PoPToP HOWTO/FAQ
----------------
Last Updated: 20021024
Send changes to: Richard de Vroede <r.devroede@linvision.com>
HOWTO/FAQ mostly compiled from PoPToP help pages and the PoPToP Mailing List
(hosted by Christopher Schulte) by Matthew Ramsay. Large contributions from
Steve Rhodes and Michael Walter.
Contents
--------
1.0 Introduction
1.1 About PoPToP
1.2 Credits
2.0 System Requirements
3.0 PPP with MSCHAPv2/MPPE Installation
4.0 PoPToP Installation
5.0 Windows Client Setup
6.0 FAQ
1.0 Introduction
----------------
1.1 About PoPToP
PoPToP is the PPTP Server solution for Linux. PoPToP allows Linux servers to
function seamlessly in the PPTP VPN environment. This enables administrators
to leverage the considerable benefits of both Microsoft and Linux. The
current pre-release version supports Windows 95/98/NT/2000 PPTP clients and
PPTP Linux clients. PoPToP is free GNU software.
PoPToP Home Page: http://www.poptop.org
1.2 Credits
PoPToP was originally started by Matthew Ramsay under the control of
Moreton Bay Ventures (http://www.moretonbay.com). Around March 1999 PoPToP
was publically released under the GNU GPL by Moreton Bay/Lineo.
PoPToP is what it is today due to the help of a number of intelligent and
experienced hackers. More specifically Kevin Thayer, David Luyer and
Peter Galbavy.
More contributors to PoPToP (in various forms) include Allan Clark, Seth
Vidal, Harald Vogt and Ron O'Hara.
And finally, credit to all the PoPToP followers who test and report
problems.
1.3 PopToP migrating from poptop.lineo.com
March 18, 2002
The main PoPToP developers left Lineo with the SnapGear spin-out. The ball
is being picked up by Daniel Djamludin. PoPToP has been actively developed
within SnapGear and a number of improvements need to be rolled out.
Henceforth from this sentence onwards you should refer to "PoPToP" as
"Poptop" for ease of use and typing.
Lineo have been asked to forward poptop.lineo.com to poptop.sourceforge.net
The sources are being gathered to go into CVS, new binaries and dev images will follow.
Source Forge looks like the best neutral ground to smooth out future upheavals.
2.0 System Requirements
-----------------------
1. A modern Linux distribution (such as Debian, Red Hat, etc.) with a recent
kernel (2.4.x recommended, 2.2.x should be ok). Note: ports exist for
Solaris, BSD and others but are not supported in this HOWTO at this
time.
2. PPP (2.4.1 recommended, 2.3.11 should be ok)
(and the MSCHAPv2/MPPE patch if you want enhanced Microsoft
compatible authentication and encryption).
3. PoPToP v1.1.3 (or download the latest release at:
http://sourceforge.net/projects/poptop
3.0 PoPToP Installation
-----------------------
Check out the documentation at http://sourceforge.net/docman/?group_id=44827
4.0 Windows Client Setup
------------------------
Install it using the add-remove programs tool. Go to windows->communications
and install VPN support.
(If you do above you may *not* need to follow the instructions below as it
will already be installed... ?
follow the instructions:
1.start->settings->control panel->network
2.Click add
3.choose adapter
4.Click add
5.select microsoft as the Manufactuarer
6.select Microsoft Virtual Private Networking Adapter
7.Click ok
8.Insert any necessary disks
9.Reboot your Machine
take a little nap here...
Once your Machine is back
1.go to dial-up networking (usually start->programs->Accessories->communications->Dial-up Networking) YMMV
2.Click make new connection
3.Name the Connection whatever you'd like.
4.Select Microsoft VPN adapter as the device
5.click next
6.type in the ip address or hostname of your pptp server
7.click next
8.click finish
9.Right-click on the intranet icon
10.select properties
11.choose server types
12.check require encrypted password
13.uncheck netbeui, ipx/spx compatible
14.click tcp/ip settings
15.turn off use IP header compression
16.turn off use default gw on remote network
17.click ok.
18.start that connection
19.type in your username and pw (yadda, yadda, yadda)
20.once it finishes its connection your up.
Note that the Win95 routine is similar but requires Dial Up Networking Update 1.3 (free from Microsoft) to be installed first.
5.0 FAQ
-------
Q&A.
INTRODUCTION
After spending the better part of two weeks developing my configuration
for a pptp sever for remote file access by Windows(tm) clients, I
thought I would pass along these notes to those who may be interested.
The basic configuration involves a Samba/PoPToP server behind a
firewall, through which clients using Win98 machines will connect using
the VPN facility built into that OS. This is diagrammed below.
_____ ___ ______ ______
| | | \ | fire | | file |
| win | ---> / net \ ---> | wall | ---> | srvr |
|_____| \__/\_/ |______| |______|
The components of the system consist of the Win98 clients running the
built-in VPN facility dialing in to their ISP's and connecting through
the firewall to the Samba server on the internal network using the pptp
protocol. The firewall uses Network Address Translation to convert an
open Internet IP address to an internal one. Sounds simple enough
right?
SIMPLE TEST SETUP
As a starting point, I configured a Win98 box to connect directly to a
PoPToP server without any authentication or encryption. This was just
to get a feel for how pptp works and verify the setup. Using the
pre-packaged rpm's was a big help here. You just rpm the thing onto the
system and fire it up, and you're in business. The diagram below
represents this simple system.
192.168.56.142 192.168.56.11
_____ ______
| | | file |
| win | ------------------> | srvr |
|_____| |______|
Emboldend by my success, I set out to turn on MS authentication and
encrytion, and this is where the fun started.
AUTHENTICATION AND ENCRYPTION
This is an area where Microsoft really shows its true colors. Turning
on password and data encryption on the Win98 VPN server configuration
was quite the eye opening experience. First with the authentication,
you will have to go through a somewhat difficult compilation of the
ppp-2.3.8 package. The worst part here is getting all the pieces
together, namely the rc4 files. This process is well documented in this
archive, so I won't go into it here.
The next realization is that Microsoft prepends the domain name to the
user name when submitting the login credentials. For example, srhodes is
now DBNET\\srhodes. If that wasn't bad enough, I found that the domain
wasn't even the one I was logged into. My best guess is that the first
domain that the computer ever logs into is stuck with it for ever. This
is a real problem if you have multiple domains that you log into. I
modified the pppd.c code to strip out the domain on MSCHAP logins, but
you can just set the user name in chap-secrets to match the windows
version.
Then I spent a whole day trying to figure out why data encryption does
not work. I tried just about everything I could think of that could be
wrong. That's when I discovered this archive, for which I am truly
grateful. It turns out that the Win9x implementation of encrytpion is
FUBAR! You have to download one of those patches from Microsoft,
MSDUN 1.4 to get the thing to work.
Windows 95
http://download.microsoft.com/download/win95/Update/17648/W95/EN-US/dun14-95.exe
Windows 98
http://download.microsoft.com/download/win98/Update/17648/W98/EN-US/dun14-98.exe
Windows 98se
http://download.microsoft.com/download/win98SE/Update/17648/W98/EN-US/dun14-SE.exe
FIREWALL CONFIGURATION
The issue with a firewall in this setup is that you need to cover two
types of protocol communication. There is one connection which is a tcp
connection on port 1723 that handles the control functions and another
connection using IP type 47, or GRE, which handles the actual data
communication. This second connection presents a problem for the
convention linux firewall, ipfwadm. You see, its only set up to handle
tcp, udp and icmp protocols. It doesn't know about GRE.
The trick around this block is to use one of the new 2.2 kernels, which
employ a new firewall called ipchains. This tool willl handle arbitrary
protocols, which can be specified by their numbers.
192.168.2.142 192.168.56.11
_____ ______ ______
| | | fire | 192.168.56.1 | file |
| win | --------------->| wall | --------------> | srvr |
|_____| 192.168.2.1 |______| |______|
You need to remember a few things before getting too deep into this.
The default gateway on win is set to 192.168.2.1, and the default
gateway on file srvr is set to 192.168.56.1. The firewall has the two
network interfaces spanning the two subnets and is configured for
IP forwarding. If you have not yet applied any firewall rules, this
configuration will work as before. The interesing part is to block out
all other access to file srvr by implementing ipchains rules.
The short story is:
ipchains -F
ipchains -P forward DENY
ipchains -I forward -p tcp -d 192.168.56.11 1723 -j ACCEPT
ipchains -A forward -p tcp -s 192.168.56.11 1723 -j ACCEPT
ipchains -A forward -p 47 -d 192.168.56.11 -j ACCEPT
ipchains -A forward -p 47 -s 192.168.56.11 -j ACCEPT
NETWORK ADDRESS TRANSLATION
The next hurdle is to configure the firewall so that it can run an open
internet IP address on the outside and allow access to an internal
address on the inside. NAT is very well suited to this task, although
you may hear otherwise from knowledgable sources. It happens to be my
preference, though certainly not the only way to skin this cat. You can
obtain the NAT software and some detailed information from
http://www.csn.tu-chemnitz.de/HyperNews/get/linux-ip-nat.html
But again, there is a problem with the GRE protocol of type 47. The
tool for configuring NAT, ipnatadm, like its half-brother ipfwadm, is
not set up to handle arbitrary protocols. Unfortunately, you'll have to
go into the code and make a slight modification if you want to use it
for this purpose. There is a procedure called parse_protocol in the
file routines.c that discriminates the type of protocol to be filtered.
The basic idea is to accept a string representing a number and use that
as the filter. Since you have to recompile the kernel anyway to get the
NAT functionality, maybe it's not so horrible, relatively speaking.
For those ambitous enough, here is the diff for the routines file, copy
this into a file called routines.diff and use the command patch -p0 <
routines.diff from within the same directory.
--- routines.c Thu Mar 25 15:41:58 1999
+++ /mnt/zip/nat/routines.c Wed Jul 21 21:09:28 1999
@@ -112,11 +112,18 @@
else if (strncmp("icmp", s, strlen(s)) == 0)
nat_set.nat.protocol = IPPROTO_ICMP;
else {
+ int number;
+ char * end;
+ number = (int)strtol(s, &end, 10);
+ nat_set.nat.protocol = number;
+ }
+ /*
+ else {
fprintf(stderr, "ipnatadm: invalid protocol \"%s\"
specified\n", s);
exit_tryhelp(2);
- /* make the compiler happy... */
return;
}
+ */
}
void parse_hostnetworkmask(char *name, struct in_addr **addrpp, __u32
*maskp, int *naddrs)
The patch is actually lifted from ipchains, which was derived from
ipfwadm, which provides the basis for ipnatadm.
Once you've got all that running, what you want to do is to set up the
NAT rules so that the incoming client thinks its talking to the
firewall, as does the outgoing file server. The short of it is:
ipnatadm -F
ipnatadm -I -i -P 6 -D 192.168.2.1 1723 -N 192.168.56.11 1723
ipnatadm -O -i -P 6 -S 192.168.56.11 1723 -M 192.168.2.1 1723
ipnatadm -I -i -P 47 -D 192.168.2.1 -N 192.168.56.11
ipnatadm -O -i -P 47 -S 192.168.56.11 -M 192.168.2.1
Here, the -P argument sets the protocol, 6 is tcp and 47 is GRE.
PPTP packets targeting the firewall are translated to the internal host
inbound and vice-versa on the way out. Very slick.
SAMBA
Here's a subject so complex you could probably devote a whole career to
it. We don't want to get too bogged down, so I'll be brief. Samba
implements the NetBIOS protocol, which has more quirks than you can
shake a stick at. One of the biggest problems is the use of subnet
broadcasting. Suffice it to say, if you want the best results, you
should set your PoPToP IP addresses to reside within the subnet on which
the file server ethernet is located. I choose 192.168.56.12 for the
server address, and it hands out IP's from 192.168.13-127.
Setting the IP forwarding on the file server to true will give you
access to other machines on the internal network.
When you go at the samba sever from Win98, you have to use encrypted
password. Look at smbpasswd and related stuff.
Finding shares on the server is not so easy. The short story here is
that browsing is implemented via broadcast packets, and broadcast
packets will not travel down a PPP link. The only way to get browsing
to work over pptp is to set Samba up as a WINS server and a Domain login
server, and configure the clients to use that WINS server and force them
to login to that Domain. Believe me, I tried just about everything to
avoid that. You will also want to set the samba server as the domain
master and preferred master for the browsing.
If you can't do that, you can set the ppp/options file to include a
ms-wins setting for the samba server. This will set the client up so
they can at least resolve host names. The only way to find a share
under this configuration is to name it explicitly. You can use the
tools menu from the Win98 file browser and say find -> computer and
enter in the name of the samba server and it will be found. I have
found that setting domain master = yes and preferred master = yes gives
a rather nice boost to the speed of name lookups on the network.
Here is my abbreviated smb.conf
[global]
workgroup = VAULT
server string = acer
log file = /var/log/samba/log.%m
max log size = 50
security = user
encrypt passwords = yes
smb passwd file = /etc/smbpasswd
socket options = TCP_NODELAY
domain master = yes
preferred master = yes
domain logons = yes
wins support = yes
dns proxy = no
[homes]
comment = Home Directories
browseable = no
writable = yes
You should also use the lmhosts option for nmbd (-H) and set up an
lmhosts file on the samba server. Make sure also the the samba server
can resolve its own name, through either /etc/hosts or DNS.
In all honesty , I went through the same simple test setup with samba as
I did for PoPToP, although its not shown here explicitly.
CONCLUSION
PoPToP is a good program, as is Samba. This configuration can work if
you put a little effort into it. I have seen a lot of questions here
and in other places about these types of systems, so I would think that
there is some demand on the part of users who want this type of
functionality. I hope these notes are useful to you if this is what you
want to do.
****************************************************************************
Q&A
I have a pptp server set up on my office LAN. I can connect to the
server and ping to it fine, but I can't ping any other hosts on the
office subnet. I have ip-forwarding turned on and I have proxyarp set
in the ppp/options file. What can be wrong?
There seem to be a lot of questions floating around about routing and
masq'ing associated with this issue.
Well, my curiosity got the best of me, so I thought I would check this
out. Shown below is my test setup for investigating this problem.
192.168.8.142 192.168.56.10 192.168.56.11 192.168.56.12
________ _______ ______ _____
| | | | | | | |
| client |------->| fire |-------->| pptp |----->| host |
| | | wall | | srvr | | |
|________| |_______| |______| |______|
H H
H 192.168.8.10 H
H H
H===================================H
192.168.5.12 pptp connection 192.168.5.11
For the sake of simplicity, we will ignore address translation issues
associated with the firewall. This assumes that the client at
192.168.8.142 is going to use 192.168.56.11 as its target address for
the pptp connection to pptp_srvr. The firewall will block all access to
the 192.168.56.0 subnet except for pptp connections associated with
pptp_srvr. This can be implemented with ipchains
ipchains -P input DENY
ipchains -P forward DENY
ipchains -A input 192.168.56.0/24 -j ACCEPT /* allow connections from
inside */
ipchains -A input -p tcp -d 192.168.56.11 1723 -j ACCEPT
ipchains -A input -p 47 -d 192.168.56.11 -j ACCEPT
ipchains -A forward -p tcp -d 192.168.56.11 1723 -j ACCEPT
ipchains -A forward -p tcp -s 192.168.56.11 1723 -j ACCEPT
ipchains -A forward -p 47 -d 192.168.56.11 -j ACCEPT
ipchains -A forward -p 47 -s 192.168.56.11 -j ACCEPT
When you connect from client to pptp_srvr, you will be able to complete
the connection and ping to pptp_srvr. However, if you attempt to ping
host, at 192.168.56.12, this will fail.
A clue to this problem can be found in the /var/tmp/messages file on
pptp_srvr. There, in the pppd messages, you will find
Cannot determine ethernet address for proxy ARP
This is due to an issue with the pppd program, which attempts to find a
hardware interface on the subnet to which the pppd client has been
assigned. In this case its looking for a hardware interface on the
192.168.5.0 subnet. It will fail to find one, and will drop the
proxyarp request.
The simplest way around this problem, and the one that is suggested in
the pppd documentation, is to set the pppd client IP assignment to be on
the local subnet. An example in this case might be 192.168.56.129.
However, it may not be possible to do that. In the case of a fully
loaded subnet, there may not be any addresses to spare. Or there may be
some security issues with giving out local subnet addresses. What to
do?
The place to look is in the arp table. If you run tcpdump on host
(192.168.56.12) during the time when client is pinging, you will see
unanswered arp requests from host attempting to find the hardware
address for 192.168.5.12. You need to proxy the hardware address of the
pptp_srvr for client in order for this request to be fulfilled. This is
the job of proxyarp. However, proxyarp has let us down in this
instance, and we need to find a workaround.
This can be done manually using the arp command on pptp_srvr. For
example, if the hardware address of the ethernet card on pptp_srvr is
00:60:08:98:14:14, you could force the arp to proxy the client pptp
address by saying
arp --set 192.168.5.12 00:60:08:98:14:13 pub
You should now be able to ping from client to host through the pptp
connection.
This can be a problem, however, in a dynamic environment when clients
are logging into and out of the pptp server on a continuous basis. One
way around this problem is to write a script that will execute upon the
initiation of each ppp connection.
The place to do this is in /etc/ppp/ip-up. This script is executed each
time a new ppp connection is started. It gets some variables passed
into it, one of which is the assigned IP address of the client. Note
that RedHat systems use ip-up.local as the place for you to make the
script. Don't forget to chmod +x !
#! /bin/bash
REMOTE_IP_ADDRESS=$5
date > /var/run/ppp.up
echo "REMOTE_IP_ADDRESS = " $REMOTE_IP_ADDRESS >> /var/run/ppp.up
arp --set $REMOTE_IP_ADDRESS 00:60:08:98:14:14 pub >> /var/run/ppp.up
exit 0
This should put you in business for accessing the remote subnet under
this scenario. I am a little bit concerned, however, because I also
built a script ip-down.local, that should remove the arp proxy when
client disconnected. It doesn't seem to do anything, however, and if I
try to delete the arp entry manually, it just spits out a cryptic error
message. The arp entries remain persistent, as far as I can tell. If
this is a problem or not, I don't know. The next few clients that log
in are treated well, so I guess its OK.
****************************************************************************
Q.
Also, after running pptpd and monitoring its log file and seeing that it
failed to open ttyp1 - I chmod +rw /dev/ttyp[0-9] and it seemed to work
somewhat. But, after I rebooted, I had to do this again. Is this normal?
A.
pptpd should be running as root (unless you have a system with a setuid
openpty() helper, which isn't very common). If it fails to open a pty/tty
pair as root then that is probably because it is in use.
Other programs which use pty/tty's will change their permissions back to
the standard ones.
****************************************************************************
Q.
sometimes when I make a connection to my pptpd server I
see a message like
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-21
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-26
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-24
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-21
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-26
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-24
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-26
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-24
Jul 2 17:30:03 ape modprobe: can't locate module ppp-compress-21
in /var/log/messages on the server. Any idea what I
can do about it?
A.
yeah, in your /lib/modules//net/ directory, there should
be files called bsd_comp.o and ppp_deflate.o.. insmod those files and
you'll be good to go.
****************************************************************************
Q.
Hi, I'm having trouble getting pptpd & mschap-v2 to work. I downloaded
all of the patches and compiled everything but whenever i try to connect
from my win98 machine, it says:
Error 691: The computer you have dialed in to has denied access because
the username and/or password is invalid on the domain.
What is this suppose to mean?
A.
Error 691 is an authentication problem probably due to the fact that MS
chap uses the domain name and username combo to authenticate. If you
look at the logs you will probably see a message saying that MS chap is
trying to authenticate user "domain\\username". I got it to work by
putting the full domain and user string in the client portion of the
chap-secrets file.
# Secrets for authentication using CHAP
# client server secret IP
addresses
workgroup\\user server password *
If anyone knows how to get it to default to a particular domain, I would
like to know.
****************************************************************************
Q.
how do I go about checking who is logged in via tunnel?
I need some way of writing the pppd data to wtmp/utmp.
(and not sessreg either)
does anyone know of any way of doing this via ppp?
A.
pppd syslogs everything to /var/log/messages (that's the default on my box
anyways) and it will say something like :
pppd[15450]: CHAP peer authentication succeeded for
you could do a tail /var/log/messages -n2000 | grep CHAP if you wanted to
see who has been logging in.
other than that, there's not much i know of. all the authentication is
provided by pppd (if you don't have an auth or a require-chap (or pap, etc.)
option, it doesn't even ask for a username.
****************************************************************************
Q.
My NT client won't connect!
A.
Try taking header and software compression off.
****************************************************************************
Q. PPTP *client* stops working.
A.
go to /var/run/pptp/ and look for a socket named x.x.x.x
delete it and try it again.
****************************************************************************
Q.
How many clients does PoPToP support?
A.
The limits under Linux are:
per-process filedescriptors
- one per client (would limit clients to 256 by default,
or 1024 with kernel recompile, or more with major libc/kernel
hackery)
- no relevant limit
ttys - currently, with a standard kernel, 256 clients
- with Unix98 ptys and a small amount of coding, 2048
ppp devices
- no limit in kernel source for ppp
processes
- 2 per client plus system processes
- standard kernel max = 512 processes, ie 256 clients
- i386 max = 4096 processes, ie 2048 clients
So it seems that 2048 will be the limit, if you fix a few things and
with a minor kernel mod (I could do all of these pretty easily and send
you a trivial kernel patch). To go above 2048 the easiest approach would
be to combine pptpctrl and pppd in one process, which would get you to
4096. Beyond there, you need to go for a select() based model, which would
be significant coding effort and require large fd-set sizes and so on.
So 4096 is the practical limit, and 2048 the easy limit.
****************************************************************************
Q.
What authentication methods (PAP/CHAP) does PoPToP work with?
A.
PoPToP uses whatever authentication methods your PPPd provides (usually
PAP and CHAP). With PPPd patches you can get MSCHAP and MSCHAPv2
authentication as well.
****************************************************************************
Q.
When running PoPToP I get the following error:
Jun 11 08:29:04 server pptpd[4875]: MGR: No more free connection slots!
What does this mean?
A.
I'd say at a guess you've only configured one IP address and you have
connected a client, and as such there are no more free connection slots should
any more clients wish to connect.
****************************************************************************
Q.
Does PoPToP suffer from the same security flaws
(http://www.counterpane.com/pptp.html) as the Windows NT PPTP server?
A.
An initial look at the article suggests that what the authors hammered was
not the PPTP protocol, but the authentication that the PPTP VPN servers on
NT offered access to via open internet. PPTP seems initially to be just
the path to the weakness, not the weakness itself. Part of their
observance of weakness deals with use of poor passwords as well, a cheap
component, simple enough to fix.
> While no flaws were found in PPTP itself, several serious flaws were
> found in the Microsoft implementation of it.
> (http://www.counterpane.com/pptp-pressrel.html)
The authors do not specifically say "this is ONLY effective against NT",
just that NT is affected. This implies that they do not recognize PoPToP,
and it may be included. The fact that PoPToP has to interOp with MS DUN's
VPN client means that it will have the same weaknesses. It can only
protect itself from DoS attacks, have immediate response to out-of-sequence
packets or illogical packets, etc.
The protocol is not considered weak in this analysis, but the weaknesses
have to be replicated in apparent behavior by PoPToP. The only thing the
developers can do with PoPToP is make it a stronger server per se -- more
able to handle the attacks when the come.
In conclusion: PoPToP suffers the same security vulnerabilities as the NT
sever (this is because it operates with Windows clients).
Update: MSCHAPv2 has been released and addresses some of the security
issues. PoPToP works with MSCHAPv2.
****************************************************************************
Q.
Does PoPToP support data encryption?
A.
Yes.. with appropriate PPPd patches. Patches are available for PPPd to
provide Microsoft compatible RC4 data encryption. The PPPd patch supports
40 and 128 bit RC4 encryption.
****************************************************************************
Q.
PoPToP or IPsec? Which is better suited to my needs?
A.
1. The difference between PoPToP and IPsec is that PoPToP is ready NOW..
and requires *no* third party software on the Windows client end
(Windows comes with a free PPTP client that is trivial to set up).
2. PoPToP is a completely *free* solution.
Update: Unfortunately not true for Mac *clients* though. The Mac client
software is around $400 US a copy.
3. PoPToP can be integrated with the latest PPPD patches that take
advantage of MSCHAPv2 and MPPE (Microsoft encryption using RC4 - 40/128
bits).
More details follow from Emir Toktar:
(Refs: A Comprehensive Guide to Virtual Private Networks, IBM.
Virtual Private Networking: An Overview White Paper - DRAFT, 3/18/98
Microsoft.)
Neither network layer-based (L2TP, PPTP,...) nor application layer-based
(IPSec,SSL,SSH) security techniques are the best choice for all
situations. There will be trade-offs. Network layer security protects the
information created by upper layer protocols, but it requires that IPSec
be implemented in the communications stack.
With network layer security, there is no need to modify existing upper
layer applications. On the other hand, if security features are already
imbedded within a given application, then the data for that specific
application will be protected while it is in transit, even in the absence
of network layer security. Therefore security functions must be imbedded
on a per-application basis.
There are still other considerations:
Authentication is provided only for the identity of tunnel endpoints, but
not for each individual packet that flows inside the tunnel. This can
expose the tunnel to man-in-the-middle and spoofing attacks.
Network layer security gives blanket protection, but this may not be as
fine-grained as would be desired for a given application. It protects
all traffic and is transparent to users and applications.
Network layer security does not provide protection once the datagram has
arrived at its destination host. That is, it is vulnerable to attack
within the upper layers of the protocol stack at the destination machine.
Application layer security can protect the information that has been
generated within the upper layers of the stack, but it offers no
protection against several common network layer attacks while the
datagram is in transit. For example, a datagram in transit would be
vulnerable to spoofing attacks against its source or destination address.
Application layer security is more intelligent (as it knows the
application) but also more complex and slower.
IPSec provides for tunnel authentication, while PPTP does not.
Layer 2 tunneling protocols inherit the user
authentication schemes of PPP, including the EAP methods discussed below.
Many Layer 3 tunneling schemes assume that the endpoints were well
known (and authenticated) before the tunnel was established. An exception
to this is IPSec ISAKMP negotiation, which provides mutual authentication
of the tunnel endpoints. (Note that most IPSec implementations support
machine-based certificates only, rather than user certificates. As a
result, any user with access to one of the endpoint machines can use
the tunnel. This potential security weakness can be eliminated when
IPSec is paired with a Layer 2 protocol such as L2TP.
Using the Extensible Authentication Protocol
(EAP), Layer 2 tunneling protocols can support a wide variety of
authentication methods, including one-time passwords, cryptographic
calculators, and smart cards. Layer 3 tunneling protocols (IPSec) can
use similar methods; for example, IPSec defines public key certificate
authentication in its ISAKMP/Oakley negotiation.
Layer 2 tunneling supports dynamic
assignment of client addresses based on the Network Control Protocol
(NCP) negotiation mechanism.
Generally, Layer 3 tunneling schemes assume that an address has already
been assigned prior to initiation of the tunnel. Schemes for assignment
of addresses in IPSec tunnel mode are currently under development and
are not yet available.
Layer 2 tunneling protocols support PPP-based
compression schemes. For example, the Microsoft implementations of both
PPTP and L2TP use Microsoft Point-to-Point Compression (MPPC). The IETF
is investigating similar mechanisms (such as IP Compression) for the
Layer 3 tunneling protocols.
Layer 2 tunneling protocols support PPP-based data
encryption mechanisms. Microsoft's implementation of PPTP supports
optional use of Microsoft Point-to-Point Encryption (MPPE), based on
the RSA/RC4 algorithm. Layer 3 tunneling protocols can use similar
methods; for example, IPSec defines several optional data encryption
methods which are negotiated during the ISAKMP/Oakley exchange.
MPPE, a Layer 2 protocol, relies on the initial key
generated during user authentication, and then refreshes it
periodically. IPSec, explicitly negotiates a common key during the
ISAKMP exchange, and also refreshes it periodically.
Layer 2 tunneling supports multiple payload
protocols, which makes it easy for tunneling clients to access their
corporate networks using IP, IPX, NetBEUI, and so forth. In contrast,
Layer 3 tunneling protocols, such as IPSec tunnel mode, typically
support only target networks that use the IP protocol. IPSec is not
multi-protocol.
IPSec will be suported by Windows 2000.
Many cases can occur, each of which needs to be examined on its own
merit. It may be desirable to employ a mix of both network layer
security techniques and application layer techniques to achieve the
desired overall level of protection. For example, you could use an upper
layer mechanism such as Secure Sockets Layer (SSL) to encrypt upper
layer data. SSL could then be supplemented with IPSec's AH protocol at
the network layer to provide per-packet data origin authentication and
protection against spoofing attacks.
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Q.
I get a 'createHostSocket: Address already in use' error! what gives?
A.
Address already in use in createHostSocket means something is already using
TCP port 1723 - maybe another pptp daemon is running?
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Q.
Does PoPToP work with Windows 2000 clients?
A.
PoPToP v0.9.5 and above should work with Windows 2000 clients.
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