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BLACKHOLE(4) Kernel Interfaces Manual BLACKHOLE(4) NAME blackhole -- quietly drop refused SCTP, TCP, or UDP packets SYNOPSIS sysctl net.inet.sctp.blackhole[={0 | 1 | 2}] sysctl net.inet.tcp.blackhole[={0 | 1 | 2 | 3}] sysctl net.inet.tcp.blackhole_local[={0 | 1}] sysctl net.inet.udp.blackhole[={0 | 1}] sysctl net.inet.udp.blackhole_local[={0 | 1}] DESCRIPTION The blackhole sysctl(8) MIB is used to control system behaviour when connection requests are received on SCTP, TCP, or UDP ports where there is no socket listening or unexpected packets are received on listening sockets. The blackhole behaviour is useful to slow down an attacker who is port- scanning a system in an attempt to detect vulnerable services. It might also slow down an attempted denial of service attack. The blackhole behaviour is disabled by default. If enabled, the lo- cally originated packets would still be responded to, unless also net.inet.tcp.blackhole_local (for TCP) and/or net.inet.udp.blackhole_local (for UDP) are enforced. SCTP Setting the SCTP blackhole MIB to a numeric value of one will prevent sending an ABORT packet in response to an incoming INIT. A MIB value of two will do the same, but will also prevent sending an ABORT packet when unexpected packets are received. TCP Normal behaviour, when a TCP SYN segment is received on a port where there is no socket accepting connections, is for the system to return a RST segment, and drop the incoming SYN segment. The connecting system will see this as a "Connection refused". By setting the TCP blackhole MIB to a numeric value of one, the incoming SYN segment is merely dropped, and no RST is sent, making the system appear as a blackhole. By setting the MIB value to two, any segment arriving on a closed port is dropped without returning a RST. Setting the MIB value to three, any segment arriving on a closed port or an unexpected segment on a listening port is dropped without sending a RST in reply. This pro- vides some degree of protection against stealth port scans. UDP Enabling blackhole behaviour turns off the sending of an ICMP port un- reachable message in response to a UDP datagram which arrives on a port where there is no socket listening. It must be noted that this behav- iour will prevent remote systems from running traceroute(8) to a sys- tem. WARNING The SCTP, TCP, and UDP blackhole features should not be regarded as a replacement for firewall solutions. Better security would consist of the blackhole sysctl(8) MIB used in conjunction with one of the avail- able firewall packages. This mechanism is not a substitute for securing a system. It should be used together with other security mechanisms. SEE ALSO ip(4), sctp(4), tcp(4), udp(4), ipf(8), ipfw(8), pfctl(8), sysctl(8) HISTORY The TCP and UDP blackhole MIBs first appeared in FreeBSD 4.0. The SCTP blackhole MIB first appeared in FreeBSD 9.1. AUTHORS Geoffrey M. Rehmet FreeBSD 15.0 September 24, 2025 BLACKHOLE(4)
NAME | SYNOPSIS | DESCRIPTION | WARNING | SEE ALSO | HISTORY | AUTHORS
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