I've been working with custom route tables on Linux, and I'm a bit confused by some of the documentation and behavior of the "ip route" command. It seems that the only valid values should be 0-255 plus the names defined in /etc/iproute2/rt_tables:
255 local
254 main
253 default
0 unspec
This would leave 1-252 for custom tables. Attempting to use an undefined table name gives an error:
$ ip route show table kermit
Error: argument "kermit" is wrong: table id value is invalid
However, it seems that I can use numbers far higher than 255 without error:
$ ip route show table 1000
[no output]
$ ip route add 10.10.10.0/24 dev eth0 table 1000
[no output]
$ ip route show table 1000
10.10.10.0/24 dev eth0 scope link
At some point, things get even weirder. Right at maxint (2^31), it "overflows" into the local table (255):
$ ip route show table 2147483647
[no output]
$ ip route show table 2147483648
[exact output of table 255 (local)]
Can anyone explain what is happening? Are there actually maxint custom routing tables that can be used?
Best Answer
As far as the 2.6 kernel is concerned, the max table is 0xFFFFFFFF (from rtnetlink.h). However, iproute2 uses a signed integer in it's filter to do the lookup so at 2^31 it thinks you specified an invalid table and defaults to showing you table 255.