3. iSCSI Security Interoperability Guidelines The following guidelines are established to meet iSCSI security requirements using IPsec in practical situations. 3.1. iSCSI Security Issues iSCSI provides for iSCSI Login, outlined in [RFC3720], which includes support for application-layer authentication. This authentication is logically between the iSCSI initiator and the iSCSI target (as opposed to between the TCP/IP communication endpoints). The intent of the iSCSI design is that the initiator and target represent the systems (e.g., host and disk array or tape system) participating in the communication, as opposed to network communication interfaces or endpoints. The iSCSI protocol and iSCSI Login authentication do not meet the security requirements for iSCSI. iSCSI Login authentication provides mutual authentication between the iSCSI initiator and target at connection origination, but does not protect control and data traffic on a per packet basis, leaving the iSCSI connection vulnerable to attack. iSCSI Login authentication can mutually authenticate the initiator to the target, but does not by itself provide per-packet authentication, integrity, confidentiality or replay protection. In
addition, iSCSI Login authentication does not provide for a protected ciphersuite negotiation. Therefore, iSCSI Login provides a weak security solution. 3.2. iSCSI and IPsec Interaction An iSCSI session [RFC3720], comprised of one or more TCP connections, is identified by the 2-tuple of the initiator-defined identifier and the target-defined identifier, <ISID, TSIH>. Each connection within a given session is assigned a unique Connection Identification, CID. The TCP connection is identified by the 5-tuple <Source IP address, Destination IP address, Protocol (TCP), Source Port, Destination Port>. An IPsec Phase 2 SA is identified by the 3-tuple <Protocol (ESP),destination address, SPI>. The iSCSI session and connection information is carried within the iSCSI Login Commands, transported over TCP. Since an iSCSI initiator may have multiple interfaces, iSCSI connections within an iSCSI session may be initiated from different IP addresses. Similarly, multiple iSCSI targets may exist behind a single IP address, so that there may be multiple iSCSI sessions between a given <source IP address, destination IP address> pair. When multiple iSCSI sessions are active between a given <initiator, target> pair, the set of TCP connections used by a given iSCSI session must be disjoint from those used by all other iSCSI sessions between the same <initiator, target> pair. Therefore a TCP connection can be associated with one and only one iSCSI session. The relationship between iSCSI sessions, TCP connections and IKE Phase 1 and Phase 2 SAs is as follows:  An iSCSI initiator or target may have more than one interface, and therefore may have multiple IP addresses. Also, multiple iSCSI initiators and targets may exist behind a single IP address. As a result, an iSCSI Session may correspond to multiple IKE Phase 1 Security Associations, though typically a single IKE Phase 1 security association will exist for an <initiator IP address, target IP address> tuple.  Each TCP connection within an iSCSI Session is protected by an IKE Phase 2 SA. The selectors may be specific to that TCP connection or may cover multiple connections. While each IKE Phase 2 SA may protect multiple TCP connections, each TCP connection is transported under only one IKE Phase 2 SA.
Given this, all the information needed for the iSCSI/IPsec binding is contained within the iSCSI Login messages from the iSCSI initiator and target. This includes the binding between an IKE Phase 1 SA and the corresponding iSCSI sessions, as well as the binding between a TCP connection, an IKE Phase 2 SA and the iSCSI connection ID. 3.3. Initiating a New iSCSI Session In order to create a new iSCSI Session, if an IKE Phase 1 SA does not already exist, then it is established by an initiator implementing iSCSI security. Subsequent iSCSI connections established within the iSCSI session will typically be protected by IKE Phase 2 SAs derived from that IKE Phase 1 SA, although additional IKE Phase 1 SAs can also be brought up. The initiator and target implementations successfully complete the IKE Phase 1 and Phase 2 negotiations before the iSCSI initiator contacts the target on well-known TCP port 3260, and sends the iSCSI Login command over the TCP connection. IPsec implementations configured with the correct policies (e.g., use ESP with non-null transform for all traffic destined for the iSCSI well-known TCP port 3260) will handle this automatically. The initiator fills in the ISID field, and leaves the TSIH field set to zero, to indicate that it is the first message of a new session establishment exchange. The initiator also fills in a CID value, that identifies the TCP connection over which the Login command is being exchanged. When the iSCSI target replies with its Login Command, both iSCSI devices will know the TSIH, and therefore the iSCSI session identifier <ISID, TSIH>. A single iSCSI session identifier may have multiple associated IKE Phase 1 SAs, and each IKE Phase 1 SA may correspond to multiple iSCSI session identifiers. Each iSCSI connection (identified by the connection identifier) corresponds to a single TCP connection (identified by the 5-tuple). Each IKE Phase 2 SA is identified by the <Protocol (ESP), destination address, SPI> combination. A Phase 2 SA may protect multiple TCP connections, and corresponds to a single IKE Phase 1 SA. Within IKE, each key refresh requires that a new security association be established. In practice there is a time interval during which an old, about-to-expire SA and newly established SA will both be valid. The IPsec implementation will choose which security association to use based on local policy, and iSCSI concerns play no role in this selection process.
3.4. Graceful iSCSI Teardown Mechanisms within iSCSI provide for both graceful and non-graceful teardown of iSCSI Sessions or individual TCP connections within a given session. The iSCSI Logout command is used to effect graceful teardown. This command allows the iSCSI initiator to request that: [a] the session be closed [b] a specific connection within the session be closed [c] a specific connection be marked for recovery When the iSCSI implementation wishes to close a session, it uses the appropriate iSCSI commands to accomplish this. After exchanging the appropriate iSCSI control messages for session closure, the iSCSI security implementation will typically initiate a half-close of each TCP connection within the iSCSI session. When the iSCSI security implementation wishes to close an individual TCP connection while leaving the parent iSCSI session active, it should half-close the TCP connection. After the expiration of the TIME_WAIT timeout, the TCP connection is closed. 3.5. Non-graceful iSCSI Teardown If a given TCP connection unexpectedly fails, the associated iSCSI connection is torn down. There is no requirement that an IKE Phase 2 delete immediately follow iSCSI connection tear down or Phase 1 deletion. Since an IKE Phase 2 SA may correspond to multiple TCP connections, such a deletion might be inappropriate. Similarly, if the IKE implementation receives a Phase 2 Delete message for a security association corresponding to a TCP connection, this does not necessarily imply that the TCP or iSCSI connection is to be torn down. If a Logout Command/Logout Response sequence marks a connection for removal from the iSCSI session, then after the iSCSI peer has executed an iSCSI teardown process for the connection, the TCP connection will be closed. The iSCSI connection state can then be safely removed. Since an IKE Phase 2 SA may be used by multiple TCP connections, an iSCSI implementation should not depend on receiving the IPsec Phase 2 delete as confirmation that the iSCSI peer has executed an iSCSI teardown process for the connection.
Since IPsec acceleration hardware may only be able to handle a limited number of active IKE Phase 2 SAs, Phase 2 delete messages may be sent for idle SAs, as a means of keeping the number of active Phase 2 SAs to a minimum. The receipt of an IKE Phase 2 delete message MUST NOT be interpreted as a reason for tearing down the corresponding iSCSI connection if no Logout Command/Logout Receive has been executed on the connection. Rather, it is preferable to leave the iSCSI connection up, and if additional traffic is sent on it, to bring up another IKE Phase 2 SA to protect it. This avoids the potential for continually bringing iSCSI connections up and down. 3.6. Application-Layer CRC iSCSI's error detection and recovery assumes that the TCP and IP checksums provide inadequate integrity protection for high speed communications. As described in [CRCTCP], when operating at high speeds, the 16-bit TCP checksum [RFC793] will not necessarily detect all errors, resulting in possible data corruption. iSCSI [RFC3720] therefore incorporates a 32-bit CRC to protect its headers and data. When a CRC check fails (i.e., CRC computed at receiver does not match the received CRC), the iSCSI PDU covered by that CRC is discarded. Since presumably the error was not detected by the TCP checksum, TCP retransmission will not occur and thus cannot assist in recovering from the error. iSCSI contains both data and command retry mechanisms to deal with the resulting situations, including SNACK, the ability to reissue R2T commands, and the retry (X) bit for commands. IPsec offers protection against an attacker attempting to modify packets in transit, as well as unintentional packet modifications caused by network malfunctions or other errors. In general, IPsec authentication transforms afford stronger integrity protection than both the 16-bit TCP checksum and the 32-bit application-layer CRC specified for use with iSCSI. Since IPsec integrity protection occurs below TCP, if an error is discovered, then the packet will be discarded and TCP retransmission will occur, so that no recovery action need be taken at the iSCSI layer. 3.6.1. Simplification of Recovery Logic Where IPsec integrity protection is known to be in place end-to-end between iSCSI endpoints (or the portion that requires additional integrity protection), portions of iSCSI can be simplified. For example, mechanisms to recover from CRC check failures are not necessary.
If the iSCSI CRC is negotiated, the recovery logic can be simplified to regard any CRC check failure as fatal (e.g., generate a SCSI CHECK CONDITION on data error, close the corresponding TCP connection on header error) because it will be very rare for errors undetected by IPsec integrity protection to be detected by the iSCSI CRC. 3.6.2. Omission of iSCSI CRC In some situations where IPsec is employed, the iSCSI CRC will not provide additional protection and can be omitted. For example, where IPsec processing as well as TCP checksum and iSCSI CRC verification are offloaded within the NIC, each of these checks will be verified prior to transferring data across the bus, so that subsequent errors will not be detected by these mechanisms. As a result, where IPsec processing is offloaded to the NIC, the iSCSI CRC is not necessary and the implementations may wish not to negotiate it. However, in other circumstances, the TCP checksum and iSCSI CRC will provide additional error coverage because they are computed and checked at a different point in the protocol stack or in devices different from those that implement the IPsec integrity checks. The resulting coverage of additional possible errors may make it desirable to negotiate use of the iSCSI CRC even when IPsec integrity protection is in use. Examples of these situations include where:  IPsec, TCP and iSCSI are implemented purely in software. Here, additional failure modes may be detected by the TCP checksum and/or iSCSI CRC. For example, after the IPsec message integrity check is successfully verified, the segment is copied as part of TCP processing, and a memory error during this process might cause the TCP checksum or iSCSI CRC verification to fail.  The implementation is an iSCSI-iSCSI proxy or gateway. Here the iSCSI CRC can be propagated from one iSCSI connection to another. In this case, the iSCSI CRC is useful to protect iSCSI data against memory, bus, or software errors within the proxy or gateway, and requesting it is desirable.  IPsec is provided by a device external to the actual iSCSI device. Here the iSCSI header and data CRCs can be kept across the part of the connection that is not protected by IPsec. For instance, the iSCSI connection could traverse an extra bus, interface card, network, interface card, and bus between the
iSCSI device and the device providing IPsec. In this case, the iSCSI CRC is desirable, and the iSCSI implementation behind the IPsec device may request it. Note that if both ends of the connection are on the same segment, then traffic will be effectively protected by the layer 2 CRC, so that negotiation of the iSCSI CRC is not necessary to protect against NIC and network errors, although it may be desirable for other reasons (e.g.,  and  above). 4. iFCP and FCIP Security Issues 4.1. iFCP and FCIP Authentication Requirements iFCP and FCIP are peer-to-peer protocols. iFCP and FCIP sessions may be initiated by either or both peer gateways. Consequently, bi- directional authentication of peer gateways MUST be provided. iFCP and FCIP are transport protocols that encapsulate SCSI and Fibre Channel frames over IP. Therefore, Fibre Channel, operating system, and user identities are transparent to the iFCP and FCIP protocols. iFCP gateways use Discovery Domain information obtained from the iSNS server to determine whether the initiating Fibre Channel N_PORT should be allowed access to the target N_PORT. N_PORT identities used in the Port Login (PLOGI) process will be considered authenticated provided that they are received over a connection whose security complies with the local security policy. There is no requirement that the identities used in authentication be kept confidential. 4.2. iFCP Interaction with IPsec and IKE A conformant iFCP Portal is capable of establishing one or more IKE Phase-1 Security Associations (SAs) to a peer iFCP Portal. A Phase-1 SA may be established when an iFCP Portal is initialized, or may be deferred until the first TCP connection with security requirements is established. An IKE Phase-2 SA protects one or more TCP connections within the same iFCP Portal. More specifically, the successful establishment of an IKE Phase-2 SA results in the creation of two uni-directional IPsec SAs fully qualified by the tuple <SPI, destination IP address, ESP>. These SAs protect the setup process of the underlying TCP connections and all their subsequent TCP traffic. Each of the TCP connections protected by an SA is either in the unbound state, or is bound to a specific iFCP session.
In summary, at any point in time:  There exist 0..M IKE Phase-1 SAs between peer iFCP portals  Each IKE Phase-1 SAs has 0..N IKE Phase-2 SAs  Each IKE Phase-2 SA protects 0..Z TCP connections The creation of an IKE Phase-2 SA may be triggered by security policy rules retrieved from an iSNS server. Alternately, the creation of an SA may be triggered by policy rules configured through a management interface, reflecting iSNS-resident policy rules. Likewise, the use of a Key Exchange payload in Quick Mode for perfect forward secrecy may be driven by security policy rules retrieved from the iSNS server, or set through a management interface. If an iFCP implementation makes use of unbound TCP connections, and such connections belong to an iFCP Portal with security requirements, then the unbound connections MUST be protected by an SA at all times just like bound connections. Upon receiving an IKE Phase-2 delete message, there is no requirement to terminate the protected TCP connections or delete the associated IKE Phase-1 SA. Since an IKE Phase-2 SA may be associated with multiple TCP connections, terminating such connections might in fact be inappropriate and untimely. To minimize the number of active Phase-2 SAs, IKE Phase-2 delete messages may be sent for Phase-2 SAs whose TCP connections have not handled data traffic for a while. To minimize the use of SA resources while the associated TCP connections are idle, creation of a new SA should be deferred until new data are to be sent over the connections. 4.3. FCIP Interaction with IPsec and IKE FCIP Entities establish tunnels with other FCIP Entities in order to transfer IP encapsulated FC frames. Each tunnel is a separate FCIP Link, and can encapsulate multiple TCP connections. The binding of TCP connections to an FCIP Link is performed using the Fibre Channel World Wide Names (WWNs) of the two FCIP Entities. FCIP Entities may have more than one interface and IP address, and it is possible for an FCIP Link to contain multiple TCP connections whose FCIP endpoint IP Addresses are different. In this case, an IKE Phase 1 SA is typically established for each FCIP endpoint IP Address pair. For the purposes of establishing an IKE Phase 1 SA, static IP addresses are typically used for identification.
Each TCP connection within an FCIP Link corresponds to an IKE Phase 2 (Quick Mode) SA. This is established prior to sending the initial TCP SYN packet and applies to the life of the connection. Phase 2 negotiation is also required for rekeying, in order to protect against replay attacks. FCIP implementations MAY provide administrative management of Confidentiality usage. These management interfaces SHOULD be provided in a secure manner, so as to prevent an attacker from subverting the security process by attacking the management interface. FCIP Entities do not require any user-level authentication, since all FCIP Entities participate in a machine-level tunnel function. FCIP uses SLP for discovery, but not to distribute security policies. 5. Security Considerations 5.1. Transport Mode Versus Tunnel Mode With respect to block storage protocols, the major differences between the IPsec tunnel mode and transport mode are as follows:  MTU considerations. Tunnel mode introduces an additional IP header into the datagram that reflects itself in a corresponding decrease in the path MTU seen by packets traversing the tunnel. This may result in a decrease in the maximum segment size of TCP connections running over the tunnel.  Address assignment and configuration. Within IPsec tunnel mode, it is necessary for inner and outer source addresses to be configured, and for inner and outer destination addresses to be discovered. Within transport mode it is only necessary to discover a single destination address and configure a single source address. IPsec tunnel mode address usage considerations are discussed in more detail below.  NAT traversal. As noted in [RFC3715], IPsec tunnel mode ESP can traverse NAT in limited circumstances, whereas transport mode ESP cannot traverse NAT. To enable NAT traversal in the general case, the IPsec NAT traversal functionality described in [RFC3715], [UDPIPsec] and [NATIKE] can be implemented. More details are provided in the next section.
 Firewall traversal. Where a block storage protocol is to traverse administrative domains, the firewall administrator may desire to verify the integrity and authenticity of each transiting packet, rather than opening a hole in the firewall for the block storage protocol. IPsec tunnel mode lends itself to such verification, since the firewall can terminate the tunnel mode connection while still allowing the endpoints to communicate end-to-end. If desired, the endpoints can in addition utilize IPsec transport mode for end-to-end security, so that they can also verify authenticity and integrity of each packet for themselves. In contrast, carrying out this verification with IPsec transport mode is more complex, since the firewall will need to terminate the IPsec transport mode connection and will need to act as an iSCSI, iFCP or FCIP gateway or TCP proxy, originating a new IPsec transport mode connection from the firewall to the internal endpoint. Such an implementation cannot provide end- to-end authenticity or integrity protection, and an application-layer CRC (iSCSI) or forwarding of the Fibre Channel frame CRC (iFCP and FCIP) is necessary to protect against errors introduced by the firewall.  IPsec-application integration. Where IPsec and the application layer protocol are implemented on the same device or host, it is possible to enable tight integration between them. For example, the application layer can request and verify that connections are protected by IPsec, and can obtain attributes of the IPsec security association. While in transport mode implementations the IPsec and application protocol implementations typically reside on the same host, with IPsec tunnel mode they may reside on different hosts. Where IPsec is implemented on an external gateway, integration between the application and IPsec is typically not possible. This limits the ability of the application to control and verify IPsec behavior. 5.1.1. IPsec Tunnel Mode Addressing Considerations IPsec tunnels include both inner and outer source as well as destination addresses. When used with IP block storage protocols, the inner destination address represents the address of the IP block storage protocol peer (e.g., the ultimate destination for the packet). The inner destination address can be discovered using SLPv2 or iSNS, or can be resolved from an FQDN via DNS, so that obtaining this address is typically not an issue.
The outer destination address represents the address of the IPsec security gateway used to reach the peer. Several mechanisms have been proposed for discovering the IPsec security gateway used to reach a particular peer. [RFC2230] discusses the use of KX Resource Records (RRs) for IPsec gateway discovery. However, while KX RRs are supported by many DNS server implementations, they have not yet been widely deployed. Alternatively, DNS SRV [RFC2782] can be used for this purpose. Where DNS is used for gateway location, DNS security mechanisms such as DNSSEC ([RFC2535], [RFC2931]), TSIG [RFC2845], and Simple Secure Dynamic Update [RFC3007] are advisable. When used with IP block storage protocols, the outer source address is configured either statically or dynamically, using mechanisms such as DHCPv4 [RFC2131], DHCPV6 [RFC3315], or stateless address autoconfiguration [RFC2373]. The inner source address SHOULD be included in the Quick Mode ID payload when the peer establishes a tunnel mode SA with the IPsec security gateway. This enables the IPsec security gateway to properly route packets back to the remote peer. The inner source address can be configured via a variety of mechanisms, depending on the scenario. Where the IP block storage peers are located within the same administrative domain, it is typically possible for the inner and outer source addresses to be the same. This will work because the outer source address is presumably assigned from within a prefix assigned to the administrative domain, and is therefore routable within the domain. Assuming that the IPsec security gateway is aware of the inner source address used by the connecting peer and plumbs a host route for it, then packets arriving at the IPsec security gateway destined for the address can be correctly encapsulated and sent down the correct tunnel. Where IP block storage peers are located within different administrative domains, it may be necessary for the inner source address to be assigned by the IPsec security gateway, effectively "joining" the remote host to the LAN attached to the IPsec security gateway. For example, if the remote peer were to use its assigned (outer) source address as the inner source address, then a number of problems might result:  Intrusion detection systems sniffing the LAN behind the IPsec security gateway would notice source addresses originating outside the administrative domain.  Reply packets might not reach their destination, since the IPsec security gateway typically does not advertise the default route, but rather only the prefix that it allocates addresses from. Since the remote peer's address does not originate from with a
prefix native to the administrative domain, it is likely that routers within the domain would not have a route for it, and would send the packet off to the router advertising the default route (perhaps a border router) instead of to the IPsec security gateway. For these reasons, for inter-domain use, assignment of inner source addresses may be needed. At present this is not a very common scenario; however, if address assignment is provided, then DHCP-based address assignment within IPsec tunnel mode [RFC3456] MUST be supported. Note that this mechanism is not yet widely deployed within IPsec security gateways; existing IPsec tunnel mode servers typically implement this functionality via proprietary extensions to IKE. 5.2. NAT Traversal As noted in [RFC3715], tunnel mode ESP can traverse NAT in a limited set of circumstances. For example, if there is only one protocol endpoint behind a NAT, "ANY to ANY" selectors are negotiated, and the receiver does not perform source address validation, then tunnel mode ESP may successfully traverse NATs. Since ignoring source address validation introduces new security vulnerabilities, and negotiation of specific selectors is desirable so as to limit the traffic that can be sent down the tunnel, these conditions may not necessarily apply, and tunnel mode NAT traversal will not always be possible. TCP carried within Transport mode ESP cannot traverse NAT, even though ESP itself does not include IP header fields within its message integrity check. This is because the 16-bit TCP checksum is calculated based on a "pseudo-header" that includes IP header fields, and the checksum is protected by the IPsec ESP message integrity check. As a result, the TCP checksum will be invalidated by a NAT located between the two endpoints. Since TCP checksum calculation and verification is mandatory in both IPv4 and IPv6, it is not possible to omit checksum verification while remaining standards compliant. In order to enable traversal of NATs existing while remaining in compliance, iSCSI, iFCP or FCIP security implementations can implement IPsec/IKE NAT traversal, as described in [RFC3715], [UDPIPsec], and [NATIKE].
The IKE [NATIKE] and IPsec [UDPIPsec] NAT traversal specifications enable UDP encapsulation of IPsec to be negotiated if a NAT is detected in the path. By determining the IP address of the NAT, the TCP checksum can be calculated based on a pseudo-header including the NAT-adjusted address and ports. If this is done prior to calculating the IPsec message integrity check, TCP checksum verification will not fail. 5.3. IKE Issues There are situations where it is necessary for IKE to be implemented in firmware. In such situations, it is important to keep the size of the IKE implementation within strict limits. An upper bound on the size of an IKE implementation might be considered to be 800 KB, with 80 KB enabling implementation in a wide range of situations. As noted in Table 5.3-1 on the next page, IKE implementations currently exist which meet the requirements. Therefore, while removal of seldom used IKE functionality (such as the nonce authentication methods) would reduce complexity, implementations typically will not require this in order to fit within the code size budget. 5.4. Rekeying Issues It is expected that IP block storage implementations will need to operate at high speed. For example, implementations operating at 1 Gbps are currently in design, with 10 Gbps implementations to follow shortly thereafter. At these speeds, a single IPsec SA could rapidly cycle through the 32-bit IPsec sequence number space. For example, a single SA operating at 1 Gbps line rate and sending 64 octet packets would exhaust the 32-bit sequence number space in 2200 seconds, or 37 minutes. With 1518 octet packets, exhaustion would occur in 14.5 hours. At 10 Gbps, exhaustion would occur in 220 seconds or 3.67 minutes. With 1518 octet packets, this would occur within 1.45 hours. In the future, it may be desirable for implementations operating at speeds of 1 Gbps or greater to implement IPsec sequence number extension, described in [Seq]. Note that depending on the transform in use, it is possible that rekeying will be required prior to exhaustion of the sequence number space. In CBC-mode ciphers the ciphertext of one block depends on the plaintext of that block as well as the ciphertext of the previous block. This implies that if the ciphertext of two blocks are identical, and the plaintext of the next block is also identical,
then the ciphertext of the next block will be identical. Thus, if identical ciphertext blocks can be found with matching subsequent blocks, an attacker can determine the existence of matching plaintext. Such "Birthday attacks" were examined by Bellare et. al. in [DESANALY]. On average, a ciphertext block of size n bits will be expected to repeat every 2^[n/2] blocks. Although a single "birthday attack" does not provide much information to an attacker, multiple such attacks might provide useful information. To make this unlikely, it is recommended that a rekey occur before 2^[n/2] blocks have been sent on a given SA. Bellare et. al. have formalized this in [DESANALY], showing that the insecurity of CBC mode increases as O(s^2/2^n), where n is the block size in bits, and s is the total number of blocks encrypted. These conclusions do not apply to counter mode.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Code size | | |Implementation | (octets) | Release | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Linux | | Pluto | 258KB | FreeSWAN | |(FreeSWAN) | | x86 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Racoon | 400KB | NetBSD 1.5 | | (KAME) | | x86 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Isakmpd | 283KB | NetBSD 1.5 | | (Erickson) | | x86 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | WindRiver | 142KB | PowerPC | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Cisco | 222KB | PowerPC | | VPN1700 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Cisco | 350K | PowerPC | | VPN3000 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Cisco | 228KB | MIPS | | VPN7200 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Table 5.3-1 - Code Size for IKE implementations The formula below sets a limit on the bytes that can be sent on a CBC SA before a rekey is required: B = (n/8) * 2^[n/2] Where: B = maximum bytes sent on the SA n = block size in bits
This means that cipher block size as well as key length needs to be considered in the rekey decision. 3DES uses a block size n = 64 bits (2^3 bytes); this implies that the SA must be rekeyed before B = (64/8) * (2^32) = 2^35 bytes are sent. At 1 Gbps, this implies that a rekey will be required every 274.9 seconds (4.6 minutes); at 10 Gbps, a rekey is required every 27.5 seconds. In terms of the sequence number space, for a 3DES encrypted message of 512 = 2^9 bytes (2^6 blocks) this implies that the key has become insecure after about 2^26 messages. 5.5. Transform Issues Since IP block storage implementations may operate at speeds of 1 Gbps or greater, the ability to offer IPsec security services at high speeds is of intense concern. Since support for multiple algorithms multiplies the complexity and expense of hardware design, one of the goals of the transform selection effort is to find a minimal set of confidentiality and authentication algorithms implementable in hardware at speeds of up to 10 Gbps, as well as being efficient for implementation in software at speeds of 100 Mbps or slower. In this specification, we primarily concern ourselves with IPsec transforms that have already been specified, and for which parts are available that can run at 1 Gbps line rate. Where existing algorithms do not gracefully scale to 10 Gbps, we further consider algorithms for which transform specifications are not yet complete, but for which parts are expected to be available for inclusion in products shipping within the next 12 months. As the state of the art advances, the range of feasible algorithms will broaden and additional mandatory-to-implement algorithms may be considered. Section 5 of [RFC2406] states: "A compliant ESP implementation MUST support the following mandatory-to-implement algorithms: - DES in CBC mode - HMAC with MD5 - HMAC with SHA-1 - NULL Authentication algorithm - NULL Encryption algorithm" The DES algorithm is specified in [FIPS46-3]; implementation guidelines are found in [FIPS74], and security issues are discussed in [DESDIFF],[DESINT], [DESCRACK]. The DES IPsec transform is defined in [RFC2405] and the 3DES in CBC mode IPsec transform is specified in [RFC2451].
The MD5 algorithm is specified in [RFC1321]; HMAC is defined in [RFC2104] and security issues with MD5 are discussed in [MD5Attack]. The HMAC-MD5 IPsec transform is specified in [RFC2403]. The HMAC- SHA1 IPsec transform is specified in [RFC2404]. In addition to these existing algorithms, NIST is currently evaluating the following modes [NSPUE2] of AES, defined in [FIPS197]: AES in Electronic Codebook (ECB) confidentiality mode AES in Cipher Block Chaining (CBC) confidentiality mode AES in Cipher Feedback (CFB) confidentiality mode AES in Output Feedback (OFB) confidentiality mode AES in Counter (CTR) confidentiality mode AES CBC-MAC authentication mode When utilizing AES modes, it may be necessary to use larger public keys; the tradeoffs are described in [KeyLen]. Additional MODP Diffie-Hellman groups for use with IKE are described in [RFC3526]. The Modes [NSPUE2] effort is also considering a number of additional algorithms, including the following: PMAC To provide authentication, integrity and replay protection, IP block storage security implementations MUST support HMAC-SHA1 [RFC2404] for authentication, and AES in CBC MAC mode with XCBC extensions SHOULD be supported [RFC3566]. HMAC-SHA1 [RFC2404] is to be preferred to HMAC-MD5, due to concerns that have been raised about the security of MD5 [MD5Attack]. HMAC- SHA1 parts are currently available that run at 1 Gbps, the algorithm is implementable in hardware at speeds up to 10 Gbps, and an IPsec transform specification [RFC2404] exists. As a result, it is most practical to utilize HMAC-SHA1 as the authentication algorithm for products shipping in the near future. AES in CBC-MAC authentication mode with XCBC extensions was selected since it avoids adding substantial additional code if AES is already being implemented for encryption; an IPsec transform document is available [RFC3566]. Authentication transforms also exist that are considerably more efficient to implement than HMAC-SHA1, or AES in CBC-MAC authentication mode. UMAC [UMAC],[UMACKR] is more efficient to implement in software and PMAC [PMAC] is more efficient to implement in hardware. PMAC is currently being evaluated as part of the NIST modes effort [NSPUE2] but an IPsec transform does not yet exist; neither does a UMAC transform.
For confidentiality, the ESP mandatory-to-implement algorithm (DES) is unacceptable. As noted in [DESCRACK], DES is crackable with modest computation resources, and so is inappropriate for use in situations requiring high levels of security. To provide confidentiality for iSCSI, iFCP, and FCIP, 3DES in CBC mode [RFC2451] MUST be supported and AES in Counter Mode [RFC3686] SHOULD be supported. For use in high speed implementations, 3DES has significant disadvantages. However, a 3DES IPsec transform has been specified and parts are available that can run at 1 Gbps, so implementing 3DES in products is practical for the short term. As described in Appendix B, 3DES software implementations make excessive computational demands at speeds of 100 Mbps or greater, effectively ruling out software-only implementations. In addition, 3DES implementations require rekeying prior to exhaustion of the current 32-bit IPsec sequence number space, and thus would not be able to make use of sequence space extensions if they were available. This means that 3DES will require very frequent rekeying at speeds of 10 Gbps or faster. Thus, 3DES is inconvenient for use at very high speeds, as well as being impractical for implementation in software at slower speeds (100+ Mbps). 5.6. Fragmentation Issues When certificate authentication is used, IKE fragmentation can be encountered. This can occur when certificate chains are used, or even when exchanging a single certificate if the key size or size of other certificate fields (such as the distinguished name and other OIDs) is large enough. Many Network Address Translators (NATs) and firewalls do not handle fragments properly, dropping them on inbound and/or outbound. Routers in the path will also frequently discard fragments after the initial one, since they typically will not contain full IP headers that can be compared against an Access List. As a result, where IKE fragmentation occurs, the endpoints will often be unable to establish an IPsec security association. The solution to this problem is to install NAT, firewall or router code load that can properly support fragments. If this cannot be done, then the following alternatives can be considered:  Obtaining certificates by other means.  Reducing the size of the certificate chain.
 Reducing the size of fields within the certificates. This includes reduction in the key size, the distinguished name or other fields. This should be considered only as a last resort. Fragmentation can become a concern when prepending IPsec headers to a frame. One mechanism that can be used to reduce this problem is to utilize path MTU discovery. For example, when TCP is used as the transport for iSCSI, iFCP or FCIP then path MTU discovery, described in [RFC1191], [RFC1435], [RFC1981], can be used to enable the TCP endpoints to discover the correct MTU, including effects due to IPsec encapsulation. However, Path MTU discovery fails when appropriate ICMP messages are not received by the host. This occurs in IPsec implementations that drop unauthenticated ICMP packets. This can result in blackholing in naive TCP implementations, as described in [RFC2923]. Appropriate TCP behavior is described in section 2.1 of [RFC2923]: "TCP should notice that the connection is timing out. After several timeouts, TCP should attempt to send smaller packets, perhaps turning off the DF flag for each packet. If this succeeds, it should continue to turn off PMTUD for the connection for some reasonable period of time, after which it should probe again to try to determine if the path has changed." If an ICMP PMTU is received by an IPsec implementation that processes unauthenticated ICMP packets, this value should be stored in the SA as proposed in [RFC2401], and IPsec should also provide notification of this event to TCP so that the new MTU value can be correctly reflected. 5.7. Security Checks When a connection is opened which requires security, IP block storage security implementations may wish to check that the connection is protected by IPsec. If security is desired and IPsec protection is removed on a connection, it is reinstated before non-protected IP block storage packets are sent. Since IPsec verifies that each packet arrives on the correct SA, as long as it can be ensured that IPsec protection is in place, then IP block storage implementations can be assured that each received packet was sent by a trusted peer. When used with IP block storage protocols, each TCP connection MUST be protected by an IKE Phase 2 SA. Traffic from one or more than one TCP connection may flow within each IPsec Phase 2 SA. IP block storage security implementations need not verify that the IP addresses and TCP port values in the packet match the socket
information that was used to setup the connection. This check will be performed by IPsec, preventing malicious peers from sending commands on inappropriate Quick Mode SAs. 5.8. Authentication Issues 5.8.1. Machine Versus User Certificates The certificate credentials provided by the iSCSI initiator during the IKE negotiation may be those of the machine or of the iSCSI entity. When machine authentication is used, the machine certificate is typically stored on the iSCSI initiator and target during an enrollment process. When user certificates are used, the user certificate can be stored either on the machine or on a smartcard. For iFCP and FCIP, the certificate credentials provided will almost always be those of the device and will be stored on the device. Since the value of a machine certificate is inversely proportional to the ease with which an attacker can obtain one under false pretenses, it is advisable that the machine certificate enrollment process be strictly controlled. For example, only administrators may have the ability to enroll a machine with a machine certificate. While smartcard certificate storage lessens the probability of compromise of the private key, smartcards are not necessarily desirable in all situations. For example, some organizations deploying machine certificates use them so as to restrict use of non-approved hardware. Since user authentication can be provided within iSCSI login (keeping in mind the weaknesses described earlier), support for machine authentication in IPsec makes it is possible to authenticate both the machine as well as the user. Since iFCP and FCIP have no equivalent of iSCSI Login, for these protocols only the machine is authenticated. In circumstances in which this dual assurance is considered valuable, enabling movement of the machine certificate from one machine to another, as would be possible if the machine certificate were stored on a smart card, may be undesirable. Similarly, when user certificate are deployed, it is advisable for the user enrollment process to be strictly controlled. If for example, a user password can be readily used to obtain a certificate (either a temporary or a longer term one), then that certificate has no more security value than the password. To limit the ability of an attacker to obtain a user certificate from a stolen password, the enrollment period can be limited, after which password access will be
turned off. Such a policy will prevent an attacker obtaining the password of an unused account from obtaining a user certificate once the enrollment period has expired. 5.8.2. Pre-Shared Keys Use of pre-shared keys in IKE Main Mode is vulnerable to man-in-the- middle attacks when used with dynamically addressed hosts (such as with iSCSI initiators). In Main Mode it is necessary for SKEYID_e to be used prior to the receipt of the identification payload. Therefore the selection of the pre-shared key may only be based on information contained in the IP header. However, where dynamic IP address assignment is typical, it is often not possible to identify the required pre-shared key based on the IP address. Thus when pre-shared key authentication is used in Main Mode along with entities whose address is dynamically assigned, the same pre- shared key is shared by a group and is no longer able to function as an effective shared secret. In this situation, neither the initiator nor Responder identifies itself during IKE Phase 1; it is only known that both parties are a member of the group with knowledge of the pre-shared key. This permits anyone with access to the group pre- shared key to act as a man-in-the-middle. This vulnerability is typically not of concern where IP addresses are typically statically assigned (such as with iFCP and FCIP), since in this situation individual pre-shared keys are possible within IKE Main Mode. However, where IP addresses are dynamically assigned and Main Mode is used along with pre-shared keys, the Responder is not authenticated unless application-layer mutual authentication is performed (e.g., iSCSI login authentication). This enables an attacker in possession of the group pre-shared key to masquerade as the Responder. In addition to enabling the attacker to present false data, the attacker would also be able to mount a dictionary attack on legacy authentication methods such as CHAP [RFC1994], potentially compromising many passwords at a time. This vulnerability is widely present in existing IPsec implementations. Group pre-shared keys are not required in Aggressive Mode since the identity payload is sent earlier in the exchange, and therefore the pre-shared key can be selected based on the identity. However, when Aggressive Mode is used the user identity is exposed and this is often considered undesirable. Note that care needs to be taken with IKE Phase 1 Identity Payload selection in order to enable mapping of identities to pre-shared keys even with Aggressive Mode. Where the ID_IPV4_ADDR or ID_IPV6_ADDR Identity Payloads are used and addresses are dynamically assigned,
mapping of identities to keys is not possible, so that group pre- shared keys are still a practical necessity. As a result, identities other than ID_IPV4_ADDR and ID_IPV6_ADDR (such as ID_FQDN or ID_USER_FQDN) SHOULD be employed in situations where Aggressive mode is utilized along with pre-shared keys and IP addresses are dynamically assigned. 5.8.3. IKE and Application-Layer Authentication In addition to IKE authentication, iSCSI implementations utilize their own authentication methods. Currently, work is underway on Fibre Channel security, so that a similar authentication process may eventually also apply to iFCP and FCIP as well. While iSCSI provides initial authentication, it does not provide per-packet authentication, integrity or replay protection. This implies that the identity verified in the iSCSI Login is not subsequently verified on reception of each packet. With IPsec, when the identity asserted in IKE is authenticated, the resulting derived keys are used to provide per-packet authentication, integrity and replay protection. As a result, the identity verified in the IKE conversation is subsequently verified on reception of each packet. Let us assume that the identity claimed in iSCSI Login is a user identity, while the identity claimed within IKE is a machine identity. Since only the machine identity is verified on a per- packet basis, there is no way for the recipient to verify that only the user authenticated via iSCSI Login is using the IPsec SA. In fact, IPsec implementations that only support machine authentication typically have no way to distinguish between user traffic within the kernel. As a result, where machine authentication is used, once an IPsec SA is opened, another user on a multi-user machine may be able to send traffic down the IPsec SA. This is true for both transport mode and tunnel mode SAs. To limit the potential vulnerability, IP block storage implementations MUST do the following:  Ensure that socket access is appropriately controlled. On a multi-user operating system, this implies that sockets opened for use by IP block storage protocols MUST be exclusive.  In the case of iSCSI, implementations MUST also ensure that application layer login credentials (such as iSCSI login credentials) are protected from unauthorized access.
If these directives are followed, then a rogue process will not be able to access an IP block storage volume. While the identity asserted within IKE is verified on a per-packet basis, to avoid interference between users on a given machine, operating system support is required. In order to segregate traffic between users when user authentication is supported, the IPsec endpoints must ensure that only traffic from that particular user is sent or received within the IPsec SA. Enforcement of this restriction is the responsibility of the operating system. In kernel mode iSCSI drivers there typically is no user context to perform user authentication. In this case the authentication is closer to machine authentication. In most operating systems device permissions would control the ability to read/write to the device prior to mounting. Once the device is mounted, ACLs set by the filesystem control access to the device. An administrator can access the blocks on the device directly (for instance, by sending pass through requests to the port driver directly such as in Windows NT). In the same way, an administrator can open a raw socket and send IPsec protected packets to an iSCSI target. The situation is analogous, and in this respect no new vulnerability is created that didn't previously exist. The Operating system's ACLs need to be effective to protect a device (whether it is a SCSI device or an iSCSI device). 5.8.4. IP Block Storage Authorization IP block storage protocols can use a variety of mechanisms for authorization. Where ID_FQDN is used as the Identity Payload, IP block storage endpoints can be configured with a list of authorized FQDNs. The configuration can occur manually, or automatically via iSNS or the iSCSI MIB, defined in [AuthMIB]. Assuming that IPsec authentication is successful, this list of FQDNs can be examined to determine authorization levels. Where certificate authentication is used, it is possible to configure IP block storage protocol endpoints with trusted roots. The trusted roots used with IP block storage protocols might be different from the trusted roots used for other purposes. If this is the case, then the burden of negotiating use of the proper certificates lies with the IPsec initiator. Note that because IKE does not deal well with certificate chains, and is typically configured with a limited set of trusted roots, it is most appropriate for intra-domain usage.
Since iSCSI supports Login authentication, it is also possible to use the identities presented within the iSCSI Login for authorization purposes. In iFCP, basic access control properties stem from the requirement that two communicating iFCP gateways be known to one or more iSNS servers before they can engage in iFCP exchanges. The optional use of discovery domains in iSNS yields access control schemas of greater complexity. 5.9. Use of AES in Counter Mode When utilizing AES modes, it may be necessary to use larger public keys; the tradeoffs are described in [KeyLen]. Additional MODP Diffie-Hellman groups for use with IKE are described in [RFC3526]. When AES in counter mode is used, it is important to avoid reuse of the counter with the same key, even across all time. Counter mode creates ciphertext by XORing generated key stream with plaintext. It is fairly easy to recover the plaintext from two messages counter mode encrypted under the same counter value, simply by XORing together the two packets. The implication of this is that it is an error to use IPsec manual keying with counter mode, except when the implementation takes heroic measures to maintain state across sessions. In any case, manual keying MUST NOT be used since it does not provide the necessary rekeying support. Another counter mode issue is it makes forgery of correct packets trivial. Counter mode should therefore never be used without also using data authentication.