Saturday, May 10, 2008

DDOS Mitigation functionality - Tips for Admin

There is some discussion going on in focus-ids mailing list on DDOS attack mitigation. That discussion prompted me to write this article. ISIC, UDPSIC, TCPSIC, ICMPSIC are some tools used to measure the network security device effectiveness of detection and mitigation of DDOS attacks. As we all know one of the main intentions of DDOS attacks is to make the service, network or target unavailable. By looking at the packets, you can't see the difference between normal genuine packets and packets generated by DDOS attacks. This makes it difficult to stop these attacks based on signature based methods.

One of the properties of many of DDOS attacks is that they try to make the discovery of source of attack difficult to find. There are two types of DDOS attacks that are common.
  • Spoofing of source IP address in the packets: DDOS attacks are generated by spoofing the source IP address of the packet. ISIC, UDPSIC, TCPSIC and ICMPSIC tools simulate these kinds of attacks. Any packet that is sent back to the source does not reach the attacker. Due to this, TCP based sessions don't get established. Note that non-TCP sessions don't have connection establishment phase.
  • Botnets : The attacker instructs the agents which were installed on compromised hosts across the globe to bombard the target. Attacker keeps changing the hosts that attack the target. Thereby, in effect making the source discovery ineffective.
Now, in addition to Botnets, there is a third kind of DDOS attack. Recent DDOS attack on by Chinese hackers is one example. Here too, sources are known, but there are many. Note that here the attack on is not generated by botnets, but supposedly by out of patriotism. Some in China felt that CNN is biased in its reporting on Olympic torch and its linkage to Tibet religious freedom. As I understand, it was simple attack, where it connects to website and accesses some URL for every 2 seconds. This attack executable was distributed and many home users in China, out of patriotism, executed it.

DDOS attack incident detection may be easier, but mitigation is difficult. If the intention of the attack is to consume the bandwidth of target site, there is nothing much the target network administrator can do. Target company/organization needs to depend on its ISP to block the flood of packets. Gathering as much information as possible and providing that information to ISP is one of the things the administrators can do.

The current trend of DDOS attacks go beyond consuming the link bandwidth. With less number of hosts participating in the DDOS attack, these attacks consume the CPU, memory bandwidth of target networks/servers. I feel the network security appliances providing DDOS attack mitigation functionality can help in this scenario. It can not only provide detection, but can stop bombardment of servers.

There are multiple products *DDOS mitigators* in the market claiming to solve some of above problems. Many IPS boxes also support this feature.

If you are hosting some servers, you can be a victim. As an administrator, I look for following features from these appliances.

DDOS attack consumes 1Mbps link by making 512 connections/sec (approximately) . Any DDOS mitigator, ideally should be able to process 512 connections in every second for 1Mbps link. If the connection is maintained for 20 seconds (which is typical), then the connection capacity needs to be 10K. For 100Mbps link, DDOS attack mitigation appliance needs to support 51200 connections/sec and should have 1M session capacity. With this capacity and connection rate, it can do better job of protecting internal networks/servers/other stateful security devices without itself getting bogged down.

DDOS mitigators are expected to limit the amount of traffic that goes to the internal servers/machines/networks etc.. Each resource in the network would have some limitations on how much traffic, connections, connection/sec it can take. Adminis, once they make a list of resources and their limitations, should be able to configure DDOS mitigators. DDOS mitigators must ensure that the resources are not flooded and it should shape the traffic accordingly. DDOS mitigators need to provide features like:
  • Ability to configure
    • connections/sec
    • Packets/sec
    • Bytes/sec
      • On per resource basis - Server/machine basis, Network basis
      • From a given source with respect to IP address range, Subnet.
  • Ability configure to filter traffic on combination of 5 tuples.
As you have observed, admins not only would like to shape the traffic to internal resources with respect to connections/sec, maximum number of connections and throughput, but also would like to have these limits from particular source(s). Yet times, I also observe that there is a requirement to limit the amount of traffic within each 5 tuple connection, between any IP address combination. Mitigators need to provide this flexibility without expecting admins to create many rules. Many times, it is not possible to create rules with all combinations of IP addresses. DDOS mitigators need to provide flexibility of creating rules with ranges, subnets along with provision to configure granularity to apply the specified traffic rates. For example, admin should be able to configure ANY to 'Internal HTTP Server IP addresses' with 10 connections/sec for every combination of source IP and destination IP. If there are 100 different sources are trying to access internal HTTP Servers, DDOS functionality should be able to rate limit the number of connections to 10/sec for each of sources independently.

As with any security device, it must also support multiple zones and provide flexibility with respect to zones. In case of hosting environments, provider may be servicing multiple customers. So, virtual instance, with each instance belonging to a customer is needed. In case of Enterprise environments, normally only one virtual instance would be used.

Flexibility is expected to be provided to disable limiting of traffic for some source networks. These networks could be networks belonging to remote offices. This feature is called white listing.

Ofcourse, it is expected that DDOS mitigators provides facilities to stop half open connections by providing TCP syn flood protection, UDP based session exhaust protection facilities, facilities to configure service inactivity timeouts for interactive protocols etc..

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