PoPToP HOWTO/FAQ
Last Updated: 1999/10/11 (by Jeff Blaize
<jblaize@netops.mediaplex.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.
1.0 Introduction
3.0 PPP with MSCHAPv2/MPPE Installation 4.0 PoPToP Installation 5.0 Windows Client Setup 6.0 Setup Notes
1.0 Introduction1.1 About PoPToPPoPToP 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 CreditsPoPToP was originally started by Matthew Ramsay under the control of Moreton Bay Ventures (http://www.moretonbay.com). Around March 1999 PoPToP was publicly released under the GNU GPL by Moreton Bay. 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, Ron O'Hara and Chris Wong. And finally, credit to all the PoPToP followers who test and report problems. 2.0 System Requirements
3.0 PPP (and MSCHAPv2/MPPE) InstallationIt is only necessary to use PPP 2.3.8 if you want Microsoft compatible MSCHAPv2/MPPE authentication and encryption. The reason for this is that the MSCHAPv2/MPPE patch currently supplied (19990813) is against PPP 2.3.8. If you don't need Microsoft compatible authentication/encryption any 2.3.x PPP source will be fine. Assuming you want Microsoft compatible authentication/encryption follow these steps:
4.0 PoPToP InstallationFollow these instructions to install PoPToP:
5.0 Windows Client SetupInstall the Windows Virtual Private Networking support using the Add/Remove Programs tool in Control Panel. In that utility go to the Windows Setup tab, and goto Communications. Check the box for Virtual Private Networking. (If you do above you may *not* need to follow the instructions below as it will already be installed... ?) follow the instructions:
take a little nap here... Once your Machine is back
Note that the Win95 routine is similar but requires Dial Up Networking Update 1.3 (free from Microsoft) to be installed first. 6.0 Setup Notes6.1 IntroductionAfter 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? 6.2 Simple Test SetupAs 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 | |_____| |______| Emboldened by my success, I set out to turn on MS authentication and encryption, and this is where the fun started. 6.3 Authentication and EncryptionThis 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 Win98 implementation of encryption is FUBAR! You have to download one of those patches from Microsoft, DUN40.exe to get the thing to work. This is for 40 bit encryption. Don't hold your breath waiting for 128 bit.
6.4 Linux Firewall Configuration (ipchains)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 interesting 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 6.5 Linux Network Address Translation (NAT)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 knowledgeable 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 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. 6.6 SAMBAHere'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. CONCLUSIONPoPToP 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. 7.0 FAQQ. 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? A.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
A.
Yeah, in your /lib/modules/ 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.
And 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:
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:
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 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. Here are some reasons I can think of:
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? Q. Does PoPToP work with Windows 2000 clients? A. PoPToP v0.9.5 and above should work with Windows 2000 clients. |