Implement IPv4 address configuration on Linux

This involved refactoring the crate structure to share code between
macOS and Linux. The new methods have not yet been implemented on
macOS, but they have todo!() placeholders.

tun/src/unix/linux/mod.rs

@@ -0,0 +1,84 @@
+use fehler::throws;
+
+use socket2::{Domain, SockAddr, Socket, Type};
+use std::fs::OpenOptions;
+use std::io::Error;
+use std::mem;
+use std::net::{Ipv4Addr, SocketAddrV4};
+use std::os::fd::RawFd;
+use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd};
+
+use super::{ifname_to_string, string_to_ifname};
+
+mod sys;
+
+#[derive(Debug)]
+pub struct TunInterface {
+    pub(crate) socket: socket2::Socket,
+}
+
+impl TunInterface {
+    #[throws]
+    pub fn new() -> TunInterface {
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .open("/dev/net/tun")?;
+
+        let iff = libc::ifreq {
+            ifr_name: [0; libc::IFNAMSIZ],
+            ifr_ifru: libc::__c_anonymous_ifr_ifru {
+                ifru_flags: (libc::IFF_TUN | libc::IFF_TUN_EXCL | libc::IFF_NO_PI) as i16,
+            },
+        };
+        unsafe { sys::tun_set_iff(file.as_raw_fd(), &iff)? };
+
+        let socket = unsafe { socket2::Socket::from_raw_fd(file.into_raw_fd()) };
+        TunInterface { socket }
+    }
+
+    #[throws]
+    pub fn name(&self) -> String {
+        let mut iff = unsafe { mem::zeroed() };
+        unsafe { sys::tun_get_iff(self.socket.as_raw_fd(), &mut iff)? };
+        ifname_to_string(iff.ifr_name)
+    }
+
+    #[throws]
+    fn ifreq(&self) -> sys::ifreq {
+        let mut iff: sys::ifreq = unsafe { mem::zeroed() };
+        iff.ifr_name = string_to_ifname(&self.name()?);
+        iff
+    }
+
+    #[throws]
+    pub fn index(&self) -> i32 {
+        let mut iff = self.ifreq()?;
+        self.perform(|fd| unsafe { sys::if_get_index(fd, &mut iff) })?;
+        unsafe { iff.ifr_ifru.ifru_ifindex }
+    }
+
+    #[throws]
+    pub fn set_ipv4_addr(&self, addr: Ipv4Addr) {
+        let addr = SockAddr::from(SocketAddrV4::new(addr, 0));
+
+        let mut iff = self.ifreq()?;
+        iff.ifr_ifru.ifru_addr = unsafe { *addr.as_ptr() };
+
+        self.perform(|fd| unsafe { sys::if_set_addr(fd, &iff) })?;
+    }
+
+    #[throws]
+    pub fn ipv4_addr(&self) -> Ipv4Addr {
+        let mut iff = self.ifreq()?;
+        self.perform(|fd| unsafe { sys::if_get_addr(fd, &mut iff) })?;
+        let addr = unsafe { *(&iff.ifr_ifru.ifru_addr as *const _ as *const sys::sockaddr_in) };
+        Ipv4Addr::from(u32::from_be(addr.sin_addr.s_addr))
+    }
+
+    #[throws]
+    fn perform<R>(&self, perform: impl FnOnce(RawFd) -> Result<R, nix::Error>) -> R {
+        let socket = Socket::new(Domain::IPV4, Type::DGRAM, None)?;
+        perform(socket.as_raw_fd())?
+    }
+}