Linux (both 2.0.x and 2.2.x), ifconfig, and routing tables.
Okay, I've asked on comp.os.linux.networking, figured I'd ask here too
since I've seen similar topics come up.
I've discovered some differences between Linux's handling of the
routing table from other Unix and Unix-style OS's, and was wondering
if anyone could shed some light on things.
I have a program I am working on (high availability/failover stuff),
that as part of it's operation does the following steps:
1. Brings down an interface with 'ifconfig interfacename down'.
2. Depending on circumstances, either brings the interface up with an
'ifconfig interfacename up', or changes it's IP address and then
brings it back up with 'ifconfig interfacename up'.
The problem I've run into is that whenever I do an 'ifconfig
interfacename down', the kernel automagically removes all the routing
table entries for that interface. If I bring the interface back up
with 'ifconfig interfacename up', I also have to run a bunch of route
commands to restore the routing table.
So the problem I have is that I need a way to either:
1. Get the kernel to leave the routing table intact when the interface
is brought down, or...
2. Find a way to save all the routing table entries involving the
interface I'm bringing down, and restore them after bringing the
interface back up?
Anyone know how do do either of these?
On every other Unix OS I've worked, an 'ifconfig interfacename down'
followed by an 'ifconfig interfacename up' restores things to their
initial state, and never changes the routing table.
For example, on one of my old SunOS boxes (with addresses remove to
protect the innocent):
# ifconfig -a
le0: flags=63<UP,BROADCAST,NOTRAILERS,RUNNING>
inet xx.xx.xx.xx netmask ffffff00 broadcast xx.xx.xx.xx
ether xx:xx:xx:xx:xx:xx
lo0: flags=49<UP,LOOPBACK,RUNNING>
inet 127.0.0.1 netmask ff000000
# netstat -r
Routing tables
Destination Gateway Flags Refcnt Use Interface
localhost localhost UH 16 562235 lo0
134.84.106.0 host-le0 U 3 50520 le0
# ifconfig le0 down
# ifconfig -a
lo0: flags=49<UP,LOOPBACK,RUNNING>
inet 127.0.0.1 netmask ff000000
# netstat -r
Routing tables
Destination Gateway Flags Refcnt Use Interface
localhost localhost UH 16 562235 lo0
So as you can see, the routing table is completely unaffected by the
'ifconfig' commands.
On linux (with 2.0.x kernel), I get the follwoing
# ifconfig -a
eth0 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
inet addr:xx.xx.xx.xx Bcast:xx.xx.xx.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:3691642 errors:0 dropped:0 overruns:0 frame:0
TX packets:3679365 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:3924 Metric:1
RX packets:11495 errors:0 dropped:0 overruns:0 frame:0
TX packets:11495 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
# netstat -r
Destination Gateway Genmask Flags MSS Window irtt Iface
localnet * 255.255.255.0 U 0 0 0 eth0
default tc8x.router.umn 0.0.0.0 UG 0 0 0 eth0
# ifconifg eth0 down
# netstat -r
Destination Gateway Genmask Flags MSS Window irtt Iface
As you can see, the ifconfig command is removing my route table entries.
Similar results are seen with the 2.2.x kernel, except the kernel
automagically handles the localnet entry when I bring the device back up.
Ideas?
--
Richard W Kaszeta PhD. Candidate and Sysadmin
bofh@me.umn.edu University of MN, ME Dept
http://www.menet.umn.edu/~kaszeta
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