Porting iptables 1.4.10 to Android

(Update 25 Aprile 2011: This port is now in Google Code called iptables4n1).

Introduction 

The Android source comes with iptables 1.3.7. But that distribution is not compatible with Linux kernel 2.6.32 or newer versions. For example, even though iptables 1.3.7 can be built into the Android Emulator and Google Dev Phone 2 (a.k.a. HTC Magic) which are based on Linux kernel 2.6.29, it won't compile with kernel 2.6.32 used in Nexus One and many newer Android devices. The main difference between iptables 1.3.7 and 1.4.x is the emergence of "xtables" in the latter version. Fortunately, Google has done a commendable job in keeping Android in sync with Linux kernel releases. This makes porting a new version of iptables to Android largely an exercise of makefile changes. I will outline the steps here as a service to the open source community.

Steps

Below are steps for porting iptables 1.4.10 to Android/Linux kernel 2.6.32.

1. Check out the Android source.
2. Download iptables 1.4.10 source from the Netfilter project.
3. Go to the checked out Android source and change to $SRC/external/iptables. Delete the content underneath that directory (or make a backup if you want) and then copy the downloaded iptables 1.4.10 content to that directory.
4. Create a new Android.mk file under $SRC/external/iptables. This is the Android makefile. You can model yours after the one from the original Android source but you need to accommodate naming changes to iptable extensions that went from libiptXXX in 1.3.7 to libxtXXX in 1.4.10.
5. Change to $SRC/external/iptables/extensions and create a new create_initext4 file there. 
6. Change back to $SRC/external/iptables and run make. Fix header inclusion issues as needed.

Sample Android.mk

ifneq ($(TARGET_SIMULATOR),true)
  BUILD_IPTABLES := 1
endif
ifeq ($(BUILD_IPTABLES),1)
LOCAL_PATH:= $(call my-dir)
#
# Build libraries
#
# libxtables
include $(CLEAR_VARS)
LOCAL_C_INCLUDES:= \
    $(LOCAL_PATH)/include/ \
    $(KERNEL_HEADERS)
LOCAL_CFLAGS:=-DNO_SHARED_LIBS
LOCAL_CFLAGS+=-DXTABLES_INTERNAL
LOCAL_CFLAGS+=-DIPTABLES_VERSION=\"1.4.10\"
LOCAL_CFLAGS+=-DXTABLES_VERSION=\"1.4.10\" # -DIPT_LIB_DIR=\"$(IPT_LIBDIR)\"
LOCAL_CFLAGS+=-DXTABLES_LIBDIR
LOCAL_SRC_FILES:= \
    xtables.c
LOCAL_MODULE_TAGS:=
LOCAL_MODULE:=libxtables
include $(BUILD_STATIC_LIBRARY)
# libip4tc
include $(CLEAR_VARS)
LOCAL_C_INCLUDES:= \
    $(KERNEL_HEADERS) \
    $(LOCAL_PATH)/include/
LOCAL_CFLAGS:=-DNO_SHARED_LIBS
LOCAL_CFLAGS+=-DXTABLES_INTERNAL
LOCAL_SRC_FILES:= \
    libiptc/libip4tc.c
LOCAL_MODULE_TAGS:=
LOCAL_MODULE:=libip4tc
include $(BUILD_STATIC_LIBRARY)
# libext4
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS:=
LOCAL_MODULE:=libext4
# LOCAL_MODULE_CLASS must be defined before calling $(local-intermediates-dir)
#
LOCAL_MODULE_CLASS := STATIC_LIBRARIES
intermediates := $(call local-intermediates-dir)
LOCAL_C_INCLUDES:= \
    $(LOCAL_PATH)/include/ \
    $(KERNEL_HEADERS) \
    $(intermediates)/extensions/
LOCAL_CFLAGS:=-DNO_SHARED_LIBS
LOCAL_CFLAGS+=-DXTABLES_INTERNAL
LOCAL_CFLAGS+=-D_INIT=$*_init
LOCAL_CFLAGS+=-DIPTABLES_VERSION=\"1.4.10\"
LOCAL_CFLAGS+=-DXTABLES_VERSION=\"1.4.10\"
PF_EXT_SLIB:=ah addrtype ecn 
PF_EXT_SLIB+=icmp #2mark
PF_EXT_SLIB+=realm
PF_EXT_SLIB+=ttl unclean DNAT LOG #DSCP ECN
PF_EXT_SLIB+=MASQUERADE MIRROR NETMAP REDIRECT REJECT #MARK
PF_EXT_SLIB+=SAME SNAT ULOG # TOS TCPMSS TTL
PF_EXT_SLIB+=TAG
EXT_FUNC+=$(foreach T,$(PF_EXT_SLIB),ipt_$(T))
# xtable stuff
NEW_PF_EXT_SLIB:=comment conntrack connmark dscp tcpmss esp
NEW_PF_EXT_SLIB+=hashlimit helper iprange length limit mac multiport
NEW_PF_EXT_SLIB+=owner physdev pkttype policy sctp standard state tcp
NEW_PF_EXT_SLIB+=tos udp CLASSIFY CONNMARK
NEW_PF_EXT_SLIB+=NFQUEUE NOTRACK
EXT_FUNC+=$(foreach N,$(NEW_PF_EXT_SLIB),xt_$(N))
# generated headers
GEN_INITEXT:= $(intermediates)/extensions/gen_initext4.c
$(GEN_INITEXT): PRIVATE_PATH := $(LOCAL_PATH)
$(GEN_INITEXT): PRIVATE_CUSTOM_TOOL = $(PRIVATE_PATH)/extensions/create_initext4 "$(EXT_FUNC)" > $@
$(GEN_INITEXT): PRIVATE_MODULE := $(LOCAL_MODULE)
$(GEN_INITEXT):
    $(transform-generated-source)
$(intermediates)/extensions/initext4.o : $(GEN_INITEXT)
LOCAL_GENERATED_SOURCES:= $(GEN_INITEXT)
LOCAL_SRC_FILES:= \
    $(foreach T,$(PF_EXT_SLIB),extensions/libipt_$(T).c) \
    $(foreach N,$(NEW_PF_EXT_SLIB),extensions/libxt_$(N).c) \
    extensions/initext4.c
LOCAL_STATIC_LIBRARIES := \
    libc
include $(BUILD_STATIC_LIBRARY)
#
# Build iptables
#
include $(CLEAR_VARS)
LOCAL_C_INCLUDES:= \
    $(LOCAL_PATH)/include/ \
    $(KERNEL_HEADERS)
LOCAL_CFLAGS:=-DNO_SHARED_LIBS
LOCAL_CFLAGS+=-DXTABLES_INTERNAL
LOCAL_CFLAGS+=-DIPTABLES_VERSION=\"1.4.10\"
LOCAL_CFLAGS+=-DXTABLES_VERSION=\"1.4.10\" # -DIPT_LIB_DIR=\"$(IPT_LIBDIR)\"
#LOCAL_CFLAGS+=-DIPT_LIB_DIR=\"$(IPT_LIBDIR)\"
LOCAL_SRC_FILES:= \
    iptables.c \
    iptables-standalone.c \
        xshared.c
LOCAL_MODULE_TAGS:=
LOCAL_MODULE:=iptables
LOCAL_STATIC_LIBRARIES := \
    libip4tc \
    libext4  \
        libxtables
include $(BUILD_EXECUTABLE)
endif

Sample create_initext4

#!/bin/sh
echo ""
for i in $1; do
    echo "extern void lib${i}_init(void);";
done;
echo "void init_extensions(void);"
echo "void init_extensions(void) {"
for i in $1; do
    echo "    lib${i}_init();";
done
echo "}"


That is all. Happy hacking!

Socket and O_NONBLOCK

It is often stated that a socket is created in blocking mode by default. However, not all implementations of the socket calls adhere to that behavior. Specialized operating systems can be particularly tricky when it comes to blocking and non-blocking mode . One should always test the O_NONBLOCK flag on a socket to be sure if it is in the blocking or nonblocking mode operation before proceeding to send/recv. Here are some example c code to demonstrate how to check and manipulate the O_NONBLOCK flag on a streaming socket.

/* !!! Error checking is omited for brevity !!! */
/* Create a streaming socket */
int sock = socket(AF_INET, SOCK_STREAM, 0);

/* Get the file descriptor flag */
int flags = fcntl(sock, F_GETFL, 0);

/* Set O_NONBLOCK if it is set
 * or unset it if it is not
 */
if ((flags & O_NONBLOCK ) == O_NONBLOCK) {
   fcntl(sock, F_SETFL, flags | O_NONBLOCK);  /* Set */
} else {
   fcntl(sock, F_SETFL, flags & (~O_NONBLOCK));  /* Unset */
}