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Forum Replies

  1. hello Rene, a question about ACL
    if I read an acl written in this way:
    access-list 100 permit ip host 131.108.1.1 any....
    the keyword "any" means :
    " permit any packet from address 131.108.1.1 to any others address if configured , in this router, more 255.255.255.255 and more all mulsticast address? (224.0.0.9 for rip for example)
    131.108.1.1 is for example the adjacent router on my fa 0/0...( and so I have to configure acl in inboud)

  2. Hi Francesco,

    An extended access-list always looks like this:

    The source and destination port is optional. For your example it will be:

    protocol = ip
    source address = 131.108.1.1 (host means using subnetmask 255.255.255.255)
    source port = not specified
    destination address = any
    destination port = not specified

    "any" really means any IP address so it'll match on destination address 0.0.0.0 - 255.255.255.255.

    When the router receives an IP packet on an interface that has an access-list then it will look for a match.

    Rene

  3. Hi John,

    That makes sense :slight_smile: In my example I didn't have NAT configured...just the IP addresses and security levels on the ASA, that's it.

    Rene

  4. Hi Asi,

    Here's a good document from Cisco that explains the "order of operation" for the ASA:

    Cisco ASA Packet Flow

    The packet tracer tool on the ASA is also great to answer this question. For example:

    ASA# packet-tracer input INSIDE tcp 192.168.1.1 50001 1.2.3.4 80

    This will show us the packet flow for a host that is using IP address 192.168.1.1 and who wants to connect to TCP port 80 on 1.2.3.4. Here's the result:

    Phase: 1
    Type: ACCESS-LIST
    Subtype: 
    Result: ALLOW
    Config:
    Implicit Rule
    Additional Information:
    MAC Access list
    
    Phase: 2
    Type: ROUTE-LOOKUP
    Subtype: Resolve Egress Interface
    Result: ALLOW
    Config:
    Additional Information:
    found next-hop 10.10.10.1 using egress ifc  OUTSIDE
    
    Phase: 3
    Type: NAT
    Subtype: 
    Result: ALLOW
    Config:
    object network INSIDE
     nat (INSIDE,OUTSIDE) dynamic interface
    Additional Information:
    Dynamic translate 192.168.1.1/50001 to 10.10.10.254/50001
    
    Phase: 4
    Type: NAT
    Subtype: per-session
    Result: ALLOW
    Config:
    Additional Information:
    
    Phase: 5
    Type: IP-OPTIONS
    Subtype: 
    Result: ALLOW
    Config:
    Additional Information:
    
    Phase: 6
    Type: SFR
    Subtype: 
    Result: ALLOW
    Config:
    class-map SFR
     match access-list SFR_REDIRECT
    policy-map global_policy
     class SFR
      sfr fail-open
    service-policy global_policy global
    Additional Information:
    
    Phase: 7
    Type: NAT
    Subtype: per-session
    Result: ALLOW
    Config:
    Additional Information:
    
    Phase: 8
    Type: IP-OPTIONS
    Subtype: 
    Result: ALLOW
    Config:
    Additional Information:
    
    Phase: 9
    Type: FLOW-CREATION
    Subtype: 
    Result: ALLOW
    Config:       
    Additional Information:
    New flow created with id 1017213, packet dispatched to next module

    You can see that the access-list is checked BEFORE NAT is applied. If your packet doesn't match the access-list then it will be dropped before NAT translation occurs.

    Hope this helps.

    Rene

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