Hi Jim,

"hold obsolete" defaults to 0, so basically, HAProxy may evince servers
from your backend quite frequently (the bigger the farm, the more chance it
happens).
Furthermore, most of those changes are "false positive" (since the server
may still be healthy).

DNS over TCP won't help.
As I stated in my previous mail, AWS DNS servers only returns 8 records per
response (they are "roundrobined"), even in TCP (I did try with "drill" DNS
client).
So, your only way to go is to use the "hold obsolete" timer.


On Thu, Jul 5, 2018 at 3:49 PM, Jim Deville <jdevi...@malwarebytes.com>
wrote:

> Hi Baptiste,
>
>
> I appreciate you taking time for this, we had tried increasing the
> response size, but I believe we left hold obsolete at defaults and that
> probably lead to flapping. How often does HAProxy re-poll DNS for this? I'm
> curious what limits this really sets for how many servers we can scale to
> with this. Also, will DNS over TCP help any? Seems like it still needs
> roughly the same settings given the round-robin responses.
>
>
> In the meantime, we will look into these settings to see if we can make
> them work as well.
>
>
> Jim
> ------------------------------
> *From:* Baptiste <bed...@gmail.com>
> *Sent:* Tuesday, July 3, 2018 9:20:53 AM
>
> *To:* Jim Deville
> *Cc:* haproxy@formilux.org; Jonathan Works
> *Subject:* Re: Issue with parsing DNS from AWS
>
> Ah yes, I also added the following "init-addr none" statement on the
> server-template line.
> This prevents HAProxy from using libc resolvers, which might end up in
> unpredictible behavior in that enviroment....
>
> Baptiste
>
> On Tue, Jul 3, 2018 at 3:18 PM, Baptiste <bed...@gmail.com> wrote:
>
> Well, I can partially reproduce the issue you're facing and I can see some
> weird behavior of AWS's DNS servers.
>
> First, by default, HAProxy only support DNS over UDP and can accept up to
> 512 bytes of payload in the DNS response.
> DNS over TCP is not yet available and accepted payload size can be
> increased using EDNS0 extension.
>
> There is a "magic" number of SRV records with AWS and default HAProxy
> accepted payload size, at around 4 SRV records, the response payload may be
> bigger than 512 bytes.
> And so, AWS DNS server does not return any data, simply returns an empty
> response, with the TRUNCATED flag.
> In such case, a client is supposed to replay the request over TCP...
>
> An other magic value with AWS DNS servers is that it won't return more
> than 8 SRV records, even if you have 10 servers in your service. (even in
> TCP)
> AWS DNS servers will simply return a round robin list of the records, some
> will disappear, some will reappear at some point in time.
>
>
> Conclusion, to make HAProxy work in such environment, you want to
> configure it that way:
> resolvers awsdns
>   nameserver dns0 NAMESERVER:53     # <=== please remove the doule quotes
>   accepted_payload_size 8192                 # <=== workaround for too
> short accepted payload
>   hold obsolete 30s                                   # <=== workaround
> for limited number of records returned by AWS
>
> You may want to read the documentation of HAProxy's resolver. There are a
> few other timeout / hold period you could tune.
>
> With the configuration above, I could easily scale from 2 to 10, back to
> 2, passing through 4, 8, etc... successfully and without any server
> flapping.
> I did not try to go higher than 10. Bear in mind the "hold obsolete"
> period is the period during which HAProxy considers a server as available
> even if the DNS server did not return it in the SRV record list.
>
> Baptiste
>
>
>
>
>
>
>
> On Tue, Jul 3, 2018 at 1:26 PM, Baptiste <bed...@gmail.com> wrote:
>
> Answering myself... I found my way in the menu to be able to allow port
> 9000 to read the stats page and to find the public IP associated to my
> "app".
> That said, I still can't get a shell on the running container, but I think
> I found an AWS documentation page for this purpose.
>
> I keep you updated.
>
> On Tue, Jul 3, 2018 at 1:06 PM, Baptiste <bed...@gmail.com> wrote:
>
> Hi Jim,
>
> I think I have something running...
> At least, terraform did not complain and I can see "stuff" in my AWS
> dashoard.
> Now, I have no idea how I can get connected to my running HAProxy
> container, neither how I can troubleshoot what's happening :)
>
> Any help would be (again) appreciated.
>
> Baptiste
>
>
>
> On Tue, Jul 3, 2018 at 11:39 AM, Baptiste <bed...@gmail.com> wrote:
>
> Hi Jim,
>
> Sorry for the long pause :)
> I was dealing with some travel, conferences and catching up on my backlog.
> So, the good news, is that this issue is now my priority :)
>
> I'll try to first reproduce it and come back to you if I have any issue
> during that step.
> (by the way, thanks for the github repo to help me speed up in that step).
>
> Baptiste
>
>
>
>
> On Mon, Jun 25, 2018 at 10:54 PM, Jim Deville <jdevi...@malwarebytes.com>
> wrote:
>
> Hi Bapiste,
>
>
> I just wanted to follow up to see if you were able to repro and perhaps
> had a patch we could try?
>
>
> Jim
> ------------------------------
> *From:* Jim Deville
> *Sent:* Thursday, June 21, 2018 1:05:49 PM
> *To:* Baptiste
> *Cc:* haproxy@formilux.org; Jonathan Works
> *Subject:* Re: Issue with parsing DNS from AWS
>
>
> Thanks for the reply, we were able to extract a minimal repro to
> demonstrate the problem: https://github.com/jg
> works/haproxy-servicediscovery
>
>
> The docker folder contains a version of the config we're using and a
> startup script to determine the local private DNS zone (AWS puts it at the
> subnet's +2).
>
>
> Jim
> ------------------------------
> *From:* Baptiste <bed...@gmail.com>
> *Sent:* Thursday, June 21, 2018 11:02:26 AM
> *To:* Jim Deville
> *Cc:* haproxy@formilux.org; Jonathan Works
> *Subject:* Re: Issue with parsing DNS from AWS
>
> and by the way, I had a quick look at the pcap file and could not find
> anything weird.
> The function you're pointing seem to say there is not enough space to
> store a server's dns name, but the allocated space is larger that your
> current records.
>
> Baptiste
>
>
>
>
>
>
>

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