Netopia R5100, R5200, R5300 HH, rrkk, oorr, wwoo, ss ww, aallll ff, uuaa, dduu, vviiiidd, ddiiii vv, nndd, ww iiii nn, ooww, HHoo, kk

Security 13-7
HH
HHoo ooww ww iiii nn nndd ddiiii vv vviiiidd dduu uuaa aallll ff ffiiii lllltt ttee eerr rrss ss ww wwoo oorr rrkk kk
As described above, a filter applies criteria to an IP packet and then takes one of three actions
AA
AA ff ffiiiilllltt ttee eerr rr’’’’ss ss aa aacc cctt ttiiiioo oonn nnss ss
■forwards the packet to the local or remote network
■Blocks (discards) the packet
■Ignores the packet
A filter forwards or blocks a packet only if it finds a match after applying its criteria. When no match occurs, the
filter ignores the packet.
AA
AA ff ffiiiilllltt ttee eerr rriiiinn nngg gg rr rruu uullllee ee
The criteria are based on information contained in the packets. A filter is simply a rule that prescribes certain
actions based on certain 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 particular filter, look at the parts of a filter.
PP
PP aa aarr rrtt ttss ss oo ooff ff aa aa ff ffiiiilllltt ttee eerr rr
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 carrying, such as TCP or UDP
PP
PP oo oorr rrtt tt nn nnuu uumm mmbb bbee eerr rrss ss
A filter can also match a packet’s port number attributes, but only if the filter’s protocol type is set to TCP or
UDP, since only those protocols use port numbers. The filter can be configured to match the following:
■The source port number (the port on the sending host that originated the packet)
■The destination port number (the port 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 |
+--------------------------------------------------------------------+