How individual filters work, A filter’s actions, A filtering rule – Netopia R5300 User Manual
Page 185: Parts of a filter, Port numbers, How individual filters work -7
Security 13-7
H
H
H
Ho
o
o
ow
w
w
w iiiin
n
n
nd
d
d
diiiivvv
viiiid
d
d
du
u
u
uaa
a
allll ffffiiiillllttttee
e
errrrssss w
w
w
wo
o
o
orrrrkk
k
k
As described above, a filter applies criteria to an IP packet and then takes one of three actions
A
A
A
A ffffiiiillllttttee
e
errrr’’’’ssss aa
a
accccttttiiiio
o
o
on
n
n
nssss
■
for wards the packet to the local or remote network
■
Blocks (discards) the packet
■
Ignores the packet
A filter for wards or blocks a packet only if it finds a match after applying its criteria. When no match occurs, the
filter ignores the packet.
A
A
A
A ffffiiiillllttttee
e
errrriiiin
n
n
ng
g
g
g rrrru
u
u
ullllee
e
e
The criteria are based on information contained in the packets. A filter is simply a rule that prescribes cer tain
actions based on cer tain conditions. For example, the following rule qualifies as a filter:
Block all Telnet attempts that originate from the remote host 199.211.211.17.
This rule applies to Telnet packets that come from a host with the IP address 199.211.211.17. If a match
occurs, the packet is blocked.
Here is what this rule looks like when implemented as a filter on the Netopia R5000 Series Router:
To understand this par ticular filter, look at the par ts of a filter.
P
P
P
Paa
a
arrrrttttssss o
o
o
offff aa
a
a ffffiiiillllttttee
e
errrr
A filter consists of criteria based on packet attributes. A typical filter can match a packet on any one of the
following attributes:
■
The source IP address (where the packet was sent from)
■
The destination IP address (where the packet is going)
■
The type of higher-layer Internet protocol the packet is carr ying, such as TCP or UDP
P
P
P
Po
o
o
orrrrtttt n
n
n
nu
u
u
um
m
m
mb
b
b
bee
e
errrrssss
A filter can also match a packet’s por t number attributes, but only if the filter’s protocol type is set to TCP or
UDP, since only those protocols use por t numbers. The filter can be configured to match the following:
■
The source por t number (the por t on the sending host that originated the packet)
■
The destination por t number (the por t on the receiving host that the packet is destined for)
+-#--Source IP Addr--Dest IP Addr-----Proto-Src.Port-D.Port--On?-Fwd-+
+--------------------------------------------------------------------+
| 1 199.211.211.17 0.0.0.0 TCP 23 Yes No |
+--------------------------------------------------------------------+