diff --git a/authfs/fd_server/src/aidl.rs b/authfs/fd_server/src/aidl.rs
new file mode 100644
index 0000000..ed3a0ea
--- /dev/null
+++ b/authfs/fd_server/src/aidl.rs
@@ -0,0 +1,240 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+use anyhow::Result;
+use log::error;
+use std::cmp::min;
+use std::collections::BTreeMap;
+use std::convert::TryInto;
+use std::fs::File;
+use std::io;
+use std::os::unix::fs::FileExt;
+use std::os::unix::io::AsRawFd;
+
+use crate::fsverity;
+use authfs_aidl_interface::aidl::com::android::virt::fs::IVirtFdService::{
+    BnVirtFdService, IVirtFdService, ERROR_FILE_TOO_LARGE, ERROR_IO, ERROR_UNKNOWN_FD,
+    MAX_REQUESTING_DATA,
+};
+use authfs_aidl_interface::binder::{
+    BinderFeatures, ExceptionCode, Interface, Result as BinderResult, Status, StatusCode, Strong,
+};
+use binder_common::new_binder_exception;
+
+fn validate_and_cast_offset(offset: i64) -> Result<u64, Status> {
+    offset.try_into().map_err(|_| {
+        new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, format!("Invalid offset: {}", offset))
+    })
+}
+
+fn validate_and_cast_size(size: i32) -> Result<usize, Status> {
+    if size > MAX_REQUESTING_DATA {
+        Err(new_binder_exception(
+            ExceptionCode::ILLEGAL_ARGUMENT,
+            format!("Unexpectedly large size: {}", size),
+        ))
+    } else {
+        size.try_into().map_err(|_| {
+            new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, format!("Invalid size: {}", size))
+        })
+    }
+}
+
+/// Configuration of a file descriptor to be served/exposed/shared.
+pub enum FdConfig {
+    /// A read-only file to serve by this server. The file is supposed to be verifiable with the
+    /// associated fs-verity metadata.
+    Readonly {
+        /// The file to read from. fs-verity metadata can be retrieved from this file's FD.
+        file: File,
+
+        /// Alternative Merkle tree stored in another file.
+        alt_merkle_tree: Option<File>,
+
+        /// Alternative signature stored in another file.
+        alt_signature: Option<File>,
+    },
+
+    /// A readable/writable file to serve by this server. This backing file should just be a
+    /// regular file and does not have any specific property.
+    ReadWrite(File),
+}
+
+pub struct FdService {
+    /// A pool of opened files, may be readonly or read-writable.
+    fd_pool: BTreeMap<i32, FdConfig>,
+}
+
+impl FdService {
+    pub fn new_binder(fd_pool: BTreeMap<i32, FdConfig>) -> Strong<dyn IVirtFdService> {
+        BnVirtFdService::new_binder(FdService { fd_pool }, BinderFeatures::default())
+    }
+
+    fn get_file_config(&self, id: i32) -> BinderResult<&FdConfig> {
+        self.fd_pool.get(&id).ok_or_else(|| Status::from(ERROR_UNKNOWN_FD))
+    }
+}
+
+impl Interface for FdService {}
+
+impl IVirtFdService for FdService {
+    fn readFile(&self, id: i32, offset: i64, size: i32) -> BinderResult<Vec<u8>> {
+        let size: usize = validate_and_cast_size(size)?;
+        let offset: u64 = validate_and_cast_offset(offset)?;
+
+        match self.get_file_config(id)? {
+            FdConfig::Readonly { file, .. } | FdConfig::ReadWrite(file) => {
+                read_into_buf(file, size, offset).map_err(|e| {
+                    error!("readFile: read error: {}", e);
+                    Status::from(ERROR_IO)
+                })
+            }
+        }
+    }
+
+    fn readFsverityMerkleTree(&self, id: i32, offset: i64, size: i32) -> BinderResult<Vec<u8>> {
+        let size: usize = validate_and_cast_size(size)?;
+        let offset: u64 = validate_and_cast_offset(offset)?;
+
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { file, alt_merkle_tree, .. } => {
+                if let Some(tree_file) = &alt_merkle_tree {
+                    read_into_buf(tree_file, size, offset).map_err(|e| {
+                        error!("readFsverityMerkleTree: read error: {}", e);
+                        Status::from(ERROR_IO)
+                    })
+                } else {
+                    let mut buf = vec![0; size];
+                    let s = fsverity::read_merkle_tree(file.as_raw_fd(), offset, &mut buf)
+                        .map_err(|e| {
+                            error!("readFsverityMerkleTree: failed to retrieve merkle tree: {}", e);
+                            Status::from(e.raw_os_error().unwrap_or(ERROR_IO))
+                        })?;
+                    debug_assert!(s <= buf.len(), "Shouldn't return more bytes than asked");
+                    buf.truncate(s);
+                    Ok(buf)
+                }
+            }
+            FdConfig::ReadWrite(_file) => {
+                // For a writable file, Merkle tree is not expected to be served since Auth FS
+                // doesn't trust it anyway. Auth FS may keep the Merkle tree privately for its own
+                // use.
+                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
+            }
+        }
+    }
+
+    fn readFsveritySignature(&self, id: i32) -> BinderResult<Vec<u8>> {
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { file, alt_signature, .. } => {
+                if let Some(sig_file) = &alt_signature {
+                    // Supposedly big enough buffer size to store signature.
+                    let size = MAX_REQUESTING_DATA as usize;
+                    let offset = 0;
+                    read_into_buf(sig_file, size, offset).map_err(|e| {
+                        error!("readFsveritySignature: read error: {}", e);
+                        Status::from(ERROR_IO)
+                    })
+                } else {
+                    let mut buf = vec![0; MAX_REQUESTING_DATA as usize];
+                    let s = fsverity::read_signature(file.as_raw_fd(), &mut buf).map_err(|e| {
+                        error!("readFsverityMerkleTree: failed to retrieve merkle tree: {}", e);
+                        Status::from(e.raw_os_error().unwrap_or(ERROR_IO))
+                    })?;
+                    debug_assert!(s <= buf.len(), "Shouldn't return more bytes than asked");
+                    buf.truncate(s);
+                    Ok(buf)
+                }
+            }
+            FdConfig::ReadWrite(_file) => {
+                // There is no signature for a writable file.
+                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
+            }
+        }
+    }
+
+    fn writeFile(&self, id: i32, buf: &[u8], offset: i64) -> BinderResult<i32> {
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { .. } => Err(StatusCode::INVALID_OPERATION.into()),
+            FdConfig::ReadWrite(file) => {
+                let offset: u64 = offset.try_into().map_err(|_| {
+                    new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, "Invalid offset")
+                })?;
+                // Check buffer size just to make `as i32` safe below.
+                if buf.len() > i32::MAX as usize {
+                    return Err(new_binder_exception(
+                        ExceptionCode::ILLEGAL_ARGUMENT,
+                        "Buffer size is too big",
+                    ));
+                }
+                Ok(file.write_at(buf, offset).map_err(|e| {
+                    error!("writeFile: write error: {}", e);
+                    Status::from(ERROR_IO)
+                })? as i32)
+            }
+        }
+    }
+
+    fn resize(&self, id: i32, size: i64) -> BinderResult<()> {
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { .. } => Err(StatusCode::INVALID_OPERATION.into()),
+            FdConfig::ReadWrite(file) => {
+                if size < 0 {
+                    return Err(new_binder_exception(
+                        ExceptionCode::ILLEGAL_ARGUMENT,
+                        "Invalid size to resize to",
+                    ));
+                }
+                file.set_len(size as u64).map_err(|e| {
+                    error!("resize: set_len error: {}", e);
+                    Status::from(ERROR_IO)
+                })
+            }
+        }
+    }
+
+    fn getFileSize(&self, id: i32) -> BinderResult<i64> {
+        match &self.get_file_config(id)? {
+            FdConfig::Readonly { file, .. } => {
+                let size = file
+                    .metadata()
+                    .map_err(|e| {
+                        error!("getFileSize error: {}", e);
+                        Status::from(ERROR_IO)
+                    })?
+                    .len();
+                Ok(size.try_into().map_err(|e| {
+                    error!("getFileSize: File too large: {}", e);
+                    Status::from(ERROR_FILE_TOO_LARGE)
+                })?)
+            }
+            FdConfig::ReadWrite(_file) => {
+                // Content and metadata of a writable file needs to be tracked by authfs, since
+                // fd_server isn't considered trusted. So there is no point to support getFileSize
+                // for a writable file.
+                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
+            }
+        }
+    }
+}
+
+fn read_into_buf(file: &File, max_size: usize, offset: u64) -> io::Result<Vec<u8>> {
+    let remaining = file.metadata()?.len().saturating_sub(offset);
+    let buf_size = min(remaining, max_size as u64) as usize;
+    let mut buf = vec![0; buf_size];
+    file.read_exact_at(&mut buf, offset)?;
+    Ok(buf)
+}
diff --git a/authfs/fd_server/src/main.rs b/authfs/fd_server/src/main.rs
index 395e2e9..3413ce6 100644
--- a/authfs/fd_server/src/main.rs
+++ b/authfs/fd_server/src/main.rs
@@ -22,235 +22,20 @@
 //! For example, `exec 9</path/to/file fd_server --ro-fds 9` starts the binder service. A client
 //! client can then request the content of file 9 by offset and size.
 
+mod aidl;
 mod fsverity;
 
 use anyhow::{bail, Result};
 use binder_common::rpc_server::run_rpc_server;
-use log::{debug, error};
-use std::cmp::min;
+use log::debug;
 use std::collections::BTreeMap;
-use std::convert::TryInto;
 use std::fs::File;
-use std::io;
-use std::os::unix::fs::FileExt;
-use std::os::unix::io::{AsRawFd, FromRawFd};
+use std::os::unix::io::FromRawFd;
 
-use authfs_aidl_interface::aidl::com::android::virt::fs::IVirtFdService::{
-    BnVirtFdService, IVirtFdService, ERROR_FILE_TOO_LARGE, ERROR_IO, ERROR_UNKNOWN_FD,
-    MAX_REQUESTING_DATA,
-};
-use authfs_aidl_interface::binder::{
-    BinderFeatures, ExceptionCode, Interface, Result as BinderResult, Status, StatusCode, Strong,
-};
-use binder_common::new_binder_exception;
+use aidl::{FdConfig, FdService};
 
 const RPC_SERVICE_PORT: u32 = 3264; // TODO: support dynamic port for multiple fd_server instances
 
-fn validate_and_cast_offset(offset: i64) -> Result<u64, Status> {
-    offset.try_into().map_err(|_| {
-        new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, format!("Invalid offset: {}", offset))
-    })
-}
-
-fn validate_and_cast_size(size: i32) -> Result<usize, Status> {
-    if size > MAX_REQUESTING_DATA {
-        Err(new_binder_exception(
-            ExceptionCode::ILLEGAL_ARGUMENT,
-            format!("Unexpectedly large size: {}", size),
-        ))
-    } else {
-        size.try_into().map_err(|_| {
-            new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, format!("Invalid size: {}", size))
-        })
-    }
-}
-
-/// Configuration of a file descriptor to be served/exposed/shared.
-enum FdConfig {
-    /// A read-only file to serve by this server. The file is supposed to be verifiable with the
-    /// associated fs-verity metadata.
-    Readonly {
-        /// The file to read from. fs-verity metadata can be retrieved from this file's FD.
-        file: File,
-
-        /// Alternative Merkle tree stored in another file.
-        alt_merkle_tree: Option<File>,
-
-        /// Alternative signature stored in another file.
-        alt_signature: Option<File>,
-    },
-
-    /// A readable/writable file to serve by this server. This backing file should just be a
-    /// regular file and does not have any specific property.
-    ReadWrite(File),
-}
-
-struct FdService {
-    /// A pool of opened files, may be readonly or read-writable.
-    fd_pool: BTreeMap<i32, FdConfig>,
-}
-
-impl FdService {
-    pub fn new_binder(fd_pool: BTreeMap<i32, FdConfig>) -> Strong<dyn IVirtFdService> {
-        BnVirtFdService::new_binder(FdService { fd_pool }, BinderFeatures::default())
-    }
-
-    fn get_file_config(&self, id: i32) -> BinderResult<&FdConfig> {
-        self.fd_pool.get(&id).ok_or_else(|| Status::from(ERROR_UNKNOWN_FD))
-    }
-}
-
-impl Interface for FdService {}
-
-impl IVirtFdService for FdService {
-    fn readFile(&self, id: i32, offset: i64, size: i32) -> BinderResult<Vec<u8>> {
-        let size: usize = validate_and_cast_size(size)?;
-        let offset: u64 = validate_and_cast_offset(offset)?;
-
-        match self.get_file_config(id)? {
-            FdConfig::Readonly { file, .. } | FdConfig::ReadWrite(file) => {
-                read_into_buf(file, size, offset).map_err(|e| {
-                    error!("readFile: read error: {}", e);
-                    Status::from(ERROR_IO)
-                })
-            }
-        }
-    }
-
-    fn readFsverityMerkleTree(&self, id: i32, offset: i64, size: i32) -> BinderResult<Vec<u8>> {
-        let size: usize = validate_and_cast_size(size)?;
-        let offset: u64 = validate_and_cast_offset(offset)?;
-
-        match &self.get_file_config(id)? {
-            FdConfig::Readonly { file, alt_merkle_tree, .. } => {
-                if let Some(tree_file) = &alt_merkle_tree {
-                    read_into_buf(tree_file, size, offset).map_err(|e| {
-                        error!("readFsverityMerkleTree: read error: {}", e);
-                        Status::from(ERROR_IO)
-                    })
-                } else {
-                    let mut buf = vec![0; size];
-                    let s = fsverity::read_merkle_tree(file.as_raw_fd(), offset, &mut buf)
-                        .map_err(|e| {
-                            error!("readFsverityMerkleTree: failed to retrieve merkle tree: {}", e);
-                            Status::from(e.raw_os_error().unwrap_or(ERROR_IO))
-                        })?;
-                    debug_assert!(s <= buf.len(), "Shouldn't return more bytes than asked");
-                    buf.truncate(s);
-                    Ok(buf)
-                }
-            }
-            FdConfig::ReadWrite(_file) => {
-                // For a writable file, Merkle tree is not expected to be served since Auth FS
-                // doesn't trust it anyway. Auth FS may keep the Merkle tree privately for its own
-                // use.
-                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
-            }
-        }
-    }
-
-    fn readFsveritySignature(&self, id: i32) -> BinderResult<Vec<u8>> {
-        match &self.get_file_config(id)? {
-            FdConfig::Readonly { file, alt_signature, .. } => {
-                if let Some(sig_file) = &alt_signature {
-                    // Supposedly big enough buffer size to store signature.
-                    let size = MAX_REQUESTING_DATA as usize;
-                    let offset = 0;
-                    read_into_buf(sig_file, size, offset).map_err(|e| {
-                        error!("readFsveritySignature: read error: {}", e);
-                        Status::from(ERROR_IO)
-                    })
-                } else {
-                    let mut buf = vec![0; MAX_REQUESTING_DATA as usize];
-                    let s = fsverity::read_signature(file.as_raw_fd(), &mut buf).map_err(|e| {
-                        error!("readFsverityMerkleTree: failed to retrieve merkle tree: {}", e);
-                        Status::from(e.raw_os_error().unwrap_or(ERROR_IO))
-                    })?;
-                    debug_assert!(s <= buf.len(), "Shouldn't return more bytes than asked");
-                    buf.truncate(s);
-                    Ok(buf)
-                }
-            }
-            FdConfig::ReadWrite(_file) => {
-                // There is no signature for a writable file.
-                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
-            }
-        }
-    }
-
-    fn writeFile(&self, id: i32, buf: &[u8], offset: i64) -> BinderResult<i32> {
-        match &self.get_file_config(id)? {
-            FdConfig::Readonly { .. } => Err(StatusCode::INVALID_OPERATION.into()),
-            FdConfig::ReadWrite(file) => {
-                let offset: u64 = offset.try_into().map_err(|_| {
-                    new_binder_exception(ExceptionCode::ILLEGAL_ARGUMENT, "Invalid offset")
-                })?;
-                // Check buffer size just to make `as i32` safe below.
-                if buf.len() > i32::MAX as usize {
-                    return Err(new_binder_exception(
-                        ExceptionCode::ILLEGAL_ARGUMENT,
-                        "Buffer size is too big",
-                    ));
-                }
-                Ok(file.write_at(buf, offset).map_err(|e| {
-                    error!("writeFile: write error: {}", e);
-                    Status::from(ERROR_IO)
-                })? as i32)
-            }
-        }
-    }
-
-    fn resize(&self, id: i32, size: i64) -> BinderResult<()> {
-        match &self.get_file_config(id)? {
-            FdConfig::Readonly { .. } => Err(StatusCode::INVALID_OPERATION.into()),
-            FdConfig::ReadWrite(file) => {
-                if size < 0 {
-                    return Err(new_binder_exception(
-                        ExceptionCode::ILLEGAL_ARGUMENT,
-                        "Invalid size to resize to",
-                    ));
-                }
-                file.set_len(size as u64).map_err(|e| {
-                    error!("resize: set_len error: {}", e);
-                    Status::from(ERROR_IO)
-                })
-            }
-        }
-    }
-
-    fn getFileSize(&self, id: i32) -> BinderResult<i64> {
-        match &self.get_file_config(id)? {
-            FdConfig::Readonly { file, .. } => {
-                let size = file
-                    .metadata()
-                    .map_err(|e| {
-                        error!("getFileSize error: {}", e);
-                        Status::from(ERROR_IO)
-                    })?
-                    .len();
-                Ok(size.try_into().map_err(|e| {
-                    error!("getFileSize: File too large: {}", e);
-                    Status::from(ERROR_FILE_TOO_LARGE)
-                })?)
-            }
-            FdConfig::ReadWrite(_file) => {
-                // Content and metadata of a writable file needs to be tracked by authfs, since
-                // fd_server isn't considered trusted. So there is no point to support getFileSize
-                // for a writable file.
-                Err(new_binder_exception(ExceptionCode::UNSUPPORTED_OPERATION, "Unsupported"))
-            }
-        }
-    }
-}
-
-fn read_into_buf(file: &File, max_size: usize, offset: u64) -> io::Result<Vec<u8>> {
-    let remaining = file.metadata()?.len().saturating_sub(offset);
-    let buf_size = min(remaining, max_size as u64) as usize;
-    let mut buf = vec![0; buf_size];
-    file.read_exact_at(&mut buf, offset)?;
-    Ok(buf)
-}
-
 fn is_fd_valid(fd: i32) -> bool {
     // SAFETY: a query-only syscall
     let retval = unsafe { libc::fcntl(fd, libc::F_GETFD) };