# Resolver does not re-resolve upstream servers after initial cache

**URL:** <https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335>\
**Category:** NGINX\
**Created:** [November 7, 2012, 9:40pm UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335 "2012-11-07T21:40:49Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\
**Post date:** [November 7, 2012, 9:40pm UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/1 "2012-11-07T21:40:49Z")

</div>

Using nginx 1.2.3-stable on Ubuntu 12.04 I have the following config:

```
http {
  resolver 172.16.0.23 valid=300s;
  resolver_timeout 10s;

  upstream myupstream {
    server example.com;
  }

  server {
    listen 80 default_server;

    location / {
      proxy_pass http://myupstream$request_uri;
      proxy_pass_request_headers on;
      proxy_set_header Host $host;
    }
  }
}

```

As I understand it, without the resolver config, nginx will resolve  
[example.com](http://example.com)’s IP once on load and cache it until it stops or fully  
reloads the config.

With the resolver config above, nginx should re-resolve the IP every  
5mins.

However, this is not happening: I can watch tcpdump -n udp port 53 but I  
see no re-resolution taking place.

I’d love to know how to fix this. Any advice appreciated thanks!

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 8, 2012, 8:34am UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/2 "2012-11-08T08:34:28Z")

</div>

On Wed, Nov 07, 2012 at 09:40:49PM +0100, Dave Nolan wrote:

> ```
> server {
> 
> ```
> 
> As I understand it, without the resolver config, nginx will resolve  
> [example.com](http://example.com)’s IP once on load and cache it until it stops or fully  
> reloads the config.
> 
> With the resolver config above, nginx should re-resolve the IP every  
> 5mins.

This is not the way how it works.

A run-time resolving only takes place if URL specified in “proxy\_pass”  
contains variables, AND the resulting server name doesn’t match any of  
the configured server groups (using the “upstream” directives). This  
is documented here: [Module ngx\_http\_proxy\_module](http://nginx.org/r/proxy_pass)

In your case, the server name is always “myupstream” and since it  
matches “upstream myupstream”, no run-time resolving takes place.

> However, this is not happening: I can watch tcpdump -n udp port 53 but I  
> see no re-resolution taking place.
> 
> I’d love to know how to fix this. Any advice appreciated thanks!

proxy\_pass http://example.com$request\_uri;

will resolve “[example.com](http://example.com)” dynamically (assuming of course there’s  
no “upstream [example.com](http://example.com)” in configuration).

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 8, 2012, 11:38am UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/3 "2012-11-08T11:38:42Z")

</div>

Ruslan E. wrote in post #1083512:

> On Wed, Nov 07, 2012 at 09:40:49PM +0100, Dave Nolan wrote:
> 
> > ```
> > server {
> > 
> > ```
> > 
> > As I understand it, without the resolver config, nginx will resolve  
> > [example.com](http://example.com)’s IP once on load and cache it until it stops or fully  
> > reloads the config.
> > 
> > With the resolver config above, nginx should re-resolve the IP every  
> > 5mins.
> 
> This is not the way how it works.
> 
> A run-time resolving only takes place if URL specified in “proxy\_pass”  
> contains variables, AND the resulting server name doesn’t match any of  
> the configured server groups (using the “upstream” directives). This  
> is documented here: [Module ngx\_http\_proxy\_module](http://nginx.org/r/proxy_pass)
> 
> In your case, the server name is always “myupstream” and since it  
> matches “upstream myupstream”, no run-time resolving takes place.

What’s the reason behind this? It feels like that, even if proxy\_pass  
defers to the server group, resolver config should be respected for  
servers defined within the group.

> > However, this is not happening: I can watch tcpdump -n udp port 53 but I  
> > see no re-resolution taking place.
> > 
> > I’d love to know how to fix this. Any advice appreciated thanks!
> 
> proxy\_pass http://example.com$request\_uri;
> 
> will resolve “[example.com](http://example.com)” dynamically (assuming of course there’s  
> no “upstream [example.com](http://example.com)” in configuration).

Thanks very much for your help.

If I switch to using [example.com](http://example.com) directly in the proxy\_pass, I lose the  
flexibility of server groups. Is there any way of dynamically  
re-resolving servers in upstream server group?

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 8, 2012, 12:07pm UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/4 "2012-11-08T12:07:46Z")

</div>

Dave Nolan Wrote:

> Thanks very much for your help.
> 
> If I switch to using [example.com](http://example.com) directly in the proxy\_pass, I lose  
> the  
> flexibility of server groups. Is there any way of dynamically  
> re-resolving servers in upstream server group?

Hi,

I can add that I lost my production servers last night because of this  
behavior.

- I use dynamic dns name for flexibility for almost all my servers
- I put one backend server to maintenance so the name was removed by dns  
(after a TTL)
- corosync manage my nginx servers… and can restart them.

You can easily understand what append :  
corosync detect a problem, fail back to another server, restart nginx  
but  
nginx can’t resolved a backend host in upstream so it failed to start  
(with  
“[emerg] host not found in upstream”).

All my nginx servers have been down because of this.

Just like you, I can’t remove my server groups but I want the  
flexibility of  
DNS resolving (Not failing at start and TTL).

–  
Guilhem Lettron  
Youscribe - [www.youscribe.com](http://www.youscribe.com)

Posted at Nginx Forum:  
[http://forum.nginx.org/read.php?2,232585,232596#msg-232596](http://forum.nginx.org/read.php?2,232585,232596#msg-232596)

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 8, 2012, 1:06pm UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/5 "2012-11-08T13:06:12Z")

</div>

On 8 Nov2012, at 15:07 , guilhem [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> All my nginx servers have been down because of this.
> 
> Just like you, I can’t remove my server groups but I want the flexibility of  
> DNS resolving (Not failing at start and TTL).

If you want the flexibility of DNS resolving and safeguard yourself  
against  
DNS failure you should either add hostnames to /etc/hosts or start  
local named/NSD/etc with appropriate slave zones.

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 9, 2012, 10:14am UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/6 "2012-11-09T10:14:40Z")

</div>

Sergey B. wrote in post #1083548:

> On 8 Nov2012, at 15:07 , guilhem [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > All my nginx servers have been down because of this.
> > 
> > Just like you, I can’t remove my server groups but I want the flexibility of  
> > DNS resolving (Not failing at start and TTL).
> 
> If you want the flexibility of DNS resolving and safeguard yourself  
> against  
> DNS failure you should either add hostnames to /etc/hosts or start  
> local named/NSD/etc with appropriate slave zones.

Sure, that kind of flexibility needs more tools than just nginx.

But actually it’s a question about consistency, right?

Even if proxy\_pass defers to the server group, resolver config should be  
respected for servers defined within the group, just like for everything  
else. I’m just interested in why it’s not, and whether there are plans  
to change it. We might be interested in sponsoring this work.

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 9, 2012, 10:24am UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/7 "2012-11-09T10:24:16Z")

</div>

Dave,

On Nov 9, 2012, at 1:14 PM, Dave Nolan wrote:

> > If you want the flexibility of DNS resolving and safeguard yourself  
> > else. I’m just interested in why it’s not, and whether there are plans  
> > to change it. We might be interested in sponsoring this work.

That’s the current (and maybe already “legacy”) design of nginx upstream  
configuration. Unfortunately there’s no quick solution but we actually  
appreciate your feedback a lot and will try to incorporate a better  
upstream design in the future releases.

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [November 9, 2012, 10:24am UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/8 "2012-11-09T10:24:51Z")

</div>

On Nov 9, 2012, at 1:14 PM, Dave Nolan wrote:

> to change it. We might be interested in sponsoring this work.

Can we get this conversation in nginx-inquiries at nginx dot com please?

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [January 8, 2015, 5:13am UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/9 "2015-01-08T05:13:42Z")

</div>

any update?

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [June 4, 2013, 5:26pm UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/10 "2013-06-04T17:26:50Z")

</div>

Andrew A. wrote in post #1083715:

> Dave,
> 
> On Nov 9, 2012, at 1:14 PM, Dave Nolan wrote:
> 
> > > If you want the flexibility of DNS resolving and safeguard yourself  
> > > else. I’m just interested in why it’s not, and whether there are plans  
> > > to change it. We might be interested in sponsoring this work.
> 
> That’s the current (and maybe already “legacy”) design of nginx upstream  
> configuration. Unfortunately there’s no quick solution but we actually  
> appreciate your feedback a lot and will try to incorporate a better  
> upstream design in the future releases.

I was wondering if there were any updates on this in more recent nginx  
versions (1.5.1 at the moment)?

This bug causes a lot of problems in cloud environments like AWS where  
IPs can change at any moment.

---

<div class="post-metadata">

**Author:** ![davenolan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/9de053/32.png) [@davenolan](https://www.ruby-forum.com/u/davenolan)\
**Post date:** [January 12, 2015, 1:28pm UTC](https://www.ruby-forum.com/t/resolver-does-not-re-resolve-upstream-servers-after-initial-cache/224335/11 "2015-01-12T13:28:34Z")

</div>

Hello!

On Thu, Jan 08, 2015 at 05:13:42AM +0100, Miroslav S. wrote:

> any update?

This is now available as a commercial feature in nginx+, see the  
“resolve” parameter here:

> **[Module ngx\_http\_upstream\_module](http://nginx.org/en/docs/http/ngx_http_upstream_module.html#server)**
>
> The ngx\_http\_upstream\_module module is used to define groups of servers that can be referenced by the proxy\_pass, fastcgi\_pass, uwsgi\_pass, scgi\_pass, memcached\_pass, and grpc\_pass directives.

–  
Maxim D.

> **[nginx](http://nginx.org/)**