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I have a VM running Debian Wheezy on which some hostname lookups take several seconds to complete, even though the resolver replies immediately. Strangely, lookups with getaddrinfo() are affected, but gethostbyname() is not.

I've switched to the Google resolvers to exclude the possibility that the local ones are broken, so my /etc/resolv.conf looks like:

search my-domain.com

My nsswitch.conf has the line:

hosts: files dns

and my /etc/hosts doesn't contain anything unusual.

If I try telnet webserver 80, it hangs for several seconds before getting a name resolution. An ltrace output [1] shows that the hang is in a getaddrinfo() call:

getaddrinfo("ifconfig.me", "telnet", { AI_CANONNAME, 0, SOCK_STREAM, 0, 0, NULL, '\000', NULL }, 0x7fffb4ffc160) = 0 <5.020621>

However, tcpdump reveals that the nameserver replied immediately, and it was only on the second reply that telnet unblocked. The replies look identical:

05:52:58.609731 IP > 54755+ A? ifconfig.me. (29)
05:52:58.609786 IP > 26090+ AAAA? ifconfig.me. (29)
05:52:58.612188 IP > 54755 4/0/0 A, A, A, A (93)

[...five second pause...]

05:53:03.613811 IP > 54755+ A? ifconfig.me. (29)
05:53:03.616424 IP > 54755 4/0/0 A, A, A, A (93)
05:53:03.616547 IP > 26090+ AAAA? ifconfig.me. (29)
05:53:03.618907 IP > 26090 0/1/0 (76)

I've checked host firewall logs and nothing on port 53 is being blocked.

What is causing the first DNS reply to be ignored?

[1] I've added a couple of lines to my ltrace.conf so I can see inside the addrinfo struct.

share|improve this question
How is the VM's NIC setup? Bridged? NAT? – slm Jul 7 '14 at 13:28
It's NATted. I'm not sure exactly where the NAT is applied (whether by ESX or further upstream); I can find out if you think it might matter. – Flup Jul 7 '14 at 13:37
I'd suspect that it's the NAT + VM that's influencing this. – slm Jul 7 '14 at 15:30
Could well be -- see my comments on Kempniu's answer below. – Flup Jul 7 '14 at 15:35
It was networky, but not specifically NAT causing this -- see my answer below. – Flup Jul 8 '14 at 7:16

The first DNS reply isn't ignored. getaddrinfo() didn't return until it received the response to the first AAAA query (ID: 26090). So the real problem here is why your machine hasn't immediately received the response to the AAAA query, while it has received the response for the A query (ID: 54755).

One of the differences between getaddrinfo() and gethostbyname() is that the former supports both IPv4 and IPv6, while the latter only supports IPv4. So when you call getaddrinfo() with ai_family set to 0 (AF_UNSPEC), it won't return until it gets a response (or hits a timeout) for both A and AAAA queries for the domain name provided. gethostbyname() only queries for an A record.

It's hard to remotely determine what may be causing your problem, especially that you've cut out some tcpdump output. Something might be selectively filtering/dropping the DNS traffic between your VM and Google Public DNS resolvers. I have tried to reproduce your problem using a KVM Debian Wheezy VM, but telnet ifconfig.me almost immediately printed the Trying <IP_address_here>... line (meaning it has already resolved the name by then).

share|improve this answer
Thanks for your detailed answer. I haven't cut anything out of the tcpdump, I only inserted a line to make it clear where the pause was. You've definitely given me something to go on though; I didn't realise the significant difference between the two library calls. – Flup Jul 7 '14 at 14:25
If no more DNS-related packets hit your machine, something is probably filtering its traffic (not necessarily on purpose). Anyway, if you find a solution, would you please share it here? – Kempniu Jul 7 '14 at 14:39
I will indeed. Having set up a test resolver, I can see conclusively that one reply packet -- the one from my question -- gets dropped every time. I suspect something in or near the VMware infrastructure is doing it, so I've contacted my counterpart who looks after that side of things. When I get to the bottom of it I'll come back and add detail. Thanks again! – Flup Jul 7 '14 at 15:25
Solution added in my own answer. Many thanks once again for your help. – Flup Jul 8 '14 at 7:16
up vote 5 down vote accepted

This was caused by an overly restrictive ruleset on a Juniper firewall that sits in front of the VMware infrastructure.

I built a test resolver so that I could see both sides of the conversation, and the missing packet identified by Kempniu in his excellent answer was indeed being dropped somewhere along the way. As noted in that answer, getaddrinfo() with no address family specified will wait for answers relating to all supported families before returning (or, in my case, timing out).

My colleague who runs the network noted that

The default behavior on the Juniper firewall is to close a DNS-related session as soon as a DNS reply matching that session is received.

So the firewall was seeing the IPv4 response, noting that it answered the VM's query, and closing the inbound path for that port. The following IPv6 reply packet was therefore dropped. I've no idea why both packets made it through the second time, but disabling this feature on the firewall fixed the problem.

This is a related extract from the Juniper KB:

Here's a scenario where DNS Reply packets are dropped:

  1. A session for DNS traffic is created when the first DNS query packet hits the firewall and there is a permitting policy configured. The default timeout is 60 sec.
  2. Immediately before the session is closed, a new DNS query is transmitted, and since it matches an existing session (since source and destination port/IP pair is always the same), it is forwarded by the firewall. Note that the session timeout is not refreshed according to any newly arriving packet.
  3. The created DNS session is aged out when the first DNS query response (reply) hits the device, regardless how much the timeout remains.
  4. When a DNS reply is passed through the firewall, the session is aged out.
  5. All subsequent DNS replies are dropped by the firewall, since no session exists.

If you're thinking of upvoting this answer, please also upvote Kempniu's answer. Without it I'd still be thrashing around trying to find some configuration problem on the VM.

share|improve this answer
+1 - nice work figuring this out! – slm Jul 8 '14 at 12:08
We have experienced the same symptoms on Debian 8.2. Ours were due to a different cause, and the "solution" was different (client-side). I blogged about our specific problem and the more general problem: philippecloutier.com/… I want to thank Flup and Kempniu, since your question and answers put me on the right track. – Philippe Cloutier Nov 27 '15 at 20:57

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