Problem

Cloning a repository from a mirror that contains submodules results in one of the following errors:
Command: git clone --recurse-submodules <Mirror URL>/git/Repository.git

fatal: clone of '<Upstream URL for Submodule>' into submodule path '<.../Repository/submodule>' failed
Failed to clone '<submodule name>'. Retry scheduled
Cloning into '<LocalPath.../Repository/submodule>'...
fatal: unable to access '<Upstream URL>': Could not resolve host: <HOST-NAME>
fatal: clone of '<Upstream URL for Submodule>' into submodule path '<.../Repository/submodule>' failed
Failed to clone '<submoodule name>' a second time, aborting

OR

Submodule '<submodule name>' (<Upstream URL for Submodule>) registered for path '<submodule name>'
Cloning into ''<LocalPath.../Repository/submodule>'...
fatal: early EOF
fatal: The remote end hung up unexpectedly
fatal: index-pack failed
error: RPC failed; curl 18 transfer closed with outstanding read data remaining
fatal: clone of '<Upstream URL for Submodule>' into submodule path '<LocalPath.../Repository/submodule>' failed
Failed to clone '<submodule name>'. Retry scheduled 
...

Cause

This can happen when users who clone from the mirror don't have access to the upstream primary server.
When creating a Submodule, Git creates a .gitmodules file. By default, the configuration uses the upstream/primary server URL.

Example of default .gitmodules file with fully qualified URL:

user@host:~/bitDC-Repos/superproject$ cat .gitmodules 
[submodule "mod1"]
path = mod1
url = https://user@bitbucket-dc/scm/git/mod1.git
[submodule "mod2"]
path = mod2
url = https://user@bitbucket-dc/scm/git/mod2.git

If the user cloning from the mirror has no access to the upstream primary server, the above URLs can not be used.

Resolution

In order for this to work correctly for both HTTPS and SSH you need to meet the following criteria :

1. All Repositories involved (SuperProject/Parent Repo and Submodule Repos) need to be on the server you are cloning from. This is true whether it is from a mirror or the upstream primary server.
2. If cloning via SSH, you need to add SSH keys to all repositories involved.

You don't need to clone the submodules locally as separate repositories.
Once all the repositories are on the server (Upstream and Mirror) and the correct access has been provided,  you can
Change URLs to Relative Paths

[submodule "mod1"]
path = mod1
url = ../mod1.git
[submodule "mod2"]
path = mod2
url = ../mod2.git 

The following command: git clone --recurse-submodules <Mirror URL>/git/Repository.git will now work.

For more information in regards to Git Submodules please refer to:

• Git - Submodules Overview
• Git - git-submodule Documentation