.\" Copyright (c) 2008 by Rhyolite Software, LLC
.\"
.\" This agreement is not applicable to any entity which sells anti-spam
.\" solutions to others or provides an anti-spam solution as part of a
.\" security solution sold to other entities, or to a private network
.\" which employs the DCC or uses data provided by operation of the DCC
.\" but does not provide corresponding data to other users.
.\"
.\" Permission to use, copy, modify, and distribute this software without
.\" changes for any purpose with or without fee is hereby granted, provided
.\" that the above copyright notice and this permission notice appear in all
.\" copies and any distributed versions or copies are either unchanged
.\" or not called anything similar to "DCC" or "Distributed Checksum
.\" Clearinghouse".
.\"
.\" Parties not eligible to receive a license under this agreement can
.\" obtain a commercial license to use DCC by contacting Rhyolite Software
.\" at sales@rhyolite.com.
.\"
.\" A commercial license would be for Distributed Checksum and Reputation
.\" Clearinghouse software.  That software includes additional features.  This
.\" free license for Distributed ChecksumClearinghouse Software does not in any
.\" way grant permision to use Distributed Checksum and Reputation Clearinghouse
.\" software
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND RHYOLITE SOFTWARE, LLC DISCLAIMS ALL
.\" WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL RHYOLITE SOFTWARE, LLC
.\" BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
.\" OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
.\" WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
.\" ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
.\" SOFTWARE.
.\"
.\" Rhyolite Software DCC 1.3.103-1.112 $Revision$
.\"
.Dd February 26, 2009
.ds volume-ds-DCC Distributed Checksum Clearinghouse
.Dt DCC 8 DCC
.Os " "
.Sh NAME
.Nm DCC
.Nd Distributed Checksum Clearinghouse
.Sh DESCRIPTION
The Distributed Checksum Clearinghouse or
.Nm
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.
.Pp
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.
.Ss 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.
.Pp
False positives or mail marked as bulk by a DCC server that
is not bulk occur only when a recipient of a message reports 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.
So in practice, you do not need to worry about DCC false positive indications
of "bulk," but not all bulk mail is unsolicited bulk mail or spam.
You must either use whitelists to distinguish solicited from unsolicited bulk
mail
or only use DCC indications of "bulk" as part of a scoring system such
as SpamAssassin.
Besides unsolicited bulk email or spam,
bulk messages include legitimate mail such as
order confirmations from merchants,
legitimate mailing lists,
and empty or test messages.
.Pp
A DCC server estimates the number copies of a
message 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.
.Pp
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.
A user of a DCC client angry about receiving a message could report it with
1,000,000 separate DCC reports
or with a single report claiming 1,000,000 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.
.Pp
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 whitelisted) customers.
.Ss What the DCC Is
A DCC server accumulates counts of cryptographic 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
.Xr 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 whitelisted and has
counts that exceed local thresholds.
.Pp
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.
.Pp
DCC servers
.Em flood
or send
reports of checksums of bulk mail to neighboring servers.
.Pp
To keep a server's database of checksums from growing without bound,
checksums are forgotten when they become old.
Checksums of bulk mail are kept longer.
See
.Xr dbclean 8 .
.Pp
DCC clients pick the nearest working DCC server using a small shared
or memory mapped file,
.Pa @prefix@/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
.Xr cdcc 8
is used to maintain this file as well as to check the health of servers.
.Ss X-DCC Headers
The DCC software includes several programs used by clients.
.Xr 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.
.Pp
.Xr Dccproc 8
adds header lines to mail presented by file name or
.Pa stdin ,
but relies on other programs
such as procmail to deal with mail with large counts.
.Xr Dccsight 8
is similar but deals with previously computed checksums.
.Pp
.Xr 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.
.Pp
DCC SMTP header lines are of one of the forms:
.Bd -literal -offset 2n
X-DCC-brand-Metrics: client server-ID; bulk cknm1=count cknm2=count ...
X-DCC-brand-Metrics: client; whitelist
.Ed
where
.Bl -hang -offset 3n -compact
.It Em whitelist
appears if the global or per-user
.Pa whiteclnt
file marks the message as good.
.It Em brand
is the "brand name" of the DCC server, such as "RHYOLITE".
.It Em client
is the name or IP address of the DCC client that added the
header line to the SMTP message.
.It Em server-ID
is the numeric ID of the DCC server that the DCC client contacted.
.It Em bulk
is present if one or more checksum counts exceeded the DCC client's
thresholds to make the message "bulky."
.It Em bulk rep
is present if the DCC reputation of the IP address of the sender is bad.
.It Em cknm1 , Ns Em cknm2 , Ns ...
are types of checksums:
.Bl -hang -offset 2n -width "Message-IDx" -compact
.It Em IP
address of SMTP client
.It Em env_From
SMTP envelope value
.It Em From
SMTP header line
.It Em Message-ID
SMTP header line
.It Em Received
last Received: header line in the SMTP message
.It Em substitute
SMTP header line chosen by the DCC client, prefixed with the name of
the header
.It Em Body
SMTP body ignoring white-space
.It Em Fuz1
filtered or "fuzzy" body checksum
.It Em Fuz2
another filtered or "fuzzy" body checksum
.It Em rep
DCC reputation of the mail sender or the estimated
probability that the message is bulk.
.El
Counts for
.Em IP , env_From , From ,
.Em Message-Id , Received ,
and
.Em substitute
checksums are omitted by the DCC client if the server
says it has no information.
Counts for
.Em Fuz1
and
.Em 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.
.It Em 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 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.
.El
.Pp
.Ss Mailing lists
Legitimate mailing list traffic differs from spam only in being solicited
by recipients.
Each client should have a private whitelist.
.Pp
DCC whitelists can also mark mail as unsolicited bulk using
blacklist entries for commonly forged values such as "From: user@public.com".
.Ss White and Blacklists
DCC server and client whitelist files share a common format.
Server files are always named
.Pa whitelist
and one is required to be in the DCC home directory
with the other server files.
Client whitelist files are
named
.Pa whiteclnt
in the DCC home directory or a subdirectory specified with the
.Fl U
option for
.Xr dccm 8 .
They specify mail that should not be reported to a DCC server or that is
always unsolicited and almost certainly bulk.
.Pp
A DCC whitelist file contains blank lines, comments starting
with "#",
and lines of the following forms:
.Bl -tag -offset 2n -width 4n -compact
.It Ar include file
Copies the contents of
.Ar file
into the whitelist.
It can occur only in the main whitelist or whiteclnt file and not in an
included file.
The file name should be absolute or relative to the DCC home directory.
.Pp
.It Ar count Em value
lines specify checksums that should be white- or blacklisted.
.Bl -inset -offset 2n -compact
.It Ar count Em env_From Ar 821-path
.It Ar count Em env_To Ar dest-mailbox
.It Ar count Em From Ar 822-mailbox
.It Ar count Em Message-ID Ar <string>
.It Ar count Em Received Ar string
.It Ar count Em Substitute Ar header string
.It Ar count Ar Hex ctype cksum
.It Ar count Em ip Ar IP-address
.El
.Pp
.Bl -tag -offset 2n -width 4n -compact
.It Ar MANY Em value
indicates that millions of targets have received messages with
the header, IP address, or checksum
.Em value .
.It Ar OK Em value
.It Ar OK2 Em value
say that messages with
the header, IP address, or checksum
.Em value
are OK and should not reported to DCC servers
or be greylisted.
.Ar OK2
says that the message is "half OK."
Two
.Ar OK2
checksums associated with a message are equivalent to one
.Ar OK .
.br
A DCC server never shares or
.Em 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.
.It Ar MX Em IP-address-or-hostname
.It Ar MXDCC Em IP-address-or-hostname
mark an address or block of addresses of trust mail relays including
MX servers, smart hosts, and bastion or DMZ relays.
The DCC clients
.Xr dccm 8 ,
.Xr dccifd 8 ,
and
.Xr dccproc 8
parse and 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
.Xr dccm 8
and
.Xr dccifd 8
as if with
.Fl a Ar DISCARD
instead of rejected.
.Ar 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 instead of reported.
.It Ar SUBMIT Em IP-address-or-hostname
marks an IP address or 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.
Since they are local addresses, DCC Reputations are not computed for them.
.El
.Pp
.Ar value
in
.Ar count Em value
lines can be
.Bl -tag -offset 2n -width 4n -compact
.It Ar dest-mailbox
is an RFC\ 821 address or a local user name.
.It Ar 821-path
is an RFC\ 821 address.
.It Ar 822-mailbox
is an RFC\ 822 address with optional name.
.It Em Substitute Ar 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 resolved host name from the
.Ar 821-path
in
the message.
.It Ar Hex ctype cksum
starts with the string
.Em Hex
followed a checksum type, and
a string of four hexadecimal numbers obtained from a DCC log file
or the
.Xr dccproc 8
command using
.Fl CQ .
The checksum type is
.Em body , Fuz1 ,
or
.Em Fuz2
or one of the preceding checksum types such as
.Em env_From .
.It Ar IP-address
is a host name, IPv4 or IPv6 address, or a block
of IP addresses in the standard xxx/mm from 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
.Pa whiteclnt
file.
A host name is converted to IP addresses with DNS,
.Pa /etc/hosts
or other mechanisms
and one checksum for each addresses added to the whitelist.
.El
.Pp
.It Ar option setting
can only be in a DCC client
.Pa whiteclnt
file used by
.Xr dccifd 8 ,
.Xr dccm 8
or
.Xr dccproc 8 .
Settings in per-user whiteclnt files override settings
in the global file.
.Ar Setting
can be any of the following:
.Bl -tag -offset 2n -width 2n -compact
.It Ar option log-all
to log all mail messages.
.It Ar option log-normal
to log only messages that meet the logging thresholds.
.It Ar option log-subdirectory-day
.It Ar option log-subdirectory-hour
.It Ar option log-subdirectory-minute
creates log files containing mail messages in subdirectories
of the form
.Ar JJJ ,
.Ar JJJ/HH ,
or
.Ar JJJ/HH/MM
where
.Ar JJJ
is the current julian day,
.Ar HH
is the current hour, and
.Ar MM
is the current minute.
See also the
.Fl l Ar logdir
option for
.Xr dccm 8 ,
.Xr dccifd 8 ,
and
.Xr dccproc 8 .
.It Ar option dcc-on
.It Ar option dcc-off
Control DCC filtering.
See the discussion of
.Fl W
for
.Xr dccm 8
and
.Xr dccifd 8 .
.It Ar option greylist-on
.It Ar option greylist-off
to control greylisting.
Greylisting for other recipients in the same SMTP transaction
can still cause greylist temporary rejections.
.Ar greylist-off
in the main whiteclnt file.
.It Ar option greylist-log-on
.It Ar option greylist-log-off
to control logging of greylisted mail messages.
.It Ar option DCC-rep-off
.It Ar option DCC-rep-on
to honor or ignore DCC Reputations computed by the DCC server.
.It Ar option DNSBL1-off
.It Ar option DNSBL1-on
.It Ar option DNSBL2-off
.It Ar option DNSBL2-on
.It Ar option DNSBL3-off
.It Ar option DNSBL3-on
honor or ignore results of DNS blacklist checks configured with
.Fl B
for
.Xr dccm 8 ,
.Xr dccifd 8 ,
and
.Xr dccproc 8 .
.It Ar option MTA-first
.It Ar option MTA-last
consider MTA determinations of spam or not-spam first so they can be overridden
by
.Pa whiteclnt
files, or last so that they can override
.Pa whiteclnt files.
.It Ar option forced-discard-ok
.It Ar option no-forced-discard
control whether
.Xr dccm 8
and
.Xr dccifd 8
are allowed to discard a message for 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,
.Xr dccm 8
and
.Xr dccifd 8
running in proxy mode temporarily reject SMTP envelope
.Em Rcpt To
values that involve differing
.Pa whiteclnt
files.
.It Ar option threshold type,rej-thold
has the same effects as
.Fl c Ar type,rej-thold
for
.Xr dccproc 8
or
.Fl t Ar type,rej-thold
for
.Xr dccm 8
and
.Xr dccifd 8 .
It is useful only in per-user whiteclnt files to override the global
DCC checksum thresholds.
.It Ar option spam-trap-accept
.It Ar option spam-trap-reject
say that mail should be reported to the DCC server as extremely
bulk or with target counts of
.Ar MANY .
Greylisting, DNS blacklist (DNSBL), and other checks are turned off.
.Ar Spam-trap-accept
tells the MTA to accept the message while
.Ar spam-trap-reject
tells the MTA to reject the message.
Use
.Ar Spam-trap-accept
for spam traps that should not be disclosed.
.Ar Spam-trap-reject
can be used  on
.Em catch-all
mailboxes that might receive legitimate mail by typographical errors
and that senders should be told about.
.El
.Pp
In the absence of explicit settings,
the default in the main whiteclnt file is equivalent to
.Bl -hang -offset 4n -width 4n -compact
.It Ar option log-normal
.It Ar option dcc-on
.It Ar option greylist-on
.It Ar option greylist-log-on
.It Ar option DCC-rep-off
.It Ar option DNSBL1-off
.It Ar option DNSBL2-off
.It Ar option DNSBL3-off
.It Ar MTA-last
.It Ar option no-forced-discard
.El
The defaults for individual recipient
.Pa whiteclnt
files are the same except as change by explicit settings
in the main file.
.El
.Pp
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 whitelisting,
except that the IP address of the SMTP client
is often not available to users of
.Xr 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 whitelisted mailing list.
.Pp
Envelope and header
.Ar From
values can be forged,
so whitelist entries for their checksums are not entirely reliable.
.Pp
Checksums of
.Ar env_To
values are never sent to DCC servers.
They are valid in only
.Pa whiteclnt
files
and used only by
.Xr dccm 8 ,
.Xr dccifd 8 ,
and
.Xr dccproc 8
when the envelope
.Em Rcpt To
value is known.
.Ss Greylists
The DCC server,
.Xr dccd 8 ,
can be used to maintain a greylist database for some DCC clients
including
.Xr dccm 8
and
.Xr dccifd 8 .
Greylisting involves temporarily refusing mail from unfamiliar
SMTP clients and is unrelated to filtering with a
Distributed Checksum Clearinghouse.
.br
See http://projects.puremagic.com/greylisting/
.Ss Privacy
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.
.Pp
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.
.Pp
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.
.Pp
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.
.Ss Security
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 DCC users
.Em must
whitelist 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.
.Pp
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 DCC server with
packets supposedly from subscribers or non-subscribers.
DCC servers defend against that attack by rate-limiting requests
from anonymous users.
.Pp
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, anonymous 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.
.Pp
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, 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.
.Pp
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.
.Ss Reliability
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 whitelisted.
.Pp
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.
.Pp
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.
.Ss 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
.Pa 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 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
.Pa ids
file as if the client had used the anonymous ID.
.Pp
Each server has a unique
.Em server-ID
less than 32768.
Servers use their IDs to identify checksums that they
.Em 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.
.Pp
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
.Pa @prefix@/flod
file described in
.Xr dccd 8 .
However, server-IDs can be mapped during flooding between
independent DCC organizations.
.Pp
.Em Passwd-IDs
are server-IDs that should not be assigned to servers.
They appear in the often publicly readable
.Pa @prefix@/flod
and specify passwords in the private
.Pa @prefix@/ids
file for the inter-server flooding protocol
.Pp
The client identified by a
.Em 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.
.Pp
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.
.Ss 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
.Xr dccm 8 ,
.Xr dccproc 8 ,
.Xr dccifd 8 ,
and
.Xr 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.
.Pp
Files that contain cleartext passwords including the shared file used by clients
must be readable only by "owner."
.Pp
The data files required by a DCC can be in a single "home" directory,
.Pa @prefix@ .
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.
.Pp
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.
.Pp
The DCC source includes "feature" m4 files to configure
sendmail to use
.Xr dccm 8
to check a DCC server about incoming mail.
.Pp
See also the INSTALL.html file.
.Ss Client Installation
Installing a DCC client starts with obtaining or compiling program binaries
for the client server data control tool,
.Xr cdcc 8 .
Installing the sendmail DCC interface,
.Xr dccm 8 ,
or
.Xr dccproc 8 ,
the general or
.Xr 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.
.Pp
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,
.Pa @prefix@
should be created.
It must be owned and writable by the UID to which cdcc is set.
.Pp
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
.Em client-ID
and corresponding password.
No client-IDs or passwords are needed touse
DCC servers that allow anonymous clients.
Use the
.Em load
or
.Em add
commands
of cdcc to create a
.Pa 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.
.Pp
If dccm is chosen,
arrange to start it with suitable arguments
before sendmail is started.
See the
.Pa homedir/dcc_conf
file and the
.Pa misc/rcDCC
script in the DCC source.
The procmail DCCM interface,
.Xr dccproc 8 ,
can be run manually or by a
.Xr procmailrc 5
rule.
.Ss Server Installation
The DCC server,
.Xr 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
.Pa @prefix@/ids
file of client-IDs,  server-IDs, and corresponding passwords.
Another is a
.Pa flod
file of peers that send and receive floods of reports of checksums
with large counts.
Both files are described
in
.Xr dccd 8 .
.Pp
The server daemon should be started when the system is rebooted,
probably before sendmail.
See the
.Pa misc/rcDCC
and
.Pa misc/start-dccd
files in the DCC source.
.Pp
The database should be cleaned regularly with
.Xr dbclean 8
such as by running the crontab job that is in the misc directory.
.Sh SEE ALSO
.Xr cdcc 8 ,
.Xr dbclean 8 ,
.Xr dcc 8 ,
.Xr dccd 8 ,
.Xr dccifd 8 ,
.Xr dccm 8 ,
.Xr dccproc 8 ,
.Xr dblist 8 ,
.Xr dccsight 8 ,
.Xr sendmail 8 .
.Sh HISTORY
Distributed Checksum Clearinghouses are based on an idea of Paul Vixie
with code designed and written at Rhyolite Software starting in 2000.
This document describes version 1.3.103.