Otdfbt

I'm trying to set up Apache as a reverse proxy. Here is the essential part of its configuration:

NameVirtualHost 10.16.10.245:9443
Listen 10.16.10.245:9443

<VirtualHost 10.16.10.245:9443>
 ServerName proxy.lan:9443

 SSLEngine on
 ...

 TraceEnable off

 SSLProxyEngine on
 ProxyPreserveHost On
 ProxyRequests Off
 ProxyVia full
 ProxyPass / http://localhost/
 ProxyPassReverse / http://localhost/
</VirtualHost>

Note, that the proxy is listening on a non standard port 9443. When I use a dummy page showing phpinfo as a backend everything works as expected. However the site I need to put behind the proxy is either too strictly or too poorly written, so the behaivior changes like this:

client -> https://proxy.lan:443 -> http://localhost = success
client -> https://proxy.lan:<ANY_OTHER_PORT> -> http://localhost = wrong redirect

The client gets redirected from https://proxy.lan:9443/ to https://proxy.lan/auth/login and obviously proxy can't serve the request cause it doesn't listen on port 443:

# wget --no-check-certificate -vS https://proxy.lan:9443
--2019-05-12 02:51:37-- https://proxy.lan:9443/
Resolving proxy.lan (proxy.lan)... 10.10.254.186
Connecting to proxy.lan (proxy.lan)|10.10.254.186|:9443... connected.
WARNING: cannot verify proxy.lan's certificate, issued by '...':
 Self-signed certificate encountered.
 WARNING: certificate common name 'backend.lan' doesn't match requested host name 'proxy.lan'.
HTTP request sent, awaiting response... 
 HTTP/1.1 302 Found
 Date: Sat, 11 May 2019 23:51:37 GMT
 Server: Apache
 Expires: Thu, 19 Nov 1981 08:52:00 GMT
 Cache-Control: no-cache, no-store, must-revalidate
 Pragma: no-cache
 Location: https://proxy.lan/auth/login
 X-Frame-Options: DENY
 X-XSS-Protection: 1; mode=block
 X-Content-Type-Options: nosniff
 Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
 Content-Security-Policy: default-src *; style-src 'self' 'unsafe-inline'; script-src 'self' 'unsafe-inline' 'unsafe-eval'
 X-Permitted-Cross-Domain-Policies: none
 Content-Length: 0
 Content-Type: text/html; charset=UTF-8
 Set-Cookie: PHPSESSID=...; path=/; secure; HttpOnly
 Via: 1.1 proxy.lan:9443 (Apache/2.2.31)
 Connection: close
Location: https://proxy.lan/auth/login [following]
--2019-05-12 02:51:37-- https://proxy.lan/auth/login
Connecting to proxy.lan (proxy.lan)|10.10.254.186|:443... failed: Connection refused.
Resolving proxy.lan (proxy.lan)... 10.10.254.186
Connecting to proxy.lan (proxy.lan)|10.10.254.186|:443... failed: Connection refused.

I can manually add the port to the resulting URL and https://proxy.lan:9443/auth/login works except for all links on the page pointing to https://proxy.lan/...

Apache environment looks like this:

HTTP_HOST proxy.lan:9443
HTTP_VIA 1.1 proxy.lan:9443 (Apache/2.2.31)
HTTP_X_FORWARDED_FOR 10.100.0.30
HTTP_X_FORWARDED_HOST proxy.lan:9443
HTTP_X_FORWARDED_SERVER proxy.lan
HTTP_CONNECTION Keep-Alive
SERVER_SIGNATURE <address>Apache Server at proxy.lan Port 9443</address>
SERVER_NAME proxy.lan
SERVER_ADDR ::1
SERVER_PORT 9443
REMOTE_ADDR ::1
...

Any ideas what can be done on the proxy side? Some rewrite rules maybe?

The ProxyPreserveHost on is causing this. If you are not sure-sure that you need it, always leave it at default off and you are good.

For the rare cases when you do need ProxyPreserveHost on, adjust your ProxyPassReverse wisely - this is the only directive that handles the redirects.

ProxyPassReverse / http://localhost/ looks wrong in your scenario, because your backend (the 80 app) doesn't seem to say "I redirect you to http://localhost/foo/bar" anywhere. If you put ProxyPassReverse / https://proxy.lan/ there is a chance it will work better - check the official docs.