1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
|
%include "default.mgp"
%default 1 bgrad
%%%%%%%%%%%%%%%%
%deffont "typewriter" tfont "MONOTYPE.TTF"
%page
%nodefault
%back "blue"
%center
%size 7
TCP state + windowtracking
%center
%size 4
by
Harald Welte <laforge@gnumonks.org>
%page
TCP state + window tracking
What? Why?
TCP is a stateful protocol, each endpoint is a state machine
What is TCP state / windowtracking?
Some intermediate System (Router/Firewall) is trying to derive the current state of the two TCP endpoints
Why does somebody want TCP state / windowtracking
Evaluation of TCP stack implementations
Hide/Protect broken implementations from a public network
%page
TCP state + windowtracking
TCP basics
states of a TCP endpoint:
LISTEN: port waiting for connection request from remote end
SYN_SENT: we've sent a SYN packet and not received anything yet
SYN_RECEIVED: We've received a SYN in reply to our SYN
ESTABLISHED: fully established TCP connection
FIN_WAIT1: waiting for FIN from remote end or ACK of sent FIN
FIN_WAIT2 waiting for FIN from remote end
TIME_WAIT: waiting for enough time to pass to be sure the remote end received the ACK of its FIN
CLOSED: no connection state at all
CLOSE_WAIT: waiting for a connection termination request from local user
CLOSING: waiting for a connection termination request acknowledgement from the remote end
LAST_ACK: Waiting for ACK of the FIN previously sent to remote end
%page
TCP state + windowtracking
TCP basics
sequence numbers
every octet has a corrsponding sequence number
sequence number is increased by one for every payload octet sent
receiver acknowledges last received contiguous sequence number (cumulative ack)
EXTENSION: selective acknowledgement (SACK) option, RFC2018
receiver can specify seperate sequencenumber blocks it has received
sliding window protocol
receiver advertises the size of the receive window
sender can only send up to 'window' number of octets which are not ACK'ed yet
EXTENSION: window scaling, RFC1323
window size of 16bit is too small for high bandwith links, thus window scaling was introduced
%page
TCP state + windowtracking
TCP state tracking
Where do we do TCP state tracking?
state tracker needs to see _all_ packets in both directions
problems with asymmetric routing!
So where's the Problem?
IP is an unreliable, best-effort protocol
If man in the middle does observe a packet, he can make no assumption on whether it actually arrives at the receiver.
%page
TCP state + window tracking
Problems
Example scenario 1
A sends SYN to B
man in the middle saves state as SYN_SENT
B sends SYN/ACK to A
man in the middle detects state transition to SYN_RECEIVED
SYN/ACK doesn't arrive at A
somebody spoofs ACK A->B to firewall
man in the middle detects state transition to ESTABLISHED
==> Any traffic between A and B will be accepted (wrong!)
%page
TCP state + window tracking
Problems
Example scenario 2
fully established TCP connection
A sends FIN to B
man in the middle saves state to FIN_WAIT1
B sends FIN/ACK to A
man in the middle saves state CLOSING/TIME_WAIT
FIN/ACK doesn't arrive at A
B retransmits FIN/ACK to A
man in the middle doesn't accept any further packets
.oO(booom)Oo.
%page
TCP state + window tracking
Problems
Example scenario 3 (FIN/RST spoofing without windowtracking)
fully established TCP connection
evil guy spoofs FIN A->B (with guessed sequence number
man in the middle saves satet as FIN_WAIT1
B ignores FIN/ACK because of wrong sequence number
A sends further segments to B
man in the middle doesn't accept further segments after FIN was sent in this direction
.oO(booom)Oo.
Solution: Real Window tracking
See paper by Guido van Rooj
%page
TCP state + window tracking
Further Problems
pickup of already established connections
window scaling sucks in this case
window tracking has to be disabled in that case
selective acknowledgements
man-in-the-middle needs to track all selectively acknowledged segments
this can draw lots of resources at the man in the middle and is prone to DoS
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
TCP state + window tracking
conntrack subsystem of netfilter
Netfilter architecture in IPv4
%font "typewriter"
%size 3
--->[1]--->[ROUTE]--->[3]--->[4]--->
| ^
| |
| [ROUTE]
v |
[2] [5]
| ^
| |
v |
%font "standard"
1=NF_IP_PRE_ROUTING
2=NF_IP_LOCAL_IN
3=NF_IP_FORWARD
4=NF_IP_POST_ROUTING
5=NF_IP_LOCAL_OUT
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
TCP state + window tracking
conntrack subsystem of netfilter
Connection tracking...
implemented seperately from NAT
enables stateful filtering
implementation
hooks into NF_IP_PRE_ROUTING to track packets
hooks into NF_IP_POST_ROUTING and NF_IP_LOCAL_IN to see if packet passed filtering rules
protocol modules (currently TCP/UDP/ICMP)
application helpers currently (FTP,IRC,H.323,talk,SNMP)
divides packets in the following four categories
NEW - would establish new connection
ESTABLISHED - part of already established connection
RELATED - is related to established connection
INVALID - (multicast, errors...)
does _NOT_ filter packets itself
can be utilized by iptables using the 'state' match
is used by NAT Subsystem
%page
TCP state + window tracking
Further Reading
RFC793,RFC2018,RFC1323: Transmission Control Protocol
http://www.netfilter.org/ - netfilter hacking howto contains some info
|
