Question

ACL: Block Public Subnet from Private

Asked by: bnoseworthy

Hello all,
On our Cisco 3750 stack the public wireless is network 192.168.32.0/24. I am trying to block it from internal networks, with the exception of DHCP. When I have no ACL applied I can pull an IP and surf the net. When I apply the ACLs below I cant pull an IP. I also want to be able to access devices on the 192.168.32.0 network from my workstation. Can you please spot what Ive missed?

int vlan 32
ip access-group 101 out

access-list 101 remark Permit access to internal DHCP servers
access-list 101 permit udp 192.168.32.0 0.0.0.255 host 10.1.5.15 eq bootpc
access-list 101 permit udp 192.168.32.0 0.0.0.255 host 10.1.5.14 eq bootpc
access-list 101 remark Deny access to internal 10.0.0.0/8 hosts
access-list 101 deny   ip any 10.0.0.0 0.255.255.255
access-list 101 remark Allow access to internet route (PIX)
access-list 101 permit ip any host 192.168.154.2
access-list 101 remark Deny access to 192.168.0.0/16 hosts
access-list 101 deny   ip any 192.168.0.0 0.0.255.255

int vlan 32
ip access-group 102 in

access-list 102 remark Allow access to vlan from management stations
access-list 102 permit ip host 10.1.10.10 192.168.32.0 0.0.0.255
access-list 102 permit udp host 10.1.5.15 192.168.32.0 0.0.0.255 eq bootpc
access-list 102 permit udp host 10.1.5.14 192.168.32.0 0.0.0.255 eq bootpc

PS
I'm not a regular poster, second question here, so I'm not sure how the points system works. I think my question is relatively easy for an expert so do I assign low points? It seems unappreciative.

Thank you in advance.

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Asked On
2009-01-07 at 07:06:49ID24031633
Tags

Cisco 3750

Topics

Network Security

,

Network Management

Participating Experts
2
Points
125
Comments
6

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Answers

 

by: Nothing_ChangedPosted on 2009-01-08 at 07:11:53ID: 23325672

The fastest way to troubleshoot what you need for just DHCP in your environment would be to put a Sniffer on a device, start a capture, then plug it into your public wireless with the ACLs disabled, and look at exactly what and to where it communicates to pull that IP you need. Odds are your implicit deny all at the end of acl 102 is dropping something you need. If you ahve a dhcp-helper address configured, be aware that this will change the look of the packets getting forwarded to your dhcp servers from their native structure if the dhcp server was on your LAN directly, so you may want to take a capture in front of your dhcp server too jsut to see how it looks there too. If you have a few short captures fro mthis and want a hand sorting it out, paste em back in and I'll have a peek.

and yes, 125 is weak but i like looking at packet captures so im in :) if you dont own a real Network General or Netscout Sniffer, use wireshark it works pretty well for low speed  captures where packet timing isnt that critical.

 

by: mickfinleyPosted on 2009-01-12 at 18:37:11ID: 23359607

instead of using eq bootpc; try eq 67 (For the DHCP problem)

Also these two entrys may be conflicting:

access-list 101 deny   ip any 10.0.0.0 0.255.255.255 (Out)

access-list 102 permit ip host 10.1.10.10 192.168.32.0 0.0.0.255 (In)

It looks like it will allow 10.1.10.10 to come in but not allow it back out.

You might try:  
  access-list 101 permit ip 192.168.32.0 0.0.0.255  host 10.1.10.10 (If RD only use eq 3389)
  access-list 101 deny ip any 10.0.0.0 0.255.255.255

 

by: mickfinleyPosted on 2009-01-12 at 18:48:32ID: 23359648

Also, the explicit deny all at the end of both acls are probably preventing internet access.  Whatever you don't specifically allow will be denied.

 

by: bnoseworthyPosted on 2009-01-12 at 19:36:50ID: 23359849

Sorry gang but I had to resolve quickly and sourced the problem out. I appreciate the input and will be implementing in a test environment for my own education.

 

by: Nothing_ChangedPosted on 2009-01-13 at 07:23:41ID: 23363587

ok good luck. incidentally, there are implicit denys at the end of every Cisco ACL, whether you put em in or not. Adding it explicitly allows you to log it and see how many hit's its getting, but aside from that it's no difference in how hte ACL works. Best practice is to include it as you have.

 

by: bnoseworthyPosted on 2009-10-24 at 05:51:17ID: 31531847

resolved

20120131-EE-VQP-002

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