From bad910084fe35cba4c40c4b04733c9e4ee56a05d Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 09:54:28 +0300
Subject: [PATCH 01/21] feat: Also return an interface ID

---
 Cargo.toml |  2 +-
 README.md  |  8 +++---
 src/lib.rs | 73 ++++++++++++++++++++++++++++++++++++++++++++----------
 3 files changed, 66 insertions(+), 17 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 41bac4e..582ebbb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,7 +7,7 @@ homepage = "https://github.com/mozilla/mtu/"
 repository = "https://github.com/mozilla/mtu/"
 authors = ["The Mozilla Necko Team <necko@mozilla.com>"]
 readme = "README.md"
-version = "0.1.1"
+version = "0.2.0"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 # Don't increase beyond what Firefox is currently using:
diff --git a/README.md b/README.md
index a6b1f41..1efff94 100644
--- a/README.md
+++ b/README.md
@@ -4,13 +4,13 @@ A crate to return the maximum transmission unit (MTU) of the local network inter
 
 ## Usage
 
-This crate exports a single function
+The main function exported by this crate is
 
 ```rust
-pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error>
+pub fn get_interface_and_mtu(remote: &SocketAddr) -> Result<(InterfaceId, usize), Error>
 ```
 
-that returns the MTU of the local network interface towards the `remote` destination, or an `Error` when the MTU could not be determined. It supports both IPv4 and IPv6.
+that returns an opaque identifier of the local network interface towards the `remote` destination together with its MTU, or an `Error` when the MTU could not be determined. It supports both IPv4 and IPv6.
 
 ## Supported Platforms
 
@@ -22,6 +22,8 @@ that returns the MTU of the local network interface towards the `remote` destina
 
 The returned MTU may exceed the maximum IP packet size of 65,535 bytes on some platforms for some remote destinations. (For example, loopback destinations on Windows.)
 
+The returned interface ID is an opaque identifier that can be used to identify the local interface. It is a hash of the interface name (on Linux and macOS) or interface index (on Windows), and has the same stability guarantees as those identifiers.
+
 ## Contributing
 
 We're happy to receive PRs that improve this crate. Please take a look at our [community guidelines](CODE_OF_CONDUCT.md) beforehand.
diff --git a/src/lib.rs b/src/lib.rs
index c2a939b..30869eb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,32 +13,38 @@ use std::{
 use log::trace;
 
 /// Prepare a default error result.
-fn default_result<T>() -> Result<T, Error> {
+fn default_result<T>() -> Result<(InterfaceId, T), Error> {
     Err(Error::new(
         ErrorKind::NotFound,
         "Local interface MTU not found",
     ))
 }
 
-/// Return the maximum transmission unit (MTU) of the local network interface towards the
-/// destination [`SocketAddr`] given in `remote`.
+type InterfaceId = u64;
+
+/// Return a unique interface ID and the maximum transmission unit (MTU) of the local network
+/// interface towards the destination [`SocketAddr`] given in `remote`.
 ///
 /// The returned MTU may exceed the maximum IP packet size of 65,535 bytes on some
 /// platforms for some remote destinations. (For example, loopback destinations on
 /// Windows.)
 ///
+/// The returned interface ID is an opaque identifier that can be used to identify the local
+/// interface. It is a hash of the interface name (on Linux and macOS) or interface index (on
+/// Windows), and has the same stability guarantees as those identifiers.
+///
 /// # Examples
 ///
 /// ```
 /// let saddr = "127.0.0.1:443".parse().unwrap();
-/// let mtu = mtu::get_interface_mtu(&saddr).unwrap();
+/// let (id, mtu) = mtu::get_interface_and_mtu(&saddr).unwrap();
 /// println!("MTU towards {:?} is {}", saddr, mtu);
 /// ```
 ///
 /// # Errors
 ///
 /// This function returns an error if the local interface MTU cannot be determined.
-pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
+pub fn get_interface_and_mtu(remote: &SocketAddr) -> Result<(InterfaceId, usize), Error> {
     #[cfg(any(target_os = "macos", target_os = "linux", target_os = "windows"))]
     #[allow(unused_assignments)] // Yes, res is reassigned in the platform-specific code.
     let mut res = default_result();
@@ -59,12 +65,12 @@ pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
 
         #[cfg(any(target_os = "macos", target_os = "linux"))]
         {
-            res = get_interface_mtu_linux_macos(&socket);
+            res = get_interface_and_mtu_linux_macos(&socket);
         }
 
         #[cfg(target_os = "windows")]
         {
-            res = get_interface_mtu_windows(&socket);
+            res = get_interface_and_mtu_windows(&socket);
         }
     }
 
@@ -73,10 +79,13 @@ pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
 }
 
 #[cfg(any(target_os = "macos", target_os = "linux"))]
-fn get_interface_mtu_linux_macos(socket: &UdpSocket) -> Result<usize, Error> {
-    use std::ffi::{c_int, CStr};
+fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
     #[cfg(target_os = "linux")]
     use std::{ffi::c_char, mem, os::fd::AsRawFd};
+    use std::{
+        ffi::{c_int, CStr},
+        hash::{DefaultHasher, Hash, Hasher},
+    };
 
     use libc::{
         freeifaddrs, getifaddrs, ifaddrs, in_addr_t, sockaddr_in, sockaddr_in6, AF_INET, AF_INET6,
@@ -126,6 +135,10 @@ fn get_interface_mtu_linux_macos(socket: &UdpSocket) -> Result<usize, Error> {
     // If we have found the interface name we are looking for, find the MTU.
     let mut res = default_result();
     if let Some(iface) = iface {
+        let mut hasher = DefaultHasher::new();
+        iface.hash(&mut hasher);
+        let id = hasher.finish();
+
         #[cfg(target_os = "macos")]
         {
             // On macOS, we need to loop again to find the MTU of that interface. We need to
@@ -147,7 +160,9 @@ fn get_interface_mtu_linux_macos(socket: &UdpSocket) -> Result<usize, Error> {
                         && name == iface
                     {
                         let data = unsafe { &*(ifa.ifa_data as *const if_data) };
-                        res = usize::try_from(data.ifi_mtu).or(res);
+                        if let Ok(mtu) = usize::try_from(data.ifi_mtu) {
+                            res = Ok((id, mtu));
+                        }
                         break;
                     }
                 }
@@ -165,7 +180,9 @@ fn get_interface_mtu_linux_macos(socket: &UdpSocket) -> Result<usize, Error> {
             if unsafe { ioctl(socket.as_raw_fd(), libc::SIOCGIFMTU, &ifr) } != 0 {
                 res = Err(Error::last_os_error());
             } else {
-                res = unsafe { usize::try_from(ifr.ifr_ifru.ifru_mtu).or(res) };
+                if let Ok(mtu) = usize::try_from(ifr.ifr_ifru.ifru_mtu) {
+                    res = Ok((id, mtu));
+                }
             }
         }
     }
@@ -175,7 +192,7 @@ fn get_interface_mtu_linux_macos(socket: &UdpSocket) -> Result<usize, Error> {
 }
 
 #[cfg(target_os = "windows")]
-fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<usize, Error> {
+fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
     use std::{ffi::c_void, slice};
 
     use windows::Win32::{
@@ -235,7 +252,12 @@ fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<usize, Error> {
             // For the matching address, find local interface and its MTU.
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
-                    res = iface.NlMtu.try_into().or(res);
+                    if Ok(mtu) = iface.NlMtu.try_into() {
+                        let mut hasher = DefaultHasher::new();
+                        iface.InterfaceIndex.hash(&mut hasher);
+                        let id = hasher.finish();
+                        res = Ok((id, mtu));
+                    }
                     break 'addr_loop;
                 }
             }
@@ -248,6 +270,31 @@ fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<usize, Error> {
     res
 }
 
+/// Return the maximum transmission unit (MTU) of the local network interface towards the
+/// destination [`SocketAddr`] given in `remote`.
+///
+/// The returned MTU may exceed the maximum IP packet size of 65,535 bytes on some
+/// platforms for some remote destinations. (For example, loopback destinations on
+/// Windows.)
+///
+/// This function is a convenience wrapper around [`get_interface_and_mtu`] that only returns the
+/// MTU. It is provided for compatibility with version 0.1 of the `mtu` crate.
+///
+/// # Examples
+///
+/// ```
+/// let saddr = "127.0.0.1:443".parse().unwrap();
+/// let mtu = mtu::get_interface_mtu(&saddr).unwrap();
+/// println!("MTU towards {:?} is {}", saddr, mtu);
+/// ```
+///
+/// # Errors
+///
+/// This function returns an error if the local interface MTU cannot be determined.
+pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
+    get_interface_and_mtu(remote).map(|(_, mtu)| mtu)
+}
+
 #[cfg(test)]
 mod test {
     use std::net::ToSocketAddrs;

From a473b55e7e8bcf573b6d36d00df9197dd41b619a Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 09:59:52 +0300
Subject: [PATCH 02/21] Fixes

---
 src/lib.rs | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 30869eb..3d0e84c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -179,10 +179,8 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
             });
             if unsafe { ioctl(socket.as_raw_fd(), libc::SIOCGIFMTU, &ifr) } != 0 {
                 res = Err(Error::last_os_error());
-            } else {
-                if let Ok(mtu) = usize::try_from(ifr.ifr_ifru.ifru_mtu) {
-                    res = Ok((id, mtu));
-                }
+            } else if let Ok(mtu) = usize::try_from(unsafe { ifr.ifr_ifru.ifru_mtu }) {
+                res = Ok((id, mtu));
             }
         }
     }
@@ -193,7 +191,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
 
 #[cfg(target_os = "windows")]
 fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
-    use std::{ffi::c_void, slice};
+    use std::{ffi::c_void, hash::DefaultHasher, slice};
 
     use windows::Win32::{
         Foundation::NO_ERROR,

From 09f2ebe82013d2f681b64713a014176e659a09f5 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 10:05:09 +0300
Subject: [PATCH 03/21] Fixes

---
 src/lib.rs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 3d0e84c..c481a80 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -191,7 +191,11 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
 
 #[cfg(target_os = "windows")]
 fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
-    use std::{ffi::c_void, hash::DefaultHasher, slice};
+    use std::{
+        ffi::c_void,
+        hash::{DefaultHasher, Hasher},
+        slice,
+    };
 
     use windows::Win32::{
         Foundation::NO_ERROR,
@@ -250,7 +254,7 @@ fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize),
             // For the matching address, find local interface and its MTU.
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
-                    if Ok(mtu) = iface.NlMtu.try_into() {
+                    if let Ok(mtu) = iface.NlMtu.try_into() {
                         let mut hasher = DefaultHasher::new();
                         iface.InterfaceIndex.hash(&mut hasher);
                         let id = hasher.finish();

From 4f4e98037bbc7362493191a892bee6405e5198d4 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 10:05:49 +0300
Subject: [PATCH 04/21] Fixes

---
 src/lib.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index c481a80..dd33181 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -190,7 +190,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
 }
 
 #[cfg(target_os = "windows")]
-fn get_interface_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
+fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
     use std::{
         ffi::c_void,
         hash::{DefaultHasher, Hasher},

From b1f75255a182040dbb7476bbae1bddddda51e70b Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 10:11:47 +0300
Subject: [PATCH 05/21] Fixes

---
 src/lib.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index dd33181..dd95d6d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -193,7 +193,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
 fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
     use std::{
         ffi::c_void,
-        hash::{DefaultHasher, Hasher},
+        hash::{DefaultHasher, Hash, Hasher},
         slice,
     };
 

From ddcd63dcd5e519a18e0f6ddf69a671707a00ce98 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 10:24:15 +0300
Subject: [PATCH 06/21] Simplify

---
 src/lib.rs | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index dd95d6d..d1c9b7e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -95,6 +95,12 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
     #[cfg(target_os = "linux")]
     use libc::{ifreq, ioctl};
 
+    fn hash_interface_name(name: &str) -> InterfaceId {
+        let mut hasher = DefaultHasher::new();
+        name.hash(&mut hasher);
+        hasher.finish()
+    }
+
     // Get the interface list.
     let mut ifap: *mut ifaddrs = ptr::null_mut();
     if unsafe { getifaddrs(&mut ifap) } != 0 {
@@ -135,10 +141,6 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
     // If we have found the interface name we are looking for, find the MTU.
     let mut res = default_result();
     if let Some(iface) = iface {
-        let mut hasher = DefaultHasher::new();
-        iface.hash(&mut hasher);
-        let id = hasher.finish();
-
         #[cfg(target_os = "macos")]
         {
             // On macOS, we need to loop again to find the MTU of that interface. We need to
@@ -161,7 +163,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
                     {
                         let data = unsafe { &*(ifa.ifa_data as *const if_data) };
                         if let Ok(mtu) = usize::try_from(data.ifi_mtu) {
-                            res = Ok((id, mtu));
+                            res = Ok((hash_interface_name(iface), mtu));
                         }
                         break;
                     }
@@ -180,7 +182,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
             if unsafe { ioctl(socket.as_raw_fd(), libc::SIOCGIFMTU, &ifr) } != 0 {
                 res = Err(Error::last_os_error());
             } else if let Ok(mtu) = usize::try_from(unsafe { ifr.ifr_ifru.ifru_mtu }) {
-                res = Ok((id, mtu));
+                res = Ok((hash_interface_name(iface), mtu));
             }
         }
     }

From c1156ca4d9ed9703f810009e6653b921294bd75f Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 11:31:44 +0300
Subject: [PATCH 07/21] Update Cargo.toml

Co-authored-by: Max Inden <mail@max-inden.de>
---
 Cargo.toml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Cargo.toml b/Cargo.toml
index 582ebbb..fcce6fb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,7 +7,7 @@ homepage = "https://github.com/mozilla/mtu/"
 repository = "https://github.com/mozilla/mtu/"
 authors = ["The Mozilla Necko Team <necko@mozilla.com>"]
 readme = "README.md"
-version = "0.2.0"
+version = "0.1.2"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 # Don't increase beyond what Firefox is currently using:

From dce0de85896c9a5a629bcd013f77caf4602365e5 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 11:48:25 +0300
Subject: [PATCH 08/21] Return interface name

---
 src/lib.rs | 49 +++++++++++++++++--------------------------------
 1 file changed, 17 insertions(+), 32 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index d1c9b7e..fc5a49b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,15 +13,13 @@ use std::{
 use log::trace;
 
 /// Prepare a default error result.
-fn default_result<T>() -> Result<(InterfaceId, T), Error> {
+fn default_result<T>() -> Result<(String, T), Error> {
     Err(Error::new(
         ErrorKind::NotFound,
         "Local interface MTU not found",
     ))
 }
 
-type InterfaceId = u64;
-
 /// Return a unique interface ID and the maximum transmission unit (MTU) of the local network
 /// interface towards the destination [`SocketAddr`] given in `remote`.
 ///
@@ -37,14 +35,14 @@ type InterfaceId = u64;
 ///
 /// ```
 /// let saddr = "127.0.0.1:443".parse().unwrap();
-/// let (id, mtu) = mtu::get_interface_and_mtu(&saddr).unwrap();
-/// println!("MTU towards {:?} is {}", saddr, mtu);
+/// let (name, mtu) = mtu::get_interface_and_mtu(&saddr).unwrap();
+/// println!("MTU towards {saddr:?} is {mtu} on {name}");
 /// ```
 ///
 /// # Errors
 ///
 /// This function returns an error if the local interface MTU cannot be determined.
-pub fn get_interface_and_mtu(remote: &SocketAddr) -> Result<(InterfaceId, usize), Error> {
+pub fn get_interface_and_mtu(remote: &SocketAddr) -> Result<(String, usize), Error> {
     #[cfg(any(target_os = "macos", target_os = "linux", target_os = "windows"))]
     #[allow(unused_assignments)] // Yes, res is reassigned in the platform-specific code.
     let mut res = default_result();
@@ -79,13 +77,10 @@ pub fn get_interface_and_mtu(remote: &SocketAddr) -> Result<(InterfaceId, usize)
 }
 
 #[cfg(any(target_os = "macos", target_os = "linux"))]
-fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
+fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(String, usize), Error> {
+    use std::ffi::{c_int, CStr};
     #[cfg(target_os = "linux")]
     use std::{ffi::c_char, mem, os::fd::AsRawFd};
-    use std::{
-        ffi::{c_int, CStr},
-        hash::{DefaultHasher, Hash, Hasher},
-    };
 
     use libc::{
         freeifaddrs, getifaddrs, ifaddrs, in_addr_t, sockaddr_in, sockaddr_in6, AF_INET, AF_INET6,
@@ -95,12 +90,6 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
     #[cfg(target_os = "linux")]
     use libc::{ifreq, ioctl};
 
-    fn hash_interface_name(name: &str) -> InterfaceId {
-        let mut hasher = DefaultHasher::new();
-        name.hash(&mut hasher);
-        hasher.finish()
-    }
-
     // Get the interface list.
     let mut ifap: *mut ifaddrs = ptr::null_mut();
     if unsafe { getifaddrs(&mut ifap) } != 0 {
@@ -163,7 +152,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
                     {
                         let data = unsafe { &*(ifa.ifa_data as *const if_data) };
                         if let Ok(mtu) = usize::try_from(data.ifi_mtu) {
-                            res = Ok((hash_interface_name(iface), mtu));
+                            res = Ok((iface.to_string(), mtu));
                         }
                         break;
                     }
@@ -182,7 +171,7 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
             if unsafe { ioctl(socket.as_raw_fd(), libc::SIOCGIFMTU, &ifr) } != 0 {
                 res = Err(Error::last_os_error());
             } else if let Ok(mtu) = usize::try_from(unsafe { ifr.ifr_ifru.ifru_mtu }) {
-                res = Ok((hash_interface_name(iface), mtu));
+                res = Ok((iface.to_string(), mtu));
             }
         }
     }
@@ -192,18 +181,15 @@ fn get_interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(InterfaceId,
 }
 
 #[cfg(target_os = "windows")]
-fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usize), Error> {
-    use std::{
-        ffi::c_void,
-        hash::{DefaultHasher, Hash, Hasher},
-        slice,
-    };
+fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Error> {
+    use std::{ffi::c_void, slice};
 
     use windows::Win32::{
         Foundation::NO_ERROR,
         NetworkManagement::IpHelper::{
-            FreeMibTable, GetIpInterfaceTable, GetUnicastIpAddressTable, MIB_IPINTERFACE_ROW,
-            MIB_IPINTERFACE_TABLE, MIB_UNICASTIPADDRESS_ROW, MIB_UNICASTIPADDRESS_TABLE,
+            if_indextoname, FreeMibTable, GetIpInterfaceTable, GetUnicastIpAddressTable,
+            MIB_IPINTERFACE_ROW, MIB_IPINTERFACE_TABLE, MIB_UNICASTIPADDRESS_ROW,
+            MIB_UNICASTIPADDRESS_TABLE,
         },
         Networking::WinSock::{AF_INET, AF_INET6, AF_UNSPEC},
     };
@@ -257,10 +243,9 @@ fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usi
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
                     if let Ok(mtu) = iface.NlMtu.try_into() {
-                        let mut hasher = DefaultHasher::new();
-                        iface.InterfaceIndex.hash(&mut hasher);
-                        let id = hasher.finish();
-                        res = Ok((id, mtu));
+                        if_indextoname(iface.InterfaceIndex, |name| {
+                            res = Ok((name, mtu));
+                        });
                     }
                     break 'addr_loop;
                 }
@@ -289,7 +274,7 @@ fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(InterfaceId, usi
 /// ```
 /// let saddr = "127.0.0.1:443".parse().unwrap();
 /// let mtu = mtu::get_interface_mtu(&saddr).unwrap();
-/// println!("MTU towards {:?} is {}", saddr, mtu);
+/// println!("MTU towards {saddr:?} is {mtu}");
 /// ```
 ///
 /// # Errors

From f53cd88f367cf672167458682a56ea764624992c Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:03:40 +0300
Subject: [PATCH 09/21] Fixes

---
 src/lib.rs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index fc5a49b..68d8d0a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -243,9 +243,10 @@ fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize),
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
                     if let Ok(mtu) = iface.NlMtu.try_into() {
-                        if_indextoname(iface.InterfaceIndex, |name| {
+                        let name = String::with_capacity(256); // IF_NAMESIZE not available?
+                        if !if_indextoname(iface.InterfaceIndex, &name).is_null() {
                             res = Ok((name, mtu));
-                        });
+                        }
                     }
                     break 'addr_loop;
                 }

From 734d33ef6aca874d5c2fdcdcc7a60ef6f1cc7dc1 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:07:14 +0300
Subject: [PATCH 10/21] Fixes

---
 src/lib.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index 68d8d0a..5da21fa 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -244,7 +244,7 @@ fn get_interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize),
                 if iface.InterfaceIndex == addr.InterfaceIndex {
                     if let Ok(mtu) = iface.NlMtu.try_into() {
                         let name = String::with_capacity(256); // IF_NAMESIZE not available?
-                        if !if_indextoname(iface.InterfaceIndex, &name).is_null() {
+                        if !if_indextoname(iface.InterfaceIndex, &mut name).is_null() {
                             res = Ok((name, mtu));
                         }
                     }

From 6b421810fb950d9118a0cecf9c350f86e20c5031 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:13:24 +0300
Subject: [PATCH 11/21] Fixes

---
 src/lib.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 66a945f..42f498b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -273,14 +273,14 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
 /// platforms for some remote destinations. (For example, loopback destinations on
 /// Windows.)
 ///
-/// This function is a convenience wrapper around [`get_interface_and_mtu`] that only returns the
+/// This function is a convenience wrapper around [`interface_and_mtu`] that only returns the
 /// MTU. It is provided for compatibility with version 0.1 of the `mtu` crate.
 ///
 /// # Examples
 ///
 /// ```
 /// let saddr = "127.0.0.1:443".parse().unwrap();
-/// let mtu = mtu::get_interface_mtu(&saddr).unwrap();
+/// let mtu = mtu::interface_mtu(&saddr).unwrap();
 /// println!("MTU towards {saddr:?} is {mtu}");
 /// ```
 ///

From 45991d401095fb5af874428fed5035048ff09c55 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:24:03 +0300
Subject: [PATCH 12/21] Fixes

---
 src/lib.rs | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 42f498b..cb0b260 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -188,6 +188,7 @@ fn interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(String, usize),
 
 #[cfg(target_os = "windows")]
 fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Error> {
+    use core::str;
     use std::{ffi::c_void, slice};
 
     use windows::Win32::{
@@ -249,9 +250,11 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
                     if let Ok(mtu) = iface.NlMtu.try_into() {
-                        let name = String::with_capacity(256); // IF_NAMESIZE not available?
+                        let name: [u8; 256]; // IF_NAMESIZE not available?
                         if !if_indextoname(iface.InterfaceIndex, &mut name).is_null() {
-                            res = Ok((name, mtu));
+                            if let Ok(name) = str::from_utf8(&name) {
+                                res = Ok((name.to_string(), mtu));
+                            }
                         }
                     }
                     break 'addr_loop;

From 78e8a785d7b5f6cdbcc7a51e920d63474e02129c Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:29:26 +0300
Subject: [PATCH 13/21] Fixes

---
 src/lib.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index cb0b260..f9204b0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -250,8 +250,8 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
                     if let Ok(mtu) = iface.NlMtu.try_into() {
-                        let name: [u8; 256]; // IF_NAMESIZE not available?
-                        if !if_indextoname(iface.InterfaceIndex, &mut name).is_null() {
+                        let name = [0u8; 256]; // IF_NAMESIZE not available?
+                        if unsafe { !if_indextoname(iface.InterfaceIndex, &mut name).is_null() } {
                             if let Ok(name) = str::from_utf8(&name) {
                                 res = Ok((name.to_string(), mtu));
                             }

From 28cba5ff9f717e021060728d78c9b128a8608eff Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:33:52 +0300
Subject: [PATCH 14/21] Docs

---
 README.md  | 4 ++--
 src/lib.rs | 6 ++----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index ff4b53c..984b07b 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ The main function exported by this crate is
 pub fn interface_and_mtu(remote: &SocketAddr) -> Result<(String, usize), Error>
 ```
 
-that returns an opaque identifier of the local network interface towards the `remote` destination together with its MTU, or an `Error` when the MTU could not be determined. It supports both IPv4 and IPv6.
+that returns the interface name and MTU of the local network interface used for transmission towards the `remote` destination, or an `Error` when the MTU could not be determined. It supports both IPv4 and IPv6.
 
 ## Supported Platforms
 
@@ -22,7 +22,7 @@ that returns an opaque identifier of the local network interface towards the `re
 
 The returned MTU may exceed the maximum IP packet size of 65,535 bytes on some platforms for some remote destinations. (For example, loopback destinations on Windows.)
 
-The returned interface ID is an opaque identifier that can be used to identify the local interface. It is a hash of the interface name (on Linux and macOS) or interface index (on Windows), and has the same stability guarantees as those identifiers.
+The returned interface name is obtained from the operating system.
 
 ## Contributing
 
diff --git a/src/lib.rs b/src/lib.rs
index f9204b0..a464bdb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -20,16 +20,14 @@ fn default_result<T>() -> Result<(String, T), Error> {
     ))
 }
 
-/// Return a unique interface ID and the maximum transmission unit (MTU) of the local network
+/// Return the interface name and the maximum transmission unit (MTU) of the local network
 /// interface towards the destination [`SocketAddr`] given in `remote`.
 ///
 /// The returned MTU may exceed the maximum IP packet size of 65,535 bytes on some
 /// platforms for some remote destinations. (For example, loopback destinations on
 /// Windows.)
 ///
-/// The returned interface ID is an opaque identifier that can be used to identify the local
-/// interface. It is a hash of the interface name (on Linux and macOS) or interface index (on
-/// Windows), and has the same stability guarantees as those identifiers.
+/// The returned interface name is obtained from the operating system.
 ///
 /// # Examples
 ///

From d12d925c3d8052a253ad7ba42942841dd2ce8853 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:34:26 +0300
Subject: [PATCH 15/21] Fixes

---
 src/lib.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index a464bdb..cdac0cf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -248,7 +248,7 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
             for iface in ifaces {
                 if iface.InterfaceIndex == addr.InterfaceIndex {
                     if let Ok(mtu) = iface.NlMtu.try_into() {
-                        let name = [0u8; 256]; // IF_NAMESIZE not available?
+                        let mut name = [0u8; 256]; // IF_NAMESIZE not available?
                         if unsafe { !if_indextoname(iface.InterfaceIndex, &mut name).is_null() } {
                             if let Ok(name) = str::from_utf8(&name) {
                                 res = Ok((name.to_string(), mtu));

From 7c8584587f47cc3d303dbc622a4e1fa6b98322e9 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:38:32 +0300
Subject: [PATCH 16/21] Pass up error

---
 src/lib.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/lib.rs b/src/lib.rs
index cdac0cf..25c0d2a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -253,6 +253,8 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
                             if let Ok(name) = str::from_utf8(&name) {
                                 res = Ok((name.to_string(), mtu));
                             }
+                        } else {
+                            res = Err(Error::last_os_error());
                         }
                     }
                     break 'addr_loop;

From 343db2a460ed277c5bcee01c80bdcc1a20c85b0b Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:49:10 +0300
Subject: [PATCH 17/21] Deprecate more

---
 README.md  |  2 +-
 src/lib.rs | 39 ++++++++++-----------------------------
 2 files changed, 11 insertions(+), 30 deletions(-)

diff --git a/README.md b/README.md
index 984b07b..b6b4428 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ A crate to return the maximum transmission unit (MTU) of the local network inter
 
 ## Usage
 
-The main function exported by this crate is
+This crate exports a single function
 
 ```rust
 pub fn interface_and_mtu(remote: &SocketAddr) -> Result<(String, usize), Error>
diff --git a/src/lib.rs b/src/lib.rs
index 25c0d2a..d968133 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -74,12 +74,6 @@ pub fn interface_and_mtu(remote: &SocketAddr) -> Result<(String, usize), Error>
     res
 }
 
-#[doc(hidden)]
-#[deprecated(since = "0.1.2", note = "Use `interface_mtu()` instead")]
-pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
-    interface_mtu(remote)
-}
-
 #[cfg(any(target_os = "macos", target_os = "linux"))]
 fn interface_and_mtu_linux_macos(socket: &UdpSocket) -> Result<(String, usize), Error> {
     use std::ffi::{c_int, CStr};
@@ -269,31 +263,18 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
     res
 }
 
-/// Return the maximum transmission unit (MTU) of the local network interface towards the
-/// destination [`SocketAddr`] given in `remote`.
-///
-/// The returned MTU may exceed the maximum IP packet size of 65,535 bytes on some
-/// platforms for some remote destinations. (For example, loopback destinations on
-/// Windows.)
-///
-/// This function is a convenience wrapper around [`interface_and_mtu`] that only returns the
-/// MTU. It is provided for compatibility with version 0.1 of the `mtu` crate.
-///
-/// # Examples
-///
-/// ```
-/// let saddr = "127.0.0.1:443".parse().unwrap();
-/// let mtu = mtu::interface_mtu(&saddr).unwrap();
-/// println!("MTU towards {saddr:?} is {mtu}");
-/// ```
-///
-/// # Errors
-///
-/// This function returns an error if the local interface MTU cannot be determined.
+#[doc(hidden)]
+#[deprecated(since = "0.1.2", note = "Use `interface_and_mtu()` instead")]
 pub fn interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
     interface_and_mtu(remote).map(|(_, mtu)| mtu)
 }
 
+#[doc(hidden)]
+#[deprecated(since = "0.1.2", note = "Use `interface_and_mtu()` instead")]
+pub fn get_interface_mtu(remote: &SocketAddr) -> Result<usize, Error> {
+    interface_and_mtu(remote).map(|(_, mtu)| mtu)
+}
+
 #[cfg(test)]
 mod test {
     use std::net::ToSocketAddrs;
@@ -306,8 +287,8 @@ mod test {
             .unwrap()
             .find(|a| a.is_ipv4() == ipv4);
         if let Some(addr) = addr {
-            match super::interface_mtu(&addr) {
-                Ok(mtu) => assert_eq!(mtu, expected),
+            match super::interface_and_mtu(&addr) {
+                Ok((_, mtu)) => assert_eq!(mtu, expected),
                 Err(e) => {
                     // Some GitHub runners don't have IPv6. Just warn if we can't get the MTU.
                     assert!(addr.is_ipv6());

From 78a11529f7926ce556e2044a559b4862f470d540 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Tue, 3 Sep 2024 12:56:45 +0300
Subject: [PATCH 18/21] Add test

---
 src/lib.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/lib.rs b/src/lib.rs
index d968133..8a38990 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -331,4 +331,12 @@ mod test {
     fn default_interface_mtu_v6() {
         check_mtu("ietf.org:443", false, 1500);
     }
+
+    #[test]
+    #[allow(deprecated)] // Purpose of the test is to cover deprecated functions.
+    fn deprecated_functions() {
+        let addr = "localhost:443".to_socket_addrs().unwrap().next().unwrap();
+        assert!(super::interface_mtu(&addr).is_ok());
+        assert!(super::get_interface_mtu(&addr).is_ok());
+    }
 }

From 1170b3bb00d7919abddb3a4f7cab51bd500f44aa Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Wed, 4 Sep 2024 09:58:07 +0300
Subject: [PATCH 19/21] Add `if_indextoname`

---
 src/lib.rs            | 4 ++--
 tests/win_bindings.rs | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index c7ffe77..7ed5c3a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -189,8 +189,8 @@ fn interface_and_mtu_windows(socket: &UdpSocket) -> Result<(String, usize), Erro
     use std::{ffi::c_void, slice};
 
     use win_bindings::{
-        FreeMibTable, GetIpInterfaceTable, GetUnicastIpAddressTable, AF_INET, AF_INET6, AF_UNSPEC,
-        MIB_IPINTERFACE_ROW, MIB_IPINTERFACE_TABLE, MIB_UNICASTIPADDRESS_ROW,
+        if_indextoname, FreeMibTable, GetIpInterfaceTable, GetUnicastIpAddressTable, AF_INET,
+        AF_INET6, AF_UNSPEC, MIB_IPINTERFACE_ROW, MIB_IPINTERFACE_TABLE, MIB_UNICASTIPADDRESS_ROW,
         MIB_UNICASTIPADDRESS_TABLE, NO_ERROR,
     };
 
diff --git a/tests/win_bindings.rs b/tests/win_bindings.rs
index 09afdc2..1367397 100644
--- a/tests/win_bindings.rs
+++ b/tests/win_bindings.rs
@@ -19,6 +19,7 @@ fn codegen_windows_bindings() {
         "Windows.Win32.NetworkManagement.IpHelper.MIB_IPINTERFACE_TABLE",
         "Windows.Win32.NetworkManagement.IpHelper.MIB_UNICASTIPADDRESS_ROW",
         "Windows.Win32.NetworkManagement.IpHelper.MIB_UNICASTIPADDRESS_TABLE",
+        "Windows.Win32.NetworkManagement.IpHelper.if_indextoname",
         "Windows.Win32.Networking.WinSock.AF_INET",
         "Windows.Win32.Networking.WinSock.AF_INET6",
         "Windows.Win32.Networking.WinSock.AF_UNSPEC",

From e1b2076df485fd85a31c6dfae0a94a792237acd0 Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Wed, 4 Sep 2024 10:33:23 +0300
Subject: [PATCH 20/21] Bindings from #18

---
 src/win_bindings.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/win_bindings.rs b/src/win_bindings.rs
index 4c4273a..dd0b822 100644
--- a/src/win_bindings.rs
+++ b/src/win_bindings.rs
@@ -28,6 +28,14 @@ pub unsafe fn GetUnicastIpAddressTable(
     windows_targets::link!("iphlpapi.dll" "system" fn GetUnicastIpAddressTable(family : ADDRESS_FAMILY, table : *mut *mut MIB_UNICASTIPADDRESS_TABLE) -> WIN32_ERROR);
     GetUnicastIpAddressTable(family, table)
 }
+#[inline]
+pub unsafe fn if_indextoname(
+    interfaceindex: u32,
+    interfacename: &mut [u8; 256],
+) -> windows_core::PSTR {
+    windows_targets::link!("iphlpapi.dll" "system" fn if_indextoname(interfaceindex : u32, interfacename : windows_core::PSTR) -> windows_core::PSTR);
+    if_indextoname(interfaceindex, core::mem::transmute(interfacename.as_ptr()))
+}
 #[repr(transparent)]
 #[derive(PartialEq, Eq, Copy, Clone, Default)]
 pub struct ADDRESS_FAMILY(pub u16);
@@ -450,3 +458,4 @@ impl core::fmt::Debug for WIN32_ERROR {
         f.debug_tuple("WIN32_ERROR").field(&self.0).finish()
     }
 }
+

From 608403389cfe662070b8846a288321709e73cd3d Mon Sep 17 00:00:00 2001
From: Lars Eggert <lars@eggert.org>
Date: Wed, 4 Sep 2024 10:34:35 +0300
Subject: [PATCH 21/21] Nit

---
 src/win_bindings.rs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/win_bindings.rs b/src/win_bindings.rs
index dd0b822..107b5c6 100644
--- a/src/win_bindings.rs
+++ b/src/win_bindings.rs
@@ -458,4 +458,3 @@ impl core::fmt::Debug for WIN32_ERROR {
         f.debug_tuple("WIN32_ERROR").field(&self.0).finish()
     }
 }
-