We've setup a L2TP VPN server with this tutorial, everything works like a charm.

The only issue is

  1. We don't want client to route all traffic using this VPN, only a particular subnet, e.g. 10.0.0.0/20

  2. On Mac, we need to set the route manually using command, but for mobile devices, seems there is no way to do so?

So, it is possible to configure for the client automatically for the subnet "10.0.0.0/20"?

  • Have you tried disabling the 'send all traffic over VPN' or similar option on the client? – Bert Feb 10 '14 at 14:53
up vote 30 down vote accepted
+50

OK, this question is asked over and over again over the Internet and most of the time there is a (semi-) incorrect answer that you cannot do what was described in the original post. Let me clarify it once and for all :)

The short answer is L2TP (and PPTP for that matter) do not have facilities to do route pushes inside the protocol, but it can be achieved outside the protocol.

Since L2TP is a Microsoft invention, the best source of information is their technical documentation (and they are quite good at it, by the way). The technical description of what I am going to explain down below can be found at VPN Addressing and Routing. The keywords for setting everything up properly (if you are going to do your own research) are: DHCPINFORM and "classless static routes".

First of all, how it works:

  1. a client connects to the VPN server
  2. after successful authentication a secure tunnel is established
  3. the client uses a DHCPINFORM message after the connection to request the DHCP Classless Static Routes option. This DHCP option contains a set of routes that are automatically added to the routing table of the requesting client (I slavishly copy-and-pasted this line directly from Microsoft documentation :) )
  4. the VPN server replies to that message with appropriate set of routes

Well, there is a caveat:

  • there is RFC-3442 describing "DHCP Classless Static Routes" and there it states that the code for this option is 121. Microsoft decided to re-invent the wheel (as always) and uses code 249 for this option. Hence, to support a wider range of clients we need to respond back with both codes

I am going to describe a typical configuration using Linux box as the VPN server (you can configure MS servers using the link to the Microsoft documentation).

To configure routes on the clients we will need the following ingredients:

  • L2TP/IPSEC (or PPTP) = for example, accel-ppp is a nice open source L2TP/PPTP server
  • DHCP server = there are many, but I am going to describe dnsmasq's configuration

The following is a dump of a working accel-ppp configuration. I am providing it in its entirety, otherwise it would be difficult to explain what goes where. If you already have your VPN working you may skip this configuration file and concentrate on the DHCP configuration described below.

[root@vpn ~]# cat /opt/accel-ppp/config/accel-ppp.conf
[modules]
log_syslog
pptp
l2tp
auth_mschap_v2
ippool
sigchld
chap-secrets
logwtmp

[core]
log-error=/var/log/accel-ppp/core.log
thread-count=4

[ppp]
verbose=1
min-mtu=1280
mtu=1400
mru=1400
check-ip=1
single-session=replace
mppe=require
ipv4=require
ipv6=deny
ipv6-intf-id=0:0:0:1
ipv6-peer-intf-id=0:0:0:2
ipv6-accept-peer-intf-id=1

[lcp]
lcp-echo-interval=30
lcp-echo-failure=3

[auth]
#any-login=0
#noauth=0

[pptp]
echo-interval=30
echo-failure=3
verbose=1

[l2tp]
host-name=access-vpn
verbose=1

[dns]
dns1=192.168.70.251
dns2=192.168.70.252

[client-ip-range]
disable

[ip-pool]
gw-ip-address=192.168.99.254
192.168.99.1-253

[log]
log-file=/var/log/accel-ppp/accel-ppp.log
log-emerg=/var/log/accel-ppp/emerg.log
log-fail-file=/var/log/accel-ppp/auth-fail.log
log-debug=/var/log/accel-ppp/debug.log
copy=1
level=3

[chap-secrets]
gw-ip-address=192.168.99.254
chap-secrets=/etc/ppp/chap-secrets

[cli]
telnet=127.0.0.1:2000
tcp=127.0.0.1:2001

[root@vpn ~]# 
===

At this point our clients can connect via L2TP (or PPTP) and communicate with the VPN server. So, the only missing part is a DHCP server that is listening on the created tunnels and that responds back with the necessary information. Below is an excerpt from the dnsmasq configuration file (I am providing DHCP related options only):

[root@vpn ~]# grep -E '^dhcp' /etc/dnsmasq.conf 
dhcp-range=192.168.99.254,static
dhcp-option=option:router
dhcp-option=121,192.168.70.0/24,192.168.99.254,192.168.75.0/24,192.168.99.254,10.0.0.0/24,192.168.99.254
dhcp-option=249,192.168.70.0/24,192.168.99.254,192.168.75.0/24,192.168.99.254,10.0.0.0/24,192.168.99.254
dhcp-option=vendor:MSFT,2,1i
[root@vpn ~]#

In the above excerpt we are pushing routes 192.168.70.0/24, 192.168.75.0/24, and 10.0.0.0/24 via 192.168.99.254 (the VPN server).

Finally, if you sniff the network traffic (e.g. on the VPN server) you will see something like the following for the response on the DHCPINFORM message:

19:54:46.716113 IP (tos 0x0, ttl 64, id 10142, offset 0, flags [none], proto UDP (17), length 333)
    192.168.99.254.67 > 192.168.99.153.68: BOOTP/DHCP, Reply, length 305, htype 8, hlen 6, xid 0xa27cfc5f, secs 1536, Flags [none]
      Client-IP 192.168.99.153
      Vendor-rfc1048 Extensions
        Magic Cookie 0x63825363
        DHCP-Message Option 53, length 1: ACK
        Server-ID Option 54, length 4: 192.168.99.254
        Domain-Name Option 15, length 18: "vpn.server.tld"
        Classless-Static-Route-Microsoft Option 249, length 24: (192.168.70.0/24:192.168.99.254),(192.168.75.0/24:192.168.99.254),(10.0.0.0/24:192.168.99.254)
        Vendor-Option Option 43, length 7: 2.4.0.0.0.1.255

P.S. I almost forgot an essential part required for the successful use of the above configuration. Well, it was described in the Microsoft docs I referred to, but who read the documentation? :) OK, clients should be configured without 'Use default gateway' on the VPN connection (on Windows it is in connection's properties -> Networking -> Internet Protocol Version 4 (TCP/IPv4) -> Properties -> Advanced -> IP Settings). On some clients there is also an option called 'Disable class based route addition' - it must be unset since it explicitly disables the functionality we are trying to implement.

  • It is my understanding that classic L2TP encapsulates PPP packets over UDP. I could be wrong but I dont think DHCP is supported over PPP, particularly sending extra routes. L2TP version 3 (still quite new in linux kernel land) lets you encapsulate ethernet frames so it might be possible there, yet I'm pretty sure mileage may vary as to how well that is supported over mobile devices. – Matthew Ife Feb 16 '14 at 10:25
  • Matthew, I don't really know how to address your question properly since you mixed up so many things into one sentence :) Well, let's start with the following: the provided configuration is working on hundreds of road-warriors I'm supervising. So, it's a working example. You can Google for DHCP over PPP and read a lot of technical documentation on how it's implemented by Cisco, Juniper, etc. If you don't want to dive into it, just imagine that L2TP encapsulates PPP over UDP, PPP is a point-to-point protocol where you can use IP, DHCP is a protocol over IP, so we are good here :) – galaxy Feb 16 '14 at 12:50
  • 1
    Also, it's quite strange to get this kind of comments when I included a link to Microsoft technical documentation for L2TP where they describe how to set things up properly using DHCPINFORM. I can understand when people don't want to read the answer (although it includes config files from a working system) since it somebody's research, but saying "I don't think DHCP is supported over PPP" when there is a technical specification from the creator of the protocol where it states that this is the way it was designed is kind of strange. – galaxy Feb 16 '14 at 13:00
  • Well to clarify my point "DHCP is not supported over PPP", I mean that IP address assignment occurs over the PPP link control protocol (point to points have no notion of a 'broadcast' address). So I think you misunderstood what I was getting at. I see now what you mean is that the DHCPINFORM Occurs inside of the tunnel after connection establishment and is nothing to do with the initial connection. I agree now that this scheme works, providing the client will send a DHCPINFORM message after the connection is setup. – Matthew Ife Feb 16 '14 at 13:40
  • Mathew, thanks :). Yes, we are not using DHCP for address assignment, it's done by IPCP (and not LCP as you said, but this is irrelevant), however, the Microsoft technical documentation states that a valid client should send the DHCPINFORM message with Option 249 to get route configuration, and this is exactly what I described in my answer. Well, somebody already voted my answer down although it's a working, valid answer :) – galaxy Feb 16 '14 at 15:32

I don't think you can push a route to the client in a L2TP/IPSEC VPN. You will have to do the configuration directly on the client.

What mobile client is it you are having trouble with? It's easier to provide some input if we know the operating system and software you are using.

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