Author: Sam Protsenko <semen.protsenko at linaro dot org>

Make Android use upstream PPP VPN code

Synopsis

This wiki page is intended to document next things:

  • Setup configuration for LCG-882 ticket ("Upstream android PPP related drivers")

  • Development status
  • Debugging notes

Hereafter is described how to create "Android <-> PC" setup:

  • BeagleBoard X15 will be acting as PPP client. It may be any other ARM board with Android and Ethernet on it.

  • PC will be acting as PPP server

Software:

  • The OS used for PC is Debian "stretch" (testing). But it should work flawlessly for Ubuntu as well.
  • On BBB X15 I used Android "Nougat"

PC and X15 should be connected via Ethernet patch cord.

This article describes setup of Legacy VPN (which is needed for LCG-882 task), for both client (Android board) and server (PC) side.

What is "Legacy VPN"?

Prior to Android 4.0, VPN support was entirely built into the platform and wasn’t extensible. Support for new VPN types could only be added as part of platform updates. To distinguish it from application-based implementations, built-in VPN support is referred to as legacy VPN.

Early Android versions supported different VPN configurations based on PPTP and L2TP/IPsec, with support for “pure-IPSec” VPNs using IPSec Xauth added in version 4.0. In addition to new built-in VPN configurations, Android 4.0 also introduced application-based VPNs by supplying the base platform class VpnService, which applications could extend in order to implement a new VPN solution.

VPN use-cases

General VPN use-case for Android devices is next: Android device connected to Internet (via WiFi), and we are creating tunnel over Internet to some corporate network. This use-case is shown on picture below.

l2tp-use-case1.png

But in this article we are gonna use another setup (it's more convenient for development): Android device connected to developer's PC via Ethernet. This use-case is shown on picture below.

l2tp-use-case2.png

Common configuration

Things described in this section are common for both PPTP and L2TP protocols.

Android

Android version I'm using: 7.0 ("Nougat")

See these instructions for AOSP build instructions (for X15 board).

Kernel

kernel version I'm using: 4.4

Kernel for Android should be built with next options enabled:

# Android kernel PPP implementation
CONFIG_PPPOPNS=y
CONFIG_PPPOLAC=y

# Upstream kernel PPP implementation
CONFIG_NET_IPGRE_DEMUX=y
CONFIG_PPP=y
CONFIG_PPP_MPPE=y
CONFIG_PPPOE=y
CONFIG_PPTP=y
CONFIG_L2TP=y
CONFIG_L2TP_DEBUGFS=y
CONFIG_L2TP_V3=y
CONFIG_L2TP_IP=y
CONFIG_L2TP_ETH=y
CONFIG_PPPOL2TP=y

# Crypto
CONFIG_CRYPTO=y
CONFIG_CRYPTO_AUTHENC=y
CONFIG_CRYPTO_MD5=y
CONFIG_CRYPTO_CBC=y
CONFIG_CRYPTO_SHA1=y
CONFIG_CRYPTO_SHA512=y
CONFIG_CRYPTO_RMD160=y
CONFIG_CRYPTO_ECHAINIV=y
CONFIG_CRYPTO_AES=y
CONFIG_CRYPTO_ANUBIS=y
CONFIG_CRYPTO_ARC4=y
CONFIG_CRYPTO_BLOWFISH=y
CONFIG_CRYPTO_BLOWFISH_COMMON=y
CONFIG_CRYPTO_CAMELLIA=y
CONFIG_CRYPTO_CAST_COMMON=y
CONFIG_CRYPTO_CAST5=y
CONFIG_CRYPTO_CAST6=y
CONFIG_CRYPTO_DES=y
CONFIG_CRYPTO_FCRYPT=y
CONFIG_CRYPTO_KHAZAD=y
CONFIG_CRYPTO_SALSA20=y
CONFIG_CRYPTO_CHACHA20=y
CONFIG_CRYPTO_SEED=y
CONFIG_CRYPTO_SERPENT=y
CONFIG_CRYPTO_TEA=y
CONFIG_CRYPTO_TWOFISH=y
CONFIG_CRYPTO_CMAC=y
CONFIG_CRYPTO_HMAC=y
CONFIG_CRYPTO_XCBC=y
CONFIG_CRYPTO_VMAC=y

# IPsec support
CONFIG_XFRM=y
CONFIG_XFRM_USER=y
CONFIG_NET_KEY=y

# Data compression (AH, ESP and IPComp)
CONFIG_INET_AH=y
CONFIG_INET_ESP=y
CONFIG_XFRM_IPCOMP=y
CONFIG_INET_IPCOMP=y

# Support IPsec in NetFilter
CONFIG_IP_NF_MATCH_AH=y
CONFIG_NETFILTER_XT_MATCH_ESP=y
CONFIG_NETFILTER_XT_MATCH_POLICY=y

# Transport, tunnel and BEET mode support
CONFIG_INET_XFRM_TUNNEL=y
CONFIG_INET_XFRM_MODE_TRANSPORT=y
CONFIG_INET_XFRM_MODE_TUNNEL=y
CONFIG_INET_XFRM_MODE_BEET=y

# IPv6 support
CONFIG_INET6_AH=y
CONFIG_INET6_ESP=y
CONFIG_INET6_IPCOMP=y
CONFIG_INET6_XFRM_TUNNEL=y
CONFIG_INET6_XFRM_MODE_TRANSPORT=y
CONFIG_INET6_XFRM_MODE_TUNNEL=y
CONFIG_INET6_XFRM_MODE_BEET=y
CONFIG_IP6_NF_MATCH_AH=y

These options enable Android implementation of PPP. Without these there will be some error (on attempt to start mtpd), related to missing PPPoX implementation in kernel.

Particularly we are interested in next two options (added in Android kernel):

Option

Meaning

Details

CONFIG_PPPOPNS

PNS (PPTP Network Server)

RFC2637

CONFIG_PPPOLAC

LAC (L2TP Access Concentrator)

RFC2661

In order to use mainline implementation of L2TP next options should be enabled in kernel:

  • CONFIG_L2TP

  • CONFIG_L2TP_DEBUGFS

  • CONFIG_L2TP_V3

  • CONFIG_L2TP_IP

  • CONFIG_L2TP_ETH

  • CONFIG_PPPOL2TP

Network details

BBB X15 doesn't have any WiFi chips on it, so I will use Ethernet connection for PPP. Because Android GUI doesn't have an option to use Ethernet, all configuration will be done from console exclusively.

Next networks and addresses will be used in this setup:

Ethernet:
    192.168.0.1   - server
    192.168.0.100 - client

PPP:
    192.168.2.1   - VPN server (PC)
    192.168.2.100 - Android (BB X15)

Network configuration (PC)

Enable IP forwarding. Edit /etc/sysctl.conf and add/change next line:

net.ipv4.ip_forward=1

and do next to enable this setting for current session:

$ sudo service procps start

Ethernet configuration for server (in /etc/network/interfaces):

auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.0.1
    netmask 255.255.255.0

NOTE: if you are using NetworkManager, you also may be needed to enable "managed" mode in NetworkManager config file.

Install PPP daemon:

$ sudo aptitude install ppp

Next CHAP authentication parameters will be used (corresponding lines will be added to /etc/ppp/chap-secrets later):

user

joe

password

test1234

Bring up Ethernet

On Android: bring up Ethernet

$ su
# ip addr add 192.168.0.100 dev eth0
# ip link set eth0 up
# ip route add 192.168.0.0/24 dev eth0
# ip route add default via 192.168.0.1 dev eth0
# ip rule add from all lookup main pref 99

On PC: if you don't see eth0 in ifconfig output, bring up the Ethernet like this:

$ sudo service networking stop
$ sudo service networking start

Test Ethernet connection

On Android:

# ip addr show dev eth0 scope global

    2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
        link/ether a0:f6:fd:ad:d9:b8 brd ff:ff:ff:ff:ff:ff
        inet 192.168.0.100/32 scope global eth0
           valid_lft forever preferred_lft forever

# ip route show

    default via 192.168.0.1 dev eth01
    192.168.0.0/24 dev eth0  scope link 

# ping 192.168.0.1

    // ping should work

Forward Internet (optional)

This section describes how to provide internet connection to Android from PC via Ethernet.

On PC: Configure IPtables to grant access for your board's subnet to public network (with Internet):

$ sudo iptables -A POSTROUTING -t nat -o wlan0 -s 192.168.0.0/24 -j MASQUERADE

where public interface is wlan0 and 192.168.0.0/24 is Ethernet network.

On Android: Configure DNS:

# ndc network interface add 100 eth0
# ndc resolver setnetdns 100 localdomain 8.8.8.8 8.8.4.4
# ndc network default set 100

Now you can test internet connection in Android browser, or just by pinging some site:

# ping google.com

Protocols

One of these protocols may be used to create VPN connection:

  • PPTP
  • L2TP

L2TP is more secure, but PPTP is easier to setup. I need both of them for my task.

PPTP protocol

Server setup

In order to test transfer over PPP, install PPTPD on PC (a.k.a server):

$ sudo aptitude install pptpd

PPTPD configuration (in /etc/pptpd.conf):

localip 192.168.2.1
remoteip 192.168.2.100-105

Add credentials for authentication using CHAP. In /etc/ppp/chap-secrets add next line:

joe      pptpd   test1234        *

Disable MPPE on server. In /etc/ppp/pptpd-options, remove next line (or comment it out):

require-mppe-128


NOTE: if previous step wasn't done, the connection will be tear down on attempt to start mtpd with next messages:

  • on Android, in output of logcat -s mtpd:* command:

I/mtpd    ( 3323): Remote server hung up
  • on PC, in /var/log/syslog:

pppd[9428]: MPPE required but peer refused


Restart networking and pptpd services on server:

$ sudo service networking stop
$ sudo service networking start
$ sudo service pptpd restart

Bring up PPTP

Now when server and client are all set to go, let's bring up PPP connection.

On Android: prepare for running L2TP:

# rm -f /data/misc/vpn/abort

On Android: bring up PPP

# mtpd eth0 pptp 192.168.0.1 1723 linkname vpn name joe password test1234 refuse-eap nodefaultroute usepeerdns idle 1800 mtu 1400 mru 1400 &

L2TP protocol

Approximate view of the big picture:

l2tp-setup-overview.png

racoon configuration

Racoon is IPSec daemon (running on PC). There are two options how to use it:

  • PSK (Pre-Shared Key)
  • RSA Certificates

We will stick to PSK as it's easier to setup and use.

First install racoon:

$ sudo aptitude install racoon

(choose "direct" method, if asked by installer).

racoon version I'm using: 0.8.2

Edit /etc/racoon/psk.txt so it has only next one line:

myhomelan d41d8cd98f00b204e980

NOTE: be sure that there is no trailing spaces after PSK in racoon config file!

Restrict permissions to that file, to avoid security errors:

$ sudo chmod 0400 /etc/racoon/psk.txt

Parameters used are as follows:

IPSec identifier

myhomelan

IPSec PSK

d41d8cd98f00b204e980

Edit /etc/racoon/racoon.conf so it has next content:

log notify;
path pre_shared_key "/etc/racoon/psk.txt";
path certificate "/etc/racoon/certs";

remote anonymous {
        exchange_mode aggressive;

        generate_policy on;
        nat_traversal on;

        dpd_delay 20;

        proposal {
                encryption_algorithm aes;
                hash_algorithm md5;
                authentication_method pre_shared_key;
                dh_group modp1024;
        }
}

sainfo anonymous {
        encryption_algorithm aes, 3des;
        authentication_algorithm hmac_sha256, hmac_md5;
        compression_algorithm deflate;
}

NOTE: pay attention to exchange_mode: it should be aggressive, since main doesn't work with Android for some reasons.

xl2tpd configuration

xl2tpd is L2TP daemon (running on PC).

NOTE: Look here first.

Install xl2tpd:

$ sudo aptitude install xl2tpd

xl2tpd version I'm using: 1.3.12

Edit /etc/xl2tpd/xl2tpd.conf so it has next content:

[global]
access control = no

[lns default]
ip range = 192.168.2.100-192.168.2.105
local ip = 192.168.2.1
require authentication = yes
require chap = yes
refuse pap = yes
length bit = yes
name = l2tpd
pppoptfile = /etc/ppp/xl2tpd-options

Edit /etc/ppp/xl2tpd-options so it has next content:

auth
nodefaultroute
proxyarp
require-chap
ms-dns 8.8.8.8
ms-dns 8.8.4.4

Add next line to /etc/ppp/chap-secrets:

joe           l2tpd   test1234                *

Bring up L2TP

On Android: prepare for running L2TP:

# rm -f /data/misc/vpn/abort

On Android: start IPSec client (in background):

# racoon eth0 192.168.0.1 udppsk myhomelan d41d8cd98f00b204e980 1701 &

On Android: start L2TP client:

# mtpd eth0 l2tp 192.168.0.1 1701 "" linkname vpn name joe password test1234 refuse-eap nodefaultroute usepeerdns idle 1800 mtu 1400 mru 1400 &

NOTE: we are passing empty param ("") instead of secret, because we don't have auth option enabled in xl2tp config file.

NOTE: If patches for upstream L2TP are applied, upstream L2TP will be chosen by default. In case it's not enabled in kernel, old Android L2TP implementation will be used as a fallback.

Check correct L2TP run

On success, you'll see next lines in terminal:

Plugin pppol2tp.so loaded.
Using interface ppp0
Connect: ppp0 <--> 
Overriding mtu 1500 to 1400
Overriding mru 1500 to mtu value 1400

Overriding mtu 1500 to 1400
CHAP authentication succeeded: Access granted
CHAP authentication succeeded
local  IP address 192.168.2.100
remote IP address 192.168.2.1
primary   DNS address 8.8.8.8
secondary DNS address 8.8.4.4

And in logcat there should be log like this:

05-11 01:37:45.851  1743  1743 I racoon  : ipsec-tools 0.7.3 (http://ipsec-tools.sf.net)
05-11 01:37:45.855  1743  1743 I racoon  : 192.168.0.100[500] used as isakmp port (fd=5)
05-11 01:37:45.855  1743  1743 I racoon  : 192.168.0.100[500] used for NAT-T
05-11 01:37:45.855  1743  1743 I racoon  : 192.168.0.100[4500] used as isakmp port (fd=6)
05-11 01:37:45.855  1743  1743 I racoon  : 192.168.0.100[4500] used for NAT-T
05-11 01:37:46.448  1768  1768 I mtpd    : Using protocol l2tp
05-11 01:37:46.448  1768  1768 I mtpd    : Connecting to 192.168.0.1 port 1701 via eth0
05-11 01:37:46.449  1743  1743 I racoon  : IPsec-SA request for 192.168.0.1 queued due to no phase1 found.
05-11 01:37:46.449  1743  1743 I racoon  : initiate new phase 1 negotiation: 192.168.0.100[500]<=>192.168.0.1[500]
05-11 01:37:46.449  1743  1743 I racoon  : begin Aggressive mode.       
05-11 01:37:46.457  1768  1768 I mtpd    : Connection established (socket = 6)
05-11 01:37:46.457  1768  1768 D mtpd    : Sending SCCRQ (local_tunnel = 51531)
05-11 01:37:46.461  1743  1743 I racoon  : received Vendor ID: RFC 3947
05-11 01:37:46.461  1743  1743 I racoon  : received broken Microsoft ID: FRAGMENTATION
05-11 01:37:46.461  1743  1743 I racoon  : received Vendor ID: DPD
05-11 01:37:46.461  1743  1743 I racoon  : Selected NAT-T version: RFC 3947
05-11 01:37:46.461  1743  1743 I racoon  : Hashing 192.168.0.100[500] with algo #1 
05-11 01:37:46.461  1743  1743 I racoon  : NAT-D payload #-1 verified
05-11 01:37:46.461  1743  1743 I racoon  : Hashing 192.168.0.1[500] with algo #1 
05-11 01:37:46.461  1743  1743 I racoon  : NAT-D payload #0 verified
05-11 01:37:46.461  1743  1743 I racoon  : NAT not detected 
05-11 01:37:46.465  1743  1743 I racoon  : couldn't find the proper pskey, try to get one by the peer's address.
05-11 01:37:46.465  1743  1743 I racoon  : Adding remote and local NAT-D payloads.
05-11 01:37:46.465  1743  1743 I racoon  : Hashing 192.168.0.1[500] with algo #1 
05-11 01:37:46.465  1743  1743 I racoon  : Hashing 192.168.0.100[500] with algo #1 
05-11 01:37:46.466  1743  1743 I racoon  : ISAKMP-SA established 192.168.0.100[500]-192.168.0.1[500] spi:60b055bd50c1e4
05-11 01:37:47.468  1743  1743 I racoon  : initiate new phase 2 negotiation: 192.168.0.100[500]<=>192.168.0.1[500]
05-11 01:37:47.470  1743  1743 W racoon  : authtype mismatched: my:hmac-sha512 peer:hmac-sha256
05-11 01:37:47.470  1743  1743 W racoon  : authtype mismatched: my:hmac-sha384 peer:hmac-sha256
05-11 01:37:47.470  1743  1743 W racoon  : authtype mismatched: my:hmac-sha peer:hmac-sha256
05-11 01:37:47.470  1743  1743 I racoon  : IPsec-SA established: ESP/Transport 192.168.0.1[0]->192.168.0.100[0] spi=88)
05-11 01:37:47.471  1743  1743 I racoon  : IPsec-SA established: ESP/Transport 192.168.0.100[500]->192.168.0.1[500] sp)

05-11 01:37:48.459  1768  1768 D mtpd    : Timeout -> Sending SCCRQ
05-11 01:37:48.460  1768  1768 D mtpd    : Received SCCRP (remote_tunnel = 44607) -> Sending SCCCN
05-11 01:37:48.461  1768  1768 D mtpd    : Received ACK -> Sending ICRQ (local_session = 2058)
05-11 01:37:48.461  1768  1768 I mtpd    : Tunnel established
05-11 01:37:48.462  1768  1768 D mtpd    : Received ICRP (remote_session = 60142) -> Sending ICCN
05-11 01:37:48.463  1768  1768 D mtpd    : Received ACK
05-11 01:37:48.463  1768  1768 I mtpd    : Session established
05-11 01:37:48.463  1768  1768 I mtpd    : Creating PPPoX tunnel socket...
05-11 01:37:48.463  1768  1768 I mtpd    : Connecting to tunnel socket...
05-11 01:37:48.463  1768  1768 I mtpd    : Creating PPPoX session socket...
05-11 01:37:48.463  1768  1768 I mtpd    : Connecting to session socket...
05-11 01:37:48.463  1768  1768 I mtpd    : Starting pppd (tunnel_fd = 7, session_fd = 8)
05-11 01:37:48.464  1768  1768 I mtpd    : Pppd started (pid = 1769)
05-11 01:37:48.480  1769  1769 I pppd    : Plugin pppol2tp.so loaded.
05-11 01:37:48.483  1769  1769 I pppd    : pppd 2.4.7 started by root, uid 0
05-11 01:37:48.484  1769  1769 I pppd    : Using interface ppp0
05-11 01:37:48.484  1769  1769 I pppd    : Connect: ppp0 <--> 
05-11 01:37:48.484  1769  1769 W pppd    : Overriding mtu 1500 to 1400
05-11 01:37:48.484  1769  1769 W pppd    : Overriding mru 1500 to mtu value 1400
05-11 01:37:51.473  1769  1769 I pppd    : CHAP authentication succeeded: Access granted
05-11 01:37:51.474  1769  1769 I pppd    : CHAP authentication succeeded
05-11 01:37:51.483  1769  1769 I pppd    : local  IP address 192.168.2.100
05-11 01:37:51.483  1769  1769 I pppd    : remote IP address 192.168.2.1
05-11 01:37:51.484  1769  1769 I pppd    : primary   DNS address 8.8.8.8
05-11 01:37:51.484  1769  1769 I pppd    : secondary DNS address 8.8.4.4

Test PPP

NOTE: if you can't bring up ppp0 interface (i.e. you can't see ppp0 in "ifconfig" output on PC) -- reflash all Android images, reboot once Android is booted, and try again. It helps somehow (haven't figured why though).

Once PPTP or L2TP is up, we can test if everything works as expected. In this test we will copy file over ppp0 interface (via TCP protocol), using netcat tool.

On Android: test that PPP connection work

# ping -I ppp0 192.168.2.1

On Android: start receiving file via PPP. You may also want to start Wireshark on PC (for ppp0 interface) to sniff TCP packets for file transfer.

# toybox nc -l -p 1234

On PC: start transmitting file (file.dat should contain some text message).

$ cat file.dat | nc -w 3 192.168.2.100 1234

Now you should see file content in console on Android side. To kill mtpd on Android, figure out mtpd process ID:

# ps | grep mtpd

And kill it:

# kill <mtpd_pid>

Testing VPN over WiFi

End user is supposed to enable VPN in Settings (UI), rather then configuring it in console. Also, Android devices usually connect to network over WiFi (instead of Ethernet way we used in this wiki). So in order to test end user use-case we will need two things:

  1. Some Android device with WiFi on board. It can be e.g. Google Pixel phone or HiKey960 development board. I will use HiKey960 further, as I have access to it.

  2. L2TP server with WiFi access point. I'll just use my old L2TP server on laptop (as described in this wiki), but will bring up WiFi Access Point using hostapd daemon.

Configuring WiFi Access Point

For Access Point we will use hostapd daemon. As DHCP server we will use dnsmasq (so that our host can provide IP address dynamically to Android device, otherwise WiFi connection won't work).

Further I will assume next parameters:

eth1

ISP network (connected to internet)

wlan2

WiFi Access Point (local network with Android device)

192.168.0.1

Access Point IP address (wlan2 on host)

joe-laptop

host name (PC/laptop)

First, check that your WiFi module actually supports Access Point feature:

# iw list

You should see lines like this:

        Supported interface modes:
                 ...
                 * AP
                 ...

"AP" means that Access Point mode is supported.

Install daemons:

# sudo aptitude install hostapd dnsmasq

hostapd version I'm using: 2.6

dnsmasq version I'm using: 2.79

In /etc/default/hostapd specify hostapd config file path:

DAEMON_CONF="/etc/hostapd/hostapd.conf"

In /etc/hostapd/hostapd.conf provide Access Point configuration:

interface=wlan2
driver=nl80211
logger_syslog=-1
logger_syslog_level=2
logger_stdout=-1
logger_stdout_level=2
ctrl_interface=/var/run/hostapd
ctrl_interface_group=0
ssid=joe-laptop
country_code=UA
hw_mode=g
channel=6
macaddr_acl=0
auth_algs=1
ieee80211n=1
wpa=2
wpa_passphrase=Boeing-737
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP

Change interface to your WiFi interface, ssid to your host name, country_code to your country and wpa_passphrase to Access Point WiFi password, accordingly.

In your /etc/network/interfaces, configure Access Point interface:

auto wlan2
iface wlan2 inet static
address 192.168.0.1
netmask 255.255.255.0

Be sure 192.168.0.1 is not used in other interface, e.g. in eth0. Also make sure that wlan2 is not managed by NetworkManager.

Configure dnsmasq in /etc/dnsmasq.conf for lending IP addresses via DHCP on wlan2 interface:

server=8.8.8.8
interface=wlan2
no-hosts
addn-hosts=/etc/hosts.dnsmasq
dhcp-range=192.168.0.2,192.168.0.10,255.255.255.0,12h
dhcp-option=3,192.168.0.1
dhcp-option=6,192.168.0.1
log-queries
log-dhcp

Create /etc/hosts.dnsmasq file and put next line into it:

192.168.0.1 joe-laptop

Restart related services:

$ sudo service networking stop
$ sudo service networking start
$ sudo service hostapd restart
$ sudo service dnsmasq restart

Forward internet from eth1 to wlan2 interface:

$ sudo iptables --table nat --append POSTROUTING --out-interface eth1 -j MASQUERADE
$ sudo iptables --append FORWARD --in-interface wlan2 -j ACCEPT

Enabling VPN on device

As a client, I used HiKey960 board with AOSP master and kernel v4.14. Build instructions: hikey-master-4.14.txt

First of all, to be on the safe side, let's put SELinux into permissive mode. On device:

# setenforce 0

In UI, connect device to your Access Point WiFi (in our case it'll be called "joe-laptop"). You should see "Connected" status. After this, you should see the Internet connection, if you have it on your laptop and forwarded it to device using iptables commands above. Use browser to check that Internet works.

Now, on device open "Settings" in UI interface. Create new VPN connection like shown on this screenshot:

vpn_settings.png

These settings will lead to starting racoon and mtpd with the same parameters as in Bring up L2TP section.

Connect to created VPN. You should see "Connected" status once it's connected. ifconfig will show you ppp0 interface. Details about connection process could be found in logcat.

Test VPN by pinging the VPN server from device:

# ping 192.168.2.1

Development status

In order to use upstream kernel implementation of L2TP, some changes were made to mtpd and pppd Android projects, as well as to kernel/configs project. All patches were merged in AOSP:

If you want to try it out:

  • build AOSP master (which has those patches merged)
  • build kernel (tested on v4.14); make sure to merge corresponding Android kernel/configs, where upstream L2TP is enabled (see patches in the link above)

Then run L2TP VPN as usual. Upstream implementation (OL2TP) will be tried first; if it's not supported by your kernel, old OLAC implementation will be used (fall-back mechanism). Check your logcat output to see details.

Debugging and development hints

Next projects may need to be debugged:

  1. xl2tpd: running on PC

  2. mtpd (part of AFS): running on Android

  3. kernel (L2TP/PPTP implementation): running on Android

Further described some debugging techniques allow us to debug those projects.

Also we can sniff packets via eth0 interface using Wireshark.

Wireshark

Change racoon encoding algorithm

In case of L2TP, traffic is encoded by racoon server (IPSec), using one of next algorithms:

  • DES
  • 3DES
  • AES

Previously we configured racoon to use AES encryption, as it's the most secure of algorithms listed above. Unfortunately, Wireshark is not able (yet) to decrypt AES encrypted packets. So in order to decrypt packets, we need to change racoon configuration to use either DES or 3DES algorithm. Let's stick to 3DES, as it's more secure.

Change /etc/racoon/racoon.conf, so that encryption_algorithm property is set to 3des in all sections. Also, you can switch log level to debug2 (most verbose debug), so some extra information can be obtained further from /var/log/syslog.

Modified config file should look like this:

log debug2;
path pre_shared_key "/etc/racoon/psk.txt";
path certificate "/etc/racoon/certs";

remote anonymous {
        exchange_mode aggressive;

        generate_policy on;
        nat_traversal on;

        dpd_delay 20;

        proposal {
                encryption_algorithm 3des;
                hash_algorithm md5;
                authentication_method pre_shared_key;
                dh_group modp1024;
        }
}

sainfo anonymous {
        encryption_algorithm 3des;
        authentication_algorithm hmac_sha1, hmac_md5;
        compression_algorithm deflate;
}

Now, restart racoon service:

# service racoon restart

Run sniffing

Start Wireshark and run sniffing on eth0 protocol.

Start racoon and mtpd commands on Android device, as it's described above.

Once packets of interest are caught -- stop sniffing.

Figure out IPSec keys

Run next command (from [8]):

# ip xfrm state

It prints next params (for both "client -> server" and "server -> client" transmissions):

  • SPI (Security Parameter Index)

  • Encryption Algorithm (we are using 3DES)
  • Encyption Key
  • Authentication Algorithm (it's HMAC with SHA-1 as hash function, with MAC truncated to 96 bits)

  • Authentication Key

Decrypt ESP packets

We only want to decrypt ESP packets (as ISAKMP packets don't have any interesting information for us).

Enable ESP decryption in Wireshark:

Edit -> Preferences -> Protocols -> ESP -> Attempt to detect/decode encrypted ESP payloads

Now add the two ESP SAs (one for each direction), as shown on image below. Use information obtained from ip xfrm state.

wireshark-esp.png

Apply changes and all ESP packets will become decrypted, changing their "Protocol" field to "L2TP", "PPP", etc, as it's shown on picture below.

wireshark-decrypted.png

See more details on [8].

Debugging x2ltpd

Enable verbose logs

Add next lines to /etc/xl2tpd/xl2tpd.conf file:

[global]
...
debug avp = yes
debug network = yes
debug state = yes
debug tunnel = yes

[lns default]
...
ppp debug = yes

It will make xl2tpd log messages more verbose. See /var/log/syslog file for xl2tpd logs.

Add debug code to xl2tpd

For further investigation one may need to change xl2tpd code (add some tracing messages, etc). We need to be sure that next requirements are met:

  • The same version of xl2tpd is used
  • Modified xl2tpd installed as Debian package
  • Previous xl2tpd removed, but config files should be left (use remove command instead of purge one)

First of all, obtain xl2tpd sources:

$ apt-get source xl2tpd

Modify xl2tpd sources (add traces, etc).

Remove old xl2tpd from your system and install build dependencies for xl2tpd:

$ sudo service xl2tpd stop
$ sudo aptitude remove xl2tpd
$ sudo aptitude build-dep xl2tpd

Build xl2tpd and create deb-package:

$ cd xl2tpd-1.3.6+dfsg/
$ sudo debuild -b -uc -us
$ cd ..

Install built package:

$ sudo dpkg -i xl2tpd_1.3.6+dfsg-3_amd64.deb
$ sudo aptitude hold xl2tpd

Finally, make sure that xl2tpd is running:

$ sudo service xl2tpd start

Now you should be able to catch your traces in /var/log/syslog:

$ sudo less /var/log/syslog

mtpd

Changes to mtpd code can be break down to next categories:

  • adding new protocols (like PPTP and L2TP from mainline kernel)
  • adding traces and other debugging stuff

Debugging mtpd

Added prints can be seen in logcat:

# logcat -s mtpd:*

Kernel (on Android side)

l2tp debugging

At this point we are only interested in modifying this file: net/l2tp/l2tp_ppp.c. The only stuff we should add there is just trace prints (to figure out tunnel id, session id, point where things go wrong, etc).

Added printings can be seen in kernel log (on Android device). Issue next command if you can't see your prints at once (i.e. your log level is not sufficient):

# dmesg

Kernel log size

If you can see only last part of kernel log, log buffer size needs to be increased. For this next option should be set as follows:

CONFIG_LOG_BUF_SHIFT=17

Development issues

"Can not find tunnel" issue: investigation

This picture shows comparison of next logs:

  • xl2tpd logs for mainline kernel L2TP implementation (on left)
  • xl2tpd logs for Android kernel L2TP implementation (on right)

We can see from picture above that st->ourtid and tunnel values should be the same (like it's happening for Android L2TP implementation). But those values are different when using code that I added to mtpd, which uses mainline kernel L2TP implementation. Because of this xl2tpd cannot find tunnel id in its list (see error in picture above). It's because I return session_fd in create_pppox() function here. In case when I return tunnel_fd instead of session_fd -- those values are the same, but it's unable to transmit any data (it's basically because mainline kernel L2TP implementation only allows to send management commands via tunnel, and data is intended to be send via session).

In original mtpd code (l2tp.c) there is only one socket being created (pppox). Further this socket FD is being passed to pppd process as pppox parameter. This commit (for Android pppd implementation) handles pppox parameter.

But for modified mtpd code (which is using mainline kernel L2TP implementation) we need to pass 3 parameters to pppd:

  • session_fd

  • tunnel_id

  • session_id

So my suspicion is that we have to port pppol2tp plugin to Android pppd implementation. After that mtpd should be modified so that it runs pppol2tp plugin and passes required parameters for L2TP to it.

Byte order issues

pppol2tp plugin had to be ported, but it didn't fix the issue with erroneous tunnel IDs. It's basically because that issue was caused by wrong byte order in some variables in code.

So there were endianness issues preventing upstream L2TP implementation from properly working. "Wrong tunnel id" issue was caused exactly by wrong byte order. In fact, it can be seen by swapping bytes in picture from this issue:

correct tunnel id = 20061 = 0x4e5d
wrong tunnel id   = 23886 = 0x5d4e

Decrypting of packets in Wireshark helped to root cause the issue.

First of all, I noticed that in L2TP packets Wireshark shows correct tunnel ID, but in logcat I see wrong values. Here is the patches that fix that issue:

Of course, those fixes didn't help me with actual issue, but helped me to figure out what was happening there.

Interesting thing is that I haven't thought about possible byte order issue in tunnel ID value before actually saw code which builds this value from bytes arrived from network (see mtpd project, l2tp_up.c file, get_attribute_raw() function).

So next I checked all variables in mtpd which were involved in network transactions, figured which byte order is used for those values and checked those variables usage throughout the whole code. When doing that I noticed, that I'm passing tunnel_id and session_id in network byte order (big endian) to pppol2tp plugin, when starting pppd. Ok, this is understandable, as in original L2TP code in mtpd they are not passing those IDs at all, so I just haven't thought about this possible issue at the time. Here is the patch that fixes this issue:

But issue still existed, so I continued to check variables. Next thing I found was wrong filling of L2TP structures for further passing to connect() call (i.e. in network byte order instead of host byte order). Why I haven't noticed that and I haven't thought of such issue when coding -- it's just because Android implementation requires just the opposite byte order, so I basically just copied the code. Btw, neither documentation mentions which byte order should be used for each field. So the last patch which finally fixes last issue is here:

With all patches listed I managed to run mtpd with mainline implementation of L2TP (like it's described here, just be sure to configure network and run racoon first: link 1, link 2) and successfully run test (copying file from PC to Android device via ppp0 interface).

pppol2tp.so is 32-bit instead of 64-bit

As part of transition my patches from Android-N to Android-O, I changed next lines in external/ppp/pppd/Android.mk:

-LOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/pppd/$(VER)
-LOCAL_UNSTRIPPED_PATH := $(TARGET_OUT_SHARED_LIBRARIES_UNSTRIPPED)/pppd/$(VER)
+LOCAL_MODULE_RELATIVE_PATH := pppd/$(VER)

So now I have two versions of pppol2tp.so plugin built, 32-bit and 64-bit one:

/system/lib/pppd/2.4.7/pppol2tp.so
/system/lib64/pppd/2.4.7/pppol2tp.so

When trying to test racoon + mtpd, next error messages appeared in logcat:

I mtpd    : Starting pppd (tunnel_fd = 8, session_fd = 9)
I mtpd    : Pppd started (pid = 2926)
pppd: dlopen failed: "/system/lib/pppd/2.4.7/pppol2tp.so" is 32-bit instead of 64-bit
pppd: Couldn't load plugin pppol2tp.so
E pppd    : dlopen failed: "/system/lib/pppd/2.4.7/pppol2tp.so" is 32-bit instead of 64-bit
E pppd    : Couldn't load plugin pppol2tp.so
I mtpd    : Received signal 17
I mtpd    : Pppd is terminated (status = 2)
D mtpd    : Sending STOPCCN
I mtpd    : Mtpd is terminated (status = 34)

It was fixed with next patch (in external/ppp/):

   1 --- a/pppd/pathnames.h
   2 +++ b/pppd/pathnames.h
   3 @@ -55,11 +55,17 @@
   4  #endif
   5  #endif /* __STDC__ */
   6  
   7 +#if defined(__LP64__)
   8 +#define LIB_DIR                "lib64"
   9 +#else
  10 +#define LIB_DIR                "lib"
  11 +#endif
  12 +
  13  #ifdef PLUGIN
  14  #ifdef __STDC__
  15 -#define _PATH_PLUGIN   DESTDIR "/lib/pppd/" VERSION
  16 +#define _PATH_PLUGIN   DESTDIR "/" LIB_DIR "/pppd/" VERSION
  17  #else /* __STDC__ */
  18 -#define _PATH_PLUGIN   "/usr/lib/pppd"
  19 +#define _PATH_PLUGIN   "/usr/" LIB_DIR "/pppd"
  20  #endif /* __STDC__ */
  21  
  22  #endif /* PLUGIN */

pppol2tp.so is not building on fresh Android build

Since PPPoL2TP plugin was added to external/ppp/pppd project, it has to be built, because pppd uses pppol2tp.so for L2TP protocol. So let's add pppol2tp plugin to LOCAL_REQUIRED_MODULES in pppd executable definition (Android.mk file):

   1  LOCAL_MODULE:= pppd
   2 +LOCAL_REQUIRED_MODULES := pppol2tp
   3  include $(BUILD_EXECUTABLE)

Another way to do that would be adding it to LOCAL_SHARED_LIBRARIES variable of pppd executable definition in external/ppp/pppd/Android.mk, but it would lead to linking pppd binary against pppol2tp.so, which we don't want, as pppol2tp.so is just a plugin and should be loaded via dlopen() only when it's needed.

Also we could add it to PRODUCT_PACKAGES variables in build/target/product/*.mk files, but it's better to specify it as a dependency of pppd, which it actually is, and not tinker with such a generic files like build/....

pppol2tp.so can't be loaded by Bionic loader

When PPPoL2TP plugin resides in /system/lib[64]/pppd/2.4.7/, it can't be loaded with dlopen() from pppd binary, as this path is not in permitted path for default linker namespace. Next errors can be observed in logcat output:

E linker  : library "/system/lib64/pppd/2.4.7/pppol2tp.so" ("/system/lib64/pppd/2.4.7/pppol2tp.so") needed or dlopened by "/system/bin/pppd" is not accessible for the namespace: [name="(default)", ld_library_paths="", default_library_paths="/system/lib64", permitted_paths="/system/lib64/drm:/system/lib64/hw:/system/framework:/system/app:/system/priv-app:/vendor/app:/vendor/framework:/oem/app:/data:/mnt/expand"]

E pppd    : dlopen failed: library "/system/lib64/pppd/2.4.7/pppol2tp.so" needed or dlopened by "/system/bin/pppd" is not accessible for the namespace "(default)"

E pppd    : Couldn't load plugin pppol2tp.so

To fix this, let's install the pppol2tp.so in root of /system/lib[64]/ without pppd/2.4.7/ sub-path, and make pppd to look there for plugins too:

   1 --- a/pppd/Android.mk
   2 +++ b/pppd/Android.mk
   3 
   4  include $(CLEAR_VARS)
   5  LOCAL_MODULE := pppol2tp
   6 -VER := $(shell awk -F '"' '/VERSION/ { print $$2; }' $(LOCAL_PATH)/patchlevel.h)
   7 -LOCAL_MODULE_RELATIVE_PATH := pppd/$(VER)
   8 
   9 --- a/pppd/pathnames.h
  10 +++ b/pppd/pathnames.h
  11  
  12  #ifdef PLUGIN
  13  #ifdef __STDC__
  14 -#define _PATH_PLUGIN   DESTDIR "/" LIB_DIR "/pppd/" VERSION
  15 +#define _PATH_PLUGIN   DESTDIR "/" LIB_DIR
  16  #else /* __STDC__ */
  17 -#define _PATH_PLUGIN   "/usr/" LIB_DIR "/pppd"
  18 +#define _PATH_PLUGIN   "/usr/" LIB_DIR
  19  #endif /* __STDC__ */

See /etc/ld.config.txt file on the board for Bionic loader configuration. In Android sources, there are three variants of ld.config.txt files (in system/core/rootdir/etc):

  1. ld.config.legacy.txt: for non-Treble-ized devices (PRODUCT_TREBLE_LINKER_NAMESPACES is false)

  2. ld.config.txt: for Treble-ized devices (PRODUCT_TREBLE_LINKER_NAMESPACES is true) but without strict linker namespace restriction (BOARD_VNDK_RUNTIME_DISABLE is true)

  3. ld.config.txt.in: for Treble-ized devices with strict linker namespace restriction

Only 3rd option can cause the described problem. The lib can't be loaded because:

  • - the default namespace is configured as 'isolated' and

    - system/${LIB}/pppd is not in the permitted paths of the default namespace

Possible solutions:

  1. Add /system/${LIB}/pppd to namespace.default.permitted.paths or

  2. Move your lib under /system/lib64 (or under other paths in permitted.paths)

I've chosen 2nd one, as it doesn't require changing any linker files, and it's gonna work in all three cases mentioned above.

Issues running L2TP+IPSec on Android-N

When trying to run racoon+mtpd on Android-N, I faced with a bunch of issues. All those issues are listed below. Solutions are provided as well.

"Network is unreachable" issue

After configuring the Ethernet connection, I tried to ping my laptop from Android:

# ping 192.168.0.1

which gave me this error: connect: Network is unreachable. The only way to overcome that was running ping -I eth0 192.168.0.1, but that of course won't cut it (as we need to rely on eth0 as default interface for further PPP work).

Turns out, all ip route commands we ran, they were changing only main routing table (ip route show is actually ip route show table main). And main routing table has lowest priority (look at ip rule show command output, it's 23000 priority or even missing there). Setting the higher priority for main routing table solves the problem:

# ip rule add from all lookup main pref 99

After that we can check it:

# ip rule show

will show us something like this:

0:      from all lookup local
99:     from all lookup main
10000:  from all fwmark 0xc0000/0xd0000 lookup legacy_system
10500:  from all oif eth0 uidrange 0-0 lookup eth0
13000:  from all fwmark 0x10063/0x1ffff lookup local_network
13000:  from all fwmark 0x10064/0x1ffff lookup eth0
14000:  from all oif eth0 lookup eth0
15000:  from all fwmark 0x0/0x10000 lookup legacy_system
16000:  from all fwmark 0x0/0x10000 lookup legacy_network
17000:  from all fwmark 0x0/0x10000 lookup local_network
19000:  from all fwmark 0x64/0x1ffff lookup eth0
22000:  from all fwmark 0x0/0xffff lookup eth0
32000:  from all unreachable

where you can see main routing table with highest priority. Now ping command works just fine.

Above fix should also allow non-root user to use eth0 interface.

PSK permissions issue

racoon refuses to start, and reason is found in /var/log/daemon.log:

ERROR: /etc/racoon/psk.txt has weak file permission

FIX: The issue is fixed by changing mode of mentioned file:

$ sudo chmod 0400 /etc/racoon/psk.txt

"authtype mismatched" issue

logcat shows next line, when trying to run racoon:

W racoon  : authtype mismatched: my:hmac-sha256 peer:hmac-sha

FIX: In /etc/racoon/racoon.conf: change this line:

authentication_algorithm hmac_sha1, hmac_md5;

to this:

authentication_algorithm hmac_sha256, hmac_md5;

Supplemental: it turned out, that this issue is not critical, in the end. VPN will work with hmac_sha1, too. But let's use hmac_sha256 just to avoid that warning.

Also, some other guys are experiencing issues with SHA2 implementation in Android: [9].

But it seems like my configuration (described at this wiki page) is not prone to that issue.

"pfkey failed" racoon issue

When running racoon + mtpd, I observe next issues in logcat:

pfkey UPDATE failed: No such file or directory
pfkey ADD failed: No such file or directory

The cause most probably (related to what I found on internet) is that AUTHENC module is not loaded in kernel. But I've got CONFIG_CRYPTO_AUTHENC=y in my .config file.

When I look into /proc/crypto, I don't see authenc module in list (but I see it in my PC's /proc/crypto). So it's not loaded somehow. /proc/crypto has entries like that: kmod="crypto-ctr(aes)", where:

  • aes is cipher

  • ctr is template

See this for details.

AUTHENC module is trying to be loaded when starting mtpd:

### cryptomgr_probe(): cra_name        = echainiv(authenc(hmac(sha256),cbc(aes)))
  XXX: goto out
  XXX: out: echainiv(authenc(hmac(sha256),cbc(aes)))
### cryptomgr_probe(): cra_name        = echainiv(authenc(hmac(sha256),cbc(aes)))
  XXX: goto out
  XXX: out: echainiv(authenc(hmac(sha256),cbc(aes)))

Which means that this code was executed (from cryptomgr_probe() function):

        tmpl = crypto_lookup_template(param->template);

        if (!tmpl) {
                pr_err("  XXX: goto out");
                goto out;
        }

When we look at echainiv(authenc(hmac(sha256),cbc(aes))), there are several components in play. Checking each of them in kernel configuration reveals that CONFIG_CRYPTO_ECHAINIV=m. Let's make it =y.

FIX: After enabling CONFIG_CRYPTO_ECHAINV, everything works. Another way to root cause that option would be to enable all crypto options in kernel, and once it works, bisect which one was missing (or just leave all crypto options enabled).

"Received signal 15" mtpd issue

When trying to start mtpd, I'm observing next messages in logcat (Android side):

D mtpd    : Timeout -> Sending SCCRQ
I mtpd    : Received signal 15
D mtpd    : Sending STOPCCN
I mtpd    : Mtpd is terminated (status = 5)

On PC side I see next output in /var/log/daemon.log:

xl2tpd[3221]: check_control: Received out of order control packet on tunnel -1 (got 1, expected 0)
xl2tpd[3221]: handle_packet: bad control packet!
xl2tpd[3221]: network_thread: bad packet
xl2tpd[3221]: build_fdset: closing down tunnel 47774

or this one:

xl2tpd[9998]: result_code_avp: avp is incorrect size.  8 < 10
xl2tpd[9998]: handle_avps: Bad exit status handling attribute 1 (Result Code) on mandatory packet.
xl2tpd[9998]: handle_packet: bad AVP handling!
xl2tpd[9998]: network_thread: bad packet
xl2tpd[9998]: build_fdset: closing down tunnel 40027

FIX: Just remove /data/misc/vpn/abort file before running raccon and mtpd.

Root causing:

  • First I noticed that re-flashing Android images fixes this issue
  • Then I narrowed it down to userdata.img re-flashing

  • Doing find /data -name '*vpn*' I found some VPN related directories

  • Removing /data/misc/vpn/ directory helped

  • By using Android Xref site I managed to find actual root cause

"Received signal 15" mtpd issue (#2)

After moving to Android-O kernel, this issue appeared again. Sometimes it stops with signal 15:

D mtpd    : Timeout -> Sending SCCRQ
I mtpd    : Received signal 15
D mtpd    : Sending STOPCCN
I mtpd    : Mtpd is terminated (status = 5)

And sometimes it's just printing "Timeout -> Sending SCCRQ" messages constantly in logcat.

Turns out that HMAC SHA-256 truncation was changed from 96 bit to 128 bit in Android-O kernel (e.g. android-4.4-o branch), as required by RFC4868. This commit changes it:

ANDROID: make PF_KEY SHA256 use RFC-compliant truncation

But my server's racoon still uses 96-bit truncation, because it relies on mainline kernel, which is using 96-bit truncation.

FIX: There are two possible solutions to this issue:

1. Change server IPSec tool from racoon to another one (such as libreswan), which implements this SHA256 truncation in userspace (and able to choose 128 bit truncation)

2. Revert mentioned patch

For now I decided to go with revert, as configuring libreswan may take a lot of time, and hacking racoon won't help either (as it relies on kernel). In future I'll still need to provide working Road Warrior libreswan configuration, because we'll need simple and reliable way to test my patches.

It also should be noted that Android-N kernel uses 96-bit truncation, thus server must use it too.

Debugging the truncation

This issue can be seen from ip xfrm state command output.

On PC:

$ sudo ip xfrm state
src 192.168.0.1 dst 192.168.0.100
        ...
        auth-trunc hmac(sha256) ... 96
        ...
src 192.168.0.100 dst 192.168.0.1
        ...
        auth-trunc hmac(sha256) ... 96

On Android:

# ip xfrm state
src 192.168.0.100 dst 192.168.0.1
        ...
        auth-trunc hmac(sha256) ... 128
        ...
src 192.168.0.1 dst 192.168.0.100
        ...
        auth-trunc hmac(sha256) ... 128
        ...

"avp is incorrect size. 8 < 10" issue

After re-flashing Android images, I was faced with another issue. From logcat:

I mtpd    : Mtpd is terminated (status = 6)

Looking at /var/log/daemon:

xl2tpd[23168]: result_code_avp: avp is incorrect size.  8 < 10

Trying to figure out what's going on, I enabled debug logging in /etc/xl2tpd/xl2tpd.conf:

debug avp = yes
debug network = yes
debug state = yes
debug tunnel = yes
...
ppp debug = yes

Restarted xl2tpd service, started mtpd again. Looking at /var/log/syslog (as it was last modified file in /var/log/):

pppd[24195]: In file /etc/ppp/xl2tpd-options: unrecognized option 'lock'

So, it seems like lock option is deprecated now.

FIX: Removing lock option from /etc/ppp/xl2tpd-options fixes the problem, and now ppp0 interface comes up, as seen in ifconfig output (on both sides, Android board and PC).

udp_xmit failed with err=-1:No such device

When you see such message in /var/log/daemon.log (when xl2tpd debugging is on), it's probably the bug in xl2tpd 1.3.11, which doesn't work well with kernel 4.15, 4.16, etc.

In logcat you'll see either:

  • hanging on D mtpd    : Timeout -> Sending SCCRQ

  • or I mtpd    : Received signal 15

See next links for details:

FIX: Build and use xl2tpd from 1.3.12 branch. WARNING: After dpkg -i xl2tpd.deb your /etc/xl2tpd/xl2tpd.conf will be rewritten by default config, so it might be wise to back it up first.

References

[1] http://www.marthijnvandenheuvel.com/2012/05/26/how-to-set-up-a-pptp-vpn-server-on-ubuntu/

[2] https://wiki.debian.org/HowTo/AndroidVPNServer

[3] http://www.brokenbitstream.com/mobile-and-vpns

[4] http://alvinalexander.com/java/jwarehouse/android/packages/VpnServices/src/com/android/server/vpn/VpnDaemons.java.shtml

[5] http://wiki.nikoforge.org/L2TP/IPSec_VPN_Setup_on_Centos_6_%2864-bit%29_for_use_with_Android_ICS_and_iOS_5_Clients

[6] "Android Security Internals" book: chapter 9 -> "Vpn Support"

[7] http://miqloproxy.com/what-is-l2tp/

[8] https://ask.wireshark.org/questions/12019/how-can-i-decrypt-ikev1-andor-esp-packets

[9] https://code.google.com/p/android/issues/detail?id=196939

LMG/Kernel/PPP (last modified 2018-10-29 14:04:29)