Lab 11 Cisco Firewall Cisco Firewall Brief overview
Lab 11 – Cisco Firewall
Cisco Firewall � Brief overview
Background: Your network is 152. 8. 0. 0/16 Everyone has the same IP address range Any address that starts with 152. 8 is considered local A local network Anything else is “outside” Rules specify source and destination IP addresses
ACLs (access control lists) � Rules are for many types of interface � 2 styles for IP ◦ Basic (in bound only from a source) ◦ Extended (in and out bound) � We’ll concentrate on the extended IP style
General format for a Cisco-like firewall configuration command (extended IP) � access-list number {permit | deny} [protocol] {any | ipaddr mask | host ipaddr} [operator port | established] [log] ◦ ◦ ◦ Bold items are entered verbatim […] items are optional {…} must be entered | denotes “or” The command must be all on one line access-list 111 permit tcp any host 152. 8. 1. 10 eq 80
Definitions: � access-list ◦ All firewall configuration ACL commands start with this keyword � number ◦ A number typically between � E. g. IP is 0 -99 or 1300 -1999, IPX is 100 -199 or 2000 -2699 � Think of it as the name of the list � Number range implies type of protocol involved � permit or deny ◦ Whether to permit or deny this packet of information if conditions match � protocol (optional) ◦ Type of protocol for this packet: IP, ICMP, SNMP, UDP or TCP � If omitted, then this command pertains to all network traffic regardless of protocol
Definitions: � Source Address ◦ Internet address of the sender of the packet ◦ Can be: �any - This access command applies to packets from any source �host ipaddr - Command applies to one particular computer � IP address of the computer � Dotted decimal format (i. e. 152. 8. 1. 2) �ipaddr mask – “sub-net” range affected � IP network address of the packet's source � Dotted-decimal format � Followed by a mask (dotted-decimal format) � When comparing the packet's source address, any address bit whose mask bit is one is ignored � 152. 8. 12. 47 0. 0. 255 � represents all IP addresses whose first 16 bits match � Destination Address ◦ Internet address of the network packet's destination �Specified in the same three formats as the source address
Definitions: � Operator (optional) ◦ Applies to TCP or UDP ports only ◦ Indicates how the port number in the packet should be compared ◦ If omitted, command applies for all ports �eq �lt �gt �neq �range equal less than greater than not equal a range of ports �est established connections �Must specify two different port numbers �Allows packets to pass through the firewall from the Internet if they are the response to a connection established from within the intranet
Definitions: � Port (optional) ◦ TCP/UDP destination port number ◦ If omitted, command applies for all port numbers ◦ Port number must be specified if an operator is given � Log (optional) ◦ Whether to log this entry to the console
Notes: � Commands are case insensitive ◦ Note: all access-list commands must fit on one line ◦ Comments can be included configuration �Comments start with an exclamation point ( ! ) � Examples: ◦ access-list 111 permit tcp any host 152. 8. 1. 10 eq 80 �Permits any computer on the Internet to connect to the computer whose �IP host address is 152. 8. 1. 1 �uses the TCP protocol �port 80 ◦ access-list 123 deny any 178. 22. 8. 9 0. 0. 255 �This will prohibit any computer from accessing a computer on the 178. 22 domain using any protocol
Access-list command order is important � When a packet arrives at your firewall ◦ it will be compared with each access-list statement in the order they appear � The first statement that applies to that packet determines if it is permitted or denied � For incoming traffic ◦ Implicit deny everything else at the end of the access -lists � For outgoing traffic ◦ Implicit permit everything else at the end of the access-lists
Firewall � IP any matches one IP address 152. 8. 1. 1 0. 0. 255 matches anything host 152. 8. 1. 10 address format: matches a (sub)network Note: the Cisco netmask is backwards from what you're used to! Called an inverse mask 0's mean "don't care"
Examples access-list 101 deny tcp any 152. 8. 0. 0. 255 eq 80 Block all incoming TCP traffic to port 80 “any” is the source address “ 152. 8. 0. 0. 255” is the destination. In this case, it refers to any IP address that starts with 152. 8. This blocks packets from entering your network from outside Protects your internal or rogue Web servers
Examples access-list 101 deny tcp 152. 8. 0. 0. 255 any eq 80 Block all outgoing TCP traffic to port 80 “ 152. 8. 0. 0. 255” is the source “any” is the destination. block all packets from the local network to the everything on port 80 outside
Examples access-list 101 allow tcp any host 152. 8. 1. 10 eq 80 access-list 101 deny tcp any 152. 8. 0. 0. 255 eq 80 Block all incoming port 80 traffic EXCEPT traffic to our Web server 152. 8. 1. 10 152. 8. 0. 0. 255 Second rule blocks packets from outside to all inside A packet going to the Web server First rule permits packets to the Web server Matches the first rule and stops being processed Packet allowed to go to the Web server A packet going anywhere else Doesn't match the first rule, so it gets caught by the second rule It is denied, or blocked
Examples We don't want employees in a certain subnet reading Reddit while at work. Block packets from 152. 8. 100. 0/24 to www. reddit. com (72. 246. 25. 35) access-list 101 deny tcp 152. 8. 100. 0. 0. 255 host 72. 246. 25. 35 Note: we're only interested in IP addresses starting with 152. 8. 100, so the netmask is 0. 0. 0. 255
Important Note � The ACLs are for an interface ◦ E. g for a specific Ethernet port (plug) � For extended IP rules need ACLs: ◦ For the outward facing ports (the internet) ◦ For the internal ones � Each rule needs source and destination addresses ◦ Remember: the firewall has two faces or sides �One to the WAN or Internet �One to the LAN side (with the switch)
Final notes The protocol for a service isn't always TCP The number after “access-list” isn't important DNS, for example, uses UDP You can leave out the protocol entirely to operate on all protocols. Can use the same number for every rule Should use a number for the type of rule Any incoming packets not covered by a rule are blocked by default Any outgoing packets not covered by a rule are allowed by default
Cisco Router Modes Mode Access Method Prompt User Exec Begin a new session Router> Privileged Exec Enter enable from user Exec Router# Global Configuration Enter configure from privileged Exec Router(config)# Mode Enter interface Fast. Ethernet number from Router(config-if)# global mode Interface Configuration • These Cisco IOS command modes are hierarchical. • When you begin a router session, you are in user EXEC mode. • You can see a list of available commands for a particular mode by entering a question mark (? ) at the prompt.
General Notes �When the 850 is reset: ◦ Get a default UID of “cisco” and PW of “cisco” ◦ One time use only! ◦ Must create a new user for use next time restart/login �Otherwise will need to reset the router and start all over again! �Minicom ◦ Don’t forget to turn line wrap on!
Lab 11 notes � When first started or reset the Cisco 850 router does absolutely nothing � Must turn on and configure services ◦ Enable NAT ◦ Enable and configure DHCP ◦ Set up ACLs (Access Control Lists) � This lab will use Minicom to configure the router through the serial port
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