What is the Distributed Checksum Clearinghouse or DCC?
The DCC or Distributed Checksum Clearinghouse is an anti-spam content filter
that runs on a variety of
The idea of the DCC is that if mail recipients could compare
the mail they receive, they could recognize unsolicited bulk mail.
A DCC server totals reports of "fuzzy" checksums of
messages from clients and answers queries about the total counts
for checksums of mail messages.
See the main DCC man page as well as the
DCC web page
and its mirror.
Is the DCC source free
The non-commercial Distributed Checksum Clearinghouse source carries a
that is free only to organizations that do not sell filtering devices or
services except to their own users and that participate in the global
ISPs that use DCC to filter mail for their
own users are intended to be covered by the free license.
You can redistribute unchanged copies of the free source, but you may not
redistribute modified, "fixed," or "improved" versions of the source
You also can't call it your own or blame anyone for the results of using it.
Organizations that do not qualify for the free license are welcome to
inquire about licenses for the commercial version by email to
or via the
The commercial version supports
Please note that organizations that do not qualify for the free DCC license
have never been allowed to use the public DCC servers.
Where can I get DCC source?
The official DCC source repositories are at
Please do not try to use ancient versions of DCC software dating from early
2005 and redistributed by third parties including some Linux packagers.
Those versions do not detect bulk mail as well as more recent versions.
Installations using those old versions also have problems using the
public DCC servers that often make it necessary to add their IP addresses
to the blacklist that protects the public DCC servers.
Even worse, all known Linux redistributions of DCC software have been
changed in ways that break things, including the
libexec/updatedcc shell script that could
otherwise be used to fetch, configure, compile, install, and restart
a current version.
When installing DCC software, please consider the installation instructions
INSTALL.html file included with
the source or in the
on line source trees.
Where can I get DCC RPMs, packages or other binary forms?
There are no official distributions of DCC binaries,
whether simple a.out files, RPM Package Manager (RPM) packages,
or BSD style ports or packages (pkg).
There are many unofficial sources of DCC binaries, including
Linux RPMs and BSD style packages.
As of 2008, the FreeBSD packages are not too far out of date and
include a working version of the
libexec/updatedcc shell script that
fetches, configures, compiles, installs, and restarts
a current version.
As far as known in 2008, all DCC RPMs offered by Linux distributors
are based on DCC software from 2005 and should not be used.
Do the fuzzy checksums ignore
Yes, they ignore many
How much bandwidth, disk space, and computing does the DCC require?
The UDP packets used by a DCC client to obtain the checksum totals
from a DCC server for a mail message generally use less bandwidth than
the DNS queries required to receive the same message.
A DCC client needs very little disk space.
Bulk messages are usually logged by DCC clients.
On systems receiving a lot of mail, the mechanisms for automatically
creating new log directories every minute, day, or hour
can keep any single log directory from becoming too large.
See the dccm
About 1.4 GBytes/day are exchanged between each pair of DCC servers.
Each server has 3 or 4 peers.
The resulting database is about 3 GBytes with the default expiration
However, while dbclean is deleting old checksums,
there are three copies of the database.
The DCC clients and server do not need many CPU cycles,
but the daily executions of dbclean
on a system with a DCC server
require a computer with at least 2 or 3 GBytes of RAM.
a DCC server prefers 4 GBytes of RAM and can use 6 GBytes.
12 to 18 GBytes of disk space are also needed.
DCC servers used by clients handling 100,000 or more messages per day
need to be larger.
Each additional 100,000 messages/day need about 100 MBytes of disk space
and system memory, given the default expiration used by
- Do I need to run a DCC server?
A mail system that processes fewer than 100,000 mail messages per day
uses less of its own bandwidth and the bandwidth of other DCC servers
by using the public
Each mail message needs a DCC transaction that requires
about 100 bytes, and so 100,000 mail messages/day imply about 10
MBytes/day of DCC client-server traffic. Each DCC server needs to
exchange "floods" or streams of checksms with 4 other servers. Each
flood is currently about 1.4 GBytes/day for a current total of about
When normally installed by the included Makefiles, DCC clients are
configured to use the
public DCC servers
without any additional configuration except opening firewalls to port UDP 6277.
Mail systems that process more than 100,000 mail messages per day
need local DCC servers connected to the global network of DCC servers.
The public DCC servers include denial of service defenses which
ignore requests in excess of about 240,000 per day per client.
It is wrong to resell the CPU cycles, network bandwidth,
disk space, and, most important, human system administration work of the
public DCC servers.
Vendors of "anti-spam appliances" or similar
that do not steal from the operators
of the public DCC servers have always run their own DCC servers.
What happens to my mail if the DCC break?
When in doubt or trouble, the DCC clients including
dccproc and dccm
deliver mail. They wait only a little while for a DCC server
to answer before giving up. They then avoid asking a server for a while
to avoid slowing down mail.
If the DCC sendmail interface or milter program, dccm, crashes,
the default parameters in misc/dcc.m4
for the sendmail.cf Xdcc line
tell sendmail to wait only about 30 seconds before
giving up and delivering the mail.
The DCC client code keeps track of the speeds of the
servers it knows about, and uses the fastest or closest.
Every hour or so it re-resolves A records
and checks the speeds of the servers it
is not using. When the current server stops working or gets significantly
slower, the client code switches to a better server.
How do I mark spam without rejecting it?
Unless given thresholds at which to reject mail,
dccproc do not reject mail.
When dccm is given a threshold by setting DCCM_REJECT_AT in
dcc_conf in the DCC home directory,
DCCM_ARGS can also be set to "-a IGNORE
so that spam is marked but not rejected.
Why doesn't the man command find the man pages?
The nroff source, formated nroff output, and HTML versions of the
man pages are in the top-level source directory.
Formatted or nroff source is installed by default somewhere in /usr/local/man
depending on the target system.
It may be necessary to add /usr/local/man to the MANPATH environment variable.
Even with that, SunOS 5.7 sometimes has trouble finding them unless
man -F is used.
Must sendmail be used with DCC?
While the sendmail milter interface, dccm
and the DCC program interface or dccifd
are the most efficient ways to report and check DCC checksums,
dccproc is also commonly used.
Can the DCC be used with smtpd?
Yes, dccproc can be used with Obtuse's smtpd.
Dave Lugo has contributed a shell script to the
which can be used to do DCC checking prior to the end of the SMTP
Can the DCC be used with Exim?
There are comments about using Dccproc with
DCC mailing list archives
including these messages:
However, those mailing list messages talked about using
dccifd was available.
Dccproc is suitable only for low mail volumes.
Can the DCC be used with SpamAssassin or other spam filters?
The DCC can be used with
well as other spam and virus filters.
Note that it is more efficient to arrange to use a DCC client daemon
such as dccm to mark passing mail and check
X-DCC header lines in the filter than to start and run
dccproc on each message.
Some commercial virus and spam filters include DCC clients that
query public DCC servers or DCC servers operated by the filter vendor
and that "flood" or exchange bulk mail checksums with public servers.
Reputable manufacturers of such devices operate their own DCC servers
connected to global network of DCC servers instead of stealing and then
selling the CPU cycles, network bandwidth, disk space, and, most important,
human system administration efforts of the public DCC servers.
How long must SpamAssassin or an MTA wait for DCC results?
DCC clients including dccproc, dccifd, and dccm can wait as long as
about 16 seconds for an answer from a DCC server.
Except when an anonymous client triggers the progressive delays that are
among the defenses against denial of service attacks in the public DCC servers,
delays are almost always less than 10 seconds.
Delays for DNS blacklists
(see dccifd -B)
How can the DCC be used with mail user agents?
- Dccproc can be used with any mail user
agent that can check mail headers.
For example, WD Baseley sent a
to the DCC
on how to configure Eudora to
act on X-DCC header lines.
Bharat Mediratta has developed DeepSix for people using mail user agents
on UNIX boxes connected remote servers such as corporate Exchange servers.
project on Sourceforge
as well as his
in the DCC mailing list.
Must I have the root password to use DCC?
No, the procmail or sendmail .forward DCC user program,
can be installed in an individual ~/bin directory.
can create a private map file used with
dccproc -h dir
dccproc -m dir/map.
Also see the DCC installation
Why don't the public DCC servers work? Do I need a client-ID?
The public DCC servers accept requests from clients using the
Incorrectly configured firewalls often cause problems.
Traceroute can be used to send UDP packets to test for interfering firewalls.
See the answer to the firewall question.
After firewalls, the most common cause of problems while trying to
use the public DCC servers is sending too many requests.
The DCC server daemon, dccd, includes
defenses against denial of service or DoS attacks.
Those defenses include progressively delaying responses
and eventually ignoring requests.
The ancient version of the DCC client software included in some
Linux redistributions tries so hard to reach the fastest server
that it can trigger those DoS defenses.
Which ports do I need to open in my firewall?
DCC traffic is like DNS traffic. You should treat port 6277
like port 53.
Allow outgoing packets to distant UDP port 6277 and incoming packets
from distant UDP port 6277.
If the command `cdcc info` says no DCC servers are answering,
you may need to adjust your firewall.
Also consider the other reasons why the
public DCC servers can ignore requests.
If you run a DCC server, open incoming connections to local TCP port 6277
from your flooding peers,
and outgoing connections to TCP port 6277 on your flooding peers.
Also open UDP port 6277 to IP address 18.104.22.168 for the DCC server status
See also the discussion of Cisco ACLs at
Why does the dccd database
grow without bound?
- Dbclean should be run every night when the
system is least busy
with the misc/cron-dccd script.
An entry like misc/crontab should be put into
the crontab file for the user that runs dccd.
The dccd database is corrupt. What should I do?
- Dbclean -R
will usually repair a broken
DCC server database.
if your server is "flooding" or exchanging checksums with other servers,
it is often quicker to stop the DCC server,
and restart dccd with the
When dccd starts, it will notice that the database has been purged
and ask its flooding peers to rewind and retransmit their checksums of
How can I stop the log directories from overflowing?
logging can be entirely
disabled by setting DCCM_LOGDIR="" or DCCIFD_LOGDIR="" in the
dcc_conf file in the DCC home directory.
Logging for individual users can be disabled by not creating or deleting
thir log directories.
However, this not only disables logging of rejected mail, but also logging
of mail that suffered system failures.
To delete old log files, run the
daily with an entry like misc/crontab
in the crontab file for the user that runs dccd
The DBCLEAN_LOGDAYS parameter in the
dcc_conf file in the DCC home directory
specifies the age of old log files.
Why do my DCC clients including
cdcc and dccproc
complain about "Resource temporarily unavailable"?
Perhaps your operating system has bugs in its implementation of
fcntl file locking, particularly for the
DCC client map file when it is on
an NFS file system.
Another common case is using an editor such as some versions of vi
that locks files on the main or a per-user
Why does dccifd or dccm complain about
thread_create() failed: 11, try again?
pthread_create(): Cannot allocate memory?
The most common cause of
thread_create() failed: 11, try again
pthread_create(): Cannot allocate memory
error messages from dccm
is a too small limit on the maximum number of processes allowed
the UID running the dccm or dccifd process.
The "maxproc" limit seen with the `limit` or `limits` shell command
should be a dozen or so larger than the sum of
the queue sizes of dccm or dccifd (or both if both are running).
See also the common question and answer about
too many simultaneous mail messages.
Why does dccm or dccifd complain about
too many simultaneous mail messages?
Dccm or dccifd can fail to create a thread to deal with an incoming
mail message if there are no available file descriptors or
Adding -d to DCCD_ARGS or DCCIFD_ARGS in
dcc_conf in the DCC home directory
sends a message to the system log that includes the limit on simultaneous mail
messages and its source, such as a process resource limit on the
number of file descriptors.
Another common limit is the maximum number of file descriptors
allowed by the select system call.
This limit can be escaped by building the sendmail milter library to
use the poll system call.
Why doesn't my DCC client pick my local DCC server?
The DCC clients including dccm
and dccproc pick the nearest and fastest
server in the list kept in the /usr/local/dcc/map
DCC servers not in that list will not be used.
That list can be viewed with the
cdcc RTT operations.
Add to the list with
or cdcc load.
A nearby server that seems slower than a more distant server will
not be chosen.
The anonymous user delay set with dccd -u
is intended to make a server appear slow to "freeloaders."
The "RTT +/-" value that can be used with
the cdcc add
and cdcc load
operations can be used to force DCC clients to prefer or avoid servers
except when absolutely necessary.
If I have a server-ID, do I need a DCC client-ID, or vice versa?
DCC server and client-IDs
serve distinct purposes.
Servers require server-IDs to identify each other in the floods of checksums
they exchange and to recognize authorized users of powerful
cdcc operations such as stop.
DCC servers require client-IDs to identify paying clients that should
be given quicker service that anonymous clients, to refuse reports from
anonymous clients, or to refuse even to answer queries from anonymous
Why does my DCC server complain about
"rejected server-IDs" among flooded checksum reports?
You have turned on IDS tracing, but do not have a
/usr/local/dcc/ids file that is complete.
You don't need and probably will not have a complete file unless you
are assigning DCC server-IDs.
Redundant paths among DCC servers exchanging
or flooding reports of checksums would cause duplicate entries in
each server's database without the mechanism that depends on every DCC server
having a unique server-ID.
With IDS tracing enabled, dccd complains
about server-IDs that are not listed in the local
Why does my DCC server refuse to accept more than
50 operations per second?
A common cause of such problems is one of the DCC server's
defenses against denial of service attacks.
A DCC server cannot know anything about anonymous clients,
or clients using client-ID 1 or without a client-ID and matching password
from the /usr/local/dcc/ids file.
As far as your server can know, an anonymous client sending many
operations is run by an unhappy sender of unsolicited bulk mail trying
to flood your server with a denial of service attack.
It is easy to tell your client its ID with the
or load operations.
The default limits can changed by
adding an dccd -R argument
can be added to DCCD_ARGS in the
dcc_conf file in the DCC home directory,
How do I keep strangers from using my DCC server?
See the dccd -u option.
How can I determine why dccm reported
a message as spam or with a recipient count of "MANY"?
Dccm is usually configured to log mail with recipient counts greater
than the -t ,log-thold,
as well as mail with some conflicts among
Each log file contains a single message, its checksums, its disposition,
and other information as described in the
dccm man page.
See also the dblist -C command.
How can I see what checksums my server has heard from its clients?
The dblist -Hv
command displays the contents of the database.
Look for records with your
with dblist -I.
How do I stop DCC false positives?
You are probably not seeing false positives.
The Distributed Checksum Clearing Houses detect both solicited
and unsolicited bulk mail, while spam is only unsolicited bulk email.
For your DCC client, dccm,
dccproc, to know to ignore bulk mail messages
that are solicited, it must be told by entries the main or a per-user
whitelist or whiteclnt file.
Why is mail from my favorite mailing list marked with an
X-DCC header line that says it is spam?
Sources of solicited bulk mail including mailing lists to which
you have subscribed should usually be in your DCC client
so that they receive no X-DCC header lines.
Why are acknowledgments of spam reports mistakenly marked as spam by DCC?
There is probably no mistake.
DCC detect bulk mail and not only unsolicited bulk mail.
Whether a bulk message is spam depends on whether you solicited or asked for it.
Some INTERNET service providers have sent literally millions of
acknowledgments of spam reports, which makes them bulk mail.
Bulk mail you want to receive should be
in your master or per-user
Why are some checksums missing from my X-DCC header lines?
If the DCC client was not able to compute a checksum for a message,
it will not ask the server about that checksum and the checksum will
not appear in the X-DCC header.
For example, if dccproc is not told and
cannot figure out the IP address of the source of the message,
that checksum will be missing.
The Fuz1 and Fuz2 checksums cannot be computed for
messages that are too small, and so will be missing for them.
A checksum will also be missing if the DCC server is configured to not count
- Do I need both server and client
The server whitelist file
used explicitly by dbclean
and implicitly by dccd
is not very useful and probably a bad idea.
The client whitelist files
are generally required.
Client whitelists apply only to the stream of mail handled by the
while server whitelists apply to reports of mail from all DCC clients
of the DCC server.
Dccproc is intended for use by individual users
with programs such as
Because the global whiteclnt file usually found in the DCC home directory
is as likely to be used as a private file,
the file name must be explicitly specified with
dccproc -w whiteclnt.
A perhaps inconvenient implication is programs such as
switch unpredictably between dccproc and dccifd
might get inconsistent results unless they invoke dccproc with the global
How do I maintain client
Start by monitoring bulk mail in the
global log directories specified with
and with DCCM_LOGDIR and DCCM_USERDIRS in the
Then add entries to whitelist files.
and the whitelists specified with
dccproc -w are maintained
with ordinary text editors.
Per-user whitelists in whiteclnt files
specified with DCCM_USERDIRS in the
are easily maintained with ordinary text editors by the system administrator.
However, it is often better to let individual users deal with their
The DCC source includes sample CGI scripts
in the cgi-bin directory in the DCC source
to let individual end-users monitor their private logs of bulk mail
and their individual whitelists.
See the README file for those scripts.
There is also a
of the cgi scripts.
When the whitelist file
used by dccm,
what must be done to tell the software about the change?
The DCC clients notice when their whiteclnt files
as well as included files change and automatically rebuild the corresponding
.dccw hash table files.
Changes to the DCC server or dccd
are not effective until after dbclean is run.
Some text editors including versions of vi lock their files.
are unable to read whitelist files while they are locked.
How do I test a whiteclnt file?
An easy way to test a DCC client whitelist or
is to feed dccproc with a test message.
For example, the following shell script would test whether the IP address
and the SMPT envelope Mail_From value email@example.com are in the
whiteclnt file in the DCC home directory:
/usr/local/bin/dccproc -QCw whiteclnt \
-a 127.0.0.1 -f firstname.lastname@example.org <<EOF
If the script produces something like
X-DCC--Metrics: calcite.rhyolite.com; whitelist
reported: 0 checksum wlist
IP: e475b896 492c60fc efecb432 6e29e3c5 ok
env_From: bef98dc1 cc6ea4d7 b8daf07c a2bfbc9e
Message-ID: 26573398 2ab927cd 681a89fa e502496d
then you know that SMTP client IP (mail sender) IP address 127.0.0.1
is whitelisted, but the SMTP envelope Mail_From value is not.
Can I use wild cards or regular expressions in DCC
No, regular expressions cannot be used,
because DCC client and server whitelists are converted to lists of checksums.
The same basic idea is used for DCC client whitelists
as for the DCC protocol.
A DCC client computes the checksums for a message, and then looks
for those checksums in the local whitelist.
Depending on the values associated with those checksums,
the DCC client asks a DCC server about them.
To use regular expressions with the DCC, consider procmail.
Procmail is included with many UNIX-like systems.
See also the
DCC clients can be configured to white- or blacklist
using called "substitute" headers.
See dccproc -S or
It is also possible to use a sendmail access_db file entries to
white- or blacklist based on portions of SMTP envelope and
client IP addresses.
For example, an access_db file line of "From:example.com OK"
can be used to tell dccm to whitelist all mail from SMTP clients
in the example.com domain.
See the -O argument to the
How do I whitelist mail from a legitimate
bulk mailer using its name or SMTP headers such as Mailing-List
Start by determining an envelope value or SMTP header that distinguishes
the bulk mail from a sample message or DCC log file.
The name of the sending computer is the mail_host value in
dccm log files.
If the distinguishing header or envelope value is not among the main
DCC whitelist values,
then a "substitute" value must be used.
An "ok substitute ..." line must be added to the whitelist file
and the DCC client program must be told with
dccproc -S or
There are example whitelist entries in the sample
Why does dccm or dccifd complain about "incompatible whitelists"?
There are several points during an SMTP transaction when an SMTP server
can reject a mail message.
Early points are when the SMTP client specifies the recipients of the
The last point is after the entire message has been received by the SMTP
Spam filters that check mail message bodies must wait until that last point.
The SMTP protocol does not allow an SMTP server to reject the
mail message for only some recipients.
The SMTP server must tell the SMTP client that the message has been
accepted for all or rejected for recipients.
This is a problem when the recipients of a single mail message have
DCC thresholds or other parameters
in their individual whitelist files
that require that the mail message be delivered to some mailboxes but
rejected for other mailboxes.
The DCC client programs solve this conflict in one of two ways.
One is telling the SMTP client
that the mail message has been accepted for all recipients and then
discarding instead of delivering the message for mailboxes with parameters
that make it spam.
This solution has the disadvantage of not informing senders of the
refusal to deliver the message.
The other solution is to temporarily reject recipients with possibly
incompatible parameters early in the SMTP transaction with the same
SMTP error status number as too many recipients for a single SMTP transaction.
This second solution has the advantage of ensuring that senders know
when their mail is rejected but the disadvantage of sometimes
requiring as many SMTP transactions as there are recipients for a mail message.
Which solution is used is determined by the
settings in the global and per-user
Unless all recipients for a mail message agree on the first solution,
perhaps by forced-discard-ok in the main
the second solution is used.
Why do legitimate mail messages have
X-DCC header lines that say they are "bulk", "many", or spam?
There are several possible causes of such problems.
The first and most obvious is that the mail is solicited bulk mail
and that the source needs to be added to your
Another possible reason is that your individual legitimate mail messages
have not been marked as spam because their Body or Fuz1
checksum counts are small, but that the IP address or other checksum
counts are large.
The IP address checksum count, for example, is the total of all reports
of addressees for that checksum.
That total is independent of the other checksums, and so counts
all reports for all messages with that source IP address.
A source of legitimate mail that has sent a message that was reported
as spam by one of its recipients will often have the totals
for the checksums of its IP address, From header, and
other values be MANY.
This is why it usually does not make sense to reject mail based on what the
DCC server reports for the IP address, From header, and other values that
are not unique to the message.
Only the last Received header line, the Message-ID line, and body checksums
can be expected to be unique and sometimes not the Message-ID
and Received header lines.
Why is legitimate mail from someone using qmail
marked as spam?
A common cause for that and similar complaints involves
null or missing Message-ID header lines.
Spam often lacks Message-ID lines or has a null or "<>" ID,
so rejecting mail with null or missing Message-IDs can be an
DCC clients treat missing Message-ID lines as if they were present but null.
The sample /usr/local/dcc/whiteclnt
whitelist file in the DCC source
includes the line:
many message-id <>
Some Mail Transfer Agents violate section 3.6.4 of RFC 2822 and
do not include Message-ID header lines in mail they send,
including some combinations of qmail and
"sendmail -bs" acting as the originating MTA,
and qmail by itself when it is generates a non-delivery message or "bounce."
Solutions to this problem include removing that line from your
or adding lines specifying the From or envelope
from values of senders of legitimate mail lacking Message-ID header lines.
Are IP address blocks
in whitelists used by
Yes, dccproc can whitelist mail
by the IP address of the immediately
preceding SMTP client,
but only if it knows that IP address.
Unless the dccproc -a
or dccproc -R
options are used, dccproc does not know the IP address.
Why is dccproc is ignoring
DCC checksums are of the entire header line or envelope value.
An entry in the whitelist file for email@example.com
will have no effect on mail with an envelope value of
The file must contain "J.Smith" firstname.lastname@example.org.
Another common cause for this problem is implied by the fact that
for an env_from whitelist entry
to have any effect, dccproc must be able to find the envelope value
in the message in a Return-Path header,
an old UNIX-style From_ header, or an -f argument.
If your mail delivery agent does not add a Return-Path header
and you do not use
then dccproc cannot know about
white or blacklist entries for envelope return addresses.
Note also that dccproc has no whitelist by default and
that dccproc -w
must be used.
What if I make a mistake with
dccproc -t many
and report legitimate mail as spam?
It is possible to delete checksums from the distributed DCC
database with the
However, it is not worth the trouble.
Unless the same (as far as the fuzzy checksums are concerned) message
is sent again, no one is likely to notice the mistake before the
report of the message's checksums expire from the DCC servers'
databases for lack of repetition.
Can the sendmail "spamfriend" mechanism tell
dccm to not check mail sent to some addresses?
Sendmail decisions to accept, reject, or discard mail are largely
independent of the decisions made by dccm.
The DCC equivalent is to add
env_to entries to the
See the sample /usr/local/dcc/whiteclnt file in the
However, if your sendmail.cf file sets the
dcc_notspam macro while processing the
envelope, then the message will by whitelisted.
This is related to the dcc_isspam macro
used by sendmail.cf modified by misc/hackmc -R
to tell dccm to report blacklisted messages as spam to the DCC server.
How do I tell dccm
to not check mail for an entire domain?
To whitelist all mail addressed to mailboxes in a domain,
add the following line to the sendmail access_DB file and rebuild
the database with the sendmail tool, makemap:
You can apply finer control by adding
a third argument to the FEATURE(dcc) macro in your sendmail.mc file
as described in
All mail for the domain can use a single "per-user"
often in the /usr/local/dcc/userdirs/esmtp/example.com, where /usr/local/dcc/userdirs
is the default value for DCCM_USERDIRSin the DCC configuration file
Making /usr/local/dcc/userdirs/esmtp a symbolic link to /usr/local/dcc/userdir/local
can be handy.
How can I avoid polluting databases of DCC servers with
checksums of my mail that is not spam?
Reports of checksums with
entries in your server's database are not flooded to its peers.
The checksums of messages whitelisted with entries in local
dccm or dccproc
whitelists are not reported to DCC servers.
It is good to add entries to DCC server and client
for localhost, your IP address blocks, and your domains if
you know that none of your users will ever send spam.
However, in the common mode in which the DCC is used, no
checksums of mail are pollution.
Checksums of genuinely private mail will have target counts of
1 or a small number, and so will not be flooded by your server to
Strangers will not see your private mail and so will not be able
to ask any DCC server about the checksums of your private mail.
On the other hand, the DCC functions best by collecting reports
of the receipt of bulk mail as soon as possible.
That implies that it is generally desirable
to send reports of all mail to a DCC server.
The DCC flooding protocol does not send checksums with counts
to other servers.
Can DCC be fed with
A spam trap is a mail address that should practically
never receive legitimate mail,
and that treats any mail that it does receive as spam.
A spam trap might a common name such as
user1 that has never been valid
and is discovered by unsolicited bulk email
dictionary attacks or guessing.
It might instead be an address hidden in a web page
or a mailbox of an account that has been disabled for many months.
Any spam trap might receive legitimate mail.
For example, a spam trap that differs from an ordinary mailbox by a
single character might receive mail intended for the ordinary mailbox.
It might be best for a system to reject mail sent to such a trap so
that legitimate mail senders know that their messages have gone astray.
A mailbox that is a long string of arbitrary letters and digits is much
less likely to receive legitimate messages and so might best accept
all messages without complaint.
There are several ways to connect
spam trap mailboxes to DCC:
dccproc -R -tMANY -cCMN,MANY -o/dev/null
will accept a message on STDIN,
look for the IP address of the sender among
Received: SMTP fields,
reports the message to the DCC server as spam and the IP address as the sender,
and exit with the default value of
dccif-test was written to test the interface to the DCC interface daemon,
When wired to a spam trap, it is more efficient than dccproc.
dccif-test -cclnt-IP-addr -oSPAM -O/dev/null
will do much the same as the dccproc example above.
- whiteclnt file option line
The best way to build a spam trap is with a
per-user whiteclnt file
option spam-trap-accept or option spam-trap-reject
With sendmail, virtual user mapping can be used to send mail to invalid
mailboxes to a single mailbox whose corresponding DCC per-user
whiteclnt file contains an
option spam-trap-accept or option spam-trap-reject
How many flooding peers does my DCC server need?
A single flooding peer delivers all reports of checksums of bulk
mail seen by any DCC server. Additional peers provided reports
sooner and so help the clients of a peer detect spews of spam sooner.
However, more peers will cause more reports to be duplicates.
A DCC server in a network of many servers should have at least three
flooding peers to ensure that the failure of a single server or network
link cannot partition the network.
Limiting the number the number of peers of any server to four or perhaps
a few more ensures that no single server is critical to the network.
To minimize the distances in the network, four peers
per server seem necessary.
An organization with more than one server can be viewed as a single
server by other organizations, with its servers flooding each other
and external peers spread among its servers.
This protects the network should the organization suffer large scale problems
while protecting the organization from single points of failure.
Do I need to tell the operators of other DCC servers
the password for controlling my server to turn on flooding?
No, you do not need to and generally should not tell other DCC server
operators the passwords for controlling your server with
the cdcc command.
Every Inter-server flood of checksums is authorized by lines in
each server's /usr/local/dcc/flod file
and authenticated by the password associated with the
passwd-ID in those lines.
The passwd-ID is a server-ID
defined in the /usr/local/dcc/ids file
that should generally be used only to authenticate floods of checksums.
How can I figure out why flooding is not working?
Many DCC server problems can be diagnosed by turning
on one or more of the tracing modes in the server with the
cdcc trace operation
or by restarting the server with
The cdcc flood list
operation displays the current flooding peers of a DCC server.
Counts of checksum reports sent and received to and from
a single peer can be displayed with
cdcc "flood stats ID"
The positions in the local database of outgoing streams of checksums
are displayed by the start of dblist -Hv.
Why didn't the RTT reported by the
cdcc info operation
change when my network topology changed?
The RTT or round trip time is an average value.
Changes in network topology, server load, and so forth are not
immediately reflected in the RTT to avoid switching DCC servers
When my clients are configured to use SOCKS, they do not
realize immediately when a server is down.
When configured to use SOCKS, DCC clients cannot "connect"
to a server and so do not receive ICMP errors and must wait for
timeouts to know the server is not answering.
This document describes DCC version 1.3.103.