DHCP Starvation attack

DHCP starvation attacks are designed to deplete all of the addresses within the DHCP scope on a particular segment. Subsequently, a legitimate user is denied an IP address requested via DHCP and thus is not able to access the network.  Yersinia is one such free hacking tool that performs automated DHCP starvation attacks. DHCP starvation may be purely a DoS mechanism or may be used in conjunction with a malicious rogue server attack to redirect traffic to a malicious computer ready to intercept traffic. Imagine a user filling up the dhcp pool and then re-directing users to their own DHCP server.

How do you fix this?
802.11 has several mechanisms built in. DHCP Proxy is one way. Port security is another. If you are running Mikrotik there are some scripts which can alert you to rogue DHCP servers, but that is an after-the-fact kind of thing.

 

Protecting your Mikrotik from DNS Amplification

There are several reasons and benefits to using your Mikrotik as a DNS caching server.  Queries to the client are just a tad faster, which makes the overall user experience seem snappier.  It also allows you to quickly change upstream DNS servers in the even of an outage, attack, etc.

There are two main avenues to think about when protecting Mikrotik from DNS.

The first is the incoming port 53 requests to the router.  You only want your customers to have access to query the Mikrotik.  In a simple scenario we have this:.

basic

ether1 is our upstream ISP connection.  Customers are other ports.  In this case if we want to block all port 53 requests from the outside world we specify the WAN interface to drop in the following code:

/ip firewall filter
add chain=input in-interface=ether1 protocol=udp dst-port=53 action=drop
add chain=input in-interface=ether1 protocol=tcp dst-port=53 action=drop

This will still allow your Mikrotik to send out DNS queries because they are sourced from a non reserved port. We are simply blocking the Mikrotik from not answering port 53 requests on the external interface.

In a later post we will talk about what to do if you have multiple wan interfaces or multiple exit paths on your router (say running OSPF)