how to print a barcode in excel 2010 Traditional IP Access Lists in Software

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Traditional IP Access Lists
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Traditional IP access lists are available in two varieties: standard and extended Standard access lists allow filtering by source address only and are thus very limited in functionality Extended access lists allow filtering by source address, destination address, and upper-layer protocols We examine standard access lists first
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Standard Access Lists
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The basic format of a standard IP access list is: Access-list [1 99] [permit|deny] [ip address] [mask] [log] NoteThe log keyword is available only in IOS 113 and later versions Each access list is given a unique number that is used to inform the IOS of the type of access list you are defining This number is also used in all subsequent references to the access list Standard IP access lists are defined within the range 1 99 In IOS version 112, named access lists were introduced, allowing you to define names for your access lists These lists were created so you can delete specific entries in the access list without recreating the entire list Additional entries, however, are still added to the end of the access list Standard IP access lists allow filtering by source IP address only In the examples that follow, we will use the following diagram as a reference point, as shown in Figure 7 6
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Figure 7 6: A Cisco routing example
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Suppose in Figure 7 6 that we want to allow only clients with node addresses 10 and 11 on segment 1 to have access to servers on segment 2 How would we accomplish this Our initial configuration is shown here: Interface ethernet 1 Ip access-group 1 out
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Ip access-list 1 permit 16010110 0000 Ip access-list 1 permit 16010111 0000 Notice a few things from this example First, the access list includes only two entries Since by default everything else is denied, this might have the unintended side effect that all other IP packets are blocked to servers on segment 2 The network mask of these entries is all zeroes, indicating an exact match (this is the default, so we could have avoided typing the wildcard mask in this example) Also notice how the access list is applied to the outgoing interface on Ethernet1 (E1) The access list could have been applied to interface Ethernet0 (E0) as an inbound access list, but this would have the undesired effect of blocking outbound traffic from all other hosts on segment 1; no hosts other than those specified to be permitted in the access list would be able to send packets outside of segment 1 To see why this is so, imagine how the packets arrive at the router A host on segment 1 with an IP address 16010112 sends a packet to the router interface E0 From the routers' perspective, this packet is incoming from the E0 segment, so it is "inbound" Therefore, it would apply the access list criteria to the packet that would then be denied by the implicit "deny all" Keeping these ideas in mind, let's modify this access list a bit In this modification, we assume that other network segments need access to the servers on segment 2 The modification is shown here: Interface ethernet 1 Ip access-group 1 out Ip access-list 1 permit 160101100000 Ip access-list 1 permit 160101110000 Ip access-list 1 deny 1601010000255 Ip access-list 1 permit any The small modifications change things quite a bit First, we now have an explicit deny statement for every address on network 1601010 This may seem like a contradiction Won't this have the effect of blocking the two nodes we wanted to allow access The answer is no, and the reason is due to the top-down processing of access lists Recall that earlier this was one of our configuration principles Access lists are not "compiled," like program code, and combined Each entry in the access list is read sequentially, from top to bottom for each packet that is processed on an interface Once a match is reached, the remaining access list entries are ignored This is why the order of the entries in an access list is so critical and why you should put the more specific entries first Recall our discussion of this topic earlier in this chapter Once the match is made for the node addresses 16010110 or 16010111, the search is completed and none of the remaining access list entries are examined Notice also that now we have added an explicit "permit any" to the end of this access list This permit statement negates the normal implicit "deny all" This feature should be used with great caution It assumes that explicit deny entries have been created for any packets you want to prevent from transiting a router interface Because the default "deny all" has been overridden, if a packet is not explicitly denied, it would be permitted access through the router interface
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Due to the sheer volume of addresses, it is usually much easier (and more secure) to use explicit permits for the packets you know you want to allow access and deny everything else Modifications can be made to the access list later to add additional permit entries if needed This example is used simply to illustrate what can be done with an access list, not necessarily what should be done It is worth noting at this point, as you may have already noticed, that standard IP access lists are not very flexible Because they only allow you to specify a filter by source IP address, in many cases they do not provide the necessary granularity that is required It's a bit like trying to squash a gnat with a sledgehammer For this reason, we will not spend much more time on standard IP access lists and will move on to the much more flexible extended IP access lists However, before moving on, we will point out three cases where the use of standard access lists is actually more beneficial than extended access lists: Limiting virtual terminal access Limiting SNMP access Routing protocol filters Limiting Virtual Terminal Access Often you'll want to limit the IP addresses that are allowed to remotely access your router This is prudent, because if someone were able to guess the user access password, they could run simple dictionary attacks against the enable password indefinitely Once the enable password is gained, the entire router is compromised One way in which you could limit virtual terminal access would be to apply an extended access list to every interface permitting telnet access to only a select few addresses This quickly becomes cumbersome, however, and there is a much simpler and cleaner way In this example, let's return to Figure 7 6 and assume we want to allow only the host 16010110 virtual terminal access to the router A simple solution is the following: Access-list 1 permit 16010110 0000 Line vty 0 4 Access-class 1 in This prevents any host other than 16010110 from accessing the router remotely, without having to apply access lists to every interface We can also limit the capability to telnet from the router once someone has gained virtual terminal access by applying an access list outbound to the virtual terminal ports: Access-list 1 deny any Line vty 0 4 Access-class 1 out This prevents a terminal line connection to any other destination What purpose would this serve Well, if telnet access is gained, an attacker might not be able to compromise the enable password
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(assuming a good password were chosen), but they could use the router as a "jumping-off point" to attack other hosts within the network Because a router is normally a trusted device in your network, this could be a very effective way to further compromise your internal hosts Of course, selective access could be allowed to hosts on a certain network: Access-list 1 permit 1601010 000255 Line vty 0 4 Access-class 1 out This allows terminal line connections to devices on the 1601010 network Limiting SNMP Access Simple Network Management Protocol (SNMP) is often used in a data network to manage network devices such as servers and routers SNMP uses a very simple authentication scheme called a community string The community string is essentially a password that allows an SNMP-speaking device to read and write information to an SNMP-capable device, such as a router There are two SNMP modes, Read-Only (RO) and Read-Write (RW) Each SNMP mode uses a different community string Although this protocol is very useful to network administrators, it is also very dangerous If you must enable SNMP access on your routers, it is often useful to limit the IP addresses that are allowed SNMP access Below is an example limiting both read and write access to station 16010110 : Access-list 1 permit 16010110 0000 Snmp-server community public RO 1 Snmp-server community private RW 1 Routing Protocol Filters Another good use of standard access lists is to filter certain network ranges when redistributing routes between different routing protocols Many times it is necessary to perform "mutual redistribution" when you need to redistribute some routes from one routing protocol into another routing protocol and a different set of routes from the second protocol into the first protocol The danger here is that if filtering is not used, a route can get redistributed from the first protocol into the second protocol and back into the first protocol This will obviously confuse routers about where particular routes are being originated A simple example should suffice to drive the point home In Figure 7 7, the router is receiving information about network 1411000 via the RIP routing protocol The router is also running OSPF and receiving information about network 1501000 via that routing protocol
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