AOIP - Anything Over IP » Intrusion Prevention

DoS TCP SYN Attack Mitigation

Jay — Fri, 21 Aug 2009 14:22:40 +0000

TCP SYN flooding is often used in conjunction with IP spoofing.

The main aim of a TCP SYN flood is to send a TCP SYN packet to a host inside your network from a spoofed IP address. The TCP SYN ACK is then sent to a machine that is not expecting one, or a machine that doesn’t exist. This causes a ‘half-opened’ connection (Embryonic Connection) to exist since the 3-way handshake has not completed with a ACK.

A Cisco router is only capable of having a certain amount of connections open to it (This is different depending on the router model) so if an attacker was to send thousands of spoofed TCP SYN packets the router would reach maximum allowed connections, even though the connections are not complete. This would prevent the router from allowing legitimate connections been created.

In order to completely fix this problem, a firewall that supports dynamic embryonic connections would need to be installed, however a router can prevent the flood of TCP SYN packets by disallowing connections from the outside coming into your network.

This means that only connections that were created from inside your network would work and all connections from the outside would be denied.

Here is the configuration breakdown. (FastEthernet 0/0 is the outside network)

AOIP.ORG(config)# access-list 101 permit tcp any 192.168.1.0 0.0.0.255 established

AOIP.ORG(config)# access-list 101 deny ip any any log

AOIP.ORG(config)# interface fa0/0

AOIP.ORG(config-if)# ip access-group 101 in

AOIP.ORG(config-if)# exit

AOIP.ORG(config)#

Mitigating SubSeven attacks

Jay — Thu, 20 Aug 2009 14:10:47 +0000

SubSeven – aka: Sub7 and Sub7Server is a backdoor program, in the form of a Trojan, used mostly for causing trouble on computer networks. It can be used for simple attacks such as hiding the mouse cursor and opening applications, but it can also be used for more serious attacks including retrieving all your personal information stored on your machine and all computer settings.

The name SubSeven was derived by reversing the word Netbus (also a famous backdoor program) and replacing with the world ‘ten’ with ‘seven’

netbus = subten = subseven

In order to mitigate this application from attacking your network the following Access-list can be configured on your routers interfaces. Most Anti-Virus programs will also prevent it from attacking your machine.

AOIP.ORG(config)# access-list 100 deny tcp any any eq 1243 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 2773 log

AOIP.ORG(config)# access-list 100 deny tcp any any range 6711 6713 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 6776 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 7000 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 7215 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 27374 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 27573 log

AOIP.ORG(config)# access-list 100 deny tcp any any eq 54283 log

AOIP.ORG(config)# access-list 100 permit ip any any

AOIP.ORG(config)# interface fa0/0

AOIP.ORG(config-if)# ip access-group 100 in

AOIP.ORG(config-if)# exit

AOIP.ORG(config)# interface fa0/1

AOIP.ORG(config-if)# ip access-group 100 in

AOIP.ORG(config-if)# exit

AOIP.ORG(config)#

Mitigating Smurf DoS Attacks

Jay — Tue, 18 Aug 2009 06:00:41 +0000

Before looking at how to mitigate a Smurf attack, let’s first understand what it is and why it’s such a problem.

A Smurf works on a weakness of IP and ICMP by sending an ICMP packet to the broadcast address of a network. For example, I could send an ICMP (Ping packet) to every computer on the network 192.168.1.0 /24

I would do this by sending an ICMP packet to the address 192.168.1.255. This would result in every computer in that network (possibly 254 machines) sending me an echo-reply message. So far, this is not the end of the world, however Smurf adds ip spoofing to the equation…

When someone does a Smurf attack, the first thing that they do is an IP Spoof to make the their IP address look like an internal address. For example, I would spoof my address to have a source IP address of 192.168.1.10. Let’s assume that the IP address 192.168.1.10 was the Domain Controller, or perhaps the E-mail or Web server of that network. If I was to now send multiple echo packets to the destination address 192.168.1.255, every machine on that network would now send a echo-reply to the source IP of 192.168.1.10 (The internal server). This means I have just caused every machine on the target network to attack the internal target machine with echo-replies. Of course this is not the end of the world if this happens once or twice, but what if I did this a few thousand, or a few hundred thousand times? What if the target network was larger and had more than 254 machines? On a larger scale this could cause the target machine to be so over loaded with echo-replies that its network card becomes saturated to a point where its prevented from doing its job… denying it from doing it’s service… “Denial of Service” (DoS). Since this attack is not coming from me directly, and I’m forcing multiple machines to attack a single host, this now becomes a “Distributed Denial of Service” (DDos) attack.

So in order for us to prevent Smurf attacks happening on our networks, we need to make sure we block directed broadcast traffic coming into our network.

Below is the configuration required to stop Smurf Attacks. Use the diagram as a reference for the ACL’s

AOIP.ORG(config)# access-list 101 deny ip any host 192.168.1.255 log

Deny the directed broadcast

AOIP.ORG(config)# access-list 101 permit ip any 192.168.1.0 0.0.0.255 log

Allow unicast traffic

AOIP.ORG(config)# interface fa0/0

AOIP.ORG(config-if)# ip access-group 101 in

Attach the ACL to the interface for inbound traffic

AOIP.ORG(config-if)# exit

Now for the other direction……

AOIP.ORG(config)# access-list 102 deny ip any host 10.0.1.255 log

Deny the directed broadcast

AOIP.ORG(config)# access-list 102 permit ip any 10.0.1.0 0.0.0.255 log

Allow unicast traffic

AOIP.ORG(config)# interface fa0/1

AOIP.ORG(config-if)# ip access-group 102 in

Attach the ACL to the interface for inbound traffic

AOIP.ORG(config-if)# exit

AOIP.ORG(config)#

In the above configuration we have now mitigated Smurf attacks from either network segments in either direction.

IP Address Spoofing Mitigation with Access Control Lists (ACL)

Jay — Mon, 17 Aug 2009 14:06:29 +0000

IP spoofing is the act of camouflaging your IP address to make it look like you are someone else. Although IP spoofing is not an attack by itself, it is the starting point of many of the most common attacks found on today’s networks.

Most, but not all, of the spoofing attacks that take place start with someone on the outside of your network spoofing their IP address to make it look like they are part of the inside of your network. Since all private networks worldwide follow the standards of RFC 1918, knowing what IP addresses a company is using inside their network is not complex.

RFC 1918 defines that the following IP addresses are usable inside private networks.

10.0.0.0 – 10.255.255.255

172.16.0.0 – 172.31.255.255

192.168.0.0 – 192.168.255.255

Based on the above, the most obvious first step is to deny traffic that has a source IP address inside the RFC 1918 block that is coming into your network from the outside interface. The outside interface should have outside (public) IP addresses as their source. There would be no valid reason for an IP address on the outside of your network falling within that range.

Further to the RFC 1918 block of addresses, we also need to block any source coming from the following:

local loopback (127.0.0.1)

network address (0.0.0.0)

broadcast address (255.255.255.255)

Private multicast range (224.0.0.0 /4 )

Here is the configuration breakdown for the above scenario

AOIP.ORG(config)# access-list 100 deny ip 10.0.0.0 0.255.255.255 any log

AOIP.ORG(config)# access-list 100 deny ip 172.16.0.0 0.15.255.255 any log

AOIP.ORG(config)# access-list 100 deny ip 192.168.0.0 0.0.255.255 any log

The above are the RFC 1918 Unicast IP addresses

AOIP.ORG(config)# access-list 100 deny ip 127.0.0.0 0.255.255.255 any log

Local loopback restriction

AOIP.ORG(config)# access-list 100 deny ip 0.0.0.0 0.255.255.255 any log

Network address restriction

AOIP.ORG(config)# access-list 100 deny ip 224.0.0.0 15.255.255.255 any log

Private multicast range restriction

AOIP.ORG(config)# access-list 100 deny ip host 255.255.255.255 any log

Broadcast source address restriction

AOIP.ORG(config)# access-list 100 permit ip any 192.168.1.0 0.0.0.255

There is an implicit deny any at the bottom of every access list, so without a permit statement, NO traffic would be allowed. In the above entry I have allowed any traffic to go to the destination address 192.168.1.0 /24 (My internal network)

AOIP.ORG(config)# interface fa0/0

For example purposes FastEthernet 0/0 is my outside interface

AOIP.ORG(config-if)# ip access-group 100 in

I have applied this ACL inbound, which will also protect the router from been attacked.

AOIP.ORG(config-if)# exit

AOIP.ORG(config)#

Not only do we want to prevent people from the outside of our network ‘pretending’ they are inside our network, we also want to prevent our inside people from spoofing to a different IP address as well. The above configuration had a focus on traffic INBOUND to our network, the below configuration is focused on preventing OUTBOUND traffic from spoofing.

AOIP.ORG(config)# access-list 105 permit ip 192.168.1.0 0.0.0.255 any

The only range that is allowed to transmit is my internal network, in this case 192.168.1.0 /24

AOIP.ORG(config)# access-list 105 deny ip any any log

Although there is an implicit ‘deny any’ at the end of all ACL’s, I have included this with the log statement so I can see how many attempts have been made by internal machines to spoof their address before leaving the network

AOIP.ORG(config)# interface fa0/1

For example purposes, interface FastEthernet 0/1 is my inside interface

AOIP.ORG(config-if)# ip access-group 105 in

I have applied this ACL inbound, which will also protect the router from been attacked.

AOIP.ORG(config-if)# exit

AOIP.ORG(config)#