libknot: add MTU getter

src/libknot/xdp/eth.c

-/*  Copyright (C) 2020 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2021 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -69,6 +69,36 @@ int knot_eth_queues(const char *devname)
 	return ret;
 }
 
+_public_
+int knot_eth_mtu(const char *devname)
+{
+	if (devname == NULL) {
+		return KNOT_EINVAL;
+	}
+
+	int fd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (fd < 0) {
+		return knot_map_errno();
+	}
+
+	struct ifreq ifr = { 0 };
+	strlcpy(ifr.ifr_name, devname, IFNAMSIZ);
+
+	int ret = ioctl(fd, SIOCGIFMTU, &ifr);
+	if (ret != 0) {
+		if (errno == EOPNOTSUPP) {
+			ret = KNOT_ENOTSUP;
+		} else {
+			ret = knot_map_errno();
+		}
+	} else {
+		ret = ifr.ifr_mtu;
+	}
+
+	close(fd);
+	return ret;
+}
+
 _public_
 int knot_eth_name_from_addr(const struct sockaddr_storage *addr, char *out,
                             size_t out_len)

src/libknot/xdp/eth.h

-/*  Copyright (C) 2020 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2021 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -18,6 +18,8 @@
 
 #include <stddef.h>
 
+#define KNOT_XDP_MAX_MTU 1792
+
 /*!
  * \brief Get number of combined queues of a network interface.
  *
@@ -29,6 +31,16 @@
  */
 int knot_eth_queues(const char *devname);
 
+/*!
+ * \brief Get value of MTU setup on a network interface.
+ *
+ * \param devname  Name of the ethdev (e.g. eth1).
+ *
+ * \retval < 0    KNOT_E* if error.
+ * \return >= 0   Interface MTU.
+ */
+int knot_eth_mtu(const char *devname);
+
 /*!
  * \brief Get the corresponding network interface name for the address.
  *