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Rules for SYN/FIN Variations kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,RST SYN,RST -j LOG --log-prefix "IPT: Bad SR Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,RST SYN,RST -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,PSH SYN,FIN,PSH -j LOG --log-prefix "IPT: Bad SFP Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,PSH SYN,FIN,PSH -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST SYN,FIN,RST -j LOG --log-prefix "IPT: Bad SFR Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST SYN,FIN,RST -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST,PSH SYN,FIN,RST,PSH -j LOG --log-prefix "IPT: Bad SFRP Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST,PSH SYN,FIN,RST,PSH -j DROP Next in Listing 2-40 you add a rule to address single FIN flag packets.

Open Program.cs. Modify the Main() method to the following (note your entity set will, by default, be prefixed with the same name as the database in the following example, mine is Book). static void Main(string[] args) { BookEntities ctx = new BookEntities(); var query = from o in ctx.Orders select o; foreach (Order order in query) { Console.WriteLine("Order: " + order.OrderID.ToString() + " " + order.Firstname + " " + order.Lastname); order.OrderItems.Load(); foreach(OrderItem orderItem in order.OrderItems) { Console.WriteLine("Adult: " + orderItem.QtyAdult.ToString() + " Child:" + orderItem.QtyChild.ToString()); } Console.WriteLine(""); } Console.ReadKey(); }

You will never find a packet that has only a FIN flag in normal TCP/IP connections; thus, any you do find are generally being used for port scans and network probing Listing 2-40 Rules for FIN-Only Flag Packets kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags FIN FIN -j LOG --log-prefix "IPT: Bad F Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags FIN FIN -j DROP These rules in Listing 2-40 select only those packets with a FIN flag, and only those packets with a FIN flag set then log and drop them Lastly you want to block so-called null packets, which have all flags present and set, and any other related Xmas-style scanning packets These are generally used for other forms of network probing used by scanning tools such as nmap Listing 2-41 shows how you can block these using the ALL and NONE special flag selectors Listing 2-41.

Rules for Null and Xmas Flag Packets kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags ALL NONE -j LOG --log-prefix "IPT: Null Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags ALL NONE -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags ALL ALL -j LOG --log-prefix "IPT: All Flags ".

kitten# iptables -A BAD_FLAGS -p tcp kitten# iptables -A BAD_FLAGS -p tcp -j LOG --log-prefix "IPT: Nmap:Xmas kitten# iptables -A BAD_FLAGS -p tcp -j DROP kitten# iptables -A BAD_FLAGS -p tcp -j LOG --log-prefix "IPT: Merry Xmas kitten# iptables -A BAD_FLAGS -p tcp

The results are shown in Figure 8-9.

--tcp-flags ALL ALL -j DROP --tcp-flags ALL FIN,URG,PSH Flags " --tcp-flags ALL FIN,URG,PSH --tcp-flags ALL SYN,RST,ACK,FIN,URG Flags " --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

Another use of malicious TCP flags is the SYN flood attack This Denial of Service attack is usually aimed at e-mail or Web servers and relies on subverting the three-way handshake connection process discussed earlier in this chapter The attacker sends a packet with the SYN flag set to the receiving host The source address of this packet is a nonexistent or uncontactable machine The receiving host replies with a packet with the SYN/ACK flags set As the source address of the packet cannot be replied to, the send fails and no ACK packet is received to fully open the connection Eventually the connection timeout is reached, and the connection closes This seems harmless enough, but on the receiving host each new connection adds connection information to a data structure in system memory This data structure has a finite size.

Normally failed connections would time out, and the data structure would be purged of the connection information But in the SYN flood attack, the attacker continues to send connection requests from nonexistent hosts until the data structure in memory overflows and no new connections are possible Generally, until the incoming SYN flood ceases, no new connections to the host are possible In some cases, the system may even halt entirely You can reduce the risk of this sort of attack using another iptables match module I discussed the state module earlier in this chapter, and now you will look at the limit module The limit module limits the rate and volume at which packets are matched to rules It is commonly used to limit traffic such as ICMP and to limit logging For example, you can limit the rate at which packets are logged (see Listing 2-42) Listing 2-42.