dccd(8) Distributed Checksum Clearinghouse dccd(8)
dccd -- Distributed Checksum Clearinghouse Daemon
dccd [-dVbfFQ] -i server-ID [-n brand] [-h homedir] -I [host-ID][,user]
[-a [server-addr][,server-port]] [-q qsize]
[-W [rate][,chg][,dbsize]] [-K [no-]type] [-T [no-]tracemode]
[-u anon-delay[,inflate]] [-C dbclean] [-L ltype,facility.level]
Dccd receives reports of checksums related to mail received by DCC
clients and queries about the total number of reports of particular
checksums. A DCC server never receives mail, address, headers, or other
information from clients, but only cryptographically secure checksums of
such information. A DCC server cannot determine the text or other infor-
mation that corresponds to the checksums it receives. It only acts as a
clearinghouse of total counts of checksums computed by clients.
Each DCC server is identified by a numeric server-ID. Each DCC client is
identified by a client-ID, either explicitly listed in the /var/dcc/ids
file or the special anonymous client-ID. Many computers are expected to
share a single client-ID. A server-ID is between 100 and 32768 while a
client-ID is between 32768 and 16777215. DCC server-IDs need be known
only to DCC servers and the people running them. The passwords associ-
ated with DCC server-IDs should be protected, because DCC servers listen
to commands authenticated with server-IDs and their associated passwords.
Each client that does not use the anonymous ID must know the client-ID
and password used by each of its servers. A single client computer can
use different passwords with different server computers. See the
A /var/dcc/whitelist of known good (or bad) sources of email prevents
legitimate mailing lists from being seen as unsolicited bulk email by DCC
clients. The whitelist used by a DCC server is built into the database
when old entries are removed by dbclean(8). Each DCC client has its own,
local whitelist, and in general, whitelists work better in DCC clients
A dccd /var/dcc/whitelist file containing IP addresses that should be in
client whiteclnt files is useful. When -T WLIST tracing is enabled (as
it is by default), dccd complains to the system log when an authenticated
client reports mail from IP addresses listed as OK, MX, or MXDCC. It is
often useful to have a /var/dcc/whitecommon file containing whitelisted
The effectiveness of a Distributed Checksum Clearinghouse increases as
the number of subscribers increases. Flooding reports of checksums among
DCC servers increases the effective number of subscribers to each server.
Each dccd daemon tries to maintain TCP/IP connections to the other
servers listed in the /var/dcc/flod file, and send them reports contain-
ing checksums with total counts exceeding thresholds. Changes in the
flod file are noticed automatically within minutes.
Controls on report flooding are specified in the flod file. Each line
specifies a hostname and port number to which reports should be flooded,
a server-ID to identify and authenticate the output stream, a server-ID
to identify and authenticate an input stream from the same server, and
flags with each ID. The ability to delete reports of checksums is handy,
but could be abused. If del is not present among the in-opts options for
the incoming ID, incoming delete requests are logged and then ignored.
Floods from DCC "brands" that count only mail to spam traps and whose
servers use the -Q option to count extremely bulk mail should be marked
with traps. They can be seen as counting millions of targets, so the
traps flag on their /var/dcc/flod file entry changes their incoming
flooded reports counts to many.
Dccd automatically checks its /var/dcc/flod and /var/dcc/ids files peri-
odically. Cdcc(8) has the commands new ids and flood check to tell dccd
to check those two files immediately. Both files are also checked for
changes after the SIGHUP signal.
The following options are available. Most of them should set by changing
the /var/dcc/dcc_conf control file.
-d enables debugging output. Additional -d options increase the number
-V displays the version of the DCC server daemon. Two or more -V
options show the options with which it was built.
-b causes the server to not detach itself from the controlling tty or
put itself into the background.
-F uses write() instead of mmap() and msync() in some cases to modify
the DCC database. It is the default on Solaris except when the
database is in a memory mapped file system. See -f.
-f uses mmap() and msync() to modify the DCC database. See -F.
-Q causes the server to treat reports of checksums as queries except
from DCC clients marked trusted in the /var/dcc/ids file with
rpt-ok. See -u to turn off access by anonymous or unauthenticated
specifies the ID of this DCC server. Each server identifies itself
as responsible for checksums that it forwards to other servers.
is an arbitrary string of letters and numbers that identifies the
organization running the DCC server. The brand is required, and
appears in the SMTP X-DCC headers generated by the DCC.
overrides the default DCC home directory, /var/dcc.
sets the UID and GID of the process or the server's name for asser-
tions of its -i server-ID flooded to peers. The default name is the
first 16 characters of the host name. If present, user must be a
valid user name.
adds an hostname or IP address to the list of local IP addresses
that the server answers. Multiple -a options can be used to specify
a subset of the available network interfaces or to use more than one
port number. The default without any -a options is to listen on all
local IP addresses. It can be useful to list some of the IP
addresses of multi-homed hosts to deal with firewalls. By default
server-port is 6277 for DCC servers and 6276 for greylist servers.
It is the UDP port at which DCC requests are received and the TCP
port for incoming floods of reports.
If server-addr is absent and if the getifaddrs(8) function is sup-
ported, separate UDP sockets are bound to each configured network
interface so that each DCC clients receives replies from the IP
addresses to which corresponding request are sent. If dccd is
started before all network interfaces are turned on or there are
interfaces that are turned on and off or change their addresses such
as PPP interfaces, then the special string @ should be used to tell
dccd to bind to an INADDR_ANY UDP socket.
Outgoing TCP connections to flood checksum reports to other DCC
servers used the IP address of a single -a option, but only if there
is single option that is not localhost. See also the /var/dcc/flod
specifies the maximum size of the queue of requests from anonymous
or unauthenticated clients. The default value is the maximum DCC
RTT in seconds times 200 or 1000.
changes dccd to a greylist server for dccm(8) or dccifd(8).
Greylisting consists of temporarily rejecting or embargoing mail
from unfamiliar combinations of SMTP client IP address, SMTP enve-
lope sender, and SMTP envelope recipient. If the SMTP client per-
sists for embargo seconds and so is probably not an open proxy,
worm-infected personal computer, or other transient source of spam,
the triple of (IP address,sender,recipient) is added to a database
similar to the usual DCC database. If the SMTP client does not try
again after embargo seconds and before window seconds after the
first attempt, the triple is forgotten. If the SMTP client persists
past the embargo, the triple is added to the database and becomes
familiar and the message is accepted. Familiar triples are remem-
bered for white seconds after the last accepted mail message. The
triple is forgotten if it is ever associated with unsolicited bulk
All three durations can be a number of minutes, hours, days, or
weeks followed by MINUTES, M, HOURS, H, DAYS, D, WEEKS or W. The
default is -G 270seconds,7days,63days. The first duration or the
embargo should be longer than open proxies can linger retransmit-
ting. The second window time should be as long as legitimate mail
servers persist in retransmitting to recognize embargoed messages
whose retransmissions were not received because of network or other
problems. The white time should be long enough to recognize and not
embargo messages from regular senders.
Usually the DCC greylist system requires that an almost identical
copy of the message be retransmitted during the embargo. If
weak-body is present, any message with the same triple of sender IP
address, sender mail address, and target mail address ends the
embargo, even if the body of the message differs.
If weak-IP is present, all mail from an SMTP client at an IP address
is accept after any message from the same IP address has been
Unlike DCC checksums, the contents of greylist databases are private
and do not benefit from broad sharing. However, large installations
can use more two or more greylist servers flooding triples among
themselves. Flooding among greylist servers is controlled by the
All greylist cooperating or flooding greylist servers must use the
same -G values.
Clients of greylist servers cannot be anonymous and must have
client-IDs and passwords assigned in the /var/dcc/ids file. This
implies that cdcc commands directed to greylist servers must specify
White- and blacklists are honored by the DCC clients. whitelisted
messages are embargoed or checked with a greylist server. The
greylist triples of blacklisted messages, messages whose DCC counts
make them spam, and other messages known to be spam are sent to a
greylist server to be removed from the greylist database and cause
an embargo on the next messages with those triples.
Messages whose checksums match greylist server whitelists are not
embargoed and the checksums of their triples are not added to the
The target counts of embargoed messages are reported to the DCC net-
work to improve the detection of bulk mail.
controls quick database cleaning. If the database is larger than
dbsize in MBytes, the database has not recently been cleand and is
not about to be cleaned, and dccd is receiving fewer than rate
requests per second, or if telling DCC clients that the database is
about to be cleaned reduces the requests/second by chg, then dccd
starts dbclean(8) for a quick database cleaning. The cleaning is
abandoned if it takes too long.
The defaults are equivalent to -W 1.0,40.0,RSS where RSS is the max-
imum dccd resident set size displayed in the system log when the
database is opened. A rate of -W 0.0 disables quick cleanings.
marks checksums of type (not) be kept or counted in the database
(unless they appear in the /var/dcc/whitelist file). Explicit set-
tings add to or remove from the initial contents of the list, which
is equivalent to -K Body -K Fuz1 -K Fuz2.
causes the server to trace or record some operations. tracemode
must be one of the following:
ADMN administrative requests from the control program, cdcc(8)
ANON errors by anonymous clients
CLNT errors by authenticated clients
RLIM rate-limited messages
QUERY all queries and reports
RIDC some messages concerning the report-ID cache that is used
to detect duplicate reports from clients
FLOOD1 messages about inter-server flooding connections
FLOOD2 messages about flooded reports
BL requests from clients in the /var/dcc/blacklist file.
DB odd database events including long chains of duplicate
WLIST reports of whitelisted checksums from authenticated, not
anonymous DCC clients
The default is ANON CLNT WLIST except for a greylist server which
uses ANON CLNT WLIST IDS.
changes the number of milliseconds anonymous or unauthenticated
clients must wait for answers to their queries and reports. The
purpose of this delay is to discourage large anonymous clients. The
anon-delay is multiplied by 1 plus the number of recent anonymous
requests from IPv4 addresses in a /24 block or IPv6 addresses a /56
block divided by the inflate value.
The string FOREVER turns off all anonymous or unauthenticated access
not only for checksum queries and reports but also cdcc(8) stats
requests. A missing value for inflate turns off inflation.
The default value is 50, except when -G is used in which case
FOREVER is assumed and required.
changes the default name or path of the program used to rebuild the
hash table when it becomes too full. The default value is
/var/dcc/libexec/dbclean. The value can include arguments as in -C
Dbclean should not be run by dccd except in emergencies such as
database corruption or hash table overflow. Dbclean(8) should be
run daily with the /var/dcc/libexec/cron-dccd cron script
specifies how messages should be logged. Ltype must be error, info,
or off to indicate which of the two types of messages are being con-
trolled or to turn off all syslog(3) messages from dccd. Level must
be a syslog(3) level among EMERG, ALERT, CRIT, ERR, WARNING, NOTICE,
INFO, and DEBUG. Facility must be among AUTH, AUTHPRIV, CRON,
DAEMON, FTP, KERN, LPR, MAIL, NEWS, USER, UUCP, and LOCAL0 through
LOCAL7. The default is equivalent to
-L info,MAIL.NOTICE -L error,MAIL.ERR
sets one or more of the four rate-limits. RL_SUB limits the number
of DCC transactions per second from subscribers or DCC clients with
known client-IDs and passwords. This limit applies to each IP
RL_ANON limits the number of DCC transactions per second from anony-
mous DCC clients. This limit applies to each IP address indepen-
dently. It is better to use -u than to change this value to exclude
RL_ALL_ANON limits the number of DCC transactions per second from
all anonymous DCC clients. This limit applies to all anonymous
clients as a group, regardless of their IP addresses.
RL_BUGS limits the number of complaints or error messages per second
for all anonymous DCC clients as a group as well as for each DCC
client by IP address.
The default is equivalent to -R 400,50,2000,0.1
/var/dcc is the DCC home directory containing data and control files.
dcc_conf is the DCC control file.
dcc_db is the database of mail checksums.
dcc_db.hash is the mail checksum database hash table.
grey_db is the database of greylist checksums.
grey_db.hash is the greylist database hash table.
flod contains lines controlling DCC flooding of the form:
host[,rport][;src[,lport]] rem-ID [passwd-ID [o-opt [i-opt]]]
where absent optional values are signaled with "-" and
host is the IP address or name of a DCC server and rport is
the name or number of the TCP port used by the remote
src and lport are the source IP address or host name and TCP
port from which the outgoing flooding connection should
come. The string * specifies any source IP address.
Incoming flooding connections must arrive at an address
and port specified with -a.
rem-id is the server-ID of the remote DCC server.
passwd-ID is a server-ID that is not assigned to a server, but
whose first password is used to sign checksum reports sent
to the remote system. Either of its passwords are
required with incoming reports. If it is absent or "-",
outgoing floods are signed with the first password of the
local server in the ids file and incoming floods must be
signed with either password of the remote server-ID.
i-opt and o-opt are comma separated lists of
off turns off flooding to the remote or local system.
no-del says checksum delete requests are refused by the
remote or local server and so turns off sending or
accepting delete requests, respectively. By default,
delete requests are sent to remote servers and
accepted in incoming floods if and only if the peers
are exchanging DCC reputations.
del says delete requests are accepted by the remote or
no-log-del turns off logging of incoming requests to
passive is used to tell a server outside a firewall to
expect a peer inside to create both of the pair of
input and output TCP connections used for flooding.
The peer inside the firewall should use SOCKS or NAT
on its flod file entry for this system.
SOCKS is used to tell a server inside a firewall that it
should create both of the TCP connections used for
flooding and that SOCKS protocol should be used. The
peer outside the firewall should use passive on its
flod file entry for this system.
NAT differs from SOCKS only by not using the SOCKS proto-
IDS->result converts server-IDs in flooded reports. IDS
may be the string `self' to specify the server's own
ID. IDS can instead be the string `all' to specify
all server-IDs or a pair of server-IDs separated by a
dash to specify an inclusive range. result can be
the string `self' to translate to the server's own
ID. `ok' sends or receives reports without transla-
tion. The string `reject' to not send outgoing or
refuse incoming reports. Only the first matching
conversion is applied. For example, when
`self->ok,all->reject' is applied to a locally gener-
ated report, the first conversion is made and the
second is ignored.
leaf=path-len does not send reports with paths longer
than path-len server-IDs. A path-len of 0 blocks
reports from this server.
IPv4 requires only IPv4 addresses to connect to this
IPv6 requires only IPv6 addresses to connect to this
vers specifies the version of the DCC flooding protocol
used by the remote DCC server with a string such as
trace1 sends information about a single peer like the
cdcc(8) command trace FLOOD1 on does for all peers.
trace2 sends information about individual flooded reports
like the cdcc(8) command trace FLOOD2 on does for all
grey_flod is the equivalent of the /var/dcc/flod file used by dccd when
it is a greylist server.
flod.map is an automatically generated file in which dccd records its
progress sending or flooding reports to DCC peers.
grey_flod.map is the equivalent of the /var/dcc/flod.map file used by
dccd when it is a greylist server.
ids contains the IDs and passwords known by the DCC server. An ids
file that can be read by others cannot be used. It contains
blank lines, comments starting with "#" and lines of the form:
id is a DCC client-ID or server-ID.
trace logs activity from clients and flooding peers using the
rpt-ok overrides -Q by saying that this client is trusted to
report only checksums for unsolicited bulk mail.
no-reps disables DCC Reputation reports from and responses to
clients using this ID.
delay=ms[*inflate] delays answers to systems using the client
id. The delay in milliseconds is multiplied by 1 plus the
number of recent requests from an IP address using id
divided by the inflate value. See -u.
pass1 is the password currently used by clients with identi-
fier id. It is a 1 to 32 character string that does not
contain blank, tab, newline or carriage return characters.
pass2 is the optional next password that those clients will
use. A DCC server accepts either password if both are
present in the file.
Both passwords can be absent if the entry not used except to
tell dccd that server-IDs in the flooded reports are valid.
The string unknown is equivalent to the null string.
whitelist contains the DCC server whitelist. It is not used directly but
is loaded into the database when dbclean(8) is run.
grey_whitelist contains the greylist server whitelist. It is not used
directly but is loaded into the database when dbclean(8) is run
blacklist if present, contains a list of IP addresses and blocks of IP
addresses of DCC clients and flooding peers that are ignored.
Each line in the file should be blank, a comment starting with
'#', or an IP address or block of IP addresses in the form
Addresses are single IPv4 or IPv6 addresses, CIDR blocks in the
usual form, or a pair of addresses separated by a hyphen (-)
specifying an inclusive range. The last line in the file that
cover an address applies. Changes to the file are automati-
cally noticed within a few minutes. An IP address is assigned
all of the flags from all of lines that apply to it. Addresses
or blocks of addresses can be preceded with ok to "punch holes"
in blacklisted blocks or specify tracing without blacklisting.
Trace logs activity. No-anon blacklists clients only when they
use the anonymous client-ID. flood-ok says that DCC flood
peering is allowed. Bad is assumed when only an address is
present. This mechanism is intended for no more than a few
dozen blocks of addresses.
dccd_clients contains client IP addresses and activity counts.
grey_clients contains greylist client IP addresses and activity counts.
dccd is usually started with other system daemons with something like the
script /var/dcc/libexec/rcDCC. That scripts uses values in
/var/dcc/dcc_conf to start the server. With the argument stop,
/var/dcc/libexec/rcDCC can be used to stop the daemon.
The database grows too large unless old reports are removed. dbclean(8)
should be run daily with the /var/dcc/libexec/cron-dccd cron(8) script
cdcc(8), dcc(8), dbclean(8), dblist(8), dccifd(8), dccm(8), dccproc(8).
dccd is based on an idea from Paul Vixie. It was designed and written at
Rhyolite Software, starting in 2000. This document describes version
June 19, 2019
Man(1) output converted with man2html modified for DCC
$Date 2001/04/29 03:22:18 $