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1 This is intended to be an example of a state-machine driven SSL application. It
2 acts as an SSL tunneler (functioning as either the server or client half,
3 depending on command-line arguments). *PLEASE* read the comments in tunala.h
4 before you treat this stuff as anything more than a curiosity - YOU HAVE BEEN
5 WARNED!! There, that's the draconian bit out of the way ...
8 Why "tunala"??
11 I thought I asked you to read tunala.h?? :-)
14 Show me
17 If you want to simply see it running, skip to the end and see some example
18 command-line arguments to demonstrate with.
21 Where to look and what to do?
24 The code is split up roughly coinciding with the detaching of an "abstract" SSL
25 state machine (which is the purpose of all this) and its surrounding application
26 specifics. This is primarily to make it possible for me to know when I could cut
27 corners and when I needed to be rigorous (or at least maintain the pretense as
28 such :-).
30 Network stuff:
32 Basically, the network part of all this is what is supposed to be abstracted out
33 of the way. The intention is to illustrate one way to stick OpenSSL's mechanisms
34 inside a little memory-driven sandbox and operate it like a pure state-machine.
35 So, the network code is inside both ip.c (general utility functions and gory
36 IPv4 details) and tunala.c itself, which takes care of application specifics
37 like the main select() loop. The connectivity between the specifics of this
38 application (TCP/IP tunneling and the associated network code) and the
39 underlying abstract SSL state machine stuff is through the use of the "buffer_t"
40 type, declared in tunala.h and implemented in buffer.c.
42 State machine:
44 Which leaves us, generally speaking, with the abstract "state machine" code left
45 over and this is sitting inside sm.c, with declarations inside tunala.h. As can
46 be seen by the definition of the state_machine_t structure and the associated
47 functions to manipulate it, there are the 3 OpenSSL "handles" plus 4 buffer_t
48 structures dealing with IO on both the encrypted and unencrypted sides ("dirty"
49 and "clean" respectively). The "SSL" handle is what facilitates the reading and
50 writing of the unencrypted (tunneled) data. The two "BIO" handles act as the
51 read and write channels for encrypted tunnel traffic - in other applications
52 these are often socket BIOs so that the OpenSSL framework operates with the
53 network layer directly. In this example, those two BIOs are memory BIOs
54 (BIO_s_mem()) so that the sending and receiving of the tunnel traffic stays
55 within the state-machine, and we can handle where this gets send to (or read
56 from) ourselves.
62 If you take a look at the "state_machine_t" section of tunala.h and the code in
63 sm.c, you will notice that nothing related to the concept of 'transport' is
64 involved. The binding to TCP/IP networking occurs in tunala.c, specifically
65 within the "tunala_item_t" structure that associates a state_machine_t object
66 with 4 file-descriptors. The way to best see where the bridge between the
67 outside world (TCP/IP reads, writes, select()s, file-descriptors, etc) and the
68 state machine is, is to examine the "tunala_item_io()" function in tunala.c.
69 This is currently around lines 641-732 but of course could be subject to change.
75 Well, although that function is around 90 lines of code, it could easily have
76 been a lot less only I was trying to address an easily missed "gotcha" (item (2)
77 below). The main() code that drives the select/accept/IO loop initialises new
78 tunala_item_t structures when connections arrive, and works out which
79 file-descriptors go where depending on whether we're an SSL client or server
80 (client --> accepted connection is clean and proxied is dirty, server -->
81 accepted connection is dirty and proxied is clean). What that tunala_item_io()
82 function is attempting to do is 2 things;
84 (1) Perform all reads and writes on the network directly into the
85 state_machine_t's buffers (based on a previous select() result), and only
86 then allow the abstact state_machine_t to "churn()" using those buffers.
87 This will cause the SSL machine to consume as much input data from the two
88 "IN" buffers as possible, and generate as much output data into the two
89 "OUT" buffers as possible. Back up in the main() function, the next main
90 loop loop will examine these output buffers and select() for writability
91 on the corresponding sockets if the buffers are non-empty.
93 (2) Handle the complicated tunneling-specific issue of cascading "close"s.
94 This is the reason for most of the complexity in the logic - if one side
95 of the tunnel is closed, you can't simply close the other side and throw
96 away the whole thing - (a) there may still be outgoing data on the other
97 side of the tunnel that hasn't been sent yet, (b) the close (or things
98 happening during the close) may cause more data to be generated that needs
99 sending on the other side. Of course, this logic is complicated yet futher
100 by the fact that it's different depending on which side closes first :-)
101 state_machine_close_clean() will indicate to the state machine that the
102 unencrypted side of the tunnel has closed, so any existing outgoing data
103 needs to be flushed, and the SSL stream needs to be closed down using the
104 appropriate shutdown sequence. state_machine_close_dirty() is simpler
105 because it indicates that the SSL stream has been disconnected, so all
106 that remains before closing the other side is to flush out anything that
107 remains and wait for it to all be sent.
109 Anyway, with those things in mind, the code should be a little easier to follow
110 in terms of "what is *this* bit supposed to achieve??!!".
113 How might this help?
116 Well, the reason I wrote this is that there seemed to be rather a flood of
117 questions of late on the openssl-dev and openssl-users lists about getting this
118 whole IO logic thing sorted out, particularly by those who were trying to either
119 use non-blocking IO, or wanted SSL in an environment where "something else" was
120 handling the network already and they needed to operate in memory only. This
121 code is loosely based on some other stuff I've been working on, although that
122 stuff is far more complete, far more dependant on a whole slew of other
123 network/framework code I don't want to incorporate here, and far harder to look
124 at for 5 minutes and follow where everything is going. I will be trying over
125 time to suck in a few things from that into this demo in the hopes it might be
126 more useful, and maybe to even make this demo usable as a utility of its own.
127 Possible things include:
129 * controlling multiple processes/threads - this can be used to combat
130 latencies and get passed file-descriptor limits on some systems, and it uses
131 a "controller" process/thread that maintains IPC links with the
132 processes/threads doing the real work.
134 * cert verification rules - having some say over which certs get in or out :-)
136 * control over SSL protocols and cipher suites
138 * A few other things you can already do in s_client and s_server :-)
140 * Support (and control over) session resuming, particularly when functioning
141 as an SSL client.
143 If you have a particular environment where this model might work to let you "do
144 SSL" without having OpenSSL be aware of the transport, then you should find you
145 could use the state_machine_t structure (or your own variant thereof) and hook
146 it up to your transport stuff in much the way tunala.c matches it up with those
147 4 file-descriptors. The state_machine_churn(), state_machine_close_clean(), and
148 state_machine_close_dirty() functions are the main things to understand - after
149 that's done, you just have to ensure you're feeding and bleeding the 4
150 state_machine buffers in a logical fashion. This state_machine loop handles not
151 only handshakes and normal streaming, but also renegotiates - there's no special
152 handling required beyond keeping an eye on those 4 buffers and keeping them in
153 sync with your outer "loop" logic. Ie. if one of the OUT buffers is not empty,
154 you need to find an opportunity to try and forward its data on. If one of the IN
155 buffers is not full, you should keep an eye out for data arriving that should be
156 placed there.
158 This approach could hopefully also allow you to run the SSL protocol in very
159 different environments. As an example, you could support encrypted event-driven
160 IPC where threads/processes pass messages to each other inside an SSL layer;
161 each IPC-message's payload would be in fact the "dirty" content, and the "clean"
162 payload coming out of the tunnel at each end would be the real intended message.
163 Likewise, this could *easily* be made to work across unix domain sockets, or
164 even entirely different network/comms protocols.
166 This is also a quick and easy way to do VPN if you (and the remote network's
167 gateway) support virtual network devices that are encapsulted in a single
168 network connection, perhaps PPP going through an SSL tunnel?
174 Please let me know if you find this useful, or if there's anything wrong or
175 simply too confusing about it. Patches are also welcome, but please attach a
176 description of what it changes and why, and "diff -urN" format is preferred.
177 Mail to email@example.com should do the trick.
183 Here is an example of how to use "tunala" ...
185 First, it's assumed that OpenSSL has already built, and that you are building
186 inside the ./demos/tunala/ directory. If not - please correct the paths and
187 flags inside the Makefile. Likewise, if you want to tweak the building, it's
188 best to try and do so in the makefile (eg. removing the debug flags and adding
189 optimisation flags).
191 Secondly, this code has mostly only been tested on Linux. However, some
192 autoconf/etc support has been added and the code has been compiled on openbsd
193 and solaris using that.
195 Thirdly, if you are Win32, you probably need to do some *major* rewriting of
196 ip.c to stand a hope in hell. Good luck, and please mail me the diff if you do
197 this, otherwise I will take a look at another time. It can certainly be done,
198 but it's very non-POSIXy.
200 See the INSTALL document for details on building.
202 Now, if you don't have an executable "tunala" compiled, go back to "First,...".
203 Rinse and repeat.
205 Inside one console, try typing;
207 (i) ./tunala -listen localhost:8080 -proxy localhost:8081 -cacert CA.pem \
208 -cert A-client.pem -out_totals -v_peer -v_strict
210 In another console, type;
212 (ii) ./tunala -listen localhost:8081 -proxy localhost:23 -cacert CA.pem \
213 -cert A-server.pem -server 1 -out_totals -v_peer -v_strict
215 Now if you open another console and "telnet localhost 8080", you should be
216 tunneled through to the telnet service on your local machine (if it's running -
217 you could change it to port "22" and tunnel ssh instead if you so desired). When
218 you logout of the telnet session, the tunnel should cleanly shutdown and show
219 you some traffic stats in both consoles. Feel free to experiment. :-)
223 - the format for the "-listen" argument can skip the host part (eg. "-listen
224 8080" is fine). If you do, the listening socket will listen on all interfaces
225 so you can connect from other machines for example. Using the "localhost"
226 form listens only on 127.0.0.1 so you can only connect locally (unless, of
227 course, you've set up weird stuff with your networking in which case probably
228 none of the above applies).
230 - ./tunala -? gives you a list of other command-line options, but tunala.c is
231 also a good place to look :-)