• Start
• Documentation
  • PostgreSQL
  • mnoGoSearch
  • Apache 2.4
  • FreeBSD handbook
  • FreeBSD Manual
  • FreeBSD Source
• Tools
• Search
• Protected Area

plain

FreeBSD manual

keyword 1 2 3 4 5 6 7 8 9 n l

download PDF document: blackhole.4.pdf

BLACKHOLE(4) FreeBSD Kernel Interfaces Manual BLACKHOLE(4)
NAME blackhole - a sysctl(8) MIB for manipulating behaviour in respect of refused SCTP, TCP, or UDP connection attempts
SYNOPSIS sysctl net.inet.sctp.blackhole[={0 | 1 | 2}] sysctl net.inet.tcp.blackhole[={0 | 1 | 2}] 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.
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 locally 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 connection. 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. This provides some degree of protection against stealth port scans.
UDP Enabling blackhole behaviour turns off the sending of an ICMP port unreachable message in response to a UDP datagram which arrives on a port where there is no socket listening. It must be noted that this behaviour will prevent remote systems from running traceroute(8) to a system.
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 available firewall packages.
This mechanism is not a substitute for securing a system. It should be used together with other security mechanisms.

AUTHORS Geoffrey M. Rehmet
FreeBSD 14.0-RELEASE-p11 November 3, 2021 FreeBSD 14.0-RELEASE-p11