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Document the sctp blackhole sysctl MIB

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PR:		184110
Submitted by:	Marie Helene Kvello-Aune <marieheleneka@gmail.com>
Reviewed by:	wblock
Approved by:	wblock (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D3528
This commit is contained in:
Allan Jude 2015-09-07 01:21:56 +00:00
parent 68bb8d6249
commit 7245b843bb
1 changed files with 24 additions and 13 deletions
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37
share/man/man4/blackhole.4
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@ -12,25 +12,35 @@
.\"
.\"
.\" $FreeBSD$
.Dd January 1, 2007
.Dd September 6, 2015
.Dt BLACKHOLE 4
.Os
.Sh NAME
.Nm blackhole
.Nd a
.Xr sysctl 8
MIB for manipulating behaviour in respect of refused TCP or UDP connection
MIB for manipulating behaviour in respect of refused SCTP, TCP, or UDP connection
attempts
.Sh SYNOPSIS
.Cd sysctl net.inet.tcp.blackhole[=[0 | 1 | 2]]
.Cd sysctl net.inet.udp.blackhole[=[0 | 1]]
.Cd sysctl net.inet.sctp.blackhole Ns Op = Ns Brq "0 | 1 | 2"
.Cd sysctl net.inet.tcp.blackhole Ns Op = Ns Brq "0 | 1 | 2"
.Cd sysctl net.inet.udp.blackhole Ns Op = Ns Brq "0 | 1"
.Sh DESCRIPTION
The
.Nm
.Xr 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.
are received on SCTP, TCP, or UDP ports where there is no socket listening.
.Pp
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.
.Ss 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.
.Ss 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.
@ -44,20 +54,15 @@ 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.
.Pp
In the UDP instance, enabling blackhole behaviour turns off the sending
.Ss 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
.Xr traceroute 8
to a system.
.Pp
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.
.Sh WARNING
The TCP and UDP blackhole features should not be regarded as a replacement
The SCTP, TCP, and UDP blackhole features should not be regarded as a replacement
for firewall solutions.
Better security would consist of the
.Nm
@ -68,6 +73,7 @@ This mechanism is not a substitute for securing a system.
It should be used together with other security mechanisms.
.Sh SEE ALSO
.Xr ip 4 ,
.Xr sctp 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr ipf 8 ,
@ -80,5 +86,10 @@ The TCP and UDP
MIBs
first appeared in
.Fx 4.0 .
.Pp
The SCTP
.Nm
MIB first appeared in
.Fx 9.1 .
.Sh AUTHORS
.An Geoffrey M. Rehmet