From 5d07ad6f30e6d92ab7e9b2f5c8c47eb9cb144905 Mon Sep 17 00:00:00 2001
From: Benjamin Saunders <ben.e.saunders@gmail.com>
Date: Sat, 2 Jul 2022 12:37:24 -0700
Subject: [PATCH] Fix Linux DF bit setting for IPv6-mapped IPv4 addresses

---
 quinn-udp/src/unix.rs | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/quinn-udp/src/unix.rs b/quinn-udp/src/unix.rs
index b68e91f70f..88fe35aaff 100644
--- a/quinn-udp/src/unix.rs
+++ b/quinn-udp/src/unix.rs
@@ -147,20 +147,21 @@ fn init(io: &std::net::UdpSocket) -> io::Result<()> {
             )
         };
 
-        if addr.is_ipv4() {
-            let rc = unsafe {
-                libc::setsockopt(
-                    io.as_raw_fd(),
-                    libc::IPPROTO_IP,
-                    libc::IP_MTU_DISCOVER,
-                    &libc::IP_PMTUDISC_PROBE as *const _ as _,
-                    mem::size_of_val(&libc::IP_PMTUDISC_PROBE) as _,
-                )
-            };
-            if rc == -1 {
-                return Err(io::Error::last_os_error());
-            }
+        // Forbid IPv4 fragmentation. Set even for IPv6 to account for IPv6 mapped IPv4 addresses.
+        let rc = unsafe {
+            libc::setsockopt(
+                io.as_raw_fd(),
+                libc::IPPROTO_IP,
+                libc::IP_MTU_DISCOVER,
+                &libc::IP_PMTUDISC_PROBE as *const _ as _,
+                mem::size_of_val(&libc::IP_PMTUDISC_PROBE) as _,
+            )
+        };
+        if rc == -1 {
+            return Err(io::Error::last_os_error());
+        }
 
+        if addr.is_ipv4() {
             let on: libc::c_int = 1;
             let rc = unsafe {
                 libc::setsockopt(