flag(n1, IFF_UP);
 }
 
+static int get_mtu(char *name)
+{
+	int idx = if_nametoindex(name);
+	return netdev_get_mtu(idx);
+}
+
 static bool create_nic(char *nic, char *br, int pid, char **cnic)
 {
 	char *veth1buf, *veth2buf;
 	veth1buf = alloca(IFNAMSIZ);
 	veth2buf = alloca(IFNAMSIZ);
-	int ret;
+	int ret, mtu;
 
 	ret = snprintf(veth1buf, IFNAMSIZ, "%s", nic);
 	if (ret < 0 || ret >= IFNAMSIZ) {
@@ -245,6 +251,16 @@ static bool create_nic(char *nic, char *br, int pid, char **cnic)
 		return false;
 	}
 
+	/* copy the bridge's mtu to both ends */
+	mtu = get_mtu(br);
+	if (mtu != -1) {
+		if (lxc_netdev_set_mtu(veth1buf, mtu) < 0 ||
+				lxc_netdev_set_mtu(veth2buf, mtu) < 0) {
+			fprintf(stderr, "Failed setting mtu\n");
+			goto out_del;
+		}
+	}
+
 	/* attach veth1 to bridge */
 	if (lxc_bridge_attach(br, veth1buf) < 0) {
 		fprintf(stderr, "Error attaching %s to %s\n", veth1buf, br);

src/lxc/network.c

@@ -297,6 +297,116 @@ out:
 	return err;
 }
 
+int netdev_get_mtu(int ifindex)
+{
+	struct nl_handler nlh;
+	struct nlmsg *nlmsg = NULL, *answer = NULL;
+	struct ip_req *ip_req;
+	struct nlmsghdr *msg;
+	int err, res;
+	int recv_len = 0, answer_len;
+	int readmore = 0;
+
+	err = netlink_open(&nlh, NETLINK_ROUTE);
+	if (err)
+		return err;
+
+	err = -ENOMEM;
+	nlmsg = nlmsg_alloc(NLMSG_GOOD_SIZE);
+	if (!nlmsg)
+		goto out;
+
+	answer = nlmsg_alloc(NLMSG_GOOD_SIZE);
+	if (!answer)
+		goto out;
+
+	/* Save the answer buffer length, since it will be overwritten
+	 * on the first receive (and we might need to receive more than
+	 * once. */
+	answer_len = answer->nlmsghdr.nlmsg_len;
+
+	ip_req = (struct ip_req *)nlmsg;
+	ip_req->nlmsg.nlmsghdr.nlmsg_len =
+		NLMSG_LENGTH(sizeof(struct ifaddrmsg));
+	ip_req->nlmsg.nlmsghdr.nlmsg_flags = NLM_F_REQUEST|NLM_F_DUMP;
+	ip_req->nlmsg.nlmsghdr.nlmsg_type = RTM_GETLINK;
+	ip_req->ifa.ifa_family = AF_UNSPEC;
+
+	/* Send the request for addresses, which returns all addresses
+	 * on all interfaces. */
+	err = netlink_send(&nlh, nlmsg);
+	if (err < 0)
+		goto out;
+
+	do {
+		/* Restore the answer buffer length, it might have been
+		 * overwritten by a previous receive. */
+		answer->nlmsghdr.nlmsg_len = answer_len;
+
+		/* Get the (next) batch of reply messages */
+		err = netlink_rcv(&nlh, answer);
+		if (err < 0)
+			goto out;
+
+		recv_len = err;
+		err = 0;
+
+		/* Satisfy the typing for the netlink macros */
+		msg = &answer->nlmsghdr;
+
+		while (NLMSG_OK(msg, recv_len)) {
+
+			/* Stop reading if we see an error message */
+			if (msg->nlmsg_type == NLMSG_ERROR) {
+				struct nlmsgerr *errmsg = (struct nlmsgerr*)NLMSG_DATA(msg);
+				err = errmsg->error;
+				goto out;
+			}
+
+			/* Stop reading if we see a NLMSG_DONE message */
+			if (msg->nlmsg_type == NLMSG_DONE) {
+				readmore = 0;
+				break;
+			}
+
+			struct ifinfomsg *ifi = NLMSG_DATA(msg);
+			if (ifi->ifi_index == ifindex) {
+				struct rtattr *rta = IFLA_RTA(ifi);
+				int attr_len = msg->nlmsg_len - NLMSG_LENGTH(sizeof(*ifi));
+				res = 0;
+				while(RTA_OK(rta, attr_len)) {
+					/* Found a local address for the requested interface,
+					 * return it. */
+					if (rta->rta_type == IFLA_MTU) {
+						memcpy(&res, RTA_DATA(rta), sizeof(int));
+						err = res;
+						goto out;
+					}
+					rta = RTA_NEXT(rta, attr_len);
+				}
+
+			}
+
+			/* Keep reading more data from the socket if the
+			 * last message had the NLF_F_MULTI flag set */
+			readmore = (msg->nlmsg_flags & NLM_F_MULTI);
+
+			/* Look at the next message received in this buffer */
+			msg = NLMSG_NEXT(msg, recv_len);
+		}
+	} while (readmore);
+
+	/* If we end up here, we didn't find any result, so signal an
+	 * error */
+	err = -1;
+
+out:
+	netlink_close(&nlh);
+	nlmsg_free(answer);
+	nlmsg_free(nlmsg);
+	return err;
+}
+
 int lxc_netdev_set_mtu(const char *name, int mtu)
 {
 	struct nl_handler nlh;

src/lxc/network.h

@@ -138,4 +138,5 @@ extern char *lxc_mkifname(char *template);
 
 extern const char *lxc_net_type_to_str(int type);
 extern int setup_private_host_hw_addr(char *veth1);
+extern int netdev_get_mtu(int ifindex);
 #endif