Langue: en

Version: 66000 (mandriva - 22/10/07)

Section: 8 (Commandes administrateur)

BSD mandoc


DCC - Distributed Checksum Clearinghouse


The Distributed Checksum Clearinghouse or 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.

Source for the server, client, and utilities is available at Rhyolite Software, LLC, http://www.rhyolite.com/dcc/ It is free for organizations that do not sell spam or virus filtering services.

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/lib/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.

X-DCC Headers

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 checksums.

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 one of the forms:

 X-DCC-brand-Metrics: client server-ID; bulk cknm1=count cknm2=count ...
 X-DCC-brand-Metrics: client; whitelist
appears if the global or per-user whiteclnt file marks the message as goo.
is the "brand name" of the DCC server, such as "RHYOLITE".
is the name or IP address of the DCC client that added the header line to the SMTP message.
is the numeric ID of the DCC server that the DCC client contacted.
is present if one or more checksum counts exceeded the DCC client's thresholds to make the message "bulky."
bulk rep
is present if the DCC reputation of the IP address of the sender is bad.
cknm1 , cknm2 , ...
are types of checksums:
address of SMTP client
SMTP envelope value
SMTP header line
SMTP header line
last Received: header line in the SMTP message
SMTP header line chosen by the DCC client, prefixed with the name of the header
SMTP body ignoring white-space
filtered or "fuzzy" body checksum
another filtered or "fuzzy" body checksum
DCC reputation of the mail sender or the estimated probability that the message is bulk.

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 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 computed.

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 is 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 unwise to reject mail based on the other counts. For example, "MAILER-DAEMON" appears to send vast quantities of mail.

Mailing lists

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: user@public.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 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:

include pathname
option setting
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


can occur only in the main whitelist file.
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
to log all mail messages.
to log only messages that meet the logging thresholds.
creates log files containing mail messages in subdirectories of the form JJJ JJJ/HH or JJJ/HH/MM where JJJ is the current julian day, HH is the current hour, and MM is the current minute. See also the -l logdir option for dccm(8), dccifd(8), and dccproc(8).
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.
to honor or ignore DCC Reputations computed by the DCC server.
honor or ignore results of DNS blacklist checks configured with -B for dccm(8) and dccifd(8).
consider MTA determinations of (not) spam first so they can be overridden by whiteclnt files, or last so that they can override whiteclnt files.
control whether dccm(8) and dccifd(8) are allowed to can discard a message to one mailbox for which it is spam when it is not spam and must be delivered to another mailbox. This can happen if a mail message is addressed to two or more mailboxes with differing whitelists. Discarding can be undesirable because false positives are not communicated to mail senders. To avoid discarding, dccm(8) and dccifd(8) temporarily reject SMTP envelope Rcpt To values that involve differing files. Also see the submit whiteclnt count described below.
threshold type,rej-thold
has the same effects as -c type,rej-thold for dccproc(8) or -t type,rej-thold for dccm(8) and dccifd(8). It is useful only in per-user whiteclnt files to override the global DCC checksum thresholds.

In the absence of explicit settings, the default in the main whiteclnt file is equivalent to

option log-normal
option dcc-on
option greylist-on
option greylist-log-on
option DCC-reps-off
option DNSBL-off
option forced-discard-nok

The defaults for each recipient are controlled by the default and explicitly set values in the main whiteclnt file.

is null and assumed to be the same as on the previous line or one of
indicating millions of targets have received messages with that checksum.
if the message is OK.
if it is "half OK." Two OK2 checksums associated with a message are equivalent to an OK
marks an IP address or block of addresses that are SMTP MX servers for your mail system. The DCC clients dccm(8), dccifd(8), and dccproc(8) skip initial Received: headers added by listed MX servers to determine the external sources of mail messages. Unsolicited bulk mail that has been forwarded through listed addresses is discarded by dccm(8) and dccifd(8) as if with -a DISCARD instead of rejected. MXDCC marks addresses that are MX servers that run DCC clients. The checksums for a mail message that has been forwarded through an address listed as MXDCC queried as if -Q had been used instead of reported.
marks an IP address or CIDR block addresses of SMTP submission clients such as web browsers that cannot tolerate 4yz temporary rejections but that cannot be trusted to not send spam. This does the equivalent of the whiteclnt option forced-discard-ok
is an
IPv4 or IPv6.
CIDR block
of 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 IPv6. There can be at most 64 CIDR blocks in a client whitelist. CIDR blocks in server whitelists cannot be larger that 65,536 entries (old Class B).
that will be converted to one or more IP addresses.
is an RFC 821 address or a local user name.
is an RFC 821 address.
is an RFC 822 address with optional name.
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 resolved host name from the 821-path in the message.
is the string hex followed by a blank and one of the preceding checksum types or body , Fuz1 or Fuz2
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 filtering with a Distributed Checksum Clearinghouse.
See http://projects.puremagic.com/greylisting/


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/lib/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 private files.

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 password.

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.

Installation Considerations

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/lib/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.

Client Installation

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/lib/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.

Server Installation

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 source.

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 1.3.48.