Reverse Proxying relative URL to root URL results in 404 errorpage

Summary

Using GitLab behind a reverse proxy doesn't seem to work if the configured URL is relative, but the server is running on a root URL

Steps to reproduce

• Install GitLab on server to run on "root" URL
• Configure relative URL according to https://docs.gitlab.com/omnibus/settings/configuration.html#enable-relative-url-in-gitlab
• Configure reverse proxy for the configured relative URL
• Browse to the configured relative URL

Example:

• Installation on internal host on https://gitlabserver.internal/ (this host is not reachable from outside the our network)
• Configure external_url to be https://our.external.webserver/gitlab
• Configure reverse proxy (Apache) on :

  ProxyPass        /gitlab https://gitlabserver.internal
  ProxyPassReverse /gitlab https://gitlabserver.internal

What is the current bug behavior?

Browsing to the configured ext_url results in a GitLab 404 error page

What is the expected correct behavior?

Browsing to the configured ext_url results the GitLab login page

Results of GitLab environment info

Expand for output related to GitLab environment info (anonymized)

System information
System:		Debian 8.9
Current User:	git
Using RVM:	no
Ruby Version:	2.3.3p222
Gem Version:	2.6.6
Bundler Version:1.13.7
Rake Version:	12.0.0
Redis Version:	3.2.5
Git Version:	2.13.5
Sidekiq Version:5.0.4
Go Version:	unknown
GitLab information
Version:	9.5.1
Revision:	c47ae37
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://our.external.webserver/gl
HTTP Clone URL:	https://our.external.webserver/gl/some-group/some-project.git
SSH Clone URL:	git@our.external.webserver:some-group/some-project.git
Using LDAP:	yes
Using Omniauth:	no
GitLab Shell
Version:	5.8.0
Repository storage paths:

• local: /var/opt/gitlab/git-data/repositories
• default: /data/gitlab/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check (anonymized and shortened due to confidentiality issues)

Checking GitLab Shell ...
GitLab Shell version >= 5.8.0 ? ... OK (5.8.0)
Repo base directory exists?
local... yes
default... yes
Repo storage directories are symlinks?
local... no
default... no
Repo paths owned by git:root, or git:git?
local... yes
default... yes
Repo paths access is drwxrws---?
local... yes
default... yes
hooks directories in repos are links: ...
10/4 ... ok
5/5 ... repository is empty
5/6 ... ok
6/7 ... repository is empty
49/8 ... ok
8/9 ... ok
50/10 ... ok
48/11 ... ok
23/13 ... ok
5/14 ... ok
10/15 ... ok
23/16 ... ok
23/17 ... ok
10/18 ... ok
10/20 ... ok
10/22 ... ok
10/23 ... ok
10/24 ... ok
10/25 ... ok
10/27 ... ok
10/28 ... ok
10/29 ... ok
10/30 ... ok
10/31 ... ok
10/32 ... ok
10/33 ... ok
10/34 ... ok
10/35 ... ok
10/36 ... ok
10/37 ... ok
10/38 ... ok
10/39 ... ok
10/40 ... ok
10/41 ... ok
10/42 ... ok
10/43 ... ok
10/44 ... ok
10/45 ... ok
10/46 ... ok
10/47 ... ok
10/48 ... ok
10/49 ... ok
10/50 ... ok
10/51 ... ok
10/54 ... ok
10/55 ... ok
10/56 ... ok
10/57 ... ok
10/58 ... ok
10/59 ... ok
10/61 ... ok
8/62 ... ok
34/63 ... ok
34/64 ... ok
34/65 ... ok
34/66 ... ok
34/67 ... ok
34/68 ... ok
34/69 ... ok
34/70 ... ok
48/71 ... ok
48/73 ... ok
48/74 ... ok
50/75 ... ok
48/76 ... ok
5/77 ... ok
5/78 ... ok
5/79 ... ok
7/80 ... ok
39/81 ... ok
41/82 ... ok
5/83 ... ok
43/84 ... ok
48/85 ... ok
8/86 ... repository is empty
43/88 ... ok
51/89 ... ok
10/90 ... ok
48/91 ... ok
48/92 ... ok
49/94 ... repository is empty
5/95 ... repository is empty
48/96 ... ok
10/97 ... ok
10/98 ... ok
53/100 ... ok
53/101 ... ok
53/102 ... ok
8/103 ... ok
48/104 ... ok
48/105 ... ok
5/106 ... ok
5/107 ... ok
50/108 ... ok
8/109 ... ok
48/110 ... ok
23/111 ... ok
50/112 ... ok
12/114 ... ok
48/116 ... ok
8/117 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: FAILED. code: 302
gitlab-shell self-check failed
Try fixing it:
Make sure GitLab is running;
Check the gitlab-shell configuration file:
sudo -u git -H editor /opt/gitlab/embedded/service/gitlab-shell/config.yml
Please fix the error above and rerun the checks.
Checking GitLab Shell ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Reply by email ...
Reply by email is disabled in config/gitlab.yml
Checking Reply by email ... Finished
Checking LDAP ...
Server: ldapmain
not verifying SSL hostname of LDAPS server 'our.ldapserver:389'
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
[...]
Checking LDAP ... Finished
Checking GitLab ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... no
Try fixing it:
sudo chown -R git /var/opt/gitlab/gitlab-rails/uploads
sudo find /var/opt/gitlab/gitlab-rails/uploads -type f -exec chmod 0644 {} ;
sudo find /var/opt/gitlab/gitlab-rails/uploads -type d -not -path /var/opt/gitlab/gitlab-rails/uploads -exec chmod 0700 {} ;
For more information see:
doc/install/installation.md in section "GitLab"
Please fix the error above and rerun the checks.
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
10/4 ... yes
5/5 ... yes
5/6 ... yes
6/7 ... yes
49/8 ... yes
8/9 ... yes
50/10 ... yes
48/11 ... yes
23/13 ... yes
5/14 ... yes
10/15 ... yes
23/16 ... yes
23/17 ... yes
10/18 ... yes
10/20 ... yes
10/22 ... yes
10/23 ... yes
10/24 ... yes
10/25 ... yes
10/27 ... yes
10/28 ... yes
10/29 ... yes
10/30 ... yes
10/31 ... yes
10/32 ... yes
10/33 ... yes
10/34 ... yes
10/35 ... yes
10/36 ... yes
10/37 ... yes
10/38 ... yes
10/39 ... yes
10/40 ... yes
10/41 ... yes
10/42 ... yes
10/43 ... yes
10/44 ... yes
10/45 ... yes
10/46 ... yes
10/47 ... yes
10/48 ... yes
10/49 ... yes
10/50 ... yes
10/51 ... yes
10/54 ... yes
10/55 ... yes
10/56 ... yes
10/57 ... yes
10/58 ... yes
10/59 ... yes
10/61 ... yes
8/62 ... yes
34/63 ... yes
34/64 ... yes
34/65 ... yes
34/66 ... yes
34/67 ... yes
34/68 ... yes
34/69 ... yes
34/70 ... yes
48/71 ... yes
48/73 ... yes
48/74 ... yes
50/75 ... yes
48/76 ... yes
5/77 ... yes
5/78 ... yes
5/79 ... yes
7/80 ... yes
39/81 ... yes
41/82 ... yes
5/83 ... yes
43/84 ... yes
48/85 ... yes
8/86 ... yes
43/88 ... yes
51/89 ... yes
10/90 ... yes
48/91 ... yes
48/92 ... yes
49/94 ... yes
5/95 ... yes
48/96 ... yes
10/97 ... yes
10/98 ... yes
53/100 ... yes
53/101 ... yes
53/102 ... yes
8/103 ... yes
48/104 ... yes
48/105 ... yes
5/106 ... yes
5/107 ... yes
50/108 ... yes
8/109 ... yes
48/110 ... yes
23/111 ... yes
50/112 ... yes
12/114 ... yes
48/116 ... yes
8/117 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.3 ? ... yes (2.3.3)
Git version >= 2.7.3 ? ... yes (2.13.5)
Active users: ... 37
Checking GitLab ... Finished