diff --git a/src/os/bsd/ipv6_addr_flags.rs b/src/os/bsd/ipv6_addr_flags.rs
index a2e6852..b9562b5 100644
--- a/src/os/bsd/ipv6_addr_flags.rs
+++ b/src/os/bsd/ipv6_addr_flags.rs
@@ -10,11 +10,24 @@ const IN6_IFF_DEPRECATED: u32 = 0x10;
 const IN6_IFF_TEMPORARY: u32 = 0x80;
 
 // `libc` does not expose `in6_ifreq` on FreeBSD/OpenBSD/NetBSD.
+//
+// Mirrors `struct in6_ifreq` from <netinet6/in6_var.h>. `SIOCGIFAFLAG_IN6` encodes
+// `sizeof(struct in6_ifreq)` (0x120 = 288 bytes on LP64), and the kernel copies that many
+// bytes in and out, so the union must be padded to the size of its largest member
+// (`icmp6_ifstat`, 34 x u64 = 272 bytes). The flags are returned in `ifru_flags6`, which
+// overlaps `ifru_addr` at the start of the union.
 #[repr(C)]
 struct In6Ifreq {
     ifr_name: [u8; libc::IFNAMSIZ],
-    ifr_addr: libc::sockaddr_in6,
-    ifr_flags: libc::c_int,
+    ifr_ifru: In6IfreqUnion,
+}
+
+#[repr(C)]
+union In6IfreqUnion {
+    ifru_addr: libc::sockaddr_in6,
+    ifru_flags6: libc::c_int,
+    // 8-byte aligned, like the u64 counters in `in6_ifstat` / `icmp6_ifstat`.
+    _size: [u64; 34],
 }
 
 pub(crate) fn get_ipv6_addr_flags(ifname: &str, addr: &Ipv6Addr) -> Ipv6AddrFlags {
@@ -25,6 +38,7 @@ pub(crate) fn get_ipv6_addr_flags(ifname: &str, addr: &Ipv6Addr) -> Ipv6AddrFlag
         }
 
         let mut req: In6Ifreq = std::mem::zeroed();
+        const _: () = assert!(std::mem::size_of::<In6Ifreq>() == 0x120);
 
         let name_bytes = ifname.as_bytes();
         let copy_len = name_bytes.len().min(libc::IFNAMSIZ - 1);
@@ -34,9 +48,9 @@ pub(crate) fn get_ipv6_addr_flags(ifname: &str, addr: &Ipv6Addr) -> Ipv6AddrFlag
             copy_len,
         );
 
-        req.ifr_addr.sin6_family = libc::AF_INET6 as libc::sa_family_t;
-        req.ifr_addr.sin6_len = std::mem::size_of::<libc::sockaddr_in6>() as u8;
-        req.ifr_addr.sin6_addr.s6_addr = addr.octets();
+        req.ifr_ifru.ifru_addr.sin6_family = libc::AF_INET6 as libc::sa_family_t;
+        req.ifr_ifru.ifru_addr.sin6_len = std::mem::size_of::<libc::sockaddr_in6>() as u8;
+        req.ifr_ifru.ifru_addr.sin6_addr.s6_addr = addr.octets();
 
         let ret = libc::ioctl(fd, SIOCGIFAFLAG_IN6, &mut req);
         libc::close(fd);
@@ -45,7 +59,7 @@ pub(crate) fn get_ipv6_addr_flags(ifname: &str, addr: &Ipv6Addr) -> Ipv6AddrFlag
             return Ipv6AddrFlags::default();
         }
 
-        let raw = req.ifr_flags as u32;
+        let raw = req.ifr_ifru.ifru_flags6 as u32;
 
         Ipv6AddrFlags {
             deprecated: raw & IN6_IFF_DEPRECATED != 0,