D.3 Reject Excess 8-Bit Characters

Much of the spam email that arrives from the Far East ( especially Korea and China) is sent with Subject : headers that contain unencoded 8-bit characters. Because 8-bit characters in the Subject : header are illegal, it is perfectly OK to reject such email.

In this section we show you how to write a routine that can reject 8-bit Subject : headers using the checkcompat ( ) routine. In the following example, the numbers to the left are for reference and are not a part of the code.

 1  int 2  checkcompat(to, e) 3          register ADDRESS *to; 4          register ENVELOPE *e; 5  { 6          char *cp, *h; 7          int len, cnt; 8    9          if (!bitnset(M_LOCALMAILER, to->q_mailer->m_flags)) 10                 return EX_OK; 11   12         if ((h = hvalue("subject", e->e_header)) =  = NULL) 13                 return EX_OK; 14         15         if ((len = strlen(h)) =  = 0) 16                 return EX_OK; 17 18         cnt = 0; 19         for (cp = h; *cp != ''; ++cp) 20         { 21                 if ((*cp & 0x80) != 0) 22                         ++cnt; 23         } 24         if ((cnt * 2) > len) 25         { 26                 e->e_flags = EF_NO_BODY_RETN; 27                 e->e_message = "553 Cannot accept eight-bit subjects"; 28                 to->q_status = "5.7.1"; 29                 return EX_UNAVAILABLE; 30         } 31         return EX_OK; 32 }

We begin (line 2) by declaring checkcompat ( ) the same way it is declared inside conf.c . The two arguments ( *to and *e ) passed to it are described in the first section of this appendix.

Our routine for rejecting 8-bit characters in the Subject : header requires four local variables (line 6). The *h will point to the value of the Subject : header. The *cp will be walked through *h looking for eight-bit characters. The len will hold the length of the Subject : header's value. The cnt will hold the count of 8-bit characters found.

Before we check the Subject : header, however, we need to make sure the message is being delivered locally. We don't need to screen outbound or relayed mail. We do that (line 9) by checking to see if the M_LOCALMAILER flag (the F=l delivery agent flag, F=l (lowercase L)) was set, which returns EX_OK if it was not.

Next , we fetch the Subject : header's value (line 12) by calling hvalue ( ). If there is no Subject : header (if hvalue returns NULL), we accept the message by returning EX_OK.

We calculate the length of the Subject : header's value (line 15) and save the length in len . If the length is zero (line 15) ”that is, if the Subject : header was present but lacked a value ”we accept the message by returning EX_OK.

If the Subject : header has a value, we next scan the value (line 19) looking for any characters that have the 8-bit set (line 21). Each such 8-bit character that is found causes the count in cnt to increment.

After all the characters in the Subject : header have been checked, we determine if more than half have the 8-bit set (line 24). If so, we reject the message with the error 553 Cannot accept eight-bit subjects .

If fewer than half of the characters have the 8-bit set, we accept the message. This is purely arbitrary. You might prefer to reject the message if even a single character has the 8-bit set.