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53 lines
1.9 KiB
53 lines
1.9 KiB
4 months ago
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This is used to send back an error packet in response to the matched
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packet: otherwise it is equivalent to
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.B DROP
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so it is a terminating TARGET, ending rule traversal.
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This target is only valid in the
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.BR INPUT ,
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.B FORWARD
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and
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.B OUTPUT
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chains, and user-defined chains which are only called from those
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chains. The following option controls the nature of the error packet
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returned:
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.TP
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\fB\-\-reject\-with\fP \fItype\fP
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The type given can be
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\fBicmp\-net\-unreachable\fP,
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\fBicmp\-host\-unreachable\fP,
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\fBicmp\-port\-unreachable\fP,
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\fBicmp\-proto\-unreachable\fP,
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\fBicmp\-net\-prohibited\fP,
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\fBicmp\-host\-prohibited\fP, or
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\fBicmp\-admin\-prohibited\fP (*),
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which return the appropriate ICMP error message (\fBicmp\-port\-unreachable\fP is
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the default). The option
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\fBtcp\-reset\fP
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can be used on rules which only match the TCP protocol: this causes a
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TCP RST packet to be sent back. This is mainly useful for blocking
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.I ident
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(113/tcp) probes which frequently occur when sending mail to broken mail
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hosts (which won't accept your mail otherwise).
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.IP
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(*) Using icmp\-admin\-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
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.PP
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\fIWarning:\fP You should not indiscriminately apply the REJECT target to
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packets whose connection state is classified as INVALID; instead, you should
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only DROP these.
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.PP
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Consider a source host transmitting a packet P, with P experiencing so much
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delay along its path that the source host issues a retransmission, P_2, with
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P_2 being successful in reaching its destination and advancing the connection
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state normally. It is conceivable that the late-arriving P may be considered
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not to be associated with any connection tracking entry. Generating a reject
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response for a packet so classed would then terminate the healthy connection.
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.PP
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So, instead of:
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.PP
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-A INPUT ... -j REJECT
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.PP
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do consider using:
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.PP
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-A INPUT ... -m conntrack --ctstate INVALID -j DROP
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-A INPUT ... -j REJECT
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