dccd(8)               Distributed Checksum Clearinghouse               dccd(8)

NNAAMMEE
     ddccccdd -- Distributed Checksum Clearinghouse Daemon

SSYYNNOOPPSSIISS
     ddccccdd [--6644ddVVbbffFFQQ] --ii _s_e_r_v_e_r_-_I_D [--nn _b_r_a_n_d] [--hh _h_o_m_e_d_i_r] --II [_h_o_s_t_-_I_D][_,_u_s_e_r]
          [--aa [_s_e_r_v_e_r_-_a_d_d_r][_,_s_e_r_v_e_r_-_p_o_r_t]] [--qq _q_s_i_z_e]
          [--GG [_o_n_,][_w_e_a_k_-_b_o_d_y_,][_w_e_a_k_-_I_P_,][_e_m_b_a_r_g_o][_,_w_i_n_d_o_w][_,_w_h_i_t_e]]
          [--WW [_r_a_t_e][_,_c_h_g][_,_d_b_s_i_z_e]] [--KK [_n_o_-]_t_y_p_e] [--TT _t_r_a_c_e_m_o_d_e]
          [--uu _a_n_o_n_-_d_e_l_a_y[_*_i_n_f_l_a_t_e]] [--CC _d_b_c_l_e_a_n] [--LL _l_t_y_p_e_,_f_a_c_i_l_i_t_y_._l_e_v_e_l]
          [--RR [_R_L___S_U_B],[_R_L___A_N_O_N],[_R_L___A_L_L___A_N_O_N],[_R_L___B_U_G_S]]

DDEESSCCRRIIPPTTIIOONN
     DDccccdd 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 or close cluster of DCC servers is identified by a
     numeric _s_e_r_v_e_r_-_I_D.  Each DCC client is identified by a _c_l_i_e_n_t_-_I_D, either
     explicitly listed in the _i_d_s file or the special anonymous client-ID.
     Many computers are expected to share a single _c_l_i_e_n_t_-_I_D.  A _s_e_r_v_e_r_-_I_D is
     less than 32768 while a _c_l_i_e_n_t_-_I_D is between 32768 and 16777215.  DCC
     server-IDs need be known only to DCC servers and the people running them.
     The passwords associated 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 sin-
     gle client computer can use different passwords with different server
     computers.  See the _i_d_s file.

     A 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 than
     servers.

     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 ddccccdd daemon tries to maintain TCP/IP connections to the other
     servers listed in the _f_l_o_d file, and send them reports containing check-
     sums with total counts exceeding thresholds.  Changes in the _f_l_o_d file
     are noticed automatically within minutes.

     Controls on report flooding are specified in the _f_l_o_d 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 _d_e_l is not present among the _i_n_-_o_p_t_s 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 --QQ option to count extremely bulk mail should be marked
     with _t_r_a_p_s.  They can be seen as counting millions of targets, so the
     _t_r_a_p_s flag on their _f_l_o_d file entry changes their incoming flooded
     reports counts to _m_a_n_y_.

     DDccccdd automatically checks its _f_l_o_d and _i_d_s files periodically.  Cdcc(8)
     has the commands nneeww iiddss and fflloooodd cchheecckk to tell ddccccdd to check those two
     files immediately.  Both files are also checked for changes after the
     SIGHUP signal.

   OOPPTTIIOONNSS
     The following options are available:

     --66   enable IPv6.  The default is equivalent to --44.  See also the IPv4
          and IPv6 options in the _f_l_o_d file description below and the _I_P_v_6 _o_n
          cdcc(8) command.

     --44   disable IPv6.  See also --66.

     --dd   enables debugging output.  Additional --dd options increase the number
          of messages.

     --VV   displays the version of the DCC server daemon.

     --bb   causes the server to not detach itself from the controlling tty or
          put itself into the background.

     --FF   uses write() instead of mmap() in some cases to modify the DCC data-
          base.  It is the default on Solaris.

     --ff   turns off --FF.

     --QQ   causes the server to treat reports of checksums as queries except
          from DCC clients marked trusted in the _i_d_s file with _r_p_t_-_o_k.  See --uu
          to turn off access by anonymous or unauthenticated clients

     --ii _s_e_r_v_e_r_-_I_D
          specifies the ID of this DCC server.  Each server identifies itself
          as responsible for checksums that it forwards to other servers.

     --nn _b_r_a_n_d
          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_-_D_C_C headers generated by the DCC.

     --hh _h_o_m_e_d_i_r
          overrides the default DCC home directory, _/_v_a_r_/_d_c_c.

     --II [_h_o_s_t_-_I_D][_,_u_s_e_r]
          changes the server's globally unique identity for flooding from the
          default value consisting of the first 16 characters of the host
          name.  or changes the UID and GID of the process _H_o_s_t_-_I_D is a string
          of up to 16 characters that replaces the first 16 characters of the
          system's hostname in assertions of the server-ID that are flooded to
          peers.  _U_s_e_r must be valid user name.

     --aa [_s_e_r_v_e_r_-_a_d_d_r][_,_s_e_r_v_e_r_-_p_o_r_t]
          adds an hostname or IP address to the list of local IP addresses
          that the server answers.  Multiple --aa 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 --aa 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
          _s_e_r_v_e_r_-_p_o_r_t 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 _s_e_r_v_e_r_-_a_d_d_r 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 ddccccdd 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
          ddccccdd to bind to an IN_ADDRANY UDP socket.

          Outgoing TCP connections to flood checksum reports to other DCC
          servers used the IP address of a single --aa option, but only if there
          is single option that is not localhost.  See also the _f_l_o_d file.

     --qq _q_s_i_z_e
          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.

     --GG [_o_n_,][_w_e_a_k_-_b_o_d_y_,][_w_e_a_k_-_I_P_,][_e_m_b_a_r_g_o][_,_w_i_n_d_o_w][_,_w_h_i_t_e]
          changes ddccccdd 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 _e_m_b_a_r_g_o _s_e_c_o_n_d_s and so is probably not an open proxy,
          worm-infected personal computer, or other transient source of spam,
          the triple of _(_I_P _a_d_d_r_e_s_s_,_s_e_n_d_e_r_,_r_e_c_i_p_i_e_n_t_) is added to a database
          similar to the usual DCC database.  If the SMTP client does not try
          again after _e_m_b_a_r_g_o seconds and before _w_i_n_d_o_w 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 _w_h_i_t_e seconds after the last accepted mail message.  The
          triple is forgotten if it is ever associated with unsolicited bulk
          email.

          All three durations can be a number of minutes, hours, days, or
          weeks followed by _M_I_N_U_T_E_S, _M, _H_O_U_R_S, _H, _D_A_Y_S, _D, _W_E_E_K_S or _W.  The
          default is --GG _2_7_0_s_e_c_o_n_d_s_,_7_d_a_y_s_,_6_3_d_a_y_s.  The first duration or the
          _e_m_b_a_r_g_o should be longer than open proxies can linger retransmit-
          ting.  The second _w_i_n_d_o_w 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 _w_h_i_t_e 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 _e_m_b_a_r_g_o.  If
          _w_e_a_k_-_b_o_d_y 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 _w_e_a_k_-_I_P is present, all mail from an SMTP client at an IP address
          is accept after any message from the same IP address has been
          accepted.

          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
          _g_r_e_y___f_l_o_d file.

          All greylist cooperating or flooding greylist servers _m_u_s_t use the
          same --GG values.

          Clients of greylist servers cannot be anonymous and must have
          client-IDs and passwords assigned in the _i_d_s file.  This implies
          that cdcc commands directed to greylist servers must specify the
          server-ID.

          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
          greylist database.

          The target counts of embargoed messages are reported to the DCC net-
          work to improve the detection of bulk mail.

     --WW [_r_a_t_e][_,_c_h_g][_,_d_b_s_i_z_e]
          controls quick database cleaning.  If the database is larger than
          _d_b_s_i_z_e, it seems that the database has not recently and is not about
          to be cleaned, ddccccdd is receiving fewer than _r_a_t_e requests per sec-
          ond, and if telling DCC clients that the database is about to be
          cleaned reduces that rate by _c_h_g%, then ddccccdd starts dbclean(8) for a
          quick database cleaning.  The cleaning is abandoned if it takes too
          long.  The default values are equivalent to --WW _1_._0_,_4_0_._0_,_R_S_S where
          _R_S_S is the maximum dccd resident set displayed the system log by --dd
          when ssttaarrttss.

     --KK [_n_o_-]_t_y_p_e
          marks checksums of _t_y_p_e (not) be kept or counted in the database
          unless they appear in the whitelist.  Explicit settings add to or
          remove from the initial contents of the list, which is equivalent to
          --KK _B_o_d_y --KK _F_u_z_1 --KK _F_u_z_2.

     --TT _t_r_a_c_e_m_o_d_e
          causes the server to trace or record some operations.  _t_r_a_c_e_m_o_d_e
          must be one of the following:
            _A_D_M_N    administrative requests from the control program, cdcc(8)
            _A_N_O_N    errors by anonymous clients
            _C_L_N_T    errors by authenticated clients
            _R_L_I_M    rate-limited messages
            _Q_U_E_R_Y   all queries and reports
            _R_I_D_C    some messages concerning the report-ID cache that is used
                    to detect duplicate reports from clients
            _F_L_O_O_D   messages about inter-server flooding connections
            _F_L_O_O_D_2  messages about flooded reports
            _I_D_S     unknown server-IDs in flooded reports
            _B_L      requests from clients in the _b_l_a_c_k_l_i_s_t file.
            _D_B      odd database events including long chains of duplicate
                    checksums
            _W_L_I_S_T   reports of whitelisted checksums from authenticated, not
                    anonymous DCC clients
          The default is _A_N_O_N _C_L_N_T.

     --uu _a_n_o_n_-_d_e_l_a_y[_*_i_n_f_l_a_t_e]
          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
          _a_n_o_n_-_d_e_l_a_y is multiplied by 1 plus the number of recent anonymous
          requests from an IP address divided by the _i_n_f_l_a_t_e value.

          The string _F_O_R_E_V_E_R turns off all anonymous or unauthenticated access
          not only for checksum queries and reports but also cdcc(8) ssttaattss
          requests.  A missing value for _i_n_f_l_a_t_e turns off inflation.

          The default value is _5_0_,_n_o_n_e, except when --GG is used in which case
          _F_O_R_E_V_E_R is assumed and required.

     --CC _d_b_c_l_e_a_n
          changes the default name or path of the program used to rebuild the
          hash table when it becomes too full.  The default value is
          _/_v_a_r_/_d_c_c_/_l_i_b_e_x_e_c_/_d_b_c_l_e_a_n in the _/_v_a_r_/_d_c_c_/_l_i_b_e_x_e_c directory.  The
          value can include arguments as in _-_C _'_$_D_C_C___L_I_B_E_X_E_C_/_d_b_c_l_e_a_n _-_F_'.

           Dbclean _s_h_o_u_l_d _n_o_t be run by ddccccdd 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

     --LL _l_t_y_p_e_,_f_a_c_i_l_i_t_y_._l_e_v_e_l
          specifies how messages should be logged.  _L_t_y_p_e must be _e_r_r_o_r, _i_n_f_o,
          or _o_f_f to indicate which of the two types of messages are being con-
          trolled or to turn off all syslog(3) messages from ddccccdd.  _L_e_v_e_l must
          be a syslog(3) level among _E_M_E_R_G, _A_L_E_R_T, _C_R_I_T, _E_R_R, _W_A_R_N_I_N_G, _N_O_T_I_C_E,
          _I_N_F_O, and _D_E_B_U_G.  _F_a_c_i_l_i_t_y must be among _A_U_T_H, _A_U_T_H_P_R_I_V, _C_R_O_N,
          _D_A_E_M_O_N, _F_T_P, _K_E_R_N, _L_P_R, _M_A_I_L, _N_E_W_S, _U_S_E_R, _U_U_C_P, and _L_O_C_A_L_0 through
          _L_O_C_A_L_7.  The default is equivalent to
                --LL _i_n_f_o_,_M_A_I_L_._N_O_T_I_C_E --LL _e_r_r_o_r_,_M_A_I_L_._E_R_R

     --RR [_R_L___S_U_B],[_R_L___A_N_O_N],[_R_L___A_L_L___A_N_O_N],[_R_L___B_U_G_S]
          sets one or more of the four rate-limits.  _R_L___S_U_B 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
          address independently.

          _R_L___A_N_O_N 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 --uu than to change this value to exclude
          anonymous clients.

          _R_L___A_L_L___A_N_O_N 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.

          _R_L___B_U_G_S 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 --RR _4_0_0_,_5_0_,_6_0_0_,_0_._1

FFIILLEESS
     /var/dcc  is the DCC home directory containing data and control files.
     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:
               _h_o_s_t[_,_r_p_o_r_t][_;_s_r_c[_,_l_p_o_r_t]] _r_e_m_-_I_D [_p_a_s_s_w_d_-_I_D [_o_-_o_p_t [_i_-_o_p_t]]]
               where absent optional values are signaled with "-" and
                _h_o_s_t is the IP address or name of a DCC server and _r_p_o_r_t is
                    the name or number of the TCP port used by the remote
                    server.
                _s_r_c and _l_p_o_r_t are the IP address or host name and TCP port
                    from which the outgoing flooding connection should come.
                    Incoming flooding connections must arrive at an address
                    and port specified with --aa.
                _r_e_m_-_i_d is the server-ID of the remote DCC server.
                _p_a_s_s_w_d_-_I_D 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 _i_d_s file and incoming floods must be
                    signed with either password of the remote server-ID.
                _i_-_o_p_t and _o_-_o_p_t are comma separated lists of
                     _o_f_f turns off flooding to the remote or local system.
                     _t_r_a_p_s indicates that the remote sending or local receiv-
                         ing system has only spam traps.
                     _n_o_-_d_e_l 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.
                     _d_e_l says delete requests are accepted by the remote or
                         local server.
                     _n_o_-_l_o_g_-_d_e_l turns off logging of incoming requests to
                         delete checksums.
                     _p_a_s_s_i_v_e 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 _S_O_C_K_S or _N_A_T
                         on its _f_l_o_d file entry for this system.
                     _S_O_C_K_S 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 _p_a_s_s_i_v_e on its
                         _f_l_o_d file entry for this system.
                     _N_A_T differs from _S_O_C_K_S only by not using the SOCKS proto-
                         col.
                     _I_D_1_-_>_I_D_2 converts server-ID _I_D_1 in flooded reports to
                         server-ID _I_D_2.  Either _I_D_1 or _I_D_2 may be the string
                         `self' to specify the server's own ID.  _I_D_1 can be
                         the string `all' to specify all server-IDs or a pair
                         of server-IDs separated by a dash to specify an
                         inclusive range.  _I_D_2 can be the string `ok' to send
                         or receive reports without translation or 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 generated report, the first con-
                         version is applied and the second is ignored.
                     _l_e_a_f_=_p_a_t_h_-_l_e_n does not send reports with paths longer
                         than _p_a_t_h_-_l_e_n server-IDs.
                     _I_P_v_4 overrides a --66 setting for this flooding peer.
                     _I_P_v_6 overrides the default or an explicit --44 setting.
                     _v_e_r_s specifies the version of the DCC flooding protocol
                         used by the remote DCC server with a string such as
                         `version2'.
                     _t_r_a_c_e sends information about a single peer like the
                         cdcc(8) command ttrraaccee FFLLOOOODD oonn does for all peers.
                     _t_r_a_c_e_2 sends information about individual flooded reports
                         like the cdcc(8) command ttrraaccee FFLLOOOODD22 oonn does for all
                         peers.
     grey_flod is the equivalent of _f_l_o_d used by ddccccdd when it is a greylist
               server.
     flod.map  is an automatically generated file in which ddccccdd records its
               progress sending or flooding reports to DCC peers.
     grey_flod.map is the equivalent of _f_l_o_d_._m_a_p _u_s_e_d _b_y ddccccdd when it is a
               greylist server.
     ids       contains the IDs and passwords known by the DCC server.  An _i_d_s
               file that can be read by others cannot be used.  It contains
               blank lines, comments starting with "#" and lines of the form:
                     _i_d[_,_r_p_t_-_o_k][_,_d_e_l_a_y_=_m_s[_*_i_n_f_l_a_t_e]] _p_a_s_s_w_d_1 [_p_a_s_s_w_d_2]
               where
                _i_d  is a DCC _c_l_i_e_n_t_-_I_D or _s_e_r_v_e_r_-_I_D.
                _R_p_t_-_o_k if present overrides --QQ by saying that this client is
                    trusted to report only checksums for unsolicited bulk
                    mail.
                _d_e_l_a_y_=_m_s[_*_i_n_f_l_a_t_e] delays answers to systems using the client
                    _i_d.  The _d_e_l_a_y in milliseconds is multiplied by 1 plus the
                    number of recent requests from an IP address using _i_d
                    divided by the _i_n_f_l_a_t_e value.  See --uu.
                _p_a_s_s_w_d_1 is the password currently used by clients with identi-
                    fier _i_d.  It is a 1 to 32 character string that does not
                    contain blank, tab, newline or carriage return characters.
                _p_a_s_s_w_d_2 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 ddccccdd that server-IDs in the flooded reports are valid.
               The string _u_n_k_n_o_w_n 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
               with --GG.
     blacklist if present, contains a list of IP addresses and blocks of IP
               addresses DCC clients 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
                     [_t_r_a_c_e_,] [_o_k_,] [_b_a_d] xxx.xxx.xxx.xxx[/yy]
               Changes to the file are automatically noticed and acted upon
               within a few minutes.  Addresses or blocks of addresses can be
               preceded with _o_k to "punch holes" in blacklisted blocks or with
               _t_r_a_c_e to log activity.  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.

EEXXAAMMPPLLEESS
     ddccccdd is usually started with other system daemons with something like the
     script _/_v_a_r_/_d_c_c_/_l_i_b_e_x_e_c_/_r_c_D_C_C.  That scripts uses values in
     /var/dcc/dcc_conf to start the server.  With the argument _s_t_o_p,
     _/_v_a_r_/_d_c_c_/_l_i_b_e_x_e_c_/_r_c_D_C_C 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 script

SSEEEE AALLSSOO
     cdcc(8), dcc(8), dbclean(8), dblist(8), dccifd(8), dccm(8), dccproc(8).
     dccsight(8),

HHIISSTTOORRYY
     ddccccdd is based on an idea from Paul Vixie.  It was designed and written at
     Rhyolite Software, starting in 2000.  This document describes version
     1.3.103.

                               February 26, 2009