Bug 17310

	In some cases, resolving a host name may loop forever (inside
	libc): After a NIS client sends the query to the NIS server,
	if the NIS server tries to use a DNS query on its own to
	resolve the name, and if the DNS query doesn't return
	immediately any result (a success or a failure, i.e. it is left 
	to expire), the query of the NIS client itself may expire
	before it gets any answer from the NIS server (which is waiting
	for the DNS query to complete). However, the libc code doesn't
	handle properly this situation, and it will enter an infinite
	loop trying to resolve the host name.
	The situation is very unplesant for programs like sendmail,
	because the result will be that a sendmail process will
	be blocked by this infinite loop and will not be able to
	deliver the email to the rest of the recepients.

Fix: Apply the following patch to src/lib/libc/yp/yplib.c, then
	recomplile libc and install it. However, this solution
	will just break the infinite loop to "only" 20 loops.
	A better solution is the fix the NIS server to cache
	its recent results, such that after the first timeout
	of the client's request, the second request will hit
	the negative answer at the server, and then the
	client will immediately return an error instead of looping
	20 times. Note that yplib.c has a number of other places
	that introduce the same potential danger for infinitive loop.
	Search for "again:", and add there a similar counter
	that would break the infinitive loop.

How-To-Repeat: 
	Compile and execute the following code. Note that the problem
	can be observed only if the particular IP address is such
	that it takes at least 30-60 seconds for "nslookup" to timeout:

	#include <stdio.h>
	#include <stdlib.h>
	#include <sys/types.h>
	#include <sys/socket.h>
	#include <netinet/in.h>
	#include <arpa/inet.h>
	#include <netdb.h>


	int
	main()
	{
		struct in_addr in;
		struct hostent *hp;
				
	/*
	  Nothing personal regarding address "200.230.88.4". It is just that
	  my DNS query fails for this address with "Server failed"
	  after approx. 30-60 seconds:

	  pavlin@xanadu[11] nslookup 200.230.88.4
	  Server:  catarina.usc.edu
	  Address:  128.125.51.47

	  *** catarina.usc.edu can't find 200.230.88.4: Server failed
	  */

	    	inet_aton("200.230.88.4", &in);
	        hp = gethostbyaddr((char *)&in, sizeof(in), AF_INET);
		if (hp) {
		    printf("OK\n");
		} else {
		    printf("FAILURE\n");
		}
		exit (0);
	}

	If the chosen IP address is appropriate, you should see the loop:
	pavlin@catarina[284] ./a.out 
	yp_match: clnt_call: RPC: Timed out
	yp_match: clnt_call: RPC: Timed out
	yp_match: clnt_call: RPC: Timed out
	yp_match: clnt_call: RPC: Timed out
	yp_match: clnt_call: RPC: Timed out
	...