pptpproxy ругается

В понедельник, в 9 утра, на debian-шлюзе в логах появились сообщения:
Dec 21 08:57:59 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:57:59 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:58:04 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:58:04 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:58:10 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:58:10 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:58:15 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:58:15 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:00 deb1 pptpproxy[2877]: new proxy connection from x.x.x.x. to 192.168.0.1 (request received on interface 0.0.0.0:1723)
Dec 21 08:59:00 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:59:00 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:02 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x3977
Dec 21 08:59:02 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:05 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:59:05 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:09 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x3977
Dec 21 08:59:09 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:13 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:59:13 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:17 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:59:17 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:21 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x3977
Dec 21 08:59:21 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:25 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:59:25 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:29 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x3977
Dec 21 08:59:29 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 08:59:33 deb1 pptpproxy[2877]: GRE thread: dropping unknown GRE packet from IP x.x.x.x, fakeCallId = 0x0
Dec 21 08:59:33 deb1 pptpproxy[2877]: GRE thread: peer not found, dropping packet
Dec 21 09:00:08 deb1 pptpproxy[2877]: end proxy connection from x.x.x.x to 192.168.0.1
На rras сервере windows 2003, который принимает pptp запросы в журнале:
Event Type: Warning
Event Source: Rasman
Event Category: None
Event ID: 20209
Date: 21.12.2009
Time: 9:02:37
User: N/A
Computer: SRV-L
Description:
A connection between the VPN server and the VPN client 192.168.0.6 has been established, but the VPN connection cannot be completed. The most common cause for this is that a firewall or router between the VPN server and the VPN client is not configured to allow Generic Routing Encapsulation (GRE) packets (protocol 47). Verify that the firewalls and routers between your VPN server and the Internet allow GRE packets. Make sure the firewalls and routers on the user's network are also configured to allow GRE packets. If the problem persists, have the user contact the Internet service provider (ISP) to determine whether the ISP might be blocking GRE packets.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

Надо уменьшить MTU на клиенте.
Или открыть на сервере icmp mtu discovery:
iptables -I OUTPUT -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

или

iptables -t mangle -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
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