From 79dbf2378420190aa5220f82469290f6dd39ce2a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophile=20Bastian?= <contact@tobast.fr>
Date: Tue, 16 Jun 2020 18:57:08 +0200
Subject: [PATCH] Set tun MTU based on VpnPacket max payload

---
 TunDevice.cpp | 25 +++++++++++++++++++++++++
 TunDevice.hpp |  6 ++++++
 UdpVpn.cpp    |  1 +
 VpnPacket.cpp |  4 ++++
 VpnPacket.hpp |  3 +++
 5 files changed, 39 insertions(+)

diff --git a/TunDevice.cpp b/TunDevice.cpp
index 33bf0a1..2767811 100644
--- a/TunDevice.cpp
+++ b/TunDevice.cpp
@@ -72,6 +72,31 @@ TunDevice::~TunDevice() {
     close(_fd);
 }
 
+uint16_t TunDevice::get_mtu() const {
+    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);  // Any socket will do
+
+    struct ifreq ifr;
+    ifr.ifr_addr.sa_family = AF_INET6;
+    strncpy(ifr.ifr_name, _dev_name.c_str(), sizeof(ifr.ifr_name)-1);
+    if (ioctl(sockfd, SIOCGIFMTU, (caddr_t)&ifr) < 0)
+        return 0;
+    close(sockfd);
+    return ifr.ifr_mtu;
+}
+
+bool TunDevice::set_mtu(uint16_t mtu) {
+    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);  // Any socket will do
+
+    struct ifreq ifr;
+    ifr.ifr_addr.sa_family = AF_INET6;
+    strncpy(ifr.ifr_name, _dev_name.c_str(), sizeof(ifr.ifr_name)-1);
+    ifr.ifr_mtu = mtu;
+    if (ioctl(sockfd, SIOCSIFMTU, (caddr_t)&ifr) < 0)
+        return false;
+    close(sockfd);
+    return true;
+}
+
 size_t TunDevice::poll_packet(char* read_buffer, size_t buf_size, int timeout) {
     int poll_rc = poll(&_poll_fd, 1, timeout);
     if(poll_rc < 0) {
diff --git a/TunDevice.hpp b/TunDevice.hpp
index a81b914..0df8b71 100644
--- a/TunDevice.hpp
+++ b/TunDevice.hpp
@@ -29,6 +29,12 @@ class TunDevice {
         const std::string& get_dev_name() const { return _dev_name; }
         int get_fd() const { return _fd; }
 
+        /** Get the interface's MTU */
+        uint16_t get_mtu() const;
+
+        /** Set the interface's MTU */
+        bool set_mtu(uint16_t mtu);
+
         /* Reads a packet from the device.
          * Timeouts after `timeout` ms, or never if `timeout < 0`.
          * Upon timeout, returns 0.
diff --git a/UdpVpn.cpp b/UdpVpn.cpp
index 6a0d4bd..1b6b2c3 100644
--- a/UdpVpn.cpp
+++ b/UdpVpn.cpp
@@ -16,6 +16,7 @@ static const size_t VPN_MTU = 1460; // TODO determine this -- issue #3
 UdpVpn::UdpVpn()
     : _stopped(false), _vpn_mtu(VPN_MTU), _tun_dev("cvpn%d"), _peer(nullptr)
 {
+    _tun_dev.set_mtu(VpnPacket::get_tunnelled_mtu(_vpn_mtu));
     _socket = socket(AF_INET6, SOCK_DGRAM, 0);
     if(_socket < 0)
         throw UdpVpn::InitializationError("Cannot create socket", errno, true);
diff --git a/VpnPacket.cpp b/VpnPacket.cpp
index 3dfcf61..f003dd4 100644
--- a/VpnPacket.cpp
+++ b/VpnPacket.cpp
@@ -30,6 +30,10 @@ VpnPacketTLV VpnPacket::first_tlv() {
     return VpnPacketTLV(*this, 0);
 }
 
+size_t VpnPacket::get_tunnelled_mtu(size_t udp_mtu) {
+    return udp_mtu - VPN_HEADER_BYTES - TLV_HEADER_BYTES;
+}
+
 uint32_t VpnPacket::get_seqno() const {
     return *(uint32_t*)(_data + DATA_SEQNO_POS);
 }
diff --git a/VpnPacket.hpp b/VpnPacket.hpp
index f4df618..dfa3284 100644
--- a/VpnPacket.hpp
+++ b/VpnPacket.hpp
@@ -56,6 +56,9 @@ class VpnPacket {
 
         VpnPacketTLV first_tlv();
 
+        /// Get the maximal payload space for a given tunnel MTU
+        static size_t get_tunnelled_mtu(size_t udp_mtu);
+
         /// Set packet peer -- used for sequence numbers
         void set_peer(VpnPeer* peer) { _peer = peer; }