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Bug#159781: marked as done (libc6: getaddrinfo doesn't do nsswitch.conf ordering correctly)



Your message dated Thu, 5 Sep 2002 21:01:41 -0400
with message-id <20020906010141.GA14796@phunnypharm.org>
and subject line Bug#159781: libc6: getaddrinfo doesn't do nsswitch.conf ordering correctly
has caused the attached Bug report to be marked as done.

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>From apenwarr@nit.ca Thu Sep 05 16:45:56 2002
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From: apenwarr@nit.ca
Subject: libc6: getaddrinfo doesn't do nsswitch.conf ordering correctly
To: submit@bugs.debian.org
X-Mailer: bug 3.3.10
Message-Id: <E17n4Ri-0004CO-00@insight.internal.nit.ca>
Date: Thu, 05 Sep 2002 17:45:50 -0400
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Package: libc6
Version: 2.2.5-14
Severity: normal

If I have this line in /etc/hosts:

   192.168.43.128 cproxy

and I type this:

   telnet cproxy
   
I show the following DNS activity in tcpdump:

   # tcpdump -vni eth0 port 53

   tcpdump: listening on eth0
   17:09:16.272347 192.168.12.100.34212 > 192.168.12.1.53:  
   	[udp sum ok] 61379+ AAAA? cproxy.internal.nit.ca. 
   	[|domain] (DF) (ttl 64, id 48677, len 68)
   17:09:16.272740 192.168.12.1.53 > 192.168.12.100.34212:  
   	[udp sum ok] 61379 NXDomain* 0/0/0 (40) (DF) (ttl 64, id 0, len 68)
   17:09:16.272875 192.168.12.100.34212 > 192.168.12.1.53:  
   	[udp sum ok] 61380+ AAAA? cproxy. 
   	[|domain] (DF) (ttl 64, id 48677, len 52)
   17:09:16.292883 192.168.12.1.53 > 192.168.12.100.34212:  
   	[udp sum ok] 61380 NXDomain* 0/0/0 (24) (DF) (ttl 64, id 0, len 52)

My nsswitch.conf contains the following line:

   hosts:          files dns

Therefore, I believe that hosts listed in /etc/hosts should, under no
circumstances, elicit a DNS lookup.

It appears to be no problem in the situation above - and indeed, it doesn't
really affect my life much in this case - BUT, when the Internet connection
is down, the DNS server takes 60-120 seconds to answer, and telnet sits
waiting all that time, even though it already knows the answer from
/etc/hosts!

When I add the following line to /etc/hosts, the DNS lookups go away,
although telnet tries first to connect to an IPv6 address (which fails
instantly and harmlessly, since I don't have IPv6 enabled):

	::1 cproxy

Although I haven't looked at the libc source, I expect the reason for this
misbehaviour is an implementation of getaddrinfo that goes something like
this:

	getaddrinfo:
		gethostbyname2(name, AF_INET6)
			nss_files_gethostbyname2(name, AF_INET6) = FAIL
			nss_dns_gethostbyname2(name, AF_INET6) = FAIL
		  ... FAIL
		gethostbyname2(name, AF_INET)
			nss_files_gethostbyname2(name, AF_INET) = OK
		  ... OK
	... OK

It should be more like this:

	getaddrinfo:
		nss_files_gethostbyname2(name, AF_INET6) = FAIL
		nss_files_gethostbyname2(name, AF_INET) = OK
	... OK
	
Or, for reference, if the name weren't in /etc/hosts, it should be like this:

	getaddrinfo:
		nss_files_gethostbyname2(name, AF_INET6) = FAIL
		nss_files_gethostbyname2(name, AF_INET) = FAIL
		nss_dns_gethostbyname2(name, AF_INET6) = FAIL
		nss_dns_gethostbyname2(name, AF_INET) = OK
	... OK

Unfortunately, this requires getaddrinfo to know about the nss rather than
just calling the nss-independent libc gethostbyname2() function, but it
looks like there's no better way to solve the problem.

Have fun,

Avery

-- System Information
Debian Release: testing/unstable
Kernel Version: Linux insight 2.4.19 #1 mer sep 4 10:52:04 EDT 2002 i686 unknown


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>From bmc@phunnypharm.org Thu Sep 05 20:01:52 2002
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Date: Thu, 5 Sep 2002 21:01:41 -0400
From: Ben Collins <bcollins@debian.org>
To: apenwarr@nit.ca, 159781-done@bugs.debian.org
Subject: Re: Bug#159781: libc6: getaddrinfo doesn't do nsswitch.conf ordering correctly
Message-ID: <20020906010141.GA14796@phunnypharm.org>
References: <E17n4Ri-0004CO-00@insight.internal.nit.ca>
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Delivered-To: 159781-done@bugs.debian.org

On Thu, Sep 05, 2002 at 05:45:50PM -0400, apenwarr@nit.ca wrote:
> Package: libc6
> Version: 2.2.5-14
> Severity: normal
> 
> If I have this line in /etc/hosts:
> 
>    192.168.43.128 cproxy
> 

Read the glibc FAQ/Bugs (upstream). This is not a bug, but is just how
getaddrinfo works, so that it can fulfill it's specifications.

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