That makes port 23 a perfect candidate for a trapped port. Telnet is obsolete since a few years now and I assume that if you have a minimum security knowledge, you never use telnet. When you think about it, you never opened all your ports, so imagine that some ports you are sure you do not need are trapped to react against any connection attempted to them.įor example, you know that any port scan attempt will target telnet port (TCP 23). In the Thylacine firewall I use a trick against SYN scan. Some IDS can use statistics to detect fast and abnormal SYN packets hitting various ports and raise an alert. Preventing SYN scan is difficult for a firewall because starting a connection with a SYN packet is the normal way.
#Unicode netmap full#
Instead of just sending a SYN scan and a RST if the port is opened, the full connect scan complete the TCP connection procedure :ģ - RST-ACK is sent to close the connection. Depending on the answer you can guess if the port is opened, closed or filtered (see part I.1).īecause SYN scan is agressive and can be detected by firewalls another more "TCP friendly" method can be used. This scan method consists in sending TCP packets with only the SYN flag on. SYN scan is the default scan mode when using nmap and it is also the most used scan technique. In the Thylacine firewall I choose another way to prevent these scans because I want these scans to be precisely logged and I want the attacker address to be temporarily banned. Now you should say, wait a minute! With this only line we could also prevent all the previously mentioned scans! That is true. If it is not the case the packet is discarded. With this line any new connection must start with only a SYN flag. Iptables -A INPUT -p tcp ! -syn -m state -state NEW -m comment -comment "Drop TCP connection not starting by SYN" -j DROP In the Thylacine firewall I chose to block any attempts of ACK scans, which means the result of an ACK scan displays that all ports are filtered. If nmap receive an RST packet port is considered unfiltered, if there is no response, to port is considered filtered.To prevent this scan approach, there are several ways (like always). If you combine that with a FIN scan or a XMAS scan you can guess which ports are opened, filtered and closed. That means the goal of the ACK scan is to guess if a port is filtered or if it is not. The thing is, if the port cannot be reached (because there is a firewall protecting it for example), then the machine won’t get the false ACK and won’t send back the RST packet. The port may be opened or closed it does not matter here.
This scan uses the fact that a machine without a firewall will send a RST packet when receiving an acknowledgment of receipt of an unknown connection. In the TCP protocol, this flag is normally set when the packet is an acknowledgment of receipt of another packet. It consists in sending TCP packets with the ACK flag set. Lets have a look at other scan techniques.
You use these kind of protections for any other kind of scan using abnormal flag settings.
Iptables -A INPUT -p tcp -tcp-flags ALL FIN -m recent -name blacklist_60 -set -m comment -comment "Drop/Blacklist FIN scan" -j DROP
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