Behavior of realip module with this config

[Paul Nickerson]

On Fri, Feb 10, 2017 at 7:33 AM, Maxim Dounin wrote:
> And real_ip_recursive switched on means that this happens
> recursively.  As a result, with the configuration in question
> nginx will use the first address in X-Forwarded-For provided, if
> any (assuming all addresses are valid).
> Note that "set_real_ip_from 0.0.0.0/0" makes client's address as
> seen by nginx easily spoofable by any client, and it is generally
> a bad idea to use it in production.

Thank you for the reply, Maxim. "set_real_ip_from 0.0.0.0/0" does indeed
seem like a bad idea in production. Thank you for calling that out.

I am confused by this statement in the documentation:
http://nginx.org/en/docs/http/ngx_http_realip_module.html
"If recursive search is enabled, the original client address that matches
one of the trusted addresses is replaced by the last non-trusted address
sent in the request header field."

The language "last non-trusted address" suggests that NGINX looks for
something in real_ip_header which does not match set_real_ip_from. But
maybe I am interpreting that incorrectly.

If set_real_ip_from were set correctly to the host's content delivery
network, load balancer, and reverse proxy infrastructures, then my
interpretation would make sense, as $remote_addr would then get set to the
client's public IP, even if the client has network address translation and
forward proxy infrastructures which append to X-Forwarded-For. But in your
answer, wouldn't $remote_addr be set to the client's private IP address if
their firewall/gateway adds that private IP address to X-Forwarded-For
while it does the NATing? That doesn't seem very useful.

This is an example situation I'm thinking of (all the IPs are random, and
are the IPs "facing" NGINX):

set_real_ip_from 10.6.1.0/24, 8.47.98.0/24;
real_ip_header X-Forwarded-For;
real_ip_recursive on;

client's computer (192.168.1.79) > client's gateway (178.150.189.138) > my
content delivery network (8.47.98.129) > my load balancer (10.6.1.56) > my
NGINX box

X-Forwarded-For = 192.168.1.79, 178.150.189.138, 8.47.98.129

I think in your answer, $remote_addr would be set to 192.168.1.25, while in
my interpretation, it would be set to 178.150.189.138. And in either case,
$realip_remote_addr is 10.6.1.56.

It would be a strangely configured client gateway / firewall / NAT / proxy
that adds to X-Forwarded-For, but it can happen.

I guess I am still confused.

 ~ Paul Nickerson

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