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BLACKHOLE(4) | Device Drivers Manual | BLACKHOLE(4) |
NAME¶
blackhole
—
a sysctl(8) MIB for
manipulating behaviour in respect of refused TCP or UDP connection
attempts
SYNOPSIS¶
sysctl net.inet.tcp.blackhole[=[0
|
1
|
2]]
sysctl net.inet.udp.blackhole[=[0
|
1]]
DESCRIPTION¶
Theblackhole
sysctl(8) MIB is used to control system behaviour
when connection requests are received on TCP or UDP ports where there is no
socket listening.
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.
In the UDP instance, 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.
The blackhole behaviour is useful to slow down anyone who is port scanning a
system, attempting to detect vulnerable services on a system. It could
potentially also slow down someone who is attempting a denial of service
attack.
WARNING¶
The TCP and UDP blackhole features should not be regarded as a replacement for firewall solutions. Better security would consist of theblackhole
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.
SEE ALSO¶
ip(4), tcp(4), udp(4), ipf(8), ipfw(8), pfctl(8), sysctl(8)HISTORY¶
The TCP and UDPblackhole
MIBs first appeared
in FreeBSD 4.0.
AUTHORS¶
Geoffrey M. RehmetJanuary 1, 2007 | Debian |