problem with nginx round robin upstream

Ruslan Ermilov ru at
Thu Aug 30 09:29:34 UTC 2012

On Thu, Aug 30, 2012 at 10:19:11AM +0300, Tsahi Asher wrote:
>    Hi
>    I have a problem with the upstream round robin mechanism.
>    in ngx_http_upstream.c ngx_http_upstream_server function the hostname that
>    is added in the configuration file under upstream directive is added to
>    the upstream server list.
>    the function uses ngx_parse_url which in turn uses ngx_parse_inet_url that
>    calls ngx_inet_resolve_host.
>    ngx_inet_resolve_host tries to resolve the host name from the
>    configuration file by calling gethostbyname. for each url gethostbyname
>    return the list of ip's that are associated with this url and these ip's
>    are added to the list of upstream servers.
>    This results in a situation that if there are three servers defined in an
>    upstream directive, each with an equal weight, but one of those server is
>    associated with two ip's (for example internal and an external ip) then
>    the round robin weight will not be equal for each server. The server which
>    has two ip's will get a double weight.
>    i can add a log if required or if my explanation is not good enough.
>    i saw that a url has a flag called one_addr that if is set to true will
>    get resolved to a single ip only, but this flag is not used.
>    adding the host ip address is one way to solve this, is there another way?

This is expected.  Here's the relevant quote from documentation [1]:

: A domain name that resolves to several IP addresses essentially defines
: multiple servers.


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