22.2. Routing ProtocolsRouting protocols are used to distribute information about where packets should be directed. Any given device can pass a packet only to another device on the same local area network. If the packet's destination is on a different local area network, the device must know what machine can move it to the next network, on the way to its destination. Most desktop machines have very little use for routing protocols -- usually, a desktop machine has a single network interface and attaches to a network with only one cross-connection to other networks. Thus, a desktop machine needs to know only "If the packet isn't bound for a machine on this network, hand it to that address over there". However, bastion hosts and Internet-connected gateways may be keeping track of much more complex situations, involving multiple network interfaces, and multiple connections from those networks to others.
Routing protocols in general are dangerous. Attackers who can send you bad routing information not only have an easy way of performing a denial of service attack (if you route your packets the wrong places, you can't talk to anybody), but also have a much easier time of intercepting data (they can get you to send data to them so they can read it on the way past). Furthermore, routing protocols tend to be old. Many of the routing protocols in use today were designed when the Internet was a kinder, gentler, and smaller place, and the idea that people might intentionally lie to you about routes had never occurred to anybody, so there is no provision for preventing it.
It's actually quite difficult to design a routing protocol that's secure and is still usable for routing on the Internet backbone, since the protocol needs to move quite large amounts of data, including frequent changes, between hosts that are already busy. Because the backbone routers are specialized devices, not general-purpose computers, and because routing problems on the backbone can cause widespread disruption, changes to backbone routing protocols have to be made very slowly and carefully.
Fortunately, the routing protocols currently used on the backbone are different from the protocols used within individual sites. We do not discuss protocols that are used between distinct entities across the backbone because these protocols do not usually cross firewalls. Instead, we discuss the protocols most commonly used for routing within networks (often called interior gateway protocols), which you may need to use across internal firewalls.
22.2.1. Routing Information Protocol (RIP)RIP is the oldest routing protocol on the Internet. In fact, it predates IP; it was taken almost verbatim from the older Xerox Network Services (XNS) system. It is also still a very commonly used routing protocol on local area IP networks. Routers (including general-purpose machines with multiple interfaces that can act as routers) use RIP to periodically broadcast which networks they know how to reach and how far away those networks are. By listening to these broadcasts, a router or host can determine what networks are reachable and choose the best (shortest) path to each. RIP servers generally just broadcast this information every 30 seconds or so for anyone interested to hear, but a RIP client can request a special update from a RIP server, which will cause the server to reply directly to the client with the information requested.
By default, RIP is completely insecure; clients simply accept any data they are sent. RIP does not provide any place in the protocol for authentication. There have been security problems with RIP clients because it is possible for RIP packets to contain not only routing information, but also the request to turn on logging to a specified file. Some Unix RIP clients were willing to accept such requests; since routing daemons have to run as root, they were then willing and able to overwrite any file on the system with a log of routing updates, which is not a useful substitute for most system files. RIP clients are no longer routinely configured to accept such requests.
any RIP implementations will allow you to configure a RIP client with slight security improvements; for instance, they will allow you to specify that you are willing to accept RIP updates only from certain IP source addresses, and/or they will allow you to declare that certain routing information cannot be modified by RIP updates. This is usually sufficient to protect clients from local misconfigurations but not sufficient to protect against active hostile acts.
A modified version of RIP, called RIP-2, provides several improvements to the routing information that's distributed and also allows for the use of passwords. Unfortunately, the normal method of using passwords is to put the same 16-character password in every packet. Again, this is sufficient to protect clients from local misconfigurations (you're unlikely to get the right password by accident) but not against hostile acts (any attacker can read the password out of any valid RIP broadcast and use it). It is easier to maintain than a list of valid IP source addresses. RIP-2 implementations that support MD5 authentication are becoming more widely available, and this authentication actually provides a reasonable amount of protection.
RIP-2 and RIP implementations can interoperate, but RIP implementations won't verify or attach the password. As a result, if you're using RIP-2 with passwords, routers that implement RIP can receive routing updates but cannot successfully send them.
126.96.36.199. Packet filtering characteristics of RIPRIP is a UDP-based service. RIP servers listen on port 520 for broadcasts from other servers and requests from clients. RIP servers generally send their broadcasts from port 520. RIP clients generally use ports above 1023.
22.2.2. Open Shortest Path First (OSPF)OSPF is a more recent protocol than RIP that uses a completely different approach to distributing routing information. The main difference between the two protocols involves the kinds of routing information they distribute, but there are also significant differences in the ways they use the network. RIP is based on UDP, while OSPF is based directly on IP; RIP uses unicast and broadcast, while OSPF uses unicast and multicast.
OSPF supports authentication, which could theoretically be quite secure -- the protocol allows for cryptographic message digests. However, the cryptographic message digest algorithm is not specified by the standard, so in practice OSPF authentication is restricted to eight-character cleartext passwords or the same degree of authentication as RIP-2. This will protect from accidental misconfigurations but not from hostile attacks.
188.8.131.52. Packet filtering characteristics of OSPFOSPF is layered directly on IP, not on TCP or UDP, and is IP protocol 89. OSPF uses both multicast and unicast packets. OSPF uses two special multicast groups, AllSPFRouters (184.108.40.206) and AllDRouters (220.127.116.11). OSPF does not have port numbers but does distinguish between different messages with a packet type identifier.
22.2.3. Internet Group Management Protocol (IGMP)IGMP is used to manage multicast groups. See Chapter 19, "Real-Time Conferencing Services", for more information about multicast. Multicasting is also increasingly being used by administrative protocols; for example, it's used for WINS server replication.
ulticast routers do not forward all multicast packets to all networks; they forward multicast packets only to places where hosts are listening for them. In order to make this decision, a multicast router has to keep track of the multicast groups in use. Since multicast packets go to all the hosts on a network segment that want them, the router doesn't need to identify all the hosts that are in a group, but it does need to know, for each network segment, what groups are of interest. IGMP is the protocol that hosts and routers use to communicate this information.
ulticast routers receive all multicast packets, regardless of the multicast address they are sent to. Hosts that use multicast receive packets only for groups they subscribe to, but all of them subscribe to a group called AllSystems (18.104.22.168). All IGMP packets are sent out with a TTL of 1, which means that they will not be forwarded through a router. This makes sense because the purpose of IGMP is to configure a router's information about a particular, directly attached network segment.
There are two parts to the IGMP process: first, hosts send out notifications, called membership reports, when they join any group other than AllSystems (and in some versions, when they leave those groups as well). Second, routers can send out periodic queries asking about group membership. A router can ask for information either about all groups, or about a particular group. In either case, hosts respond with membership reports, just like the ones they send when they initially join groups. The protocol is designed so that only one host per group will respond. All the router needs to know is whether or not there is interest in the group; it doesn't need to know how many hosts are interested.
22.214.171.124. Packet filtering characteristics of IGMPIGMP is based directly on IP and is protocol 2. IGMP does not have port numbers; IGMP packets are distinguished by packet types. IGMP is a datagram protocol that does not have an ACK bit or its equivalent. All IGMP packets are sent with a TTL of 1 and will not pass through a packet filter unless it is completely transparent or is specially configured to pass IGMP without decreasing the TTL. You would pass IGMP through a packet filter only if the packet filter was between a multicast router and the hosts it serves, and the packet filter was also going to pass all other multicast. In this situation, it would probably be better to either tunnel multicast or configure the packet filter to be a multicast router.
22.2.4. Router Discovery/ICMP Router Discovery Protocol (IRDP)Router discovery, also known as ICMP Router Discovery Protocol (IRDP), is not actually a routing protocol. It is a mechanism that hosts can use to find routers, independent of any routing protocol. Router discovery allows a host to ask what routers are on its network, using something called a router solicitation, and get back an answer from each router. The routers can also provide information about relative desirability. That is, each router sends a number indicating how good it thinks it is, configured by the administrator who set up the router. The host can then use this information to pick a router. That's all the information the host gets from the router; there's no information about what networks the router is connected to, for instance.
In addition to responding to requests from hosts, routers send out the same information periodically. Whether or not it's been requested, it is called a router announcement, and hosts are supposed to treat unsolicited announcements the same way they treat announcements they've asked for.
Router discovery is a useful way for hosts to find default routers without needing to implement complicated routing protocols. However, it contains no authentication information at all, allowing attackers to send out router announcements that will divert traffic. If the attacker is on the network being attacked, those announcements could divert traffic to where the attacker could read or modify it. If the attacker doesn't have a point of presence on the network, there's less benefit to the attacker. Denial of service attacks are certainly possible, and in a few cases, an attacker might be able to divert traffic to another network that the attacker was present on.
Router discovery is not widely implemented, and most hosts that use it do so as a supplement to other ways of configuring routing. You may therefore have hosts that are using router discovery without knowing it. These hosts will already have routes configured by some other means (using RIP announcements, DHCP, or simply having some human type them in somewhere). How these hosts treat router announcements is entirely implementation dependent. Many of them will use announced routers instead of routers they knew about from other sources; others will apply a ranking based on the information in the announcement; and some of them will prefer preconfigured routers to announced routers.
There is absolutely no reason for router discovery to ever go through a router. Router discovery is intended only to convey information about the local network. It is therefore safe and advisable to filter it out in all packet filtering routers. You will also want to turn off router discovery on bastion hosts, in order to be sure that they are not going to pay attention to invalid announcements if other bastion hosts are compromised.
126.96.36.199. Packet filtering characteristics of router discoveryRouter discovery is based on ICMP. Router discovery is supposed to be multicast-based, but it is allowable (although strongly discouraged) for hosts to use broadcast instead of multicast.
22.2.5. Proxying Characteristics of Routing ProtocolsBecause routing protocols are used by a host to develop routing tables that are specific to where that host is in the network, it doesn't make any sense to proxy routing protocols to hosts elsewhere in the network.
22.2.6. Network Address Translation Characteristics of Routing ProtocolsRouting protocols transfer information that's highly dependent on network addresses and is certain to be pointless if it passes through a network address translator. It doesn't make any sense to run network address translation on routing protocols.
22.2.7. Summary of Recommendations for Routing Protocols
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