Chapter 30 IDP

Table 149 IP v4 Packet Headers (continued)

HEADER

DESCRIPTION

Time To Live

This is a counter that decrements every time it passes through a

 

router. When it reaches zero, the datagram is discarded. It is used

 

to prevent accidental routing loops.

 

 

Protocol

The protocol indicates the type of transport packet being carried,

 

for example, 1 = ICMP; 2= IGMP; 6 = TCP; 17= UDP.

 

 

Header Checksum

This is used to detect processing errors introduced into the packet

 

inside a router or bridge where the packet is not protected by a link

 

layer cyclic redundancy check. Packets with an invalid checksum

 

are discarded by all nodes in an IP network.

 

 

Source IP Address

This is the IP address of the original sender of the packet.

 

 

Destination IP

This is the IP address of the final destination of the packet.

Address

 

 

 

Options

IP options is a variable-length list of IP options for a datagram that

 

define IP Security Option, IP Stream Identifier, (security and

 

handling restrictions for the military), Record Route (have each

 

router record its IP address), Loose Source Routing (specifies a

 

list of IP addresses that must be traversed by the datagram),

 

Strict Source Routing (specifies a list of IP addresses that must

 

ONLY be traversed by the datagram), Timestamp (have each

 

router record its IP address and time), End of IP List and No IP

 

Options.

 

 

Padding

Padding is used as a filler to ensure that the IP packet is a multiple

 

of 32 bits.

 

 

30.8 Configuring Custom Signatures

Select Configuration > Anti-X > IDP > Custom Signatures. The first screen shows a summary of all custom signatures created. Click the SID or Name heading to sort. Click the Add icon to create a new signature or click the Edit icon to edit an existing signature. You can also delete custom signatures here or save them to your computer.

498

 

ZyWALL USG 50 User’s Guide