Troubleshooting AWS Client VPN: Authorization rules for Active Directory groups not working as expected
Problem
I have configured authorization rules for my Active Directory groups, but they are
not working as I expected. I have added an authorization rule for
0.0.0.0/0
to authorize traffic for all networks, but traffic still
fails for specific destination CIDRs.
Cause
Authorization rules are indexed on network CIDRs. Authorization rules must grant
Active Directory groups access to specific network CIDRs. Authorization rules for
0.0.0.0/0
are handled as a special case, and are therefore
evaluated last, regardless of the order in which the authorization rules are
created.
For example, say that you create five authorization rules in the following order:
-
Rule 1: Group 1 access to
10.1.0.0/16
-
Rule 2: Group 1 access to
0.0.0.0/0
-
Rule 3: Group 2 access to
0.0.0.0/0
-
Rule 4: Group 3 access to
0.0.0.0/0
-
Rule 5: Group 2 access to
172.131.0.0/16
In this example, Rule 2, Rule 3, and Rule 4 are evaluated last. Group 1 has access to
10.1.0.0/16
only, and Group 2 has access to 172.131.0.0/16
only. Group 3 does not have access to 10.1.0.0/16
or
172.131.0.0/16
, but it has access to all other networks. If you remove
Rules 1 and 5, all three groups have access to all networks.
Client VPN uses longest prefix matching when evaluating authorization rules. See Route priority in the Amazon VPC User Guide for more details.
Solution
Verify that you create authorization rules that explicitly grant Active Directory
groups access to specific network CIDRs. If you add an authorization rule for
0.0.0.0/0
, keep in mind that it will be evaluated last, and that
previous authorization rules may limit the networks to which it grants
access.