85 lines
2.7 KiB
Groff
85 lines
2.7 KiB
Groff
.\"
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.\" blackhole - drop refused TCP or UDP connects
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\"
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.\" $FreeBSD$
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.Dd January 1, 2007
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.Dt BLACKHOLE 4
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.Os
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.Sh NAME
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.Nm blackhole
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.Nd a
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.Xr sysctl 8
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MIB for manipulating behaviour in respect of refused TCP or UDP connection
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attempts
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.Sh SYNOPSIS
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.Cd sysctl net.inet.tcp.blackhole[=[0 | 1 | 2]]
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.Cd sysctl net.inet.udp.blackhole[=[0 | 1]]
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.Sh DESCRIPTION
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The
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.Nm
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.Xr sysctl 8
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MIB is used to control system behaviour when connection requests
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are received on TCP or UDP ports where there is no socket listening.
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.Pp
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Normal behaviour, when a TCP SYN segment is received on a port where
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there is no socket accepting connections, is for the system to return
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a RST segment, and drop the connection.
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The connecting system will
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see this as a
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.Dq Connection refused .
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By setting the TCP blackhole
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MIB to a numeric value of one, the incoming SYN segment
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is merely dropped, and no RST is sent, making the system appear
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as a blackhole.
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By setting the MIB value to two, any segment arriving
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on a closed port is dropped without returning a RST.
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This provides some degree of protection against stealth port scans.
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.Pp
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In the UDP instance, enabling blackhole behaviour turns off the sending
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of an ICMP port unreachable message in response to a UDP datagram which
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arrives on a port where there is no socket listening.
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It must be noted that this behaviour will prevent remote systems from running
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.Xr traceroute 8
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to a system.
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.Pp
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The blackhole behaviour is useful to slow down anyone who is port scanning
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a system, attempting to detect vulnerable services on a system.
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It could potentially also slow down someone who is attempting a denial
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of service attack.
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.Sh WARNING
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The TCP and UDP blackhole features should not be regarded as a replacement
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for firewall solutions.
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Better security would consist of the
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.Nm
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.Xr sysctl 8
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MIB used in conjuction with one of the available firewall packages.
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.Pp
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This mechanism is not a substitute for securing a system.
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It should be used together with other security mechanisms.
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.Sh SEE ALSO
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.Xr ip 4 ,
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.Xr tcp 4 ,
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.Xr udp 4 ,
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.Xr ipf 8 ,
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.Xr ipfw 8 ,
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.Xr pfctl 8 ,
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.Xr sysctl 8
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.Sh HISTORY
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The TCP and UDP
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.Nm
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MIBs
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first appeared in
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.Fx 4.0 .
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.Sh AUTHORS
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.An Geoffrey M. Rehmet
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