INN - cancels
What are "cancels"
RFC 1036 says among other things:
"cancel <Message-ID> If a message with the given Message-ID is present on the local system, the message is cancelled. This mechanism allows a user to cancel a message after the message has been distributed over the network."
Cancels are a way to remove an article from a group. Most important use is to remove an article that you didn't intend to send to this group, for example a private mail. Don't forget a group is mostly public.
But, basically, anybody having the necessary knowledge can issue a cancel to remove an article from a group, even if he's not the article author. This may lead to "cancel war" where an user try to remove articles from an opponent. Of course this is bad.
...and how forgive them
"In order not to actually process any cancel or supersedes messages, you can start innd with the -C flag, or add this flag to the innflags parameter. " and "-C This flag tells innd to accept and propagate but not actually process cancel or supersedes messages. This is intended for sites concerned about abuse of cancels, or that wish to use another cancel mechanism with stronger authentication". (on this page).
Send a cancel message
This is not obvious and even pretty complicated because the way most clients works.
When an usenet client read a message, at first it only read the headers and display the subject message, but as soon as you click on the header to read the message, it is downloaded to your computer and only you can remove it.
So there are often two kind of "remove article" command: delete the article from your client, works only for you and on any article, even not yours. But then no other people is concerned and they can read the message at will.
The second kind is the true "cancel" message. This one try to remove the message from any server. But...
- you have to be the owner of the message. This is the rule, but some servers allows cancels on any messages :-(.
- some server refuse to acknowledge cancels. If your message got to these servers, it will be there forever (as long as the server keeps messages).
- Cancel-lock/cancel-key aim to solve this problem but not all the clients knows it and not all the servers accept it. Hopefully my server do (if not it's some misconfiguration I will try to fix if you warn me).
- Thunderbird: "Delete Message" in context menu is of the first kind (only local), "Cancel Message" from the "Message" menu is a real cancel. To have this option you may have to trick Thunderbird. The cancel also remove the article from view, even if it's not really cancelled.
- Mesnews: explains the fact clearly: option "Remove the message from the server" (I have the french version, I dunno the exact wording) for the cancel. Warn if the cancel is forgiven.
- Pan: "Article menu, Remove Article" is local only, "Postage menu, Cancel Article" is the real cancel. Warn if the cancel is forgiven.
"test" groups are aimed at such trials. Try not to use other groups for this.