2 .\" blackhole - drop refused TCP or UDP connects
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22 MIB for manipulating behaviour in respect of refused TCP or UDP connection
25 .Cd sysctl net.inet.tcp.blackhole[=[0 | 1 | 2]]
26 .Cd sysctl net.inet.udp.blackhole[=[0 | 1]]
31 MIB is used to control system behaviour when connection requests
32 are received on TCP or UDP ports where there is no socket listening.
34 Normal behaviour, when a TCP SYN segment is received on a port where
35 there is no socket accepting connections, is for the system to return
36 a RST segment, and drop the connection.
37 The connecting system will
39 .Dq Connection refused .
40 By setting the TCP blackhole
41 MIB to a numeric value of one, the incoming SYN segment
42 is merely dropped, and no RST is sent, making the system appear
44 By setting the MIB value to two, any segment arriving
45 on a closed port is dropped without returning a RST.
46 This provides some degree of protection against stealth port scans.
48 In the UDP instance, enabling blackhole behaviour turns off the sending
49 of an ICMP port unreachable message in response to a UDP datagram which
50 arrives on a port where there is no socket listening.
51 It must be noted that this behaviour will prevent remote systems from running
55 The blackhole behaviour is useful to slow down anyone who is port scanning
56 a system, attempting to detect vulnerable services on a system.
57 It could potentially also slow down someone who is attempting a denial
60 The TCP and UDP blackhole features should not be regarded as a replacement
61 for firewall solutions.
62 Better security would consist of the
65 MIB used in conjunction with one of the available firewall packages.
67 This mechanism is not a substitute for securing a system.
68 It should be used together with other security mechanisms.
84 .An Geoffrey M. Rehmet