blackhole -- a sysctl(8) MIB for manipulating behaviour in respect of
refused TCP or UDP connection attempts
sysctl net.inet.tcp.blackhole[=[0 | 1 | 2]]
sysctl net.inet.udp.blackhole[=[0 | 1]]
The blackhole 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.
The TCP and UDP blackhole features should not be regarded as a replacement
for ipfw(8) as a tool for firewalling a system. In order to create
a highly secure system, ipfw(8) should be used for protection, not the
blackhole feature.
This mechanism is not a substitute for securing a system. It should be
used together with other security mechanisms.
ip(4), tcp(4), udp(4), ipfw(8), sysctl(8)
Geoffrey M. Rehmet
The TCP and UDP blackhole MIBs first appeared in FreeBSD 4.0.
FreeBSD 5.2.1 August 17, 1999 FreeBSD 5.2.1 [ Back ] | 