lkl tools: allow setting the MTU of an interface

tools/lkl/include/lkl.h

@@ -170,6 +170,15 @@ int lkl_if_up(int ifindex);
  */
 int lkl_if_down(int ifindex);
 
+/**
+ * lkl_if_set_mtu - set MTU on interface
+ *
+ * @ifindex - the ifindex of the interface
+ * @mtu - the requested MTU size
+ * @returns - return 0 if no error: otherwise negative value returns
+ */
+int lkl_if_set_mtu(int ifindex, int mtu);
+
 /**
  * lkl_if_set_ipv4 - set IPv4 address on interface
  *

tools/lkl/lib/hijack/init.c

@@ -35,6 +35,7 @@ hijack_init(void)
 {
 	int ret, i, dev_null, nd_id = -1, nd_ifindex = -1;
 	char *tap = getenv("LKL_HIJACK_NET_TAP");
+	char *mtu_str = getenv("LKL_HIJACK_NET_MTU");
 	char *ip = getenv("LKL_HIJACK_NET_IP");
 	char *netmask_len = getenv("LKL_HIJACK_NET_NETMASK_LEN");
 	char *gateway = getenv("LKL_HIJACK_NET_GATEWAY");
@@ -105,6 +106,14 @@ hijack_init(void)
 				nd_id, lkl_strerror(nd_ifindex));
 	}
 
+	if (nd_ifindex >= 0 && mtu_str) {
+		int mtu = atoi(mtu_str);
+
+		ret = lkl_if_set_mtu(nd_ifindex, mtu);
+		if (ret < 0)
+			fprintf(stderr, "failed to set MTU: %s\n", lkl_strerror(ret));
+	}
+
 	if (nd_ifindex >= 0 && ip && netmask_len) {
 		unsigned int addr = inet_addr(ip);
 		int nmlen = atoi(netmask_len);

tools/lkl/lib/net.c

@@ -65,6 +65,28 @@ int lkl_if_down(int ifindex)
 	return err;
 }
 
+int lkl_if_set_mtu(int ifindex, int mtu)
+{
+	struct lkl_ifreq ifr;
+	int err, sock;
+
+	sock = lkl_sys_socket(LKL_AF_INET, LKL_SOCK_DGRAM, 0);
+	if (sock < 0)
+		return sock;
+
+	err = ifindex_to_name(sock, &ifr, ifindex);
+	if (err < 0)
+		return err;
+
+	ifr.lkl_ifr_mtu = mtu;
+
+	err = lkl_sys_ioctl(sock, LKL_SIOCSIFMTU, (long)&ifr);
+
+	lkl_sys_close(sock);
+
+	return err;
+}
+
 int lkl_if_set_ipv4(int ifindex, unsigned int addr, unsigned int netmask_len)
 {
 	struct lkl_ifreq ifr;