Source Address Validation in Intra-domain Networks Gap Analysis, Problem Statement, and Requirements

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Table of Contents

1. Introduction

Source Address Validation (SAV) is important for defending against source address spoofing attacks. A multi-fence architecture called Source Address Validation Architecture (SAVA) [RFC5210] was proposed to implement SAV at three levels: access network SAV, intra-domain SAV, and inter-domain SAV. When SAV has not been adopted by every source/host, the multi-fence architecture helps enhance the effectiveness of SAV across the whole Internet by preventing or mitigating source address spoofing.¶

Specifically, access network SAV can ensure that a host must use the source IP address assigned to the host. By deploying access network SAV, hosts in the corresponding access network cannot forge a source address of another host. There are many mechanisms for SAV in access networks. Static ACL rules can be manually configured for validation by specifying which source addresses are acceptable or unacceptable. Dynamic ACL is another efficient mechanism which is associated with authentication servers (e.g., RADIUS and DIAMETER). The servers receive access requests and then install or enable ACL rules on the device to permit particular users' packets. SAVI [RFC7039] represents a kind of mechanism enforcing that the valid source IP address of a host matches the link-layer property of the host's network attachment. For example, SAVI solution for DHCP [RFC7513] creates a binding between a DHCPv4/DHCPv6-assigned IP address and a link-layer property (like MAC address or switch port) on a SAVI device. IP Source Guard (IPSG) [IPSG] combined with DHCP snooping is an implementation of SAVI solution for DHCP. Cable Source-Verify [cable-verify] also shares some features of SAVI and is used in cable modem networks. Cable modem termination system (CMTS) devices with Cable Source-Verify maintain the bindings of the CPE's IP address, the CPE's MAC address, and the corresponding cable modem identifier. When receiving a packet from a host, the device can check the validity of source IP address according to the bindings.¶

Given numerous access networks managed by different operators throughout the world, it is difficult to require all access networks to deploy SAV simultaneously. Therefore, intra-domain SAV and inter-domain SAV are needed to block source-spoofed data packets from access networks as close to the source as possible. Intra-domain SAV and inter-domain SAV perform SAV at the granularity of IP prefixes, which is coarser than the granularity of access network SAV (i.e., IP address), as an IP prefix covers a range of IP addresses.¶

This document focuses on the analysis of intra-domain SAV. In contrast to inter-domain SAV, intra-domain SAV does not require collaboration between different ASes. Intra-domain SAV rules can be generated by the AS itself. Consider an AS X which provides its host networks or customer networks with the connectivity to the rest of the Internet. Intra-domain SAV for AS X aims at achieving two goals without collaboration with other ASes: i) blocking source-spoofed packets originated from its host networks or customer networks using a source address of other networks; and ii) blocking source-spoofed packets coming from other ASes using a source address of AS X.¶

Figure 1 illustrates the goals and function of intra-domain SAV with two cases. Case i shows that the host network or customer network of AS X originates source-spoofed packets using a source address of other networks. If AS X deploys intra-domain SAV, the spoofed packets can be blocked by host-facing routers or customer-facing routers of AS X (i.e., Goal i). Case ii shows that AS X receives source-spoofed packets using a source address of AS X from other ASes (e.g., AS Y). If AS X deploys intra-domain SAV, the spoofed packets from AS Y can be blocked by AS border routers of AS X (i.e., Goal ii).¶

Case i: The host network or customer network of AS X originates
        packets spoofing source addresses of other networks
Goal i: If AS X deploys intra-domain SAV,
        the spoofed packets can be blocked by AS X

                                    Spoofed packets
                                    using source addresses
  +-------------------------------+ of other networks     +------+
  | Host/Customer Network of AS X |---------------------->| AS X |
  +-------------------------------+                       +------+

Case ii: AS X receives packets spoofing source addresses of AS X
         from other ASes (e.g., AS Y)
Goal ii: If AS X deploys intra-domain SAV,
         the spoofed packets can be blocked by AS X

           Spoofed packets
           using source addresses
  +------+ of AS X               +------+
  | AS X |<----------------------| AS Y |
  +------+                       +------+

The scope of intra-domain SAV includes all IP-encapsulated scenarios:¶

• Native IP forwarding: including both forwarding based on global routing table and CE site forwarding of VPN.¶

• IP-encapsulated Tunnel (IPsec, GRE, SRv6, etc.): focusing on the validation of the outer layer IP address.¶

• Validating both IPv4 and IPv6 addresses.¶

Scope does not include:¶

• Non-IP packets: including MPLS label-based forwarding and other non-IP-based forwarding.¶

There are many mechanisms for intra-domain SAV. This document provides the gap analysis of existing intra-domain SAV mechanisms. According to the gap analysis, the document concludes the main problems of existing intra-domain SAV mechanisms and describes the requirements for future ones.¶

2. Existing Mechanisms

This section briefly introduces existing intra-domain SAV mechanisms. Particularly, ingress filtering (i.e., BCP38 [RFC2827] and BCP84 [RFC3704]) is the best current practice for intra-domain SAV.¶

• ACL-based ingress filtering [RFC2827] is a typical mechanism for intra-domain SAV. It requires that network operators manually configure ACL rules on intra-domain routers to block or permit data packets using specific source addresses. This mechanism can be used on interfaces of host-facing or customer-facing routers facing an intra-domain host/customer network to prevent the corresponding host/customer network from spoofing source prefixes of other networks [manrs-antispoofing]. In addition, it is also usually used on interfaces of AS border routers facing an external AS to block data packets using disallowed source addresses, such as internal source addresses owned by the local AS [nist-rec]. In any application scenario, ACL rules must be updated in time to be consistent with the latest filtering criteria when the network changes.¶

• Strict uRPF [RFC3704] is another typical intra-domain SAV mechanism. It is typically used on interfaces of host-facing or customer-facing routers facing an intra-domain host/customer network. Routers deploying strict uRPF accept a data packet only when i) the local FIB contains a prefix covering the packet's source address and ii) the corresponding outgoing interface for the prefix in the FIB matches the packet's incoming interface. Otherwise, the packet will be blocked.¶

• Loose uRPF [RFC3704] uses a pretty looser validation method which loses the directionality. A packet will be accepted if the router's local FIB contains a prefix covering the packet's source address regardless of the interface from which the packet is received. In fact, interfaces of AS border routers facing an external AS may use loose uRPF to block incoming data packets using non-global addresses [nist-rec].¶

• Carrier Grade NAT has some operations on the source addresses of packets, but it is not an anti-spoofing tool, as described in [manrs-antispoofing]. If the source address of a packet is in the INSIDE access list, the NAT rule can translate the source address to an address in the pool OUTSIDE. The NAT rule cannot determine whether the source address is spoofed or not. In addition, the packet using a spoofed source address will still be forwarded if the spoofed source address is not included in the INSIDE access list. Therefore, Carrier Grade NAT cannot help identify and block source-spoofed data packets.¶

3. Gap Analysis

Towards the two goals of intra-domain SAV in Figure 1, intra-domain SAV is commonly deployed on host-facing routers, customer-facing routers, and AS border routers. This section elaborates the key problems of SAV on host-facing or customer-facing routers and SAV on AS border routers, respectively. Since existing intra-domain SAV mechanisms either require high operational overhead or have limitations in accuracy, they will improperly block data packets using a legitimate source address (i.e., improper block) or improperly permit data packets using a spoofed source address (i.e., improper permit).¶

 +---------------------------------------------------------------+
 |                                                           AS  |
 |                         +----------+                          |
 |                         | Router 3 |                          |
 |FIB of Router 1          +----------+  FIB of Router 2         |
 |Dest           Next_hop    /      \    Dest           Next_hop |
 |192.0.2.128/25 Network 1  /        \   192.0.2.0/25   Network 1|
 |192.0.2.0/25   Router 3  /          \  192.0.2.128/25 Router 3 |
 |                  +----------+     +----------+                |
 |                  | Router 1 |     | Router 2 |                |
 |                  +-----+#+--+     +-+#+------+                |
 |                        /\           /                         |
 |     Traffic with        \          / Traffic with             |
 |     source IP addresses  \        /  destination IP addresses |
 |     of 192.0.2.0/25       \      \/  of 192.0.2.0/25          |
 |                      +----------------+                       |
 |                      |  Host/Customer |                       |
 |                      |    Network 1   |                       |
 |                      | (192.0.2.0/24) |                       |
 |                      +----------------+                       |
 |                                                               |
 +---------------------------------------------------------------+

 The legitimate traffic originated from Network 1 with source IP
 addresses of 192.0.2.0/25 will be improperly blocked by Router 1
 if Router 1 uses strict uRPF.

          +-------------------------+
          |          AS Y           | AS Y announces the route
          |   (where the anycast    | for anycast prefix P3
          |    server is located)   | in BGP
          +-----------+-------------+
                      |
                      |
          +-----------+-------------+
          |       Other ASes        |
          +-------------------------+
             /                    \
            /                      \
           /                        \
+------------------+   +---------------------------------------+
|      AS Z        |   |         +----------+             AS X |
| (where the user  |   |         | Router 4 |                  |
|    is located)   |   |         +----------+                  |
+------------------+   |              |                        |
                       |              |                        |
                       |         +----+-----+                  |
                       |         | Router 5 |                  |
                       |         +----#-----+                  |
                       |              /\    DSR responses with |
                       |              |     source IP addresses|
                       |              |     of P3              |
                       |       +---------------+               |
                       |       | Host/Customer |               |
                       |       |   Network 2   |               |
                       |       |     (P2)      |               |
                       |       +---------------+               |
                       | (where the edge server is located)    |
                       +---------------------------------------+
DSR response packets from edge server in Network 2 with
source IP addresses of P3 (i.e., the anycast prefix) will
be improperly blocked by Router 5 if Router 5 uses strict uRPF.

 Packets with +              Packets with +
 spoofed P1/P2|              spoofed P1/P2|
+-------------|---------------------------|---------+
|   AS        \/                          \/        |
|         +--+#+-----+               +---+#+----+   |
|         | Router 3 +---------------+ Router 4 |   |
|         +----------+               +----+-----+   |
|          /        \                     |         |
|         /          \                    |         |
|        /            \                   |         |
| +----------+     +----------+      +----+-----+   |
| | Router 1 |     | Router 2 |      | Router 5 |   |
| +----------+     +----------+      +----+-----+   |
|        \             /                  |         |
|         \           /                   |         |
|          \         /                    |         |
|       +---------------+         +-------+-------+ |
|       |   Customer    |         |     Host      | |
|       |   Network     |         |   Network     | |
|       |     (P1)      |         |     (P2)      | |
|       +---------------+         +---------------+ |
|                                                   |
+---------------------------------------------------+

4. Problem Statement

Accurate validation and low operational overhead are two important design goals of intra-domain SAV mechanisms. However, as analyzed above, existing intra-domain SAV mechanisms have problems of inaccurate validation or high operational overhead.¶

ACL-based ingress filtering relies on manual configurations and thus requires high operational overhead in dynamic networks. To guarantee accuracy of ACL-based SAV, network operators have to manually update the ACL-based filtering rules in time when the prefix or topology changes. Otherwise, improper block or improper permit problems may appear.¶

Strict uRPF can automatically update SAV rules, but may improperly block legitimate traffic under asymmetric routing scenario or hidden prefix scenario. The root cause is that strict uRPF uses the router's local FIB to determine the valid incoming interface for a specific source address, which may not match the real incoming direction from the source, due to the existence of asymmetric routes. Hence, it may mistakenly consider a valid incoming interface as invalid, resulting in improper block problems; or it may mistakenly consider an invalid incoming interface as valid, resulting in improper permit problems.¶

Loose uRPF is also an automated SAV mechanism but its SAV rules are overly loose. Most spoofed packets will be improperly permitted by loose uRPF.¶

5. Requirements for New SAV Mechanisms

This section lists the requirements which can be a guidance for narrowing the gaps of existing intra-domain SAV mechanisms. The requirements can be fully or partially fulfilled when designing new intra-domain SAV mechanisms. The new intra-domain SAV mechanisms SHOULD avoid data-plane packet modification. Existing architectures or protocols or mechanisms can be used in the new SAV mechanisms to achieve better SAV function.¶

6. Security Considerations

The new intra-domain SAV mechanisms should not introduce additional security vulnerabilities or confusion to the existing intra-domain architectures or control or management plane protocols.¶

Similar to the security scope of intra-domain routing protocols, intra-domain SAV mechanisms should ensure integrity and authentication of protocol messages that deliver the required SAV information, and consider avoiding unintentional misconfiguration. It is not necessary to provide protection against compromised or malicious intra-domain routers which poison existing control or management plane protocols. Compromised or malicious intra-domain routers may not only affect SAV, but also disrupt the whole intra-domain routing domain. Security solutions to prevent these attacks are beyond the capability of intra-domain SAV.¶

7. IANA Considerations

This document does not request any IANA allocations.¶

8. Acknowledgements

Many thanks to the valuable comments from: Jared Mauch, Barry Greene, Fang Gao, Kotikalapudi Sriram, Anthony Somerset, Yuanyuan Zhang, Igor Lubashev, Alvaro Retana, Joel Halpern, Aijun Wang, Michael Richardson, Li Chen, Gert Doering, Mingxing Liu, Libin Liu, John O'Brien, Roland Dobbins, Xiangqing Chang, Tony Przygienda, Yingzhen Qu, Changwang Lin, etc.¶

9. References

Authors' Addresses