Sendmail, 3rd Edition

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 ...