2003-10-06 08:59:54 UTC
properties of the Internet or any other large network to create
a reliable, volatile distributed data storage of a large capacity.
Juggling with packets: floating data storage
"Your dungeon is built on an incline. Angry monsters can't play
Wojciech Purczynski <***@isec.pl>
Michal Zalewski <***@coredump.cx>
1) Juggle with oranges!
Most of us, the authors of this paper, have attempted to juggle with
three or more apples, oranges, or other fragile ballistic objects.
The effect is usually rather pathetic, but most adept juggler padawans
sooner or later learn to do it without inflicting excessive collateral
A particularly bright juggler trainee may notice that, as long as he
continues to follow a simple procedure, at least one of the objects is
in the air at all times, and that he has to hold at most two objects
in his hands at once. Yet, each and every apple goes thru his hands
every once in a while, and it is possible for him to recover it at
He may then decide the entire process is extremely boring and go back
to his computer. And while checking his mail, an educated juggler may
notice that a typical network service has but one duty: to accept and
process data coming from a remote system, and take whatever steps it
deems appropriate based on its interpretation of the data. Many of
those services do their best to behave robustly and be fault-tolerant
- and, why not, to supply useful feedback about the transaction.
In some cases, the mere fact a service is attempting to process the
data and reply conforming to the protocol can be used in the ways
authors never dreamed of. One of more spectacular examples, which our
fellow juggler may be familiar with, is a research done at the
University of Notre Dame, titled "Parasitic Computing" and published
in letters to "Nature". The researchers have attempted to use TCP/IP
checksum generation algorithm to perform boolean operations necessary
to solve non-polynomial problems.
Nevertheless, such attempts were quite impractical in the real world,
concludes our hero; the cost of preparing and delivering a trivia to
be solved far exceeds any eventual gain - the sender has to perform
operations of comparable computational complexity to even deliver the
request. The computing power of such a device is puny! - we hear him
A real juggler would focus on a different kind of outsourced data
processing, one that is much closer to his domain of expertise. Why,
distributed parasitic data storage, of course. - What if I write
a single letter on every orange, and then start juggling? I can then
store more orange-bytes than my physical capacity (the number of
oranges I can hold in my hands) is! How brilliant... But, but, would
it work without oranges?
2) The same, without oranges
The basic premise for this paper is the observation that for all
network communications, there is a non-zero (and often considerable)
delay between the act of sending an information and receiving a reply.
The effect should be contributed to the physical constrains of the
medium, and to the data processing times in all computer equipment.
A packet storing a piece of data, just like an orange with a message
written on it, once pushed travels for a period of time before coming
back to the source - and for this period of time, we can safely
forget its message without losing data.
As such, the Internet has a non-zero momentary data storage capacity.
It is possible to push out a piece of information and effectively have
it stored until echoed back. By establishing a mechanism for cyclic
transmission and reception of chunks of data to and from a number of
remote hosts, it is possible to maintain an arbitrary amount of data
constantly `on the wire', thus establishing a high-capacity volatile
The medium can be used for memory-expensive operations, as a regular
storage, or for handling certain types of sensitive data that are
expected not to leave a physical trail on a hard disk or other
Since it is not considered a bad programming practice to send back as
much relevant information as the sender had sent to the service, and
many services or stacks maintain a high level of verbosity, based on
our juggling experience, we conclude it is not only possible, but also
feasible to establish this kind of storage, even over a low-end
As opposed to traditional methods of parasitic data storage (P2P
abuse, open FTP servers, binary Usenet postings), the use of this
medium may or may not leave a trail of data (depending on a solution
used) and specifically does not put any single system under
a noticable load. The likehood of this technique being detected,
considered an abuse, and the data being removed, is much lower.
3) Class A data storage: memory buffers
Class A data storage uses the capacity inherent to communication
delays during the transmission and processing of live data. The
information remains cached in the memory of a remote machine, and is
not likely to be swapped out to a disk device.
Examples rely on sending a message that is known to result in
partial or full echo of the original request, and include:
- SYN+ACK, RST+ACK responses to SYN packets and other bounces,
- ICMP echo replies,
- DNS lookup responses and cache data. It is possible to store some
information in a lookup request and have it bounced back with
a NXDomain reply; or to store data in NS cache,
- Cross-server chat network message relaying. Relaying text messages
across IRC servers and so on can exhibit considerable latency,
- HTTP, FTP, web proxy or SMTP error or status replies,
Most important properties of class A storage are:
- Low latency (miliseconds to minutes), making it more useful
for near random access memory applications,
- Lower per-system capacity (usually kilobytes), making it less
suitable for massive storage,
- Only one chance to receive, or few retransmits, making it
less reliable in case of a network failure,
- Data not likely to be stored on a non-volatile medium or swapped
out, increasing privacy and deniability.
In particular, when using higher level protocols, additional features
appear which may solve some of above disadvantages. It is possible to
establish a connection to a service such as SMTP, FTP, HTTP or any
other text-based service, and send a command that is known to result
in an acknowledgment or error message being echoed along with part of
the original data. We do not, however, send full formatted message,
leaving some necessary characters unsent. In most cases end-of-line
characters are required in order the command to be completed. In this
state, our data is already stored on remote service waiting for
a complete command or until connection timeout occurs.
To prevent timeouts, either on TCP or application level, no-op packets
need to be sent periodically. \0 character interpreted as an empty
string has no effect on many services but is sufficient to reset TCP
and service timeout timers. A prominent example of an application
vulnerable to this attack is Microsoft Exchange.
The attacker can sustain the connection for an arbitrary amount of
time, with a piece of data already stored at the other end. To recover
the information, the command must be completed with missing \r\n and
then response is send to the client.
A good example is SMTP VRFY command:
220 inet-imc-01.redmond.corp.microsoft.com Microsoft.com ESMTP Server
Thu, 2 Oct 2003 15:13:22 -0700
252 2.1.5 Cannot VRFY user, but will take message for
It is possible to store just over 300 bytes, including non-printable
characters, this way - and have it available almost instantly.
More data may be stored if HTTP TRACE method is used with data passed
in arbitrary HTTP headers, depending on the server software.
Sustained connections may give us arbitrarily high latency, thus
making large storage capacity.
This type of storage is naturally more suited for privacy-critical
applications or low-latency lower to medium capacity storage
(immediate RAM-extending storage for information that should leave no
The storage is not suitable for critical data that should be preserved
at all costs, due to the risk of data being lost on network failure.
4) Class B data storage: disk queues
Class B data storage uses "idle" data queues that are used to store
information for an extended period of time (often on the disk).
Particularly on MTA systems mail messages are queued for up to 7 days
(or more, depending on the configuration). This feature may give us
a long delay between sending data to store on remote host and
receiving it back.
As a typical SMTP server prevents to relay mail from the client to
himself, mail bounces may be used to get data returned after long
period of time.
An example attack scenario:
1. The user builds a list of SMTP servers (perhaps ones that provide
a reasonable expectation of being beyond the reach of his foes),
2. The user blocks (with block/drop, not reject) all incoming
connections to his port 25.
3. For each server, the attacker has to confirm its delivery timeouts
and the IP from which the server connects back while trying to
return a bounce. This is done by sending an appropriate probe to
an address local to the server (or requesting a DSN notification
for a valid address) and checking how long the server tries to
connect back before giving up. The server does not have to be an
4. After confirming targets, the attacker starts sending data at
a pace chosen so that the process is spread evenly over the
period of one week. The data should be divided so that there
is one chunk per each server. Every chunk is sent to a separate
server to immediately generate a bounce back to the sender.
5. The process of maintaining the data boils down to accepting
an incoming connection and receiving the return at most a week
from the initial submission, just before the entry is about
to be removed from the queue. This is done by allowing this
particular server to go thru the firewall. Immediately after
receiving a chunk, it is relayed back.
6. To access any portion of data, the attacker has to look up which
MTA is holding this specific block, then allow this IP to connect
and deliver the bounce. There are three possible scenarios:
- If the remote MTA supports ETRN command, the delivery can be
- If the remote MTA was in the middle of a three-minute run in
attempt to connect to a local system (keeps retrying thanks to
the fact its SYN packets are dropped, not rejected with RST+ACK),
the connection can be established in a matter of seconds,
- Otherwise, it is necessary to wait between 5 minutes and
an hour, depending on queue settings.
This scheme can be enhanced using DNS names instead of IPs for users
on dynamic IP or to provide an additional protection (or when it is
necessary to cut the chain immediately).
Important properties of class B storage:
- High per-system capacity (megabytes), making it a perfect solution
for storing large files and so on,
- Higher access latency (minutes to hours), likening it to a tape
device, not RAM (with exceptions of SMTP hosts that accept ETRN
command to immediately re-attempt delivery),
- Very long lifetime, increasing per-user capacity and reliability,
- Plenty of delivery attempts, making it easy to recover the data
even after temporary network or hardware problems,
- Likely to leave a trace on the storage devices, making it a less
useful solution for fully deniable storage (although it would
still require examining a number of foreign systems, which does
not have to be feasible).
Class B storage is suitable for storing regular file archives,
large append-only buffers, encrypted resources (with a proper
selection of hosts, it remains practically deniable), etc.
5) Discreet class A storage
In certain situations, it might be necessary to advise a solution for
discreet data storage that is not stored on the machine itself, and
makes it possible to deny the presence of this information on any
The basic requirement is that the data is:
- not being delivered back until a special key sequence
- permanently discarded without leaving any record on any
non-volatile storage media in absence of keep-alive
It is possible to use class A storage to implement this functionality
using the sustained command method discussed above. The proper TCP
sequence number is necessary to release the data, and until this
sequence is delivered, the data is not sent back or disclosed to any
party. If the client node goes offline, the data is discarded and
The sequence number is thus the key to the stored information,
and, granted the lifetime of the data is fairly short when
keep-alive \0s stop coming, it is often an adequate protection.
6) User-accessible capacity
In this section, we attempt to estimate the capacity available to
a single user.
To maintain a constant amount of data "outsourced" to the network, it
is required to receive and send it back on a regular basis.
The time data may be stored remotely is defined by maximum lifetime
Tmax of a single packet (including packet queuing and processing
delays). The maximum amount of data that may be sent is limited by
maximum available network bandwith (L).
Thus, the maximum capacity can be defined as:
Cmax [bytes] = L [bytes/second] * Tmax [seconds] / Psize * Dsize
Dsize - size of a packet required to store an initial portion of
data on remote host
Psize - size of a packet required to sustain the information
stored on remote host
Psize and Dsize are equal and thus can be omitted whenever the
entire chunk of data is bounced back and forth, and differ only
for "sustained command" scenarios. The smallest TCP/IP packet to
accomplish this would have 41 bytes. The maximum amount of data
that can be sustained using HTTP headers is about 4096 bytes.
That all, in turn, gives us the following chart:
Bandwith | Class A | Class B
28.8 kbps | 105 MB | 2 GB
256 kbps | 936 MB | 18 GB
2 Mbps | 7.3 GB | 147 GB
100 Mbps | 365 GB | 7 TB
7) Internet as a whole
In this section, we attempt to estimate the theoretical momentary
capacity of the Internet as a whole.
To estimate theoretical class A storage capacity of the Internet,
we assumed that:
- ICMP messages are the best compromise between storage
capacity and preserving remote system's resources,
- An average operating system has a packet input queue capable
of holding at least 64 packets,
- The default path MTU is around 1500 (the most common MTU).
We have taken the estimated number of hosts on the Internet from ISC
survey for 2003, which listed a count of 171,638,297 systems with
reverse DNS entries. Not all IPs with reverse DNS have to be
operational. To take this into account, we have used ICMP echo
response ratio calculated from the last survey that performed such
a test (1999). The data suggested that approximately 20% of visible
systems were alive. This sets the number of systems ready to respond
ICMP requests at roughly 34,000,000.
By multiplying the number of systems that reply to ICMP echo request
by the average packet cache size and maximum packet size (minus
headers), we estimate the total theoretical momentary capability for
class A ICMP storage to be at approximately 3 TB.
To estimate theoretical class B storage capacity, we use the example
of MTA software. There is no upper cap for the amount of data we
feed to a single host. While it is safe to assume only messages
under approximately one megabyte are not going to cause noticable
system load and other undesirable effects, we assume the average
maximum queue size to be at 500 MB.
Own research suggests that roughly 15% of systems that respond to
ping requests have port 25 open. We thus estimate the population of
SMTP servers to be 3% (15% of 20%) of the total host count, that is
just over 5,000,000 hosts.
This gives a total storage space capacity of 2500 TB.
8) Legal notice
The authors reserve right to a method and system for cheap data
storage for developing countries.
A do-it-yourself kit (long wire, speaker + microphone, shovel and
a driver disk) will provide an affordable, portable and reusable way
for extending storage memory on portable systems.
It is estiamted that, after digging a 100 ft well, it is possible to
achieve over six kilobits of extra RAM storage at 20 kHz.
We are currently looking for distributors.
The authors would like to thank Lance Spitzner for a brief review
of this paper and some useful suggestions and comments.
You may always find this paper under following locations:
iSEC Security Research
Full-Disclosure - We believe in it.
iSEC Security Research
Full-Disclosure - We believe in it.