UDP packets. To defeat various restrictions which may be imposed on us
by network providers (like the prohibition of incoming TCP
connections) we're sticking with UDP for everything this time,
-including key setup.
+including key setup. This means we have to handle retries, etc.
Other new features include being able to deal with subnets hidden
behind changing 'real' IP addresses, and the ability to choose
** Configuration and structure
+[The original plan]
+
The network is made up from a number of 'sites'. These are collections
of machines with private IP addresses. The new secnet code runs on
machines which have interfaces on the private site network and some
tunnel endpoint, but this is not vital. Individual tunnels are
identified by their two endpoint names.
+[The new plan]
+
+It appears that people want to be able to use secnet on mobile
+machines like laptops as well as to interconnect sites. In particular,
+they want to be able to use their laptop in three situations:
+
+1) connected to their internal LAN by a cable; no tunnel involved
+2) connected via wireless, using a tunnel to protect traffic
+3) connected to some other network, using a tunnel to access the
+internal LAN.
+
+They want the laptop to keep the same IP address all the time.
+
+Case (1) is simple.
+
+Case (2) requires that the laptop run a copy of secnet, and have a
+tunnel configured between it and the main internal LAN default
+gateway. secnet must support the concept of a 'soft' tunnel where it
+adds a route and causes the gateway to do proxy-ARP when the tunnel is
+up, and removes the route again when the tunnel is down.
+
+The usual prohibition of packets coming in from one tunnel and going
+out another must be relaxed in this case (in particular, the
+destination address of packets from these 'mobile station' tunnels may
+be another tunnel as well as the host).
+
+(Quick sanity check: if chiark's secnet address was in
+192.168.73.0/24, would this work properly? Yes, because there will be
+an explicit route to it, and proxy ARP will be done for it. Do we want
+packets from the chiark tunnel to be able to go out along other
+routes? No. So, spotting a 'local' address in a remote site's list of
+networks isn't sufficient to switch on routing for a site. We need an
+explicit option. NB packets may be routed if the source OR the
+destination is marked as allowing routing [otherwise packets couldn't
+get back from eg. chiark to a laptop at greenend]).
+
+[the even newer plan]
+
+secnet sites are configured to grant access to particular IP address
+ranges to the holder of a particular public key. The key can certify
+other keys, which will then be permitted to use a subrange of the IP
+address range of the certifying key.
+
+This means that secnet won't know in advance (i.e. at configuration
+time) how many tunnels it might be required to support, so we have to
+be able to create them (and routes, and so on) on the fly.
+
+** VPN-level configuration
+
+At a high level we just want to be able to indicate which groups of
+users can claim ownership of which ranges of IP addresses. Assuming
+these users (or their representatives) all have accounts on a single
+machine, we can automate the submission of keys and other information
+to make up a 'sites' file for the entire VPN.
+
+The distributed 'sites' file should be in a more restricted format
+than the secnet configuration file, to prevent attackers who manage to
+distribute bogus sites files from taking over their victim's machines.
+
+The distributed 'sites' file is read one line at a time. Each line
+consists of a keyword followed by other information. It defines a
+number of VPNs; within each VPN it defines a number of locations;
+within each location it defines a number of sites. These VPNs,
+locations and sites are turned into a secnet.conf file fragment using
+a script.
+
+Some keywords are valid at any 'level' of the distributed 'sites'
+file, indicating defaults.
+
+The keywords are:
+
+vpn n: we are now declaring information to do with VPN 'n'. Must come first.
+
+location n: we are now declaring information for location 'n'.
+
+site n: we are now declaring information for site 'n'.
+endsite: we're finished declaring information for the current site
+
+restrict-nets a b c ...: restrict the allowable 'networks' for the current
+ level to those in this list.
+end-definitions: prevent definition of further vpns and locations, and
+ modification of defaults at VPN level
+
+dh x y: the current VPN uses the specified group; x=modulus, y=generator
+
+hash x: which hash function to use. Valid options are 'md5' and 'sha1'.
+
+admin n: administrator email address for current level
+
+key-lifetime n
+setup-retries n
+setup-timeout n
+wait-time n
+renegotiate-time n
+
+address a b: a=dnsname, b=port
+networks a b c ...
+pubkey x y z: x=keylen, y=encryption key, z=modulus
+mobile: declare this to be a 'mobile' site
+
+** Logging etc.
+
+There are several possible ways of running secnet:
+
+'reporting' only: --version, --help, etc. command line options and the
+--just-check-config mode.
+
+'normal' run: perform setup in the foreground, and then background.
+
+'failed' run: setup in the foreground, and terminate with an error
+before going to background.
+
+'reporting' modes should never output anything except to stdout/stderr.
+'normal' and 'failed' runs output to stdout/stderr before
+backgrounding, then thereafter output only to log destinations.
+
** Protocols
*** Protocol environment:
Definitions:
-A is the originating gateway machine
-B is the destination gateway machine
+A is the originating gateway machine name
+B is the destination gateway machine name
+A+ and B+ are the names with optional additional data, currently ignored
PK_A is the public RSA key of A
PK_B is the public RSA key of B
PK_A^-1 is the private RSA key of A
Note that 'i' may be re-used from one session to the next, whereas 'n'
is always fresh.
+The protocol version selection stuff is not yet implemented.
+
Messages:
1) A->B: *,iA,msg1,A,B,nA
-2) B->A: iA,iB,msg2,B,A,nB,nA
+2) B->A: iA,iB,msg2,B+,A+,nB,nA
(The order of B and A reverses in alternate messages so that the same
code can be used to construct them...)
-3) A->B: {iB,iA,msg3,A,B,nA,nB,g^x mod m}_PK_A^-1
+3) A->B: {iB,iA,msg3,A+,B+,nA,nB,g^x mod m}_PK_A^-1
If message 1 was a replay then A will not generate message 3, because
it doesn't recognise nA.
If message 2 was from an attacker then B will not generate message 4,
because it doesn't recognise nB.
-4) B->A: {iA,iB,msg4,B,A,nB,nA,g^y mod m}_PK_B^-1
+4) B->A: {iA,iB,msg4,B+,A+,nB,nA,g^y mod m}_PK_B^-1
At this point, A and B share a key, k. B must keep retransmitting
message 4 until it receives a packet encrypted using key k.
sent when a key times out, or the tunnel is forcibly terminated for
some reason.
-XXX not yet implemented.
+**** Protocol sub-goal 3: send a packet
+
+8) i?,i?,msg0,(send-packet/msg9,packet)_k
-8) i?,i?,NAK/msg8
+**** Other messages
+
+9) i?,i?,NAK (NAK is encoded as zero)
If the link-layer can't work out what to do with a packet (session has
-gone away, etc.) it can transmit a NAK back to the sender. The sender
-can then try to verify whether the session is alive by sending ping
-packets, and forget the key if it isn't. Potential denial-of-service
-if the attacker can stop the ping/pong packets getting through (the
-key will be forgotten and another key setup must take place), but if
-they can delete packets then we've lost anyway...
+gone away, etc.) it can transmit a NAK back to the sender.
-The attacker can of course forge NAKs since they aren't protected. But
-if they can only forge packets then they won't be able to stop the
-ping/pong working. Trust in NAKs can be rate-limited...
+This can alert the sender to the situation where the sender has a key
+but the receiver doesn't (eg because it has been restarted). The
+sender, on receiving the NAK, will try to initiate a key exchange.
-Alternative idea (which is actually implemented): if you receive a
-packet you can't decode, because there's no key established, then
-initiate key setup...
+Forged (or overly delayed) NAKs can cause wasted resources due to
+spurious key exchange initiation, but there is a limit on this because
+of the key exchange retry timeout.
-Keepalives are probably a good idea.
+10) i?,i?,msg8,A,B,nA,nB,msg?
-**** Protocol sub-goal 3: send a packet
+This is an obsolete form of NAK packet which is not sent by any even
+vaguely recent version of secnet. (In fact, there is no evidence in
+the git history of it ever being sent.)
-9) i?,i?,msg0,(send-packet/msg9,packet)_k
+This message number is reserved.
~ianmdlvl / secnet.git / blobdiff