Known issues with Opportunistic Encryption Claudia Schmeing ------------------------------------------ This is an overview of known issues with OE, current August 2002. This document supplements: FreeS/WAN Quickstart Guide doc/quickstart.html Opportunism HOWTO doc/opportunism.howto Opportunism spec doc/opportunism.spec Internet Draft doc/draft-richardson-ipsec-opportunistic.txt * Use the most recent Linux FreeS/WAN release from ftp.xs4all.nl to try OE. Right now, that's 2.00pre1. In the FreeS/WAN 1 series, use at least 1.98b. DESIGN LIMITATIONS * Because Opportunistic Encryption relies on DNS: - to authenticate one FreeS/WAN to another, and - to prove that we have the right to protect traffic for a given IP, this authentication/authorization is only as strong as your DNS is secure. Without secure DNS, OE protects against passive snooping only. Because the public key and gateway information that FreeS/WAN gets from DNS is not authenticated, a man-in-the-middle attack is still possible. We hope that as DNSsec is widely adopted, OE with strong authentication will become more widespread. However, our software does not distinguish between strongly and weakly authenticated OE. This information might be useful for defining local security policy. * Denial of service attacks are possible against OE. If you rely on OE rather than VPN to connect several offices, a determined attacker could prevent you from communicating securely. * OE challenges the notion that all IPsec peers are "friends". Strangers can now tunnel _through_ your regular public interface, to get packets to another (ipsecN) interface. This calls for a re-visit to firewall policy. * OE is only as reliable as your DNS is. If your DNS service is flaky, you will not be able to reliably establish OE connections to known OE-capable peers. If you ping a peer, but your FreeS/WAN does not find a TXT record signifying the peer's ability to respond to OE negotiation), FreeS/WAN will not try to opportunistically initiate, and communication will fallback to clear. For more secure and reliable DNS, we recommend that you run DNS within your security perimeter, either on your security gateway, or on a machine to which you have a VPN connection. It is also possible to have your DNS server located elsewhere on your LAN, though this may cause lag on startup. See also "Getting DNS through" in opportunism.howto http://lists.freeswan.org/pipermail/design/2002-April/002285.html . CURRENT ISSUES * Currently, all OE connections are "fallback-to-clear". Consequently, you cannot rely on OE to protect sensitive traffic, without having your firewall drop any cleartext packets to and from the affected hosts. In the 3.x series, ipsec.conf will have a facility to set default policy for groups of peer IPs. * In some situations, the local FreeS/WAN reports that you have a connection but your peer has failed to clear. While an initial negotiation achieved tunnel that both sides knew about, if one side later goes down, the other may not know. The result is a one-way connection; highly undesirable, since it gives a false sense of security. Only by snooping packets will you see that the return packets are not encrypted. Workaround: Alter the _updown script to block cleartext packets from a host we expect to send encrypted. This is not specific to OE; a similar problem can occur for VPNs. We are currently encountering two examples of this behaviour: on rekey, and on peer restart. The rekey problem: If one side (A) has a shorter _keylife_ than the other, and that side also has _rekey=no_, then when the keylife has expired, it will expect that its peer (B) will make a new conn to replace the existing one. Unfortunately, B has no idea. B continues to send out encrypted packets on the original connection, while A passes the return packets along in the clear. There is a proposed patch for (A) here: http://lists.freeswan.org/pipermail/design/2002-July/003114.html The restart problem: If A and B have an OE connection, but A is rebooted, normally A will try to re-connect to B and (if it has no DNS-related failures) it will succeed. But, if A is set up for responder-only OE, you will have a one-way connection until B notices that its original tunnel has expired. For details see: http://lists.freeswan.org/pipermail/design/2002-May/002582.html http://lists.freeswan.org/pipermail/design/2002-June/002610.html TIP: If an OE connection isn't behaving, you can recreate it with ipsec whack --oppohere sourceIPaddress --oppothere targetIPaddress * There is no good clean facility to delete OE connections. Available are: ipsec auto --status to list connections ipsec whack --deletestate to delete by state#. * You may experience seeming gaps at rekey time. Once you generate traffic, you will find that the OE connection returns. By default, OE connections are not rekeyed; if they were we'd have a mountain of useless connections. As a consequence, if your OE connection is idle at rekey time, it will go down until you generate further traffic. To ensure prompt rekeying, you can run a ping thorough the OE tunnel. There is an old issue with an Assertion failure at rekey time. * At the moment, you can only run active OE on one physical interface. Active means --routed, to trap outbound packets. It is this route that is a problem. Untested theory: you can have multiple active OE conns, for different source addresses, but they all have to point their traffic out the single interface. When responding: you can only define one OE connection (per host or subnet) in ipsec.conf, and that conn will apply to one interface. Normally this will be the public interface which your default route uses; it is, however, configurable. Theoretically, it might make sense to select between multiple OE conns based on some criterion, such as address ranges. This might be useful for local OE, or in a complex routing scenario. Currently, Pluto expects only one OE connection. If you add another, Pluto may choose randomly between them, producing unpredictable results. * Building OE conns between nodes on a LAN is not possible. This is a side effect of conflicts about ARP entries in the rt_cache and our "stupid routing tricks". There is no known workaround at this time. "Stupid routing tricks" are an ongoing issue, and should go away in KLIPS 3, planned for fall 2002. See these explanations: http://lists.freeswan.org/pipermail/design/2002-April/002285.html http://lists.freeswan.org/pipermail/design/2002-August/003249.html * CNAME lookups can fail. In some instances, Opportunistic Encryption does not work with reverse delegation. A common method of reverse delegation is to use CNAMES. When delegating DNS, a provider uses a CNAME to indicate the domain which is now authoritative for the reverse lookup. Because of errors in BIND implementations, Linux FreeS/WAN may not properly follow a well constructed CNAME trail ending in a KEY. If you were to follow the same trail manually, you would find the correct KEY. When a DNS server queried by Linux FreeS/WAN follows a CNAME, it seems to forget what record type it is looking for, and it returns a PTR, despite the fact that a KEY was requested. We have seen that this affects KEY lookups. We suspect that it affects all non-PTR records, including KEY and TXT. Fix (untested): Set up a DNS locally (within your security perimeter) which includes a fix. We believe the problem is fixed in Bind 9, and if not, we can create a patch to fix a Bind 9 implementation, and submit it to the maintainers. Workaround: Meanwhile, if you are experiencing this bug, you can send your provider KEY and TXT records for direct insertion into the reverse ZONE files, rather than asking your provider to delegate authority using CNAME. People who own IP blocks, rather than leasing them, may not experience this problem. If you were assigned IPs more than five years ago, you may own your IPs. * To make OE operation smoother, we may need a script that runs and warns if we have the reverse DNS records, but not the software running. The reverse records advertise that we can do OE, but when the software is not running this is false advertising. * Coaxial OE doesn't work in practise. This includes OE-in-WAVEsec. If you have coaxial connections (one within another) you should have use of one of these encrypted links, but it is not clear which KLIPS will prefer. In particular, the behaviour may not be symmetrical. Worse yet, it just seems to trip over itself and be generally unworkable. Weird but predictable: If you have both a gateway and a host who advertise (via DNS) an ability to do OE you need to be serious about doing host-based OE, or you will be stuck in initiator-only mode. If your host advertises but does not run OE, then when a peer tries to connect to your host, it will fail to clear. The peer will then not try to encrypt traffic bound for that host as it travels to the gateway. To remedy the situation, restart ipsec on the peer (or otherwise flush out the %pass eroute), and ping the peer from your host to initiate OE. OLD ISSUES * Failure to look up own host name is a show stopper. Solved in 1.98 and 1.98b. Solution: new setting %dnsondemand. Usage: leftrsasigkey=%dnsondemand # now in sample ipsec.conf rightrsasigkey=%dnsondemand. From man ipsec.conf: The value %dnsondemand means the key is to fetched from DNS at the time it is needed. If Linux FreeS/WAN can't get the key for your public interface from DNS, it will not keep trying, and you will not be able to do OE. The error message is: May 14 09:40:24 road Pluto[21210]: failure to fetch key for 193.110.157.18 from DNS: failure querying DNS for KEY of 18.157.110.193.in-addr.arpa.: Host name lookup failure Workaround: 1 or 2 1. Supply a key in the conn. leftrsasigkey=0s... 2. Fix the KEY lookup failure and try again. * Assertion failure at OE rekey time. Fixed in 2.0pre0. Patch for 1.98b posted at http://lists.freeswan.org/pipermail/design/2002-August/003347.html * 1.91 to 1.94 have serious problems with %trap and %hold bugs. These bugs, introduced while coding the support structure for OE, affect both OE and VPN connections.