Provided by: dcc-common_1.2.74-2_i386
DCC - Distributed Checksum Clearinghouse
The Distributed Checksum Clearinghouse or DCC is a cooperative,
distributed system intended to detect "bulk" mail or mail sent to many
people. It allows individuals receiving a single mail message to
determine that many other people have received essentially identical
copies of the message and so reject or discard the message.
Freely redistributable source for the server, client, and utilities is
available at Rhyolite Software, http://www.rhyolite.com/dcc/
How the DCC Is Used
The DCC can be viewed as a tool for end users to enforce their right to
"opt-in" to streams of bulk mail by refusing bulk mail except from
sources in a "whitelist." Whitelists are the responsibility of DCC
clients, since only they know which bulk mail they solicited.
The only false positives (mail marked as "bulk" by a DCC server that is
not) occur when one of the recipients of a message report it to a DCC
server as having been received many times or when the "fuzzy" checksums
of differing messages are the same. The fuzzy checksums ignore aspects
of messages in order to compute identical checksums for substantially
identical messages. The fuzzy checksums are designed to ignore only
differences that do not affect meanings.
It is not reasonable to worry about third parties reporting your incoming
or outgoing mail to a DCC server as bulk unless you give them copies. If
you trust yourself and your correspondents to not report your mutual mail
as bulk, then false positives are not a concern.
A DCC server computes a lower bound on the total number of addresses to
which a message has been sent by counting checksums reported by DCC
clients. Each client must decide which bulk messages are unsolicited and
what degree of "bulkiness" is objectionable. Client DCC software marks,
rejects, or discards mail that is bulk according to local thresholds on
target addresses from DCC servers and unsolicited according to local
whitelists. DCC servers are usually configured to receive reports from
as many targets as possible, including sources that cannot be trusted to
not exaggerate the number of copies of a message they see. An end user
of a DCC client angry about receiving a message could report it with
10,000,000 separate DCC packets or with a single report claiming as many
targets. An unprincipled user could subscribe a "spam trap" to mailing
lists such as those of the IETF or CERT. Such abuses of the system area
not problems, because much legitimate mail is "bulk." You cannot reject
bulk mail unless you have a whitelist of sources of legitimate bulk mail.
The DCC can also be used by an Internet service provider to detect bulk
mail coming from its own customers. In such circumstances, the DCC
client might be configured to only log bulk mail from unexpected (not
white-listed) sources. See the -N option for dccm(8) or dccifd(8).
What the DCC Is
A DCC server accumulates counts of cryptographically secure checksums of
messages but not the messages themselves. It exchanges reports of
frequently seen checksums with other servers. DCC clients send reports
of checksums related to incoming mail to a nearby DCC server running
dccd(8). Each report from a client includes the number of recipients for
the message. A DCC server accumulates the reports and responds to
clients the the current total number of recipients for each checksum.
The client adds an SMTP header to incoming mail containing the total
counts. It then discards or rejects mail that is not "white-listed" and
has counts that exceed local thresholds.
A special value of the number of addressees is "MANY" and means it is
certain that this message was bulk and might be unsolicited, perhaps
because it came from a locally blacklisted source or was addressed to an
invalid address or "spam trap." The special value "MANY" is merely the
largest value that fits in the fixed sized field containing the count of
addressees. That "infinity" accumulated total can be reached with
millions of independent reports as well as with one or two.
DCC servers share or flood reports of checksums that are seen frequently.
Each server has its own threshold for determining "frequently," because a
message sent to 50 addressees in a domain with 60 mailboxes is more
likely to be unsolicited bulk advertising than a message sent to 100
addressees in a domain with 600,000 mailboxes.
To keep a server’s database of checksums from growing without bound,
checksums are forgotten when they become old. Checksums with large
totals are kept longer. See dbclean(8).
DCC clients pick the nearest working DCC server using a small shared or
memory mapped file, /var/dcc/map. It contains server names, port
numbers, passwords, recent performance measures, and so forth. This file
allows clients to use quick retransmission timeouts and to waste little
time on servers that have temporarily stopped working or become
unreachable. The utility program cdcc(8) is used to maintain this file
as well as to check the health of servers.
The DCC includes several programs used by clients. Dccm(8) uses the
sendmail "milter" interface to query a DCC server, add header lines to
incoming mail, and reject mail whose total checksum counts are high.
Dccm is intended to be run with SMTP servers using sendmail.
Dccproc(8) adds header lines to mail presented by file name or stdin, but
relies on other programs such as procmail to deal with mail with large
counts. Dccsight(8) is similar but deals with previously computed
Dccifd(8) is similar to dccproc but is not run separately for each mail
message and so is far more efficient. It receives mail messages via a
socket somewhat like dccm, but with a simpler protocol that can be used
by Perl scripts or other programs.
DCC SMTP header lines are of the form:
X-DCC-brand-Metrics: chost server-ID; bulk cknm1=count cknm2=count ...
brand is the "brand name" of the DCC server, such as "RHYOLITE".
chost is the name or IP address of the DCC client that added the
header line to the SMTP message.
server-ID is the numeric ID of the DCC server that the DCC client
bulk is present if one or more checksum counts exceeded the DCC
client’s thresholds to make the message "bulky."
cknm1,cknm2,... are types of checksums, and one of
IP address of SMTP client
env_From SMTP envelope value
From SMTP header line
Message-ID SMTP header line
Received last Received: header line in the SMTP message
substitute SMTP header line chosen by the DCC client,
prefixed with the name of the header
Body SMTP body ignoring white-space
Fuz1 filtered or "fuzzy" body checksum
Fuz2 another filtered or "fuzzy" body checksum
Counts for IP, env_From, From, Message-Id, Received, and
substitute checksums are omitted by the DCC client if the
server says it has no information. Counts for Body, Fuz1, and
Fuz2 are omitted if the message body is empty or contains too
little of the right kind of information for the checksum to be
count is the total number of recipients of messages with that
checksum reported directly or indirectly to the DCC server.
The special count "MANY" means that DCC client have claimed
that the message is directed at millions of recipients.
"MANY" imples the message definitely bulk, but not necessarily
unsolicited. The special counts "OK" and "OK2" mean the
checksum has been marked "good" or "half-good" by DCC servers.
An example header line is:
X-DCC-RHYOLITE-Metrics: calcite.rhyolite.com 101; Body=16 Fuz1=16 Fuz2=16
DCC clients commonly accept any mail regardless of other checksum counts
with at least one "OK" or at least two "OK2" counts among IP, env_from,
and From checksum counts. It is common to reject other mail with large
(including "MANY") counts among Received, Body, Fuz1, and Fuz2 counts.
It is generally not wise to reject mail based on the other counts. For
example, "MAILER-DAEMON" appears to send vast quantities of mail.
Legitimate mailing list traffic differs from spam only in being solicited
by recipients. Each client should have a private whitelist.
DCC whitelists can also mark mail as unsolicited bulk using blacklist
entries for commonly forged marks such as "From: email@example.com".
Systems that send many essentially identical copies of solicited mail
such as "auto-responders," should be in the DCC servers whitelists
because their messages are often substantially identical and so "bulk."
White and Blacklists
DCC server and client whitelist files share a common format. Server
files are always named whitelist and one is required to be in the DCC
home directory with the other server files. Client whitelist files are
commonly named whiteclnt in the DCC home directory or a subdirectory
specified with the -U option for dccm(8). They specify mail that should
not be reported to a DCC server or that is unsolicited bulk.
A DCC whitelist file contains blank lines, comments starting with "#",
and lines of the forms:
count ip hostname
count env_From 821-path
count env_To dest-mailbox
count From 822-mailbox
count substitute header string
count Message-ID <string>
count Received string
count hex_type hex_cksum
include can occur only in the main whitelist file.
pathname should be absolute or relative to the DCC home directory.
option setting can only be in a DCC client whitelist or whiteclnt
file and affect only dccifd(8) and dccm(8). Settings in per-
user whiteclnt files override settings in the global file.
Setting can be
log-all to log all mail messages.
to log only messages that meet the logging
dcc-off Control DCC filtering. See the discussion of -W
for dccm(8) and dccifd(8).
to control greylisting. Greylisting for other
recipients in the same SMTP transaction can still
cause greylist temporary rejections. greylist-off
in the main whiteclnt file.
to control logging of greylisted mail messages.
honor or ignore results of DNS blacklist checks
configured with -B for dccm(8) and dccifd(8).
The default in the main whiteclnt file is equivalent to
count is null and assumed to be the same as on the previous line or
MANY indicating millions of targets have received messages
with that checksum.
OK if the message is OK.
OK2 if it is "half OK." Two OK2 checksums associated
with a message are generally equivalent to an OK.
hostname is an
address IPv4 or IPv6.
block of 2 to 1024 IPv4 or IPv6 addresses in the
standard form xxx.yyy.zzz.www/mm with mm limited
for server whitelists to 16 for IPv4 or 112 for
name that will be converted to one or more IP
dest-mailbox is an RFC 821 address or a local user name.
821-path is an RFC 821 address.
822-mailbox is an RFC 822 address with optional name.
header is the name of an SMTP header such as "Sender" or the name
of one of two SMTP envlope values, "HELO" or "Mail_Host" for
the sendmail resolved host name from the 821-path in the
hex_type is the string hex followed by a blank and one of the
preceding checksum types or body, Fuz1, or Fuz2.
hex_cksum is a string of four hexadecimal numbers obtained from a
DCC log file.
A DCC server never shares or floods reports containing checksums marked
in its whitelist with OK or OK2 to other servers. A DCC client does not
report or ask its server about messages with a checksum marked OK or OK2
in the client whitelist. This is intended to allow a DCC client to keep
private mail so private that even its checksums are not disclosed.
Checksums of the IP address of the SMTP client sending a mail message are
practically unforgeable, because it is impractical for an SMTP client to
"spoof" its address or pretend to use some other IP address. That would
make the IP address of the sender useful for white-listing, except that
the IP address of the SMTP client is often not available to users of
dccproc(8). In addition, legitimate mail relays make whitelist entries
for IP addresses of little use. For example, the IP address from which a
message arrived might be that of a local relay instead of the home
address of a white-listed mailing list.
Envelope and header From values can be forged, so whitelist entries for
their checksums are not completely reliable.
Checksums of env_To values are never sent to DCC servers. They are valid
in only whiteclnt files and used only by dccm(8), dccifd(8), and other
DCC clients with access to the envelope Rcpt To value. They are another
mechanism used by DCC clients to protect the privacy of some mail.
The DCC server, dccd(8), can be used to maintain a greylist database for
some DCC clients including dccm(8) and dccifd(8). Greylisting involves
temporarily refusing mail from unfamiliar SMTP clients and is unrelated
to Distributed Checksum Clearinghouses.
Because sending mail is a less private act than receiving it, and because
sending bulk mail is usually not private at all and cannot be very
private, the DCC tries first to protect the privacy of mail recipients,
and second the privacy of senders of mail that is not bulk.
DCC clients necessarily disclose some information about mail they have
received. The DCC database contains checksums of mail bodies, header
lines, and source addresses. While it contains significantly less
information than is available by "snooping" on Internet links, it is
important that the DCC database be treated as containing sensitive
information and to not put the most private information in the DCC
database. Given the contents of a message, one might determine whether
that message has been received by a system that subscribes to the DCC.
Guesses about the sender and addressee of a message can also be validated
if the checksums of the message have been sent to a DCC server.
Because the DCC is distributed, organizations can operate their own DCC
servers, and configure them to share or "flood" only the checksums of
bulk mail that is not in local whitelists.
DCC clients should not report the checksums of messages known to be
private to a DCC server. For example, checksums of messages local to a
system or that are otherwise known a priori to not be unsolicited bulk
should not be sent to a remote DCC server. This can accomplished by
adding entries for the sender to the client’s local whitelist file.
Client whitelist files can also include entries for email recipients
whose mail should not be reported to a DCC server.
Additional privacy protections are provided by the thresholds at which
DCC servers exchange or flood reports. These thresholds are primarily
intended to reduce the traffic among DCC servers using the observation
that the vast majority of messages are sent to a handful of addressees
and so are useless to other DCC servers. A DCC server’s peer reporting
thresholds also ensure that checksums shared with peer DCC servers are
"bulk" and so intrinsically not private.
Whenever considering security, one must first consider the risks. The
worst DCC security problems are unauthorized commands to a DCC service,
denial of the DCC service, and corruption of DCC data. The worst that
can be done with remote commands to a DCC server is to turn it off or
otherwise cause it to stop responding. The DCC is designed to fail
gracefully, so that a denial of service attack would at worst allow
delivery of mail that would otherwise be rejected. Corruption of DCC
data might at worst cause mail that is already somewhat "bulk" by virtue
of being received by two or more people to appear have higher recipient
numbers. Since all DCC users must "white-list" all sources of legitimate
bulk mail, this is also not a concern. Such security risks should be
addressed, but only with defenses that don’t cost more than the possible
damage from an attack..
The DCC must contend with senders of unsolicited bulk mail who resort to
unlawful actions to express their displeasure at having their advertising
blocked. Because the DCC protocol is based on UDP, an unhappy advertiser
could try to flood a clearinghouse server with packets supposedly from
subscribers or non-subscribers. DCC servers defend against that attack
by rate-limit requests from non-subscribers.
Also because of the use of UDP, clients must be protected against forged
answers to their queries. Otherwise an unsolicited bulk mail advertiser
could send a stream of "not spam" answers to an SMTP client while
simultaneously sending mail that would otherwise be rejected. This is
not a problem for authenticated clients of the DCC because they share a
secret with the DCC. Unauthenticated DCC clients do not share any
secrets with the DCC, except for unique and unpredictable bits in each
query or report sent to the DCC. Therefore, DCC servers
cryptographically sign answers to unauthenticated clients with bits from
the corresponding queries. This protects against attackers that do not
have access to the stream of packets from the DCC client.
The passwords or shared secrets used in the DCC client and server
programs are "cleartext" for several reasons. In any shared secret
authentication system, at least one party must know the secret or keep
the secret in cleartext. You could encrypt the secrets in a file, but
because they are used by programs, you would need a cleartext copy of the
key to decrypt the file somewhere in the system, making such a scheme
more expensive but no more secure than a file of cleartext passwords.
Asymmetric systems such as that used in UNIX allow one party to not know
the secrets, but they must be and are designed to be computationally
expensive when used in applications like the DCC that involve thousands
or more authentication checks per second. Moreover, because of
"dictionary attacks," asymmetric systems are now little more secure than
keeping passwords in cleartext. An adversary can compare the hash values
of combinations of common words with /etc/passwd hash values to look for
bad passwords. Worse, by the nature of a client/server protocol like
that used in the DCC or a UNIX shell login, clients must have the
cleartext password. Since it is among the more numerous and much less
secure clients that adversaries would seek files of DCC passwords, it
would be a waste to complicate the DCC server with an asymmetric system
like that used by UNIX.
The DCC protocol is vulnerable to dictionary attacks to recover
passwords. An adversary could capture some DCC packets, and then check
to see if any of the 100,000 to 1,000,000 passwords in so called "cracker
dictionaries" applied to a packet generated the same signature. This is
a concern only if DCC passwords are poorly chosen, such as any
combination of words in an English dictionary. There are ways to prevent
this vulnerability regardless of how badly passwords are chosen, but they
are computationally expensive and require additional network round trips.
Since DCC passwords are created and typed into files once and do not need
to be remembered by people, it is cheaper and quite easy to simply choose
good passwords that are not in dictionaries.
It is better to fail to filter unsolicited bulk mail than to fail to
deliver legitimate mail, so DCC clients fail in the direction of assuming
that mail is legitimate or even white-listed.
A DCC client sends a report or other request and waits for an answer. If
no answer arrives within a reasonable time, the client retransmits.
There are many things that might result in the client not receiving an
answer, but the most important is packet loss. If the client’s request
does not reach the server, it is easy and harmless for the client to
retransmit. If the client’s request reached the server but the server’s
response was lost, a retransmission to the same server would be
misunderstood as a new report of another copy of the same message unless
it is detected as a retransmission by the server. The DCC protocol
includes transactions identifiers for this purpose. If the client
retransmitted to a second server, the retransmission would be
misunderstood by the second server as a new report of the same message.
Each request from a client includes a timestamp to aid the client in
measuring the round trip time to the server and to let the client pick
the closest server. Clients monitor the speed of all of the servers they
know including those they are not currently using, and use the quickest.
Client and Server-IDs
Servers and clients use numbers or IDs to identify themselves. ID 1 is
reserved for anonymous, unauthenticated clients. All other IDs are
associated with a pair of passwords in the ids file, the current and next
or previous and current passwords. Clients included their client IDs in
their messages. When they are not using the anonymous ID, they digitally
sign their messages to servers with the first password associated with
their client-ID. Servers treat messages with signatures that match
neither of the passwords for the client-ID in their own ids file as if
the client had used the anonymous ID.
Each server has a unique server-ID less than 32768. Servers use their
IDs to identify checksums that they flood to other servers. Each server
expects local clients sending administrative commands to use the server’s
ID and sign administrative commands with the associated password.
Server-IDs must be unique among all systems that share reports by
"flooding." All servers must be told of the IDs all other servers whose
reports can be received in the local /var/dcc/flod file described in
dccd(8). However, server-IDs can be mapped during flooding between
independent DCC organizations.
Passwd-IDs are server-IDs that should not be assigned to servers but used
to specify passwords used in the inter-server flooding protocol. They
are used in publicly readable configuration files to specify passwords in
The client identified by a client-ID might be a single computer with a
single IP address, a single but multi-homed computer, or many computers.
Client-IDs are not used to identify checksum reports, but the
organization operating the client. A client-ID need only be unique among
clients using a single server. A single client can use different client-
IDs for different servers, each client-ID authenticated with a separate
An obscure but important part of all of this is that the inter-server
flooding algorithm depends on server-IDs and timestamps attached to
reports of checksums. The inter-server flooding mechanism requires
cooperating DCC servers to maintain reasonable clocks ticking in UTC.
Clients include timestamps in their requests, but as long as their
timestamps are unlikely to be repeated, they need not be very accurate.
DCC clients on a computer share information about which servers are
currently working and their speeds in a shared memory segment. This
segment also contains server host names, IP addresses, and the passwords
needed to authenticate known clients to servers. That generally requires
that dccm(8), dccproc(8), dccifd(8), and cdcc(8) execute with an UID that
can write to the DCC home directory and its files. The sendmail
interface, dccm, is a daemon that can be started by an "rc" or other
script already running with the correct UID. The other two, dccproc and
cdcc need to be set-UID because they are used by end users. They
relinquish set-UID privileges when not needed.
Files that contain cleartext passwords including the shared file used by
clients must be readable only by "owner."
The data files required by a DCC can be in a single "home" directory,
often /var/dcc. Distinct DCC servers can run on a single computer,
provided they use distinct UDP port numbers and home directories. It is
possible and convenient for the DCC clients using a server on the same
computer to use the same home directory as the server.
The DCC source distribution includes sample control files. They should
be modified appropriately and then copied to the DCC home directory.
Files that contain cleartext passwords must not be publicly readable.
The DCC source includes "feature" m4 files to configure sendmail to use
dccm(8) to check a DCC server about incoming mail.
See also the INSTALL.txt or INSTALL.html file.
Installing a DCC client starts with obtaining or compiling program
binaries for the client server data control tool, cdcc(8). Installing
the sendmail DCC interface, dccm(8), or dccproc(8), the general or
procmail(1) interface is the main part of the client installation.
Connecting the DCC to sendmail with dccm is most powerful, but requires
administrative control of the system running sendmail.
As noted above, cdcc and dccproc should be set-UID to a suitable UID.
Root or 0 is thought to be safe for both, because they are careful to
release privileges except when they need them to read or write files in
the DCC home directory. A DCC home directory should be created, often in
/var/dcc. It must be owned and writable by the UID to which cdcc is set.
After the DCC client programs have been obtained, contact the operator(s)
of the chosen DCC server(s) to obtain each server’s hostname, port
number, and a client-ID and corresponding password. No client-IDs or
passwords are needed touse DCC servers that allow anonymous clients. Use
the load or add commands of cdcc to create a map file in the DCC home
directory. It is usually necessary to create a client whitelist file of
the format described above. To accommodate users sharing a computer but
not ideas about what is solicited bulk mail, the client whitelist file
can be any valid path name and need not be in the DCC home directory.
If dccm is chosen, arrange to start it with suitable arguments before
sendmail is started. See the homedir/dcc_conf file and the misc/rcDCC
script in the DCC source. The procmail DCCM interface, dccproc(8), can
be run manually or by a procmailrc(5) rule.
The DCC server, dccd(8), also requires that the DCC home directory exist.
It does not use the client shared or memory mapped file of server
addresses, but it requires other files. One is the ids file of client-
IDs, server-IDs, and corresponding passwords. Another is a flod file of
peers that send and receive floods of reports of checksums with large
counts. Both files are described in dccd(8).
The server daemon should be started when the system is rebooted, probably
before sendmail. See the misc/rcDCC and misc/start-dccd files in the DCC
The database should be cleaned regularly with dbclean(8) such as by
running the crontab job that is in the misc directory.
cdcc(8), dbclean(8), dcc(8), dccd(8), dccifd(8), dccm(8), dccproc(8),
dblist(8), dccsight(8), sendmail(8).
The Distributed Checksum Clearinghouse is based on an idea of Paul Vixie
with code designed and written at Rhyolite Software starting in 2000.
This describes version 1.2.74.