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Using DCC

DCC is the Distributed Checksum Clearinghouse. . DCC is a HashSharingSystem.

Enabling DCC

To make SpamAssassin use DCC, uncomment the following line in the v310.pre file so the plugin gets loaded:

loadplugin Mail::SpamAssassin::Plugin::DCC

I've installed DCC but I never seem to get DCC hits, even though I should. Why?

If you have installed DCC, and are running spamassassin or spamd without the "-L" switch, you should be seeing hits in spam reports.

Check your firewall setup. DCC uses UDP packets when replying, which are blocked by most firewalls by default. As a result, it requires that you open your firewall for DCC reply packets on UDP port 6277. Here's sample firewall rules required:

      allow udp local gt 1023 to remote 6277
      allow udp remote 6277 to local gt 1023

If you're running a large site, processing upwards of tens of thousands of messages a day, the DCC maintainers have requested that you consider setting up your own DCC server as described in dccd(8), and arrange to peer with the rest of the public servers, to reduce their load.

IPTables Setup:

Assuming you allow all outbound packets out of your machine, you only need to add an INPUT rule to your /etc/sysconfig/iptables file. Add the following line in your INPUT chain, above any REJECT rules:

-A <chain-name> -p udp -m udp --dport 1024:65535 --sport 6277 -j ACCEPT

DCC does not give hits in spamd

If DCC gives hits when run with "spamassassin" from the command-line, but not from spamd, you may have a PATH issue where the DCC client cannot be found in spamd's path.

Add the path to your dccproc tool to the file:

    dcc_path /usr/local/bin/dccproc

Detailed instructions

Very detailed instructions on configuring DCC for a personal Unix account are in SingleUserUnixInstall.


Using SpamAssassin to submit spam is described in ReportingSpam.

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