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Firewalls Original slides prepared by Theo Benson Unix Firewalls • FreeBSD: ipfw • Linux: ipfw → ipchains → iptables • MacOS X: ipfw ipfw example rules: # SSH # Allow ssh from unc.edu hosts /sbin/ipfw -f add allow tcp from 152.2.0.0/16 to any 22 setup /sbin/ipfw -f add allow tcp from 152.19.0.0/16 to any 22 setup /sbin/ipfw -f add allow tcp from 152.23.0.0/16 to any 22 setup Stateful Firewalls • A bit more complicated • Keep track of transport layer connections (e.g., TCP, UDP) that may comprise multiple packets • Often allow only connections initiated from behind the firewall How are they deployed? The firewall is the gatekeeper The Internet AKA “Everything evil” “circle of trust” Only one way in or out into the circle Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Similar to streaming a Video … Loading Youtube Browser HTTP Requests Get: image.png HTTP Requests Get: video.avi Network Allowing Outbound Connections Only SYN The Internet AKA “Everything evil” “circle of trust” • Why would someone from the outside want to start a connection? Allowing Outbound Connections Only SYN The Internet AKA “Everything evil” “circle of trust” • Why would someone from the outside want to start a connection? – They would if you were running a web-server, an email-server, a gaming server …. Pretty much any ‘server’ service. – Firewall configuration may allow “punching holes” to specific addresses/ports Traversing Firewalls • Two hosts behind separate firewalls may try to fool their firewalls by simultaneously establishing outbound connections. • An external server may help coordinate which source ports, sequence numbers, to use. (E.g., STUN protocol.) Network Address Translation (NAT) Src: 192.168.1.100:32532 192.168.1.100 Src: 128.2.205.42:45323 128.2.205.42 • For outbound packets, the translator replaces (typically) private address with it’s own public address, and rewrites the source port. • Translator remembers the mapping. • For inbound packets, the reverse translation is performed. NAT versus Firewall • A network address translator is not intrinsically a firewall, but – Often the two are combined in one device – Traffic cannot be sent directly to private addresses used behind a NAT from the public Internet – A NAT may block incoming connections by necessity because it does not know which private address to forward the traffic to What Happens When you Connect to a Website? Loading SoundCloud Browser Network HTTP Requests Get: image.png HTTP Requests Get: sound.mp3 What happens if the virus/worm is hidden in an email? Picture? Or if the security exploit is in an HTML page? Deep Packet Inspection • Examine payload (data) portion of packet as well as headers IP Header TCP/UDP Header Payload Application Level Firewall • Why are they needed? • Attackers are tricky – When exploiting security vulnerabilities – Attacks span multiple packets • Need a system to scan across multiple packets for Virus/Worm/Vulnerability exploits Application Level Firewalls • Similar to Packet-filters except: – Supports regular expression – Search across different packets for a match – Reconstructs objects (images,pictures) from packets and scans objects. Application Level Firewalls • Similar to Packet-filters except: – Supports regular expression – Searches across different packets for a match – Reconstructs objects (images,pictures) from packets and scans objects. Appy reg-ex to the object: HTTP Requests Get: image.png Application Level Firewalls • Similar to Packet-filters except: – Supports regular expression – Searches across different packets for a match – Reconstructs objects (images,pictures) from packets and scans objects. HTTP Requests Get: image.png Why doesn’t everyone use App level firewalls? • Object re-assembly requires a lot of memory • Regular-expressions require a lot of CPU • App level firewalls are a lot more expensive – And also much slower – So you need more -- a lot more. How do you Attack the Firewall? • Most Common: Denial-of-Service attacks – Figure out a bug in the Firewall code – Code causes it to handle a packet incorrectly – Send a lot of ‘bug’ packets and no one can use the firewall