diff --git a/sandboxed_api/sandbox2/BUILD.bazel b/sandboxed_api/sandbox2/BUILD.bazel
index bac3822..7a94c20 100644
--- a/sandboxed_api/sandbox2/BUILD.bazel
+++ b/sandboxed_api/sandbox2/BUILD.bazel
@@ -89,7 +89,7 @@ cc_library(
         ":syscall",
         ":util",
         "//sandboxed_api/util:status",
-        "//sandboxed_api/util:statusor",
+        "@com_google_absl//absl/base",
         "@com_google_absl//absl/memory",
         "@com_google_absl//absl/strings",
     ],
@@ -127,6 +127,36 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "network_proxy_server",
+    srcs = ["network_proxy_server.cc"],
+    hdrs = ["network_proxy_server.h"],
+    copts = sapi_platform_copts(),
+    deps = [
+        ":comms",
+        "//sandboxed_api/sandbox2/util:fileops",
+        "@com_google_absl//absl/memory",
+        "@com_google_glog//:glog",
+    ],
+)
+
+cc_library(
+    name = "network_proxy_client",
+    srcs = ["network_proxy_client.cc"],
+    hdrs = ["network_proxy_client.h"],
+    copts = sapi_platform_copts(),
+    visibility = ["//visibility:public"],
+    deps = [
+        ":comms",
+        "//sandboxed_api/sandbox2/util:strerror",
+        "//sandboxed_api/util:status",
+        "@com_google_absl//absl/memory",
+        "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/synchronization",
+        "@com_google_glog//:glog",
+    ],
+)
+
 cc_library(
     name = "ipc",
     srcs = ["ipc.cc"],
@@ -136,6 +166,8 @@ cc_library(
         ":comms",
         ":logserver",
         ":logsink",
+        ":network_proxy_client",
+        ":network_proxy_server",
         "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/memory",
         "@com_google_absl//absl/strings",
@@ -154,7 +186,6 @@ cc_library(
         ":regs",
         ":syscall",
         ":violation_cc_proto",
-        "//sandboxed_api/sandbox2/network_proxy:filtering",
         "//sandboxed_api/sandbox2/util:bpf_helper",
         "//sandboxed_api/util:flags",
         "@com_google_absl//absl/base:core_headers",
@@ -275,10 +306,10 @@ cc_library(
         ":client",
         ":executor",
         ":comms",
-        ":forkserver_cc_proto",
-        ":global_forkserver",
         ":violation_cc_proto",
         ":forkserver",
+        ":forkserver_cc_proto",
+        ":global_forkserver",
         ":ipc",
         ":limits",
         ":logsink",
@@ -290,6 +321,7 @@ cc_library(
         ":result",
         ":syscall",
         ":util",
+        ":network_proxy_client",
         "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/container:flat_hash_set",
@@ -301,17 +333,12 @@ cc_library(
         "@com_google_absl//absl/time",
         "@com_google_absl//absl/types:optional",
         "@org_kernel_libcap//:libcap",
-        "//sandboxed_api/sandbox2/network_proxy:client",
-        "//sandboxed_api/sandbox2/network_proxy:filtering",
-        "//sandboxed_api/sandbox2/network_proxy:server",
         "//sandboxed_api/sandbox2/unwind",
         "//sandboxed_api/sandbox2/unwind:unwind_cc_proto",
         "//sandboxed_api/sandbox2/util:bpf_helper",
         "//sandboxed_api/sandbox2/util:file_base",
         "//sandboxed_api/sandbox2/util:fileops",
-        "//sandboxed_api/sandbox2/util:strerror",
         "//sandboxed_api/util:raw_logging",
-        "//util/functional",
         "//sandboxed_api/util:status",
         "//sandboxed_api/util:statusor",
 
@@ -337,7 +364,7 @@ cc_library(
     deps = [
         ":comms",
         ":logsink",
-        "//sandboxed_api/sandbox2/network_proxy:client",
+        ":network_proxy_client",
         "//sandboxed_api/sandbox2/util:file_helpers",
         "//sandboxed_api/sandbox2/util:fileops",
         "//sandboxed_api/sandbox2/util:strerror",
diff --git a/sandboxed_api/sandbox2/CMakeLists.txt b/sandboxed_api/sandbox2/CMakeLists.txt
index fa8c515..b2b58fd 100644
--- a/sandboxed_api/sandbox2/CMakeLists.txt
+++ b/sandboxed_api/sandbox2/CMakeLists.txt
@@ -15,7 +15,6 @@
 add_subdirectory(examples)
 add_subdirectory(unwind)
 add_subdirectory(util)
-add_subdirectory(network_proxy)
 
 # sandboxed_api/sandbox2:bpfdisassembler
 add_library(sandbox2_bpfdisassembler STATIC
@@ -122,6 +121,36 @@ target_link_libraries(sandbox2_logsink
   PUBLIC glog::glog
 )
 
+# sandboxed_api/sandbox2:network_proxy_server
+add_library(sandbox2_network_proxy_server STATIC
+  network_proxy_server.cc
+  network_proxy_server.h
+)
+add_library(sandbox2::network_proxy_server ALIAS sandbox2_network_proxy_server)
+target_link_libraries(sandbox2_network_proxy_server PRIVATE
+  absl::memory
+  glog::glog
+  sandbox2::comms
+  sandbox2::fileops
+  sapi::base
+)
+
+# sandboxed_api/sandbox2:network_proxy_client
+add_library(sandbox2_network_proxy_client STATIC
+  network_proxy_client.cc
+  network_proxy_client.h
+)
+add_library(sandbox2::network_proxy_client ALIAS sandbox2_network_proxy_client)
+target_link_libraries(sandbox2_network_proxy_client PRIVATE
+  absl::strings
+  absl::synchronization
+  glog::glog
+  sandbox2::comms
+  sandbox2::strerror
+  sapi::base
+  sapi::status
+)
+
 # sandboxed_api/sandbox2:ipc
 add_library(sandbox2_ipc STATIC
   ipc.cc
diff --git a/sandboxed_api/sandbox2/client.cc b/sandboxed_api/sandbox2/client.cc
index d07c9b1..235ec19 100644
--- a/sandboxed_api/sandbox2/client.cc
+++ b/sandboxed_api/sandbox2/client.cc
@@ -40,6 +40,7 @@
 #include "absl/strings/str_join.h"
 #include "absl/strings/str_split.h"
 #include "sandboxed_api/sandbox2/comms.h"
+#include "sandboxed_api/sandbox2/network_proxy_client.h"
 #include "sandboxed_api/sandbox2/sanitizer.h"
 #include "sandboxed_api/sandbox2/util/strerror.h"
 #include "sandboxed_api/util/raw_logging.h"
diff --git a/sandboxed_api/sandbox2/client.h b/sandboxed_api/sandbox2/client.h
index 3518ff8..d1a4c2c 100644
--- a/sandboxed_api/sandbox2/client.h
+++ b/sandboxed_api/sandbox2/client.h
@@ -25,7 +25,7 @@
 
 #include "sandboxed_api/sandbox2/comms.h"
 #include "sandboxed_api/sandbox2/logsink.h"
-#include "sandboxed_api/sandbox2/network_proxy/client.h"
+#include "sandboxed_api/sandbox2/network_proxy_client.h"
 
 namespace sandbox2 {
 
diff --git a/sandboxed_api/sandbox2/examples/network_proxy/BUILD.bazel b/sandboxed_api/sandbox2/examples/network_proxy/BUILD.bazel
index ad15d87..2a3faed 100644
--- a/sandboxed_api/sandbox2/examples/network_proxy/BUILD.bazel
+++ b/sandboxed_api/sandbox2/examples/network_proxy/BUILD.bazel
@@ -14,10 +14,10 @@
 
 # The 'network proxy' example demonstrates how to use network proxy server.
 
-load("//sandboxed_api/bazel:build_defs.bzl", "sapi_platform_copts")
-
 licenses(["notice"])
 
+load("//sandboxed_api/bazel:build_defs.bzl", "sapi_platform_copts")
+
 # Executor
 cc_binary(
     name = "networkproxy_sandbox",
@@ -27,7 +27,6 @@ cc_binary(
     deps = [
         "//sandboxed_api/sandbox2",
         "//sandboxed_api/sandbox2:comms",
-        "//sandboxed_api/sandbox2/network_proxy:filtering",
         "//sandboxed_api/sandbox2/util:bpf_helper",
         "//sandboxed_api/sandbox2/util:fileops",
         "//sandboxed_api/sandbox2/util:runfiles",
@@ -43,7 +42,7 @@ cc_binary(
     deps = [
         "//sandboxed_api/sandbox2:client",
         "//sandboxed_api/sandbox2:comms",
-        "//sandboxed_api/sandbox2/network_proxy:client",
+        "//sandboxed_api/sandbox2:network_proxy_client",
         "//sandboxed_api/sandbox2/util:fileops",
         "@com_google_absl//absl/strings:str_format",
     ],
diff --git a/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_bin.cc b/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_bin.cc
index d859f67..cd57021 100644
--- a/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_bin.cc
+++ b/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_bin.cc
@@ -14,7 +14,7 @@
 #include "absl/strings/str_format.h"
 #include "sandboxed_api/sandbox2/client.h"
 #include "sandboxed_api/sandbox2/comms.h"
-#include "sandboxed_api/sandbox2/network_proxy/client.h"
+#include "sandboxed_api/sandbox2/network_proxy_client.h"
 #include "sandboxed_api/sandbox2/util/fileops.h"
 
 static ssize_t ReadFromFd(int fd, uint8_t* buf, size_t size) {
@@ -113,6 +113,5 @@ int main(int argc, char** argv) {
   if (!CommunicationTest(client.get())) {
     return 4;
   }
-
   return 0;
 }
diff --git a/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_sandbox.cc b/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_sandbox.cc
index ef80b00..46aae97 100644
--- a/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_sandbox.cc
+++ b/sandboxed_api/sandbox2/examples/network_proxy/networkproxy_sandbox.cc
@@ -42,8 +42,6 @@ std::unique_ptr<sandbox2::Policy> GetPolicy(absl::string_view sandboxee_path) {
       .AllowSyscall(__NR_munmap)
       .AllowSyscall(__NR_getpid)
       .AddNetworkProxyHandlerPolicy()
-      .AllowTcMalloc()
-      .AllowIPv6("::1")
       .AddLibrariesForBinary(sandboxee_path)
       .BuildOrDie();
 }
@@ -131,7 +129,10 @@ int main(int argc, char** argv) {
       // is capable of enabling sandboxing on its own).
       ->set_enable_sandbox_before_exec(false)
       // Set cwd to / to get rids of warnings connected with file namespace.
-      .set_cwd("/");
+      .set_cwd("/")
+      // Enable built-in network proxy.
+      .ipc()
+      ->EnableNetworkProxyServer();
   executor
       ->limits()
       // Remove restrictions on the size of address-space of sandboxed
diff --git a/sandboxed_api/sandbox2/ipc.cc b/sandboxed_api/sandbox2/ipc.cc
index 7a25ff5..7c77654 100644
--- a/sandboxed_api/sandbox2/ipc.cc
+++ b/sandboxed_api/sandbox2/ipc.cc
@@ -17,12 +17,16 @@
 #include "sandboxed_api/sandbox2/ipc.h"
 
 #include <sys/socket.h>
-#include <thread>
+#include <unistd.h>
+
+#include <thread>  // NOLINT(build/c++11)
 
 #include <glog/logging.h>
 #include "absl/memory/memory.h"
 #include "sandboxed_api/sandbox2/logserver.h"
 #include "sandboxed_api/sandbox2/logsink.h"
+#include "sandboxed_api/sandbox2/network_proxy_client.h"
+#include "sandboxed_api/sandbox2/network_proxy_server.h"
 
 namespace sandbox2 {
 
@@ -99,4 +103,16 @@ void IPC::EnableLogServer() {
   log_thread.detach();
 }
 
+void IPC::EnableNetworkProxyServer() {
+  int fd = ReceiveFd(NetworkProxyClient::kFDName);
+
+  auto proxy_server = [fd]() {
+    NetworkProxyServer network_proxy_server(fd);
+    network_proxy_server.Run();
+  };
+
+  std::thread proxy_thread{proxy_server};
+  proxy_thread.detach();
+}
+
 }  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/ipc.h b/sandboxed_api/sandbox2/ipc.h
index 69f8421..52c4704 100644
--- a/sandboxed_api/sandbox2/ipc.h
+++ b/sandboxed_api/sandbox2/ipc.h
@@ -23,6 +23,7 @@
 #include <tuple>
 #include <vector>
 
+#include "absl/base/macros.h"
 #include "absl/strings/string_view.h"
 #include "sandboxed_api/sandbox2/comms.h"
 
@@ -59,6 +60,10 @@ class IPC final {
   // Client::SendLogsToSupervisor in the sandboxee.
   void EnableLogServer();
 
+  // Enable network proxy server, this will start a thread in the sandbox
+  // that waits for connection requests from the sandboxee.
+  void EnableNetworkProxyServer();
+
  private:
   friend class Executor;
   friend class Monitor;
diff --git a/sandboxed_api/sandbox2/monitor.cc b/sandboxed_api/sandbox2/monitor.cc
index 9cc251f..1e62f86 100644
--- a/sandboxed_api/sandbox2/monitor.cc
+++ b/sandboxed_api/sandbox2/monitor.cc
@@ -53,7 +53,6 @@
 #include "sandboxed_api/sandbox2/limits.h"
 #include "sandboxed_api/sandbox2/mounts.h"
 #include "sandboxed_api/sandbox2/namespace.h"
-#include "sandboxed_api/sandbox2/network_proxy/server.h"
 #include "sandboxed_api/sandbox2/policy.h"
 #include "sandboxed_api/sandbox2/regs.h"
 #include "sandboxed_api/sandbox2/result.h"
@@ -141,9 +140,6 @@ Monitor::~Monitor() {
   if (log_file_) {
     std::fclose(log_file_);
   }
-  if (network_proxy_server_) {
-    network_proxy_thread_.join();
-  }
 }
 
 namespace {
@@ -202,10 +198,6 @@ void Monitor::Run() {
   // sandbox master/monitor, which ptrace_attach'es to the child.
   int clone_flags = CLONE_UNTRACED;
 
-  if (policy_->allowed_hosts_) {
-    EnableNetworkProxyServer();
-  }
-
   // Get PID of the sandboxee.
   pid_t init_pid = 0;
   Namespace* ns = policy_->GetNamespace();
@@ -357,14 +349,6 @@ void Monitor::MainLoop(sigset_t* sset) {
       KillSandboxee();
     }
 
-    if (network_proxy_server_ &&
-        network_proxy_server_->violation_occurred_.load(
-            std::memory_order_acquire) &&
-        !network_violation_) {
-      network_violation_ = true;
-      KillSandboxee();
-    }
-
     // It should be a non-blocking operation (hence WNOHANG), so this function
     // returns quickly if there are no events to be processed.
     // Prioritize main pid to avoid resource starvation
@@ -417,10 +401,7 @@ void Monitor::MainLoop(sigset_t* sset) {
       VLOG(1) << "PID: " << ret << " terminated with signal: "
               << util::GetSignalName(WTERMSIG(status));
       if (ret == pid_) {
-        if (network_violation_) {
-          SetExitStatusCode(Result::NETWORK_VIOLATION, 0);
-          result_.SetNetworkViolation(network_proxy_server_->violation_msg_);
-        } else if (external_kill_) {
+        if (external_kill_) {
           SetExitStatusCode(Result::EXTERNAL_KILL, 0);
         } else if (timed_out_) {
           SetExitStatusCode(Result::TIMEOUT, 0);
@@ -813,10 +794,7 @@ void Monitor::EventPtraceExit(pid_t pid, int event_msg) {
   // 3) Regular signal/other exit cause.
   if (pid == pid_) {
     VLOG(1) << "PID: " << pid << " main special exit";
-    if (network_violation_) {
-      SetExitStatusCode(Result::NETWORK_VIOLATION, 0);
-      result_.SetNetworkViolation(network_proxy_server_->violation_msg_);
-    } else if (external_kill_) {
+    if (external_kill_) {
       SetExitStatusCode(Result::EXTERNAL_KILL, 0);
     } else if (timed_out_) {
       SetExitStatusCode(Result::TIMEOUT, 0);
@@ -948,14 +926,4 @@ void Monitor::LogSyscallViolationExplanation(const Syscall& syscall) const {
   }
 }
 
-void Monitor::EnableNetworkProxyServer() {
-  int fd = ipc_->ReceiveFd(NetworkProxyClient::kFDName);
-
-  network_proxy_server_ = absl::make_unique<NetworkProxyServer>(
-      fd, &policy_->allowed_hosts_.value(), pthread_self());
-
-  network_proxy_thread_ = std::thread(&NetworkProxyServer::Run,
-  network_proxy_server_.get());
-}
-
 }  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/monitor.h b/sandboxed_api/sandbox2/monitor.h
index 069d93e..61358c8 100644
--- a/sandboxed_api/sandbox2/monitor.h
+++ b/sandboxed_api/sandbox2/monitor.h
@@ -26,13 +26,11 @@
 #include <cstdio>
 #include <ctime>
 #include <memory>
-#include <thread>
 
 #include "absl/synchronization/notification.h"
 #include "sandboxed_api/sandbox2/comms.h"
 #include "sandboxed_api/sandbox2/executor.h"
 #include "sandboxed_api/sandbox2/ipc.h"
-#include "sandboxed_api/sandbox2/network_proxy/server.h"
 #include "sandboxed_api/sandbox2/notify.h"
 #include "sandboxed_api/sandbox2/policy.h"
 #include "sandboxed_api/sandbox2/regs.h"
@@ -141,10 +139,6 @@ class Monitor final {
   // Processes stop path.
   void EventPtraceStop(pid_t pid, int stopsig);
 
-  // Enable network proxy server, this will start a thread in the sandbox
-  // that waits for connection requests from the sandboxee.
-  void EnableNetworkProxyServer();
-
   // Internal objects, owned by the Sandbox2 object.
   Executor* executor_;
   Notify* notify_;
@@ -174,8 +168,6 @@ class Monitor final {
   std::atomic<int64_t> deadline_millis_{0};
   // Was external kill sent to the sandboxee
   bool external_kill_ = false;
-  // Network violation occurred and process of killing sandboxee started
-  bool network_violation_ = false;
   // Is the sandboxee timed out
   bool timed_out_ = false;
   // Should we dump the main sandboxed PID's stack?
@@ -186,12 +178,6 @@ class Monitor final {
   // Log file specified by
   // --sandbox_danger_danger_permit_all_and_log flag.
   FILE* log_file_ = nullptr;
-
-  // Handle to the class responsible for proxying and validating connect()
-  // requests.
-  std::unique_ptr<NetworkProxyServer> network_proxy_server_;
-
-  std::thread network_proxy_thread_;
 };
 
 }  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/network_proxy_client.cc b/sandboxed_api/sandbox2/network_proxy_client.cc
new file mode 100644
index 0000000..5e214e9
--- /dev/null
+++ b/sandboxed_api/sandbox2/network_proxy_client.cc
@@ -0,0 +1,217 @@
+// Copyright 2019 Google LLC
+//
+// 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.
+
+#include "sandboxed_api/sandbox2/network_proxy_client.h"
+
+#include <linux/net.h>
+#include <linux/seccomp.h>
+#include <stdio.h>
+#include <syscall.h>
+#include <ucontext.h>
+
+#include <cerrno>
+#include <iostream>
+
+#include <glog/logging.h>
+#include "absl/memory/memory.h"
+#include "absl/strings/str_cat.h"
+#include "sandboxed_api/sandbox2/util/strerror.h"
+#include "sandboxed_api/util/canonical_errors.h"
+#include "sandboxed_api/util/status.h"
+#include "sandboxed_api/util/status_macros.h"
+
+namespace sandbox2 {
+
+#ifndef SYS_SECCOMP
+constexpr int SYS_SECCOMP = 1;
+#endif
+
+#if defined(__x86_64__)
+constexpr int kRegResult = REG_RAX;
+constexpr int kRegSyscall = REG_RAX;
+constexpr int kRegArg0 = REG_RDI;
+constexpr int kRegArg1 = REG_RSI;
+constexpr int kRegArg2 = REG_RDX;
+#endif
+#if defined(__powerpc64__)
+constexpr int kRegResult = 3;
+constexpr int kRegSyscall = 0;
+constexpr int kRegArg0 = 3;
+constexpr int kRegArg1 = 4;
+constexpr int kRegArg2 = 5;
+#endif
+
+constexpr char NetworkProxyClient::kFDName[];
+
+int NetworkProxyClient::ConnectHandler(int sockfd, const struct sockaddr* addr,
+                                       socklen_t addrlen) {
+  sapi::Status status = Connect(sockfd, addr, addrlen);
+  if (status.ok()) {
+    return 0;
+  }
+  LOG(ERROR) << "ConnectHandler() failed: " << status.message();
+  return -1;
+}
+
+sapi::Status NetworkProxyClient::Connect(int sockfd,
+                                         const struct sockaddr* addr,
+                                         socklen_t addrlen) {
+  absl::MutexLock lock(&mutex_);
+
+  // Check if socket is SOCK_STREAM
+  int type;
+  socklen_t type_size = sizeof(int);
+  int result = getsockopt(sockfd, SOL_SOCKET, SO_TYPE, &type, &type_size);
+  if (result == -1) {
+    return sapi::FailedPreconditionError("Invalid socket FD");
+  }
+  if (type_size != sizeof(int) || type != SOCK_STREAM) {
+    errno = EINVAL;
+    return sapi::InvalidArgumentError(
+        "Invalid socket, only SOCK_STREAM is allowed");
+  }
+
+  // Send sockaddr struct
+  if (!comms_.SendBytes(reinterpret_cast<const uint8_t*>(addr), addrlen)) {
+    errno = EIO;
+    return sapi::InternalError("Sending data to network proxy failed");
+  }
+
+  SAPI_RETURN_IF_ERROR(ReceiveRemoteResult());
+
+  // Receive new socket
+  int s;
+  if (!comms_.RecvFD(&s)) {
+    errno = EIO;
+    return sapi::InternalError("Receiving data from network proxy failed");
+  }
+  if (dup2(s, sockfd) == -1) {
+    close(s);
+    return sapi::InternalError("Processing data from network proxy failed");
+  }
+  return sapi::OkStatus();
+}
+
+sapi::Status NetworkProxyClient::ReceiveRemoteResult() {
+  int result;
+  if (!comms_.RecvInt32(&result)) {
+    errno = EIO;
+    return sapi::InternalError("Receiving data from the network proxy failed");
+  }
+  if (result != 0) {
+    errno = result;
+    return sapi::InternalError(
+        absl::StrCat("Error in network proxy server: ", StrError(errno)));
+  }
+  return sapi::OkStatus();
+}
+
+namespace {
+
+static NetworkProxyHandler* g_network_proxy_handler = nullptr;
+
+void SignalHandler(int nr, siginfo_t* info, void* void_context) {
+  g_network_proxy_handler->ProcessSeccompTrap(nr, info, void_context);
+}
+
+}  // namespace
+
+sapi::Status NetworkProxyHandler::InstallNetworkProxyHandler(
+    NetworkProxyClient* npc) {
+  if (g_network_proxy_handler) {
+    return sapi::AlreadyExistsError(
+        "Network proxy handler is already installed");
+  }
+  g_network_proxy_handler = new NetworkProxyHandler(npc);
+  return sapi::OkStatus();
+}
+
+void NetworkProxyHandler::InvokeOldAct(int nr, siginfo_t* info,
+                                       void* void_context) {
+  if (oldact_.sa_flags & SA_SIGINFO) {
+    if (oldact_.sa_sigaction) {
+      oldact_.sa_sigaction(nr, info, void_context);
+    }
+  } else if (oldact_.sa_handler == SIG_IGN) {
+    return;
+  } else if (oldact_.sa_handler == SIG_DFL) {
+    sigaction(SIGSYS, &oldact_, nullptr);
+    raise(SIGSYS);
+  } else if (oldact_.sa_handler) {
+    oldact_.sa_handler(nr);
+  }
+}  // namespace sandbox2
+
+void NetworkProxyHandler::ProcessSeccompTrap(int nr, siginfo_t* info,
+                                             void* void_context) {
+  ucontext_t* ctx = (ucontext_t*)(void_context);
+  if (info->si_code != SYS_SECCOMP) {
+    InvokeOldAct(nr, info, void_context);
+    return;
+  }
+  if (!ctx) return;
+
+#if defined(__x86_64__)
+  auto* registers = ctx->uc_mcontext.gregs;
+#elif defined(__powerpc64__)
+  auto* registers = ctx->uc_mcontext.gp_regs;
+  using ppc_gpreg_t = std::decay<decltype(registers[0])>::type;
+#endif
+
+  int syscall = registers[kRegSyscall];
+
+  int sockfd;
+  const struct sockaddr* addr;
+  socklen_t addrlen;
+
+  if (syscall == __NR_connect) {
+    sockfd = static_cast<int>(registers[kRegArg0]);
+    addr = reinterpret_cast<const struct sockaddr*>(registers[kRegArg1]);
+    addrlen = static_cast<socklen_t>(registers[kRegArg2]);
+#if defined(__powerpc64__)
+  } else if (syscall == __NR_socketcall &&
+             static_cast<int>(registers[kRegArg0]) == SYS_CONNECT) {
+    ppc_gpreg_t* args = reinterpret_cast<ppc_gpreg_t*>(registers[kRegArg1]);
+
+    sockfd = static_cast<int>(args[0]);
+    addr = reinterpret_cast<const struct sockaddr*>(args[1]);
+    addrlen = static_cast<socklen_t>(args[2]);
+#endif
+  } else {
+    InvokeOldAct(nr, info, void_context);
+    return;
+  }
+
+  sapi::Status result = network_proxy_client_->Connect(sockfd, addr, addrlen);
+  if (result.ok()) {
+    registers[kRegResult] = 0;
+  } else {
+    registers[kRegResult] = -errno;
+  }
+}
+
+void NetworkProxyHandler::InstallSeccompTrap() {
+  sigset_t mask;
+  sigemptyset(&mask);
+  sigaddset(&mask, SIGSYS);
+
+  struct sigaction act = {};
+  act.sa_sigaction = &SignalHandler;
+  act.sa_flags = SA_SIGINFO;
+
+  CHECK_EQ(sigaction(SIGSYS, &act, &oldact_), 0);
+  CHECK_EQ(sigprocmask(SIG_UNBLOCK, &mask, nullptr), 0);
+}
+
+}  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/network_proxy_client.h b/sandboxed_api/sandbox2/network_proxy_client.h
new file mode 100644
index 0000000..111a1c1
--- /dev/null
+++ b/sandboxed_api/sandbox2/network_proxy_client.h
@@ -0,0 +1,75 @@
+// Copyright 2019 Google LLC
+//
+// 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.
+
+#ifndef SANDBOXED_API_SANDBOX2_NETWORK_PROXY_CLIENT_H_
+#define SANDBOXED_API_SANDBOX2_NETWORK_PROXY_CLIENT_H_
+
+#include <netinet/in.h>
+
+#include "absl/synchronization/mutex.h"
+#include "sandboxed_api/sandbox2/comms.h"
+#include "sandboxed_api/util/status.h"
+
+namespace sandbox2 {
+
+class NetworkProxyClient {
+ public:
+  static constexpr char kFDName[] = "sb2_networkproxy";
+
+  explicit NetworkProxyClient(int fd) : comms_(fd) {}
+
+  NetworkProxyClient(const NetworkProxyClient&) = delete;
+  NetworkProxyClient& operator=(const NetworkProxyClient&) = delete;
+
+  // Establishes a new network connection.
+  // Semantic is similar to a regular connect() call.
+  // Arguments are sent to network proxy server, which sends back a connected
+  // socket.
+  sapi::Status Connect(int sockfd, const struct sockaddr* addr,
+                       socklen_t addrlen);
+  // Same as Connect, but with same API as regular connect() call.
+  int ConnectHandler(int sockfd, const struct sockaddr* addr,
+                     socklen_t addrlen);
+
+ private:
+  Comms comms_;
+  sapi::Status ReceiveRemoteResult();
+
+  // Needed to make the Proxy thread safe.
+  absl::Mutex mutex_;
+};
+
+class NetworkProxyHandler {
+ public:
+  // Installs the handler that redirects connect() syscalls to the trap
+  // function. This function exchange data with NetworkProxyServer that checks
+  // if this connection is allowed and sends the connected socket to us.
+  // In other words, this function just use NetworkProxyClient class.
+  static sapi::Status InstallNetworkProxyHandler(NetworkProxyClient* npc);
+  void ProcessSeccompTrap(int nr, siginfo_t* info, void* void_context);
+
+ private:
+  NetworkProxyHandler(NetworkProxyClient* npc) : network_proxy_client_(npc) {
+    InstallSeccompTrap();
+  }
+  void InvokeOldAct(int nr, siginfo_t* info, void* void_context);
+  void InstallSeccompTrap();
+
+  struct sigaction oldact_;
+  NetworkProxyClient* network_proxy_client_;
+};
+
+}  // namespace sandbox2
+
+#endif  // SANDBOXED_API_SANDBOX2_NETWORK_PROXY_CLIENT_H_
diff --git a/sandboxed_api/sandbox2/network_proxy_server.cc b/sandboxed_api/sandbox2/network_proxy_server.cc
new file mode 100644
index 0000000..e0f7552
--- /dev/null
+++ b/sandboxed_api/sandbox2/network_proxy_server.cc
@@ -0,0 +1,95 @@
+// Copyright 2019 Google LLC
+//
+// 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.
+
+#include "sandboxed_api/sandbox2/network_proxy_server.h"
+
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <syscall.h>
+#include <cerrno>
+#include <cstring>
+
+#include <glog/logging.h>
+#include "absl/memory/memory.h"
+#include "sandboxed_api/sandbox2/util/fileops.h"
+
+namespace sandbox2 {
+
+NetworkProxyServer::NetworkProxyServer(int fd)
+    : comms_{absl::make_unique<Comms>(fd)}, fatal_error_{false} {}
+
+void NetworkProxyServer::ProcessConnectRequest() {
+  std::vector<uint8_t> addr;
+  if (!comms_->RecvBytes(&addr)) {
+    fatal_error_ = true;
+    return;
+  }
+
+  const struct sockaddr_in* saddr =
+      reinterpret_cast<const sockaddr_in*>(addr.data());
+
+  // Only IPv4 TCP and IPv6 TCP are supported.
+  if (!((addr.size() == sizeof(sockaddr_in) && saddr->sin_family == AF_INET) ||
+        (addr.size() == sizeof(sockaddr_in6) &&
+         saddr->sin_family == AF_INET6))) {
+    SendError(EINVAL);
+    return;
+  }
+
+  int new_socket = socket(saddr->sin_family, SOCK_STREAM, 0);
+  if (new_socket < 0) {
+    SendError(errno);
+    return;
+  }
+
+  file_util::fileops::FDCloser new_socket_closer(new_socket);
+
+  int result = connect(
+      new_socket, reinterpret_cast<const sockaddr*>(addr.data()), addr.size());
+
+  if (result == 0) {
+    NotifySuccess();
+    if (!fatal_error_) {
+      if (!comms_->SendFD(new_socket)) {
+        fatal_error_ = true;
+        return;
+      }
+    }
+  }
+}
+
+void NetworkProxyServer::Run() {
+  while (!fatal_error_) {
+    ProcessConnectRequest();
+  }
+  LOG(INFO)
+      << "Clean shutdown or error occurred, shutting down NetworkProxyServer";
+}
+
+void NetworkProxyServer::SendError(int saved_errno) {
+  if (!comms_->SendInt32(saved_errno)) {
+    fatal_error_ = true;
+  }
+}
+
+void NetworkProxyServer::NotifySuccess() {
+  if (!comms_->SendInt32(0)) {
+    fatal_error_ = true;
+  }
+}
+
+}  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/network_proxy_server.h b/sandboxed_api/sandbox2/network_proxy_server.h
new file mode 100644
index 0000000..55f6c5b
--- /dev/null
+++ b/sandboxed_api/sandbox2/network_proxy_server.h
@@ -0,0 +1,53 @@
+// Copyright 2019 Google LLC
+//
+// 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.
+
+#ifndef SANDBOXED_API_SANDBOX2_NETWORK_PROXY_SERVER_H_
+#define SANDBOXED_API_SANDBOX2_NETWORK_PROXY_SERVER_H_
+
+#include <memory>
+
+#include "sandboxed_api/sandbox2/comms.h"
+
+namespace sandbox2 {
+
+// This is a proxy server that spawns connected sockets on requests.
+// Then it sends the file descriptor to the requestor. It is used to get around
+// limitations created by network namespaces.
+class NetworkProxyServer {
+ public:
+  explicit NetworkProxyServer(int fd);
+
+  NetworkProxyServer(const NetworkProxyServer&) = delete;
+  NetworkProxyServer& operator=(const NetworkProxyServer&) = delete;
+
+  // Starts handling incoming connection requests.
+  void Run();
+
+ private:
+  // Notifies the network proxy client about the error and sends its code.
+  void SendError(int saved_errno);
+
+  // Notifies the network proxy client that no error occurred.
+  void NotifySuccess();
+
+  // Serves connection requests from the network proxy client.
+  void ProcessConnectRequest();
+
+  std::unique_ptr<Comms> comms_;
+  bool fatal_error_;
+};
+
+}  // namespace sandbox2
+
+#endif  // SANDBOXED_API_SANDBOX2_NETWORK_PROXY_SERVER_H_
diff --git a/sandboxed_api/sandbox2/policy.h b/sandboxed_api/sandbox2/policy.h
index d4a0ca1..b507f22 100644
--- a/sandboxed_api/sandbox2/policy.h
+++ b/sandboxed_api/sandbox2/policy.h
@@ -31,7 +31,6 @@
 #include "absl/base/macros.h"
 #include "absl/types/optional.h"
 #include "sandboxed_api/sandbox2/namespace.h"
-#include "sandboxed_api/sandbox2/network_proxy/filtering.h"
 #include "sandboxed_api/sandbox2/syscall.h"
 #include "sandboxed_api/sandbox2/violation.pb.h"
 
@@ -105,9 +104,6 @@ class Policy final {
   // Get a policy which would allow the Monitor module to track all syscalls.
   std::vector<sock_filter> GetTrackingPolicy() const;
 
-  // Contains a list of hosts the sandboxee is allowed to connect to.
-  absl::optional<AllowedHosts> allowed_hosts_;
-
   friend class Monitor;
   friend class PolicyBuilder;
   friend class PolicyBuilderPeer;  // For testing
diff --git a/sandboxed_api/sandbox2/policybuilder.cc b/sandboxed_api/sandbox2/policybuilder.cc
index 99b4b97..c731e97 100644
--- a/sandboxed_api/sandbox2/policybuilder.cc
+++ b/sandboxed_api/sandbox2/policybuilder.cc
@@ -39,7 +39,6 @@
 #include "sandboxed_api/sandbox2/namespace.h"
 #include "sandboxed_api/sandbox2/util/bpf_helper.h"
 #include "sandboxed_api/sandbox2/util/path.h"
-#include "sandboxed_api/sandbox2/util/strerror.h"
 #include "sandboxed_api/util/canonical_errors.h"
 #include "sandboxed_api/util/status_macros.h"
 
@@ -48,7 +47,7 @@ namespace {
 
 }  // namespace
 
-PolicyBuilder& PolicyBuilder::AllowSyscall(uint32_t num) {
+PolicyBuilder& PolicyBuilder::AllowSyscall(unsigned int num) {
   if (handled_syscalls_.insert(num).second) {
     user_policy_.insert(user_policy_.end(), {SYSCALL(num, ALLOW)});
   }
@@ -69,7 +68,8 @@ PolicyBuilder& PolicyBuilder::AllowSyscalls(SyscallInitializer nums) {
   return *this;
 }
 
-PolicyBuilder& PolicyBuilder::BlockSyscallWithErrno(uint32_t num, int error) {
+PolicyBuilder& PolicyBuilder::BlockSyscallWithErrno(unsigned int num,
+                                                    int error) {
   if (handled_syscalls_.insert(num).second) {
     user_policy_.insert(user_policy_.end(), {SYSCALL(num, ERRNO(error))});
   }
@@ -545,17 +545,17 @@ PolicyBuilder& PolicyBuilder::AllowDynamicStartup() {
   });
 }
 
-PolicyBuilder& PolicyBuilder::AddPolicyOnSyscall(uint32_t num,
+PolicyBuilder& PolicyBuilder::AddPolicyOnSyscall(unsigned int num,
                                                  BpfInitializer policy) {
   return AddPolicyOnSyscalls({num}, policy);
 }
 
 PolicyBuilder& PolicyBuilder::AddPolicyOnSyscall(
-    uint32_t num, const std::vector<sock_filter>& policy) {
+    unsigned int num, const std::vector<sock_filter>& policy) {
   return AddPolicyOnSyscalls({num}, policy);
 }
 
-PolicyBuilder& PolicyBuilder::AddPolicyOnSyscall(uint32_t num, BpfFunc f) {
+PolicyBuilder& PolicyBuilder::AddPolicyOnSyscall(unsigned int num, BpfFunc f) {
   return AddPolicyOnSyscalls({num}, f);
 }
 
@@ -700,7 +700,6 @@ sapi::StatusOr<std::unique_ptr<Policy>> PolicyBuilder::TryBuild() {
 
   StoreDescription(pb_description.get());
   output->policy_builder_description_ = std::move(pb_description);
-  output->allowed_hosts_ = std::move(allowed_hosts_);
   already_built_ = true;
   return std::move(output);
 }
@@ -849,15 +848,6 @@ PolicyBuilder& PolicyBuilder::CollectStacktracesOnKill(bool enable) {
 }
 
 PolicyBuilder& PolicyBuilder::AddNetworkProxyPolicy() {
-  if (allowed_hosts_) {
-    SetError(sapi::FailedPreconditionError(
-        "AddNetworkProxyPolicy or AddNetworkProxyHandlerPolicy can be called "
-        "at most once"));
-    return *this;
-  }
-
-  allowed_hosts_ = AllowedHosts();
-
   AllowFutexOp(FUTEX_WAKE);
   AllowFutexOp(FUTEX_WAIT);
   AllowFutexOp(FUTEX_WAIT_BITSET);
@@ -930,36 +920,4 @@ void PolicyBuilder::StoreDescription(PolicyBuilderDescription* pb_description) {
   }
 }
 
-PolicyBuilder& PolicyBuilder::AllowIPv4(const std::string& ip_and_mask,
-                                        uint32_t port) {
-  if (!allowed_hosts_) {
-    SetError(sapi::FailedPreconditionError(
-        "AddNetworkProxyPolicy or AddNetworkProxyHandlerPolicy must be called "
-        "before adding IP rules"));
-    return *this;
-  }
-
-  sapi::Status status = allowed_hosts_->AllowIPv4(ip_and_mask, port);
-  if (!status.ok()) {
-    SetError(status);
-  }
-  return *this;
-}
-
-PolicyBuilder& PolicyBuilder::AllowIPv6(const std::string& ip_and_mask,
-                                        uint32_t port) {
-  if (!allowed_hosts_) {
-    SetError(sapi::FailedPreconditionError(
-        "AddNetworkProxyPolicy or AddNetworkProxyHandlerPolicy must be called "
-        "before adding IP rules"));
-    return *this;
-  }
-
-  sapi::Status status = allowed_hosts_->AllowIPv6(ip_and_mask, port);
-  if (!status.ok()) {
-    SetError(status);
-  }
-  return *this;
-}
-
 }  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/policybuilder.h b/sandboxed_api/sandbox2/policybuilder.h
index c4997a7..b9d48db 100644
--- a/sandboxed_api/sandbox2/policybuilder.h
+++ b/sandboxed_api/sandbox2/policybuilder.h
@@ -31,7 +31,6 @@
 #include "absl/memory/memory.h"
 #include "absl/strings/string_view.h"
 #include "sandboxed_api/sandbox2/mounts.h"
-#include "sandboxed_api/sandbox2/network_proxy/filtering.h"
 #include "sandboxed_api/sandbox2/policy.h"
 #include "sandboxed_api/util/statusor.h"
 
@@ -502,10 +501,6 @@ class PolicyBuilder final {
   // Not recommended
   PolicyBuilder& SetRootWritable();
 
-  // Allows connections to this IP.
-  PolicyBuilder& AllowIPv4(const std::string& ip_and_mask, uint32_t port = 0);
-  PolicyBuilder& AllowIPv6(const std::string& ip_and_mask, uint32_t port = 0);
-
  private:
   friend class PolicyBuilderPeer;  // For testing
   friend class StackTracePeer;
@@ -547,9 +542,6 @@ class PolicyBuilder final {
   // This function returns a PolicyBuilder so that we can use it in the status
   // macros
   PolicyBuilder& SetError(const sapi::Status& status);
-
-  // Contains list of allowed hosts.
-  absl::optional<AllowedHosts> allowed_hosts_;
 };
 
 }  // namespace sandbox2
diff --git a/sandboxed_api/sandbox2/result.cc b/sandboxed_api/sandbox2/result.cc
index a83e457..1fa194f 100644
--- a/sandboxed_api/sandbox2/result.cc
+++ b/sandboxed_api/sandbox2/result.cc
@@ -79,10 +79,6 @@ std::string Result::ToString() const {
                             Syscall(GetSyscallArch(), reason_code()).GetName(),
                             "] Stack: ", GetStackTrace());
       break;
-    case sandbox2::Result::NETWORK_VIOLATION:
-      result = absl::StrCat("NETWORK VIOLATION - Violating network policy: ",
-                            network_violation_);
-      break;
     case sandbox2::Result::SIGNALED:
       result = absl::StrCat("Process terminated with a SIGNAL - Signal: ",
                             util::GetSignalName(reason_code()),
@@ -133,8 +129,6 @@ std::string Result::StatusEnumToString(StatusEnum value) {
       return "SETUP_ERROR";
     case sandbox2::Result::VIOLATION:
       return "VIOLATION";
-    case sandbox2::Result::NETWORK_VIOLATION:
-      return "NETWORK_VIOLATION";
     case sandbox2::Result::SIGNALED:
       return "SIGNALED";
     case sandbox2::Result::TIMEOUT:
diff --git a/sandboxed_api/sandbox2/result.h b/sandboxed_api/sandbox2/result.h
index cbda608..77cb043 100644
--- a/sandboxed_api/sandbox2/result.h
+++ b/sandboxed_api/sandbox2/result.h
@@ -45,8 +45,6 @@ class Result {
     SETUP_ERROR,
     // Syscall violation
     VIOLATION,
-    // Network policy violation
-    NETWORK_VIOLATION,
     // Process terminated with a signal
     SIGNALED,
     // Process terminated with a timeout
@@ -121,10 +119,6 @@ class Result {
     syscall_ = std::move(syscall);
   }
 
-  void SetNetworkViolation(std::string network_violation) {
-    network_violation_ = std::move(network_violation);
-  }
-
   StatusEnum final_status() const { return final_status_; }
   uintptr_t reason_code() const { return reason_code_; }
 
@@ -143,8 +137,6 @@ class Result {
 
   const std::string& GetProgName() const { return prog_name_; }
 
-  const std::string& GetNetworkViolation() const { return network_violation_; }
-
   void SetProgName(const std::string& name) { prog_name_ = name; }
 
   const std::string& GetProcMaps() const { return proc_maps_; }
@@ -187,8 +179,6 @@ class Result {
   std::string prog_name_;
   // /proc/pid/maps of the main process.
   std::string proc_maps_;
-  // IP and port if network violation occurred
-  std::string network_violation_;
   // Final resource usage as defined in <sys/resource.h> (man getrusage), for
   // the Monitor thread.
   rusage rusage_monitor_;