Merge #776

776: Add file descriptor support for tuntap devices r=Dirbaio a=blechschmidt

Android exposes a file descriptor to a TUN interface through the [VpnService](https://developer.android.com/reference/android/net/VpnService) interface. This commit adds support for operating on such a file descriptor directly.

Co-authored-by: B. Blechschmidt <git@blechschmidt.io>

src/phy/sys/tuntap_interface.rs

@@ -6,8 +6,7 @@ use std::os::unix::io::{AsRawFd, RawFd};
 #[derive(Debug)]
 pub struct TunTapInterfaceDesc {
     lower: libc::c_int,
-    ifreq: ifreq,
-    medium: Medium,
+    mtu: usize,
 }
 
 impl AsRawFd for TunTapInterfaceDesc {
@@ -29,15 +28,23 @@ impl TunTapInterfaceDesc {
             lower
         };
 
-        Ok(TunTapInterfaceDesc {
-            lower,
-            ifreq: ifreq_for(name),
-            medium,
-        })
+        let mut ifreq = ifreq_for(name);
+        Self::attach_interface_ifreq(lower, medium, &mut ifreq)?;
+        let mtu = Self::mtu_ifreq(medium, &mut ifreq)?;
+
+        Ok(TunTapInterfaceDesc { lower, mtu })
+    }
+
+    pub fn from_fd(fd: RawFd, mtu: usize) -> io::Result<TunTapInterfaceDesc> {
+        Ok(TunTapInterfaceDesc { lower: fd, mtu })
     }
 
-    pub fn attach_interface(&mut self) -> io::Result<()> {
-        let mode = match self.medium {
+    fn attach_interface_ifreq(
+        lower: libc::c_int,
+        medium: Medium,
+        ifr: &mut ifreq,
+    ) -> io::Result<()> {
+        let mode = match medium {
             #[cfg(feature = "medium-ip")]
             Medium::Ip => imp::IFF_TUN,
             #[cfg(feature = "medium-ethernet")]
@@ -45,11 +52,11 @@ impl TunTapInterfaceDesc {
             #[cfg(feature = "medium-ieee802154")]
             Medium::Ieee802154 => todo!(),
         };
-        self.ifreq.ifr_data = mode | imp::IFF_NO_PI;
-        ifreq_ioctl(self.lower, &mut self.ifreq, imp::TUNSETIFF).map(|_| ())
+        ifr.ifr_data = mode | imp::IFF_NO_PI;
+        ifreq_ioctl(lower, ifr, imp::TUNSETIFF).map(|_| ())
     }
 
-    pub fn interface_mtu(&mut self) -> io::Result<usize> {
+    fn mtu_ifreq(medium: Medium, ifr: &mut ifreq) -> io::Result<usize> {
         let lower = unsafe {
             let lower = libc::socket(libc::AF_INET, libc::SOCK_DGRAM, libc::IPPROTO_IP);
             if lower == -1 {
@@ -58,7 +65,7 @@ impl TunTapInterfaceDesc {
             lower
         };
 
-        let ip_mtu = ifreq_ioctl(lower, &mut self.ifreq, imp::SIOCGIFMTU).map(|mtu| mtu as usize);
+        let ip_mtu = ifreq_ioctl(lower, ifr, imp::SIOCGIFMTU).map(|mtu| mtu as usize);
 
         unsafe {
             libc::close(lower);
@@ -69,7 +76,7 @@ impl TunTapInterfaceDesc {
 
         // SIOCGIFMTU returns the IP MTU (typically 1500 bytes.)
         // smoltcp counts the entire Ethernet packet in the MTU, so add the Ethernet header size to it.
-        let mtu = match self.medium {
+        let mtu = match medium {
             #[cfg(feature = "medium-ip")]
             Medium::Ip => ip_mtu,
             #[cfg(feature = "medium-ethernet")]
@@ -81,6 +88,10 @@ impl TunTapInterfaceDesc {
         Ok(mtu)
     }
 
+    pub fn interface_mtu(&self) -> io::Result<usize> {
+        Ok(self.mtu)
+    }
+
     pub fn recv(&mut self, buffer: &mut [u8]) -> io::Result<usize> {
         unsafe {
             let len = libc::read(

src/phy/tuntap_interface.rs

@@ -28,8 +28,7 @@ impl TunTapInterface {
     /// no special privileges are needed. Otherwise, this requires superuser privileges
     /// or a corresponding capability set on the executable.
     pub fn new(name: &str, medium: Medium) -> io::Result<TunTapInterface> {
-        let mut lower = sys::TunTapInterfaceDesc::new(name, medium)?;
-        lower.attach_interface()?;
+        let lower = sys::TunTapInterfaceDesc::new(name, medium)?;
         let mtu = lower.interface_mtu()?;
         Ok(TunTapInterface {
             lower: Rc::new(RefCell::new(lower)),
@@ -37,6 +36,19 @@ impl TunTapInterface {
             medium,
         })
     }
+
+    /// Attaches to a TUN/TAP interface specified by file descriptor `fd`.
+    ///
+    /// On platforms like Android, a file descriptor to a tun interface is exposed.
+    /// On these platforms, a TunTapInterface cannot be instantiated with a name.
+    pub fn from_fd(fd: RawFd, medium: Medium, mtu: usize) -> io::Result<TunTapInterface> {
+        let lower = sys::TunTapInterfaceDesc::from_fd(fd, mtu)?;
+        Ok(TunTapInterface {
+            lower: Rc::new(RefCell::new(lower)),
+            mtu,
+            medium,
+        })
+    }
 }
 
 impl Device for TunTapInterface {