diff --git a/litebox_common_linux/src/lib.rs b/litebox_common_linux/src/lib.rs
index 73ab21d31..abd6eee80 100644
--- a/litebox_common_linux/src/lib.rs
+++ b/litebox_common_linux/src/lib.rs
@@ -1778,6 +1778,10 @@ pub struct UserMsgHdr<Platform: litebox::platform::RawPointerProvider> {
     pub msg_name: Platform::RawConstPointer<u8>,
     /// size of socket address structure
     pub msg_namelen: u32,
+    /// Explicit padding to match the 4-byte gap that Linux's naturally-aligned
+    /// `struct user_msghdr` has between `msg_namelen` and `msg_iov` on 64-bit.
+    #[cfg(target_pointer_width = "64")]
+    _pad: u32,
     /// ptr to an array of `iovec` structures
     pub msg_iov: Platform::RawConstPointer<IoVec<Platform::RawMutPointer<u8>>>,
     /// number of elements in msg_iov
@@ -1788,6 +1792,10 @@ pub struct UserMsgHdr<Platform: litebox::platform::RawPointerProvider> {
     pub msg_controllen: usize,
     /// flags on received message
     pub msg_flags: SendFlags,
+    /// Explicit trailing padding to match the 4-byte gap after `msg_flags` in
+    /// Linux's naturally-aligned `struct user_msghdr` on 64-bit (total size 56).
+    #[cfg(target_pointer_width = "64")]
+    _pad2: u32,
 }
 
 impl<Platform: litebox::platform::RawPointerProvider> Clone for UserMsgHdr<Platform> {
@@ -1795,11 +1803,15 @@ impl<Platform: litebox::platform::RawPointerProvider> Clone for UserMsgHdr<Platf
         Self {
             msg_name: self.msg_name,
             msg_namelen: self.msg_namelen,
+            #[cfg(target_pointer_width = "64")]
+            _pad: 0,
             msg_iov: self.msg_iov,
             msg_iovlen: self.msg_iovlen,
             msg_control: self.msg_control,
             msg_controllen: self.msg_controllen,
             msg_flags: self.msg_flags,
+            #[cfg(target_pointer_width = "64")]
+            _pad2: 0,
         }
     }
 }
diff --git a/litebox_shim_linux/src/syscalls/net.rs b/litebox_shim_linux/src/syscalls/net.rs
index 2d2ba0435..0db39678b 100644
--- a/litebox_shim_linux/src/syscalls/net.rs
+++ b/litebox_shim_linux/src/syscalls/net.rs
@@ -1868,6 +1868,12 @@ mod tests {
     extern crate alloc;
     extern crate std;
 
+    // Compile-time layout check: UserMsgHdr must match Linux's struct user_msghdr.
+    const _USER_MSG_HDR_SIZE: () = assert!(
+        core::mem::size_of::<litebox_common_linux::UserMsgHdr<crate::Platform>>()
+            == core::mem::size_of::<libc::msghdr>()
+    );
+
     const TUN_IP_ADDR: [u8; 4] = [10, 0, 0, 2];
     const TUN_IP_ADDR_STR: &str = "10.0.0.2";
     const TUN_DEVICE_NAME: &str = "tun99";
@@ -2044,14 +2050,12 @@ mod tests {
                         iov_len: buf2.len(),
                     },
                 ];
-                let hdr = litebox_common_linux::UserMsgHdr {
-                    msg_name: ConstPtr::from_usize(0),
-                    msg_namelen: 0,
-                    msg_iov: ConstPtr::from_usize(iovec.as_ptr() as usize),
-                    msg_iovlen: iovec.len(),
-                    msg_control: ConstPtr::from_usize(0),
-                    msg_controllen: 0,
-                    msg_flags: SendFlags::empty(),
+                let hdr = {
+                    use zerocopy::FromZeros as _;
+                    let mut h = litebox_common_linux::UserMsgHdr::<crate::Platform>::new_zeroed();
+                    h.msg_iov = ConstPtr::from_usize(iovec.as_ptr() as usize);
+                    h.msg_iovlen = iovec.len();
+                    h
                 };
                 assert_eq!(
                     task.do_sendmsg(client_fd, &hdr, SendFlags::empty())