Here's how ypserv abuses named

[Craig Leres]

This afternoon, I noticed a lot of nameserver traffic on my ethernet. A
host was making an request, getting an answer, and then asking the same
question again. Over and over. After a little investigation, I think I
understand the problem and can offer a solution.

Let's say someone wants to send mail to a host that has a MX record but no
A record. (For example, DEC is setup so that any mx query in the domain
dec.com returns decwrl.dec.com.) Unfortunately, he try this from a Sun
that is running a non-mx sendmail.

Here's where things go wrong. Sendmail does a gethostbyname() which turns
into a yp request. Ypserv (which is running with -i) forks and the child
makes an A record request of the nameserver. The nameserver comes back
with the status NO_ADDRESS (an alias for NO_DATA) which means that there
are valid records for the name (i.e. the MX record) but that the requested
record does not exist. The ypserv child exists and after some short
timeout, the parent ypserv forks off another to repeat the process.

To demonstrate the bug, try something similar to:

	% ypmatch fnord.dec.com hosts.byname

This should hang for a long time (perhaps forever) while ypserv
(repeatedly) puts the DNS (Domain Name System) through its paces.

It seems obvious that if ypserv gets NO_ADDRESS, it should just return the
error and not retry. Appended is a context diff to the 3.5 version of
usr.etc/ypserv/ypserv_proc.c (your line numbers may vary) which implements
this fix. I'm not sure how this problem relates to 4.0.

		Craig
------
RCS file: RCS/ypserv_proc.c,v
retrieving revision 1.3
diff -c -r1.3 ypserv_proc.c
*** /tmp/,RCSt1a01990	Fri Mar  3 04:05:34 1989
--- ypserv_proc.c	Fri Mar  3 04:06:01 1989
***************
*** 172,178 ****
  	if (h == NULL) {
  		extern int h_errno;

! 		if (h_errno == TRY_AGAIN || h_errno == NO_ADDRESS) exit(0);
  		*statusp = YP_NOKEY;
  		return(CHILD_PROC);
  	}
--- 172,181 ----
  	if (h == NULL) {
  		extern int h_errno;

! 		/* Bail if there's a chance we'll get the info next time */
! 		if (h_errno == TRY_AGAIN)
! 			exit(0);
! 
  		*statusp = YP_NOKEY;
  		return(CHILD_PROC);
  	}