2 .\" blackhole - drop refused TCP or UDP connects
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22 MIB for manipulating behaviour in respect of refused SCTP, TCP, or UDP connection
25 .Cd sysctl net.inet.sctp.blackhole Ns Op = Ns Brq "0 | 1 | 2"
26 .Cd sysctl net.inet.tcp.blackhole Ns Op = Ns Brq "0 | 1 | 2"
27 .Cd sysctl net.inet.udp.blackhole Ns Op = Ns Brq "0 | 1"
32 MIB is used to control system behaviour when connection requests
33 are received on SCTP, TCP, or UDP ports where there is no socket listening.
35 The blackhole behaviour is useful to slow down an attacker who is port-scanning
36 a system in an attempt to detect vulnerable services.
37 It might also slow down an attempted denial of service attack.
39 Setting the SCTP blackhole MIB to a numeric value of one
40 will prevent sending an ABORT packet in response to an incoming INIT.
41 A MIB value of two will do the same, but will also prevent sending an ABORT packet
42 when unexpected packets are received.
44 Normal behaviour, when a TCP SYN segment is received on a port where
45 there is no socket accepting connections, is for the system to return
46 a RST segment, and drop the connection.
47 The connecting system will
49 .Dq Connection refused .
50 By setting the TCP blackhole
51 MIB to a numeric value of one, the incoming SYN segment
52 is merely dropped, and no RST is sent, making the system appear
54 By setting the MIB value to two, any segment arriving
55 on a closed port is dropped without returning a RST.
56 This provides some degree of protection against stealth port scans.
58 Enabling blackhole behaviour turns off the sending
59 of an ICMP port unreachable message in response to a UDP datagram which
60 arrives on a port where there is no socket listening.
61 It must be noted that this behaviour will prevent remote systems from running
65 The SCTP, TCP, and UDP blackhole features should not be regarded as a replacement
66 for firewall solutions.
67 Better security would consist of the
70 MIB used in conjunction with one of the available firewall packages.
72 This mechanism is not a substitute for securing a system.
73 It should be used together with other security mechanisms.
95 .An Geoffrey M. Rehmet