December 2005
Intermediate to advanced
1066 pages
33h 38m
English
Content preview from Understanding Linux Network InternalsBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Policy Routing and Its Effects on Routing Table Definitions
When the kernel is compiled with support for Policy Routing, an administrator can configure up to 255 independent routing tables. To support this optional feature, while keeping the routing subsystem lean and simple when Policy Routing is not used, the Linux developers have added some complexity to the source code that you should understand before trying to read the files.
Variable and Structure Definitions
With Policy Routing, the pointers to the 255 routing tables are stored in the
fib_tables array, defined in net/ipv4/fib_frontend.c and shown in Figure 34-1.
struct fib_table *fib_tables[RT_TABLE_MAX+1];
Note that the two routing tables ip_fib_main_table and ip_fib_local_table
are defined as two elements of fib_tables in
include/net/ip_fib.h:
#ifndef CONFIG_IP_MULTIPLE_TABLES extern struct fib_table *ip_fib_local_table; extern struct fib_table *ip_fib_main_table; ... ... ... #else #define ip_fib_local_table (fib_tables[RT_TABLE_LOCAL]) #define ip_fib_main_table (fib_tables[RT_TABLE_MAIN]) ... ... ... #endif
When the first route is added to a new routing table, the table is initialized with
fib_hash_init. When Policy Routing is not
configured, this function is called only at boot time and therefore is tagged with the
_ _init macro.[*] But with Policy Routing, a new routing table can be created at any time, so
fib_hash_init cannot be so tagged. This explains
its conditional prototype definition:
#ifdef CONFIG_IP_MULTIPLE_TABLES ...
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