Definition Statements (TCP/IP Network Administration, 3rd Edition)

B.7. Definition Statements

Definition statements are general configuration statements that relate to more than one protocol. Definition statements must appear before any protocol statements in gated.conf. The three definition statements are:

autonomoussystem asn [loops n] ;: Defines the autonomous system number (asn) used by BGP or EGP. The loops number defines the number of times this autonomous system may appear in an AS path for path vector protocols, such as BGP. The default value for n is 1.
routerid address ;: Defines the router identifier used by BGP and OSPF. Use the address of your primary OSPF or BGP interface. By default, gated uses the address of the first interface it encounters.
martians {host address [allow]; address [mask mask | masklen number] [allow] ; default [allow] ; } ;: Changes the list of addresses about which all routing information is ignored. Sometimes a misconfigured system sends out obviously invalid destination addresses. These invalid addresses, called martians, are rejected by the routing software. This command allows changes to the list of martian addresses. A martian address can be specified as a host address by using the host keyword before the address, or as a network address by simply specifying the address.

An address mask can be defined for a network address. The mask can be defined in dotted decimal notation using the mask keyword or as a numeric prefix length using the masklen keyword. The address masks mask 255.255.0.0 and masklen 16 are equivalent. If no address mask is specified, the natural mask is used. Specifying an address in the martians statement adds the address to the martians list. The allow keyword is used to remove an address from the martians list. When an address is removed from the martians list, it then becomes a valid address for routing.

gated contains a standard martian list of addresses that are known to be invalid. This is the default martian list. The option default allow removes all of the standard entries from the martians list and permits unrestricted routing. Don't do this if you're on a connected network.

Here is a sample of each definition statement:

autonomoussystem 249 ; 
routerid 172.16.12.2 ; 
martians { 
        host 0.0.0.26 ; 
        192.168.0.0 masklen 16 allow ; } ;

The statements in the sample perform the following functions:

The autonomoussystem statement tells gated to use AS number 249 for its BGP or EGP packets.
The routerid statement tells gated to use 172.16.12.2 as the router identifier for OSPF and BGP.
The martians statement prevents routes to 0.0.0.26 from being included in the table, but it allows routes to the private IP addresses in the range 192.168.0.0 to 192.168.255.255.