The Latest In Denial Of Service Attacks:  “Smurfing”
Description and Information to Minimize Effects
Craig A. Huegen <>
Cisco Systems, Inc.
NANOG 11 Interprovider Operations BOF

Description of “Smurfing”
Newest DoS attack
Network-based, fills access pipes
Uses ICMP echo/reply packets with broadcast networks to multiply traffic
Requires the ability to send spoofed packets
Abuses “bounce-sites” to attack victims
Traffic multiplied by a factor of 50 to 200

Description of Smurfing (cont’d)

Multiplied Bandwidth
Perpetrator has T1 bandwidth available (typically a cracked account), and uses half of it (768 Kbps) to send spoofed packets, half to bounce site 1, half to bounce site 2
Bounce site 1 has a switched co-location network of 80 hosts and T3 connection to net
Bounce site 2 has a switched co-location network of 100 hosts and T3 connection to net
(384 Kbps * 80 hosts) = 30 Mbps outbound traffic for bounce site 1
(384 Kbps * 100 hosts) = 37.5 Mbps outbound traffic for bounce site 2
Victim is pounded with 67.5 Mbps (!) from half a T1!

Profiles of Participants
Typical Perpetrators
Cracked superuser account on well-connected enterprise network
Superuser account on university residence hall network (Ethernet)
Typical PPP dial-up account (for smaller targets)
Typical Bounce Sites
Large co-location subnets
Large switched enterprise subnets
Typically scanned for large numbers of responding hosts
Typical Victims
IRC Users, Operators, and Servers
Providers who eliminate troublesome users’ accounts

Prevention Techniques
How to prevent your network from being the source of the attack:
Apply filters to each customer network
Allow only those packets with source addresses within the customer’s assigned netblocks
Apply filters to your upstreams
Allow only those packets with source addresses within your netblocks to protect others
Deny those packets with source addresses within your netblocks to protect yourself
This also prevents other forms of attacks as well

Prevention Techniques
How to prevent being a “bounce site”:
Turn off directed broadcasts to subnets with 5 hosts or more
Cisco:  Interface command “no ip directed-broadcast”
Proteon:  IP protocol configuration “disable directed-broadcast”
Bay Networks:  Set a false static ARP address for bcast address
Use access control lists (if necessary) to prevent ICMP echo requests from entering your network
Probably not an elegant solution; makes troubleshooting difficult
Encourage vendors to turn off replies for ICMP echos to broadcast addresses
Host Requirements RFC-1122 Section states “An ICMP Echo Request destined to an IP broadcast or IP multicast address MAY be silently discarded.”
Patches are available for free UNIX-ish operating systems.

Prevention Techniques
If you do become a bounce site:
Trace the traffic streams to the edge of your network, and work with your upstream or peer in order to track the stream further
MCI’s DoSTracker tool
Manual tracing/logging tips

Prevention Techniques
How to suppress an attack if you’re the victim:
Implement ACL’s at network edges to block ICMP echo responses to your high-visibility hosts, such as IRC servers
Again, will impair troubleshooting -- “ping” breaks
Will still allow your access pipes to fill
Work with upstream providers to determine the help they can provide to you
Blocking ICMP echoes for high-visibility hosts from coming through your access pipes
Tracing attacks

Prevention Techniques
Technical help tips for Cisco routers:
BugID CSCdj35407 - “fast drop” ACL code
This bug fix optimizes the way that packets denied by an ACL are dropped within IOS, reducing CPU utilization for large amounts of denied traffic.
First major release of integration is 11.1(14)CA
Not available in 11.2 yet, but coming
BugID CSCdj35856 - ACL logging throttles
This bug fix places a throttle in IOS which will allow a user to specify the rate at which logging will take place of packets which match a condition in an ACL where “log” or “log-input” is specified.
First maintenance release of integration is 11.1(14.1)CA
Not available in 11.2 yet, but coming

White paper on “smurf” attacks:
Ingress filtering:
MCI’s DoSTracker tool:
Other DoS attacks:
“Defining Strategies to Protect Against TCP SYN Denial of Service Attacks”
“Defining Strategies to Protect Against UDP Diagnostic Port Denial of Service Attacks”

Craig Huegen