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Date: Mon, 20 Oct 2014 15:45:23 -0400 (EDT)
From: Alan Stern <stern@...land.harvard.edu>
To: James Carlson <carlsonj@...kingcode.com>
cc: James Chapman <jchapman@...alix.com>, <linux-ppp@...r.kernel.org>,
<netdev@...r.kernel.org>
Subject: Re: Routing BUG with ppp over l2tp
On Mon, 20 Oct 2014, James Carlson wrote:
> On 10/20/14 12:39, Alan Stern wrote:
> > As far as I can tell, the problem is caused by bad routing. The kernel
> > gets confused because the IP address assigned by the VPN server to the
> > server's end of the ppp tunnel is the _same_ as the server's actual IP
> > address.
>
> Indeed! That's pretty darned lame behavior by that peer. It would
> probably be workable if you had a virtual router instance and were able
> to put the L2TP connection in one routing instance and the PPP
> connection in another routing instance, but that's likely not at all
> simple to achieve.
I'd like to find the simplest solution. Ideally it should "just work",
like the Windows and OS-X clients do.
> > Unfortunately, I can't work around this problem by reconfiguring the
> > VPN server -- there's no way to tell it to use a different IP address
> > for its end of the VPN tunnel. Furthermore, the server works just fine
> > with clients running Windows or OS-X.
>
> Really? That seems ... improbable.
I guess that depends on how you judge probabilities. :-)
As evidence to convince you, here's a log of a session on a rather old
Mac Powerbook G4 running OS 10.4.11. The situation isn't exactly the
same as with my Linux system, because for this test the client and the
VPN server are on the same subnet -- I don't think that should make any
difference. The client's IP address is 140.247.233.41, the server's is
.37, and the router to the outside world is .33. The client's ppp IP
address (assigned by the server) is 10.170.30.1.
The following commands were carried out while the VPN was connected:
------------------------------------------------------------------------
michael-burns-powerbook-g4:~ stern$ netstat -rn -f inet
Routing tables
Internet:
Destination Gateway Flags Refs Use Netif Expire
default 140.247.233.37 UGSc 2 4 ppp0
10 ppp0 USc 1 0 ppp0
127 127.0.0.1 UCS 0 0 lo0
127.0.0.1 127.0.0.1 UH 12 2278 lo0
140.247.233.32/27 link#4 UCS 2 0 en0
140.247.233.33 0:8:e3:ff:fc:b8 UHLW 0 0 en0 1198
140.247.233.37 0:1e:f7:15:53:a8 UHLW 3 10 en0 1153
140.247.233.37/32 link#4 UCS 1 0 en0
140.247.233.41 127.0.0.1 UHS 0 0 lo0
169.254 link#4 UCS 0 0 en0
michael-burns-powerbook-g4:~ stern$ ping -c1 -n 10.160.0.2
PING 10.160.0.2 (10.160.0.2): 56 data bytes
64 bytes from 10.160.0.2: icmp_seq=0 ttl=64 time=1.368 ms
--- 10.160.0.2 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 1.368/1.368/1.368/nan ms
michael-burns-powerbook-g4:~ stern$ ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 140.247.233.41 netmask 0xffffffe0 broadcast 140.247.233.63
ether 00:03:93:12:da:48
media: autoselect (100baseTX <full-duplex>) status: active
supported media: none autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 100baseTX <half-duplex> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback>
michael-burns-powerbook-g4:~ stern$ ifconfig ppp0
ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280
inet 10.170.30.1 --> 140.247.233.37 netmask 0xff000000
------------------------------------------------------------------------
I don't understand (and can't be bothered to look up) those arcane
symbols in the netstat output. The IP address used for the ping test
(10.160.0.2) is a system on the VPN's private network.
Here's comparable output for a connection from a computer running
Windows 7 (same IP addresses as before):
------------------------------------------------------------------------
C:\Users\stern>netstat -rn
===========================================================================
Interface List
20...........................Rowland VPN
10...00 1a 6b 57 30 02 ......Intel(R) 82566DM Gigabit Network Connection
1...........................Software Loopback Interface 1
11...00 00 00 00 00 00 00 e0 Microsoft ISATAP Adapter
12...00 00 00 00 00 00 00 e0 Teredo Tunneling Pseudo-Interface
19...00 00 00 00 00 00 00 e0 Microsoft 6to4 Adapter
21...00 00 00 00 00 00 00 e0 Microsoft ISATAP Adapter #2
===========================================================================
IPv4 Route Table
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 140.247.233.33 140.247.233.41 4491
0.0.0.0 0.0.0.0 On-link 10.170.30.1 11
10.170.30.1 255.255.255.255 On-link 10.170.30.1 266
127.0.0.0 255.0.0.0 On-link 127.0.0.1 4531
127.0.0.1 255.255.255.255 On-link 127.0.0.1 4531
127.255.255.255 255.255.255.255 On-link 127.0.0.1 4531
140.247.233.32 255.255.255.224 On-link 140.247.233.41 4491
140.247.233.37 255.255.255.255 On-link 140.247.233.41 4236
140.247.233.41 255.255.255.255 On-link 140.247.233.41 4491
140.247.233.63 255.255.255.255 On-link 140.247.233.41 4491
224.0.0.0 240.0.0.0 On-link 127.0.0.1 4531
224.0.0.0 240.0.0.0 On-link 140.247.233.41 4492
224.0.0.0 240.0.0.0 On-link 10.170.30.1 11
255.255.255.255 255.255.255.255 On-link 127.0.0.1 4531
255.255.255.255 255.255.255.255 On-link 140.247.233.41 4491
255.255.255.255 255.255.255.255 On-link 10.170.30.1 266
===========================================================================
Persistent Routes:
Network Address Netmask Gateway Address Metric
0.0.0.0 0.0.0.0 140.247.233.33 Default
===========================================================================
C:\Users\stern>ping -n 1 10.160.0.2
Pinging 10.160.0.2 with 32 bytes of data:
Reply from 10.160.0.2: bytes=32 time<1ms TTL=64
Ping statistics for 10.160.0.2:
Packets: Sent = 1, Received = 1, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
C:\Users\stern>ipconfig /all
Windows IP Configuration
Host Name . . . . . . . . . . . . : Windows-test
Primary Dns Suffix . . . . . . . : rowland.org
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : rowland.org
PPP adapter Rowland VPN:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Rowland VPN
Physical Address. . . . . . . . . :
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv4 Address. . . . . . . . . . . : 10.170.30.1(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.255
Default Gateway . . . . . . . . . : 0.0.0.0
DNS Servers . . . . . . . . . . . : 10.160.0.2
10.160.0.3
Primary WINS Server . . . . . . . : 10.160.0.2
NetBIOS over Tcpip. . . . . . . . : Disabled
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Intel(R) 82566DM Gigabit Network Connection
Physical Address. . . . . . . . . : 00-1A-6B-57-30-02
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::1426:1891:bf83:3982%10(Preferred)
IPv4 Address. . . . . . . . . . . : 140.247.233.41(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.224
Default Gateway . . . . . . . . . : 140.247.233.33
DHCPv6 IAID . . . . . . . . . . . : 234887787
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-1B-A2-3E-B1-00-1A-6B-57-30-02
DNS Servers . . . . . . . . . . . : 8.8.8.8
NetBIOS over Tcpip. . . . . . . . : Enabled
------------------------------------------------------------------------
Although the Windows ipconfig output doesn't show the IP address of the
server side of the ppp tunnel, it does show up in a Details window
under the Network control panel, and it is indeed set to
140.247.233.37.
> > So it looks like the problem has to be fixed either in the kernel or in
> > the way pppd sets up its routing entry. Can you guys help?
>
> I think the easiest solution is to configure pppd to lie to the kernel
> about the remote address. Who cares what the remote address is on a
> point-to-point link anyway?
>
> There's currently no option to do this, but the code change in ipcp_up()
> in pppd/ipcp.c would be rather simple. Just make the "noremoteip" code
> run all the time:
>
> /* Deliberately falsify the remote address. We don't care. */
> ho->hisaddr = htonl(0x0aa00002);
>
> As long as you don't need to contact that specific remote server using
> the badly-assigned "internal" VPN address and can live with the fact
> that you'll either go through the regular Internet to that address or be
> forced to use some other address configured on that server, you should
> be good.
>
> (The address I used above is 10.160.0.2. That was one of the internal
> DNS server addresses provided in the log you posted. It's not necessary
> that the address used here is exactly that, but it may well be helpful.)
That might work. But using a nonstandard version of pppd would be
awkward, and I would prefer to avoid it.
> If you can't do that for some reason, then I suppose it would be
> possible to use IP Chains (or whatever the packet-modification tool du
> jure is used in your Linux distribution) to nail up an exception so that
> the outside packets go to the outside interface and the inside ones go
> to the PPP interface. Doing that likely requires selecting on (at
> least!) source address, so it's messy and ugly and possibly error-prone,
> but it might be doable.
That sounds like a fairly easy thing to try. But it would still
require manual intervention instead of just working. Fixing the kernel
would be preferable, IMO.
> Otherwise, contact the maintainer of that VPN server. It's just plain
> old broken, and life's too short for broken software.
It is an old Cisco security appliance, no doubt well past End-Of-Life.
I'm starting to think it might be preferable to throw the thing away
and start up a VPN server on the department's firewall (which is a
Linux box) instead.
Alan Stern
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