These extensions can be used if `--protocol tcp' is specified. It
provides the following options:
.TP
.BR "--source-port " "[!] \fIport\fP[:\fIport\fP]"
Source port or port range specification. This can either be a service
name or a port number. An inclusive range can also be specified,
using the format
.IR port : port .
If the first port is omitted, "0" is assumed; if the last is omitted,
"65535" is assumed.
If the second port greater then the first they will be swapped.
The flag
.B --sport
is a convenient alias for this option.
.TP
.BR "--destination-port " "[!] \fIport\fP[:\fIport\fP]"
Destination port or port range specification.  The flag
.B --dport
is a convenient alias for this option.
.TP
.BR "--tcp-flags " "[!] \fImask\fP \fIcomp\fP"
Match when the TCP flags are as specified.  The first argument is the
flags which we should examine, written as a comma-separated list, and
the second argument is a comma-separated list of flags which must be
set.  Flags are: 
.BR "SYN ACK FIN RST URG PSH ALL NONE" .
Hence the command
.nf
 ip6tables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
.fi
will only match packets with the SYN flag set, and the ACK, FIN and
RST flags unset.
.TP
.B "[!] --syn"
Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits
cleared.  Such packets are used to request TCP connection initiation;
for example, blocking such packets coming in an interface will prevent
incoming TCP connections, but outgoing TCP connections will be
unaffected.
It is equivalent to \fB--tcp-flags SYN,RST,ACK,FIN SYN\fP.
If the "!" flag precedes the "--syn", the sense of the
option is inverted.
.TP
.BR "--tcp-option " "[!] \fInumber\fP"
Match if TCP option set.