Actually there is a simple solution. I needed a lot of time to get to this so it's worth sharing.
There is a special type of Access list called reflexive.
This kind of access list will allow traffic from one VLAN to another only if the communication is established in other direction before that.
It can't be used for IP traffic but only for every protocol separatelly so you will need to use more rows in ACL to allow TCp, ICPM etc, but it will solve your problem.
Here is how is done:
Let's say that you have two VLANs: VLAN 10 and VLAN 20.
VLAN 10 INTERFACE = 10.10.10.1 /24
VLAN 20 INTERFACE = 10.10.20.1 /24
VLAN 10 can access VLAN 20 but,
VLAN 20 can't access VLAN 10. (That was the whole problem, to allow access only in one direction).
To be able to do so, you need to let the traffic from VLAN 10 go to VLAn 20 but you need also to let this communication to go back to VLAn 10 in order to close the communication bidirectional functionality.
Almost every communication needs to get back to source in order to make the circle functional.
But, if you allow this communicaton to go back to VLAN 10, you will alow all the communication in both ways, and this is the problem that we can solve using reflexise ACLs.
We will make extended named ACL with name EASYONE:
ip access-list extended EASYONE
permit tcp 10.10.20.0 0.0.0.255 10.10.10.0 0.0.0.255 established
(The work established at end of this ACL row means that this TCP traffic from VLAN 20 to VLAN 10 will only be allowed when it's from some communication that was started from VLAN 10, a going back traffic)
permit icmp 10.10.20.0 0.0.0.255 10.10.10.0 0.0.0.255 echo-reply
(This echo-reply row will allow VLAN 20 to reply to ping and other ICMP requests)
deny ip 10.10.20.0 0.0.0.255 10.10.10.0 0.0.0.255
permit ip any any
(This row will deny all other traffic from VLAN 20 directed to VLAN 10 but with permit ip any any it will allow VLAN 20 to go let say to gatway and further to internet and other VLANs)
Finally, we will put the ACL EASYONE to VLAN 20 L3 interface
interface vlan 20
ip access-group EASYONE in
To conclude the config without comments, indeed easy now when is done:
ip access-list extended EASYONE
permit tcp 10.10.20.0 0.0.0.255 10.10.10.0 0.0.0.255 established
permit icmp 10.10.20.0 0.0.0.255 10.10.10.0 0.0.0.255 echo-reply
deny ip 10.10.20.0 0.0.0.255 10.10.10.0 0.0.0.255
permit ip any any
exit
interface vlan 20
ip access-group EASYONE in
The credit for the solution goes to my mentor and friend Sandra who did the configuration and lab for it but more than that she came out with the established word at end of the ACL and whole reflexive ACL solution.
For this detailed article and more about Cisco, Juniper, etc configurations from real life please visit blog howdoesinternetwork.com and enjoy networking as we do.
Valter Popeskic
