[VulnWatch] Algorithmic Complexity Attacks and the Linux Networking Code
From: Florian Weimer (fw_at_deneb.enyo.de)
To: firstname.lastname@example.org Date: Sat, 17 May 2003 23:12:58 +0200
Algorithmic Complexity Attacks and the Linux Networking Code
The Linux networking code makes extensive use of hash tables to
implement caches to support packet classification. One of these
caches, the routing cache, can be used to mount effective denial of
service attacks, using an algorithmic complexity attack.
The Linux Routing Cache
The routing cache (or "dst cache") caches routing decisions for a
traffic flow. A traffic flow consists of packets which have the
same IPv4 source and destination address and the same TOS value in
the IP header. These flows are unidirectional; for a two-way
communication, two flows exist, one in each direction. Even if the
cache is also called "dst cache" for historical reasons, the cache
covers more than just destination addresses.
When a packet arrives, the kernel must route it. The IP routing
code checks for a suitable traffic flow and reuses the cached
routing decisions, if possible. Otherwise, it makes a new routing
decision and creates a new traffic flow, by updating the routing
cache accordingly. This routing occurs on single-homed host with
disabled IP forwarding as well as on full-table routers.
The routing cache is implemented as a hash table, in a rather
particular way. The bucket count is an integral power of two which
is fixed on system boot and scaled according to the amount of
physical RAM. The hash function is GF(2)-linear (which means that
it is easy to find collisions). Collision chaining is used to
store different entries which hash to the same bucket. A garbage
collection mechanism ensures that the size of the cache stays below
the configured maximum entry count. This entry count is scaled
with available system memory, too.
Note that there are additional hash tables in the networking code.
For example, IP connection tracking adds an additional hash table
(which uses a different, but still rather weak hash table).
Our attack is targeted at a host and uses packets with carefully
chosen source addresses and TOS values to trigger collisions in the
lower bits of the routing cache hash function. (Note that these
collisions have nothing to do with colliding packets on the wire.)
As a result, all these packets create distinct flows which are
stored in a linear list hooked to a single bucket to a hash table.
In essence, this reduces the hash table to a linear list, and
finding entries becomes extremely expensive when the list is very
long. (This effect is detailed in the paper cited below.)
The effectiveness of the attack depends significantly on the
maximum size of routing cache. As described above, the default
maximum size depends on the amount of physical RAM present in the
machine. Therefore, machines with more RAM are more vulnerable if
they operate in the default configuration. For example, we were
able to freeze a machine with four gigabytes of RAM with a stream
of about 400 packets per second. (The same machine remained
unaffected when we used random source addresses instead of source
addresses that lead to collisions in the hash function.)
Of course, the hash function is extremely simple, but this is not
source of the problem. Even though it is possible to find
collisions by solving a rather small system of linear equations
over the field of two elements, the main problem results from the
fact that the attacker can determine the hash bucket for traffic
flows (and send packets based on that information).
Red Hat published a security advisory which includes a patch (see
below) which changes the hash function to a non-linear, keyed hash
function. While the the hash function is not cryptographically
strong, it is certainly much more complicated (if not even
impossible) for an attacker to trigger collisions. (As an
additional protection, the key is changed every ten minutes.) In
our experience, the patch, applied to stock Linux 2.4.20, works
reasonably well in typical denial of service situations.
If you cannot apply the patch and are confronted with an attack of
this type, there are two options to protect machines: setting rate
limits using iptables, or decreasing the routing cache size.
Choosing suitable rate limits is very complicated, so it is not
recommended. You can decrease the routing cache size using the
/proc interface. (If the total size of the cache is reduced, the
maximum length of a collision chain is reduced, too, and this
particular attack is no longer possible.) As root, run the
# echo 4096 > /proc/sys/net/ipv4/route/max_size
# echo 2048 > /proc/sys/net/ipv4/route/gc_thresh
(On most systems, you can edit /etc/sysctl.conf to make these
However, note that this approach of decreasing the cache size has a
severe impact on routing performance if the number of parallel
flows processed by the machine exceeds the maximum routing cache
Frequently Asked Questions
* How significant is this problem?
The Linux IP stack is not very robust against various types of
denial of service attacks. As a result, this problem is
unlikely to have any practical consequences. We recommend to
apply the patch to fix this problem during routine maintenance
and not to change the maximum routing cache size preventively
because of the potential performance impact.
* Are other vendors affected by the problem?
At this time, we do not believe that Cisco IOS routers or
machines running Solaris or one of the BSD variants are
affected. However, only source code inspection can reveal if a
product is affected, and vendors are encouraged to verify that
their products are unaffected. Flow-based routing using hash
tables is particularly prone to this vulnerability, and
implementations of this principle should therefore be
* Is exploit code available publicly?
To our knowledge, this kind of attack is currently (May 2003)
not in the wild, and no widely available attack tools support
* Does this attack affect only affects routers?
No, it is also relevant for hosts. The routing cache includes
both source and destination addresses, and it is possible to
spoof source addresses accordingly. However, routers are at
somewhat greater risk because to attack them, you can choose
the destination addresses in a way that trigger collisions
which does not require root privileges (or special IP packet
generation code) on the attacking host.
* I've read that it is impossible to spoof source addresses on the
current Internet, thanks to ingress filtering, so this attack is
not a problem, right?
While proper ingress (and egress) filtering is a standard
practice to reduce source address spoofing, it isn't
universally applied throughout the Internet. A lot of denial
of service attacks still use spoofed source addresses and
arrive at the intended victim.
* Is it possible to use rate limits to counter the attack?
It is possible, but not recommended. To protect machines which
large amounts of memory in default configurations, ridiculously
low rate limits would be required which would enable denial of
service attacks on their own. Note that all rate limits which
protect the routing cache have to be applied in the PREROUTING
chain, as the standard INPUT chain is processed after a packet
has already updated the routing cache.
* Netfilter connection tracking uses a huge hash table as well.
Is it affected?
Yes, we believe that it is affected by the essentially same
problem. The Red Hat patch corrects Netfilter connection
* Will the Red Hat patch fix other performance issues with the
Unfortunately, the answer is no. We now have multiple reports
that Linux routers break down according to the inefficiency of
the routing cache under stress, at incredible low packet rates.
These problems continue to exist and are likely to persist
until the kernel developers eliminate the routing cache.
* Why took it so long before this bug was fixed?
Kernel developers were contacted at the beginning of April,
when the issue was independently discovered in the Linux
kernel, not in February, when the first technical report was
written by Scott Crosby and Dan Wallach.
* Scott A. Crosby, Dan S. Wallach, Denial of Service via
Algorithmic Complexity Attacks
* Red Hat, Updated 2.4 kernel fixes security vulnerabilities and
* The patch for Linux 2.4.20 that has been published by Red Hat
* Most current version of this document