fds: Make many fds nonblocking

[secnet.git] / NOTES

diff --git a/NOTES b/NOTES
index 09c083e0c8ed6c49cde3e61ea6685fdb52c448c3..f5ebc652045e20818d83188fd6f919e8deb11b53 100644 (file)
--- a/NOTES
+++ b/NOTES
@@ -174,8 +174,9 @@ quite stable so the feature doesn't gain us much.
 
 Definitions:
 
 
 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, see below

 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
 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


@@ -193,21 +194,86 @@ i? is appropriate index for receiver
 Note that 'i' may be re-used from one session to the next, whereas 'n'
 is always fresh.
 
 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.
+The optional additional data after the sender's name consists of some
+initial subset of the following list of items:
+ * A 32-bit integer with a set of capability flags, representing the
+   abilities of the sender.
+ * In MSG3/MSG4: a 16-bit integer being the sender's MTU, or zero.
+   (In other messages: nothing.)  See below.
+ * More data which is yet to be defined and which must be ignored
+   by receivers.
+The optional additional data after the receiver's name is not
+currently used.  If any is seen, it must be ignored.
+
+Capability flag bits must be in one the following two categories:
+
+1. Early capability flags must be advertised in MSG1 or MSG2, as
+   applicable.  If MSG3 or MSG4 advertise any "early" capability bits,
+   MSG1 or MSG3 (as applicable) must have advertised them too.  Sadly,
+   advertising an early capability flag will produce MSG1s which are
+   not understood by versions of secnet which predate the capability
+   mechanism.
+
+2. Late capability flags are advertised in MSG2 or MSG3, as
+   applicable.  They may also appear in MSG1, but this is not
+   guaranteed.  MSG4 must advertise the same set as MSG2.
+
+No capability flags are currently defined.  Unknown capability flags
+should be treated as late ones.
+
+
+MTU handling
+
+In older versions of secnet, secnet was not capable of fragmentation
+or sending ICMP Frag Needed.  Administrators were expected to configure
+consistent MTUs across the network.
+
+It is still the case in the current version that the MTUs need to be
+configured reasonably coherently across the network: the allocated
+buffer sizes must be sufficient to cope with packets from all other
+peers.
+
+However, provided the buffers are sufficient, all packets will be
+processed properly: a secnet receiving a packet larger than the
+applicable MTU for its delivery will either fragment it, or reject it
+with ICMP Frag Needed.
+
+The MTU additional data field allows secnet to advertise an MTU to the
+peer.  This allows the sending end to handle overlarge packets, before
+they are transmitted across the underlying public network.  This can
+therefore be used to work around underlying network braindamage
+affecting large packets.
+
+If the MTU additional data field is zero or not present, then the peer
+should use locally-configured MTU information (normally, its local
+netlink MTU) instead.
+
+If it is nonzero, the peer may send packets up to the advertised size
+(and if that size is bigger than the peer's administratively
+configured size, the advertiser promises that its buffers can handle
+such a large packet).
+
+A secnet instance should not assume that just because it has
+advertised an mtu which is lower than usual for the vpn, the peer will
+honour it, unless the administrator knows that the peers are
+sufficiently modern to understand the mtu advertisement option.  So
+secnet will still accept packets which exceed the link MTU (whether
+negotiated or assumed).
+

 
 Messages:
 
 
 Messages:
 

-1) A->B: *,iA,msg1,A,B,protorange-A,nA
+1) A->B: *,iA,msg1,A+,B+,nA
+
+i* must be encoded as 0.  (However, it is permitted for a site to use
+zero as its "index" for another site.)

 
 

-2) B->A: iA,iB,msg2,B,A,chosen-protocol,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...)
 
 
 (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,protorange-A,chosen-protocol,nA,nB,g^x mod m}_PK_A^-1
+3) A->B: {iB,iA,msg3,A+,B+,[chosen-transform],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 1 was a replay then A will not generate message 3, because
 it doesn't recognise nA.


@@ -215,18 +281,11 @@ 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.
 
 If message 2 was from an attacker then B will not generate message 4,
 because it doesn't recognise nB.
 

-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
+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.
 
 
 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
 5) A: iB,iA,msg5,(ping/msg5)_k
 
 6) B: iA,iB,msg6,(pong/msg6)_k


@@ -273,3 +332,37 @@ vaguely recent version of secnet.  (In fact, there is no evidence in
 the git history of it ever being sent.)
 
 This message number is reserved.
 the git history of it ever being sent.)
 
 This message number is reserved.

+
+11) *,*,PROD,A,B
+
+Sent in response to a NAK from B to A.  Requests that B initiates a
+key exchange with A, if B is willing and lacks a transport key for A.
+(If B doesn't have A's address configured, implicitly supplies A's
+public address.)
+
+This is necessary because if one end of a link (B) is restarted while
+a key exchange is in progress, the following bad state can persist:
+the non-restarted end (A) thinks that the key is still valid and keeps
+sending packets, but B either doesn't realise that a key exchange with
+A is necessary or (if A is a mobile site) doesn't know A's public IP
+address.
+
+Normally in these circumstances B would send NAKs to A, causing A to
+initiate a key exchange.  However if A and B were already in the
+middle of a key exchange then A will not want to try another one until
+the first one has timed out ("setup-time" x "setup-retries") and then
+the key exchange retry timeout ("wait-time") has elapsed.
+
+However if B's setup has timed out, B would be willing to participate
+in a key exchange initiated by A, if A could be induced to do so.
+This is the purpose of the PROD packet.
+
+We send no more PRODs than we would want to send data packets, to
+avoid a traffic amplification attack.  We also send them only in state
+WAIT, as in other states we wouldn't respond favourably.  And we only
+honour them if we don't already have a key.
+
+With PROD, the period of broken communication due to a key exchange
+interrupted by a restart is limited to the key exchange total
+retransmission timeout, rather than also including the key exchange
+retry timeout.