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Filters and Network Address Translation
162
BLACK BOX
®
Advanced Console Server
Iptables can use extended packet matching modules. These are loaded in two ways:
implicitly, when -p or - -protocol is specified, or with the -m or - -match option, followed by
the matching module name; after these, various extra command line options become
available, depending on the specific module.
TCP Extension
These extensions are loaded if the protocol specified is tcp or “-m tcp” is specified. It
provides the following options:
-x
- -exact
Expand numbers. Display the exact value of the packet and byte counters,
instead of only the rounded number in K's (multiples of 1000) M's (multiples
of 1000K) or G's (multiples of 1000M). This option is only relevant for the -L
command.
- -line-
numbers
When listing rules, add line numbers to the beginning of each rule, corre-
sponding to that rule's position in the chain.
- -source-port [!] [port[:port]]
Source port or port range specification. This can
either be a service name or a port number. Inclusive
range can also be specified, using the format
port:port. If the first port is omitted, "0" is assumed; if
the last is omitted, “65535” is assumed. If the second
port is greater then the first they will be swapped. The
flag - -sport is an alias for this option.
- -destination-port [!] [port[:port]]
Destination port or port range specification. The flag
- -dport is an alias for this option.
- -tcp-flags [!] mask comp
Match when the TCP flags are as specified. The first
argument is the flags which we should examine, writ-
ten as a comma-separated list, and the second argu-
ment is a comma-separated list of flags which must be
set. Flags are: SYN ACK FIN RST URG PSH ALL NONE.
Hence the command iptables
-A FORWARD -p tcp - -tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the
ACK, FIN and RST flags unset.