This is used to send back an error packet in response to the matched
packet: otherwise it is equivalent to
.B DROP
so it is a terminating TARGET, ending rule traversal.
This target is only valid in the
.BR INPUT ,
.B FORWARD
and
.B OUTPUT
chains, and user-defined chains which are only called from those
chains.  The following option controls the nature of the error packet
returned:
.TP
.BI "--reject-with " "type"
The type given can be
.nf
.B " icmp-net-unreachable"
.B " icmp-host-unreachable"
.B " icmp-port-unreachable"
.B " icmp-proto-unreachable"
.B " icmp-net-prohibited"
.B " icmp-host-prohibited or"
.B " icmp-admin-prohibited (*)"
.fi
which return the appropriate ICMP error message (\fBport-unreachable\fP is
the default).  The option
.B tcp-reset
can be used on rules which only match the TCP protocol: this causes a
TCP RST packet to be sent back.  This is mainly useful for blocking 
.I ident
(113/tcp) probes which frequently occur when sending mail to broken mail
hosts (which won't accept your mail otherwise).
.TP
(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT