-#* Design of new, multi-subnet secnet protocol
+* Design of new, multi-subnet secnet protocol
-Like the first version, we're tunnelling IP packets inside 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.
+Like the first (1995/6) version, we're tunnelling IP packets inside
+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. 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.
-The configuration is held in memory as a data structure as follows:
+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.
-The root is a Dictionary. Dictionaries hold (key,value) pairs. Keys
-are atoms. Values are lists, dictionaries or closures. Lists can hold
-the following types: string, number.
+** VPN-level configuration
-Closures cannot be constructed directly; they are added to the
-'default' dictionary before the configuration file is read. Invocation
-of a closure can return any type of value.
+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.
-Configuration file format: the file describes a dictionary.
+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.
-key value;
+Some keywords are valid at any 'level' of the distributed 'sites'
+file, indicating defaults.
-value is item[,item...]
+The keywords are:
-item can be "string", number, path (looks up in dictionary),
-{dictionary}, value(value), value{dictionary}. If item is a list it
-is copied into the list - we can't have lists of lists.
+vpn n: we are now declaring information to do with VPN 'n'. Must come first.
-A path is [/]key[\[index\]][/key[\[index\]]...], defining a lookup
-from the current dictionary (or parents) or the root. If a key refers
-to a list of more than one item then an index number (base 0) in
-square brackets can be used to specify the list item number.
+location n: we are now declaring information for location 'n'.
-Items of the form value1(value2) invoke executable value1 with an
-argument of value2. The return value can be a string or dictionary,
-but not a list. (Invocation happens after the entire configuration
-file has been read.)
+site n: we are now declaring information for site 'n'.
+endsite: we're finished declaring information for the current site
-Items of the form value{dict} invoke executable value with an argument
-of a single-element list, containing dict. It's just syntactic sugar
-for value({dict}).
+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
-When a key is used (rather than defined) it is looked up in the
-current dictionary, and if it isn't found it is looked up in the
-(lexical) parent, until the root is reached.
+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
-What sorts of crypto-related things do we need to define?
+** Logging etc.
-sources of randomness
-block algorithms
-block cipher modes?
-hash functions
-padding functions
-public key signature algorithms
-public key crypto key stores
-key setup algorithms
+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
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: I'm not
+yet convinced it's a good idea. Instead, the initiator could try
+using its preferred protocol (which starts with a different magic
+number) and fall back if there's no reply.
+
Messages:
-1) A->B: *,iA,msg1,A,B,nA
+1) A->B: *,iA,msg1,A,B,protorange-A,nA
-2) B->A: iA,iB,msg2,B,A,nB,nA
+2) B->A: iA,iB,msg2,B,A,chosen-protocol,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,protorange-A,chosen-protocol,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
+If an attacker is trying to manipulate the chosen protocol, B can spot
+this when it sees A's message 3.
+
+4) B->A: {iA,iB,msg4,B,A,protorange-B,chosen-protocol,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.
+A can abandon the exchange if the chosen protocol is not the one that
+it would have chosen knowing the acceptable protocol ranges of A and
+B.
+
5) A: iB,iA,msg5,(ping/msg5)_k
6) B: iA,iB,msg6,(pong/msg6)_k
sent when a key times out, or the tunnel is forcibly terminated for
some reason.
-8) i?,i?,NAK/msg8
+8) i?,i?,NAK (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
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...
-Alternative idea: if you receive a packet you can't decode, because
-there's no key established, then initiate key setup...
+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...
+
+Keepalives are probably a good idea.
**** Protocol sub-goal 3: send a packet
9) i?,i?,msg0,(send-packet/msg9,packet)_k
+
+Some messages may take a long time to prepare (software modexp on slow
+machines); this is a "please wait" message to indicate that a message
+is in preparation.
+
+10) i?,i?,msg8,A,B,nA,nB,msg?
~ian / secnet.git / blobdiff