nginx: Add an option which defaults loadbalancer requests to https.

In some cases, it is not possible to configure the load-balancer to
add an X-Forwarded-Proto header.  If Zulip is serving its traffic over
HTTP, it will rightly error out, since it cannot guarantee that its
response will be served over an encrypted connection.

Add a new `loadbalancer.rejects_http_requests` settings which serves
as a way for the operator to swear that the load-balancer will *never*
serve responses from Zulip over an unencrypted connection.  In most
cases, this is because the load-balancer is configured to have port 80
always serve an HTTP 301 redirect to the same URL over HTTPS.

Properly configuring the proxy to send `X-Forwarded-Proto` is always a
better solution than using this configuration parameter, so use of
this should be viewed as a last resort.

docs/production/reverse-proxies.md

@@ -244,6 +244,11 @@ things you need to be careful about when configuring it:
    connection mechanism from your proxy to Zulip. Note that the proxies _must_
    set the header, overriding any existing values, not add a new header.
 
+   If your proxy _cannot_ set the `X-Forwarded-Proto` header, you can opt to do
+   all HTTP-to-HTTPS redirection at the load-balancer level, and set
+   [`loadbalancer.rejects_http_requests` in `zulip.conf`][no-proto-header]; but
+   note the important security caveats for that in its documentation.
+
 1. Configure your proxy to pass along the `Host:` header as was sent
    from the client, not the internal hostname as seen by the proxy.
    If this is not possible, you can set `USE_X_FORWARDED_HOST = True`
@@ -272,4 +277,5 @@ things you need to be careful about when configuring it:
    with multiple IPs for your Zulip machine; sometimes this happens with
    IPv6 configuration).
 
+[no-proto-header]: system-configuration.md#rejects_http_requests
 [nginx-proxy-longpolling-config]: https://github.com/zulip/zulip/blob/main/puppet/zulip/files/nginx/zulip-include-common/proxy_longpolling

docs/production/system-configuration.md

@@ -370,6 +370,16 @@ Comma-separated list of IP addresses or netmasks of external load balancers
 whose `X-Forwarded-For` and `X-Forwarded-Proto` should be respected. These can
 be individual IP addresses, or CIDR IP address ranges.
 
+#### `rejects_http_requests`
+
+Set to a true value if incoming requests from load loadbalancer's IP addresses
+which do not contain an `X-Forwarded-Proto` should be assumed to have come into
+them over HTTPS. This setting _is a security vulnerability_ unless the load
+balancer unilaterally rejects unencrypted HTTP connections, or responds to them
+with 301 status codes. Note that Zulip's HSTS headers are not sufficient
+protection here, since API clients do not respect them; the load balancer _must
+not_ send any requests to Zulip which came in unencrypted.
+
 ### `[http_proxy]`
 
 #### `host`

puppet/zulip/manifests/nginx.pp

@@ -56,6 +56,7 @@ class zulip::nginx {
   }
 
   $loadbalancers = split(zulipconf('loadbalancer', 'ips', ''), ',')
+  $lb_rejects_http_requests = zulipconf('loadbalancer', 'rejects_http_requests', false)
   file { '/etc/nginx/zulip-include/trusted-proto':
     ensure  => file,
     require => Package[$zulip::common::nginx],

puppet/zulip/templates/nginx/trusted-proto.template.erb

@@ -31,14 +31,19 @@ geo $remote_addr $is_from_proxy {
 # We set $trusted_x_forwarded_proto in two steps because `geo` does
 # not support variable interpolation in the value, but does support
 # CIDR notation, which the loadbalancer list may use.
-map $is_x_forwarded_proto_trusted $trusted_x_forwarded_proto {
-    0 $scheme;
-    1 $http_x_forwarded_proto;
+map "$is_x_forwarded_proto_trusted:$http_x_forwarded_proto" $trusted_x_forwarded_proto {
+    "~^0:" $scheme;
+<%- if @lb_rejects_http_requests -%>
+    "~^1:$" "https";
+<% end -%>
+    "~^1:" $http_x_forwarded_proto;
 }
 
 map "$is_from_proxy:$is_x_forwarded_proto_trusted:$http_x_forwarded_proto" $x_proxy_misconfiguration {
     "~^0:0:" "Incorrect reverse proxy IPs set in Zulip (try $remote_addr?); see https://zulip.readthedocs.io/en/latest/production/reverse-proxies.html";
+<%- if not @lb_rejects_http_requests -%>
     "~^0:1:$" "No X-Forwarded-Proto header sent from trusted proxy $realip_remote_addr; see example configurations in https://zulip.readthedocs.io/en/latest/production/reverse-proxies.html";
+<% end -%>
     default "";
 }
 <% end %>