Here's the other one Eric ----------------------------------------------------------------------------- From: hughes@soda.berkeley.edu (Eric Hughes) Newsgroups: alt.hackers Subject: a new feature of the remailer Message-ID: <HUGHES.92Oct9095848@soda.berkeley.edu> Date: 9 Oct 92 13:58:48 GMT Distribution: alt Organization: /accounts/hughes/.organization Lines: 191 Here is the third in the series of articles on the remailer. Remember, these are meant to be fairly tutorial. Eric ----------------------------------------------------------------------------- There's a new feature in the remailing software. Some people can't add arbitrary header fields because of mailer or gateway restrictions. This restricts them from using the remailer. I have added a facility to allow new header fields to be pasted onto the end of a header when the mail arrives. This effectively happens before processing by the remailer software. These new fields exist during transit in the message body, where they remain untouched. Only after the message is delivered to my account does this operator take effect. Syntax: If the first line of the body is the two characters "::", then the following lines are appended to the header, up to the next blank line. Here's how it works. First of all, here's my new .maildelivery file: ------- cut here ------- # # field pattern action/ string # result (quote included spaces) # Request-Remailing-To "" pipe R "perl remailer/remail.perl" Request-Remailing-To "" file R remailer/archive * "" pipe R "/usr/local/lib/mh/rcvtty -biff" * "" pipe ? "perl remailer/incoming.header.perl" ------- cut here ------- Comments are indicated by #. The Request-Remailing-To lines have been there. The second of the makes an archive for debugging purposes. It will go eventually. The third field, "*", indicates all fields, it runs 'rcvtty' on my mail; this replaces the function of biff, since mail is getting piped to slocal now, disabling biff. The last line is the important one. It says "If the mail hasn't been delivered by now, run the incoming header rewrite script on it. If that doesn't work, continue trying to deliver it." Now here's the trick. slocal has no way of taking the output of the rewrite and continuing to process it. (It should. It would make this whole job easy.) So in order to continue processing, you need to redeliver the mail. You could invoke sendmail and mail it back to yourself, but that would mangle the existing header. So the thing to do is to recursively invoke slocal from within the perl script. Here's the perl script to do all this: ------- cut here ------- # First read in the whole header. # We check for the Second-Pass: line to detect infinite loops. while (<>) { last if /^$/ ; exit 1 if /^Second-Pass:/ ; $header .= $_ ; } # We have just read the last line in the header. # Now we check to see if there is a pasting operator. if ( ( $_ = <> ) && /^::$/ ) { while (<>) { last if /^$/ ; $header .= $_ ; } } else { # There is either an empty body or no pasting operator # Thus exit with a return code of 1 to indicate that # the mail has not been delivered. exit 1 ; } # There was a header pasting operator. # So we open 'slocal' as a pipe, effectively redelivering the mail # back to ourselves. #open( OUTPUT, ">foo" ) ; open( OUTPUT, "| /usr/local/lib/mh/slocal -user hughes" ) ; select( OUTPUT ) ; # print a "From " line to satisfy slocal @weekdays = ( "Sun","Mon","Tue","Wed","Thu","Fri", "Sat" ) ; @months = ( "Jan","Feb","Mar","Apr","May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec" ) ; ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime ; printf "From hughes %s %s ", @weekdays[ $wday ], @months[ $mon ] ; printf "%2d %02d:%02d:%02d 19%d\n", $mday, $hour, $min, $sec, $year ; # Now just print out the message print $header ; print "Second-Pass:\n" ; print "\n" ; while (<>) { } continue { print ; } ------- cut here ------- Here's how the perl script works. The first loop reads lines from the existing header. When it sees a blank line (regexp /^$/) it terminates the loop. If it sees a field "Second-Pass", it knows it has filtered this message before and exits with a return code indicating that the mail has not been delivered. The variable $header is appended with the current header line. $header contains the whole header when the loop terminates. Properly speaking, the Second-Pass test is not necessary to detect infinite loops. Since the pasting operator gets removed during the rewrite, the script won't return an exit status of 0 more times than the pasting operator appears. But should something get screwed up, such as a different module adding pasting commands (how? I don't know), the Second-Pass test should prevent infinite recursion. The next statement reads another line from the input file. This line is the first line of the message body. If this line is the pasting operator, then header lines are accumulated in $header as before until a blank line. The difference is that these header lines are being read from the body of the message. If there is no pasting operator, the script exits undelivered. At this point we now have to redeliver the message back to ourselves. We first open slocal as the output pipe. The next section is a kludge. It turns out that slocal strips off the out-of-band "From " (no colon) line that the mail delivery system uses. In other words, the message which slocal pipes into its pipes is not identical to the message it itself received. This means that slocal cannot be directly recursed. What this section does is to create a "From " line to make slocal happy. It calls localtime() and then formats those numbers into the proper form. It turns out that slocal will deliver this mail without the "From " line, even to /usr/spool/mail, but it doesn't do so properly. On my system, in added some delimiters which I think I've tracked down to the 'mtstailor' file, namely mmdelivery1 and mmdelivery2. Since these are not null on my system, there's some garbage added which screws up separation of the spool file into messages. Adding a "From " line fixes that. This misbehavior may not be so surprising, considering that slocal was "meant" to be invoked only in a .forward file. Now we print the variable $header which contains the whole header, including newlines. Using a single string removes the need for an array. We added the Second-Pass line and a blank line for the end of the header. The final loop prints out the rest of the message body. There is another way to proceed to get the same functionality. One could write a filter to translate the first occurrence only of \n\n::\n into \n. We could then pass the message through this filter before slocal saw it. And for now, that would do the same thing. But suppose we want more that one rewrite rule active? Then you would only be able to apply each rewrite rule exactly once in fixed order. You want to be able to rewrite a message and then apply all the rewrite rules again. At least one other rewrite rule is planned: automatic decryption. Since decrypting a message will completely change the body, and since some of the header fields may need to be hidden, you have to be able to decrypt the body and then paste on header lines. But since you need to indicate an encrypted body by a header line (well, not really, but it's more reliable), and since some people can't add these header lines, you need to paste lines before encryption as well. Thus the rewrite rules need to be applied asyncronously and hence I'm using a fairly complex slocal scheme to do a simple filter. Eventually I hope to write an equivalent to slocal which knows about message rewrites and simple filters, but that's for later. ----------------------------------------------------------------------------- To follow this project (and others soon to be started), send mail to cypherpunks-request@toad.com and I'll add you to the mailing list. Eric