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 " icmp6-no-route"
.B " no-route"
.B " icmp6-adm-prohibited"
.B " adm-prohibited"
.B " icmp6-addr-unreachable"
.B " addr-unreach"
.B " icmp6-port-unreachable"
.B " port-unreach"
.fi
which return the appropriate IPv6-ICMP error message (\fBport-unreach\fP is
the default). Finally, 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).