Ian! D. Allen
2004-Sep-22 17:18 UTC
ping failure on dual-home using -I without default route
My Linux workstation (Mandrake 10.1 kernel 2.6.8.1) is dual-homed to two
ADSL Internet providers. Card eth0 (192.168.9.250) is the default route
and leads to an SMC router (192.168.9.254). Card eth1 (192.168.1.250)
leads to a Linksys router (192.168.1.1). I''m not doing any NAT or
PPPoE
in the workstation - the SMC and Linksys handle it all.
If I remove all general default routes from my configuration and rely
only on the specific default routes configured in separate tables for
eth0 and eth1 (see below), then these commands both work:
# nc -s 192.168.1.250 foo.bar # works out eth1
# mtr -a 192.168.1.250 foo.bar # works out eth1
but this one fails:
# ping -I eth1 foo.bar # fails out eth1
For example:
# ping -I eth1 linux.org
PING linux.org (198.182.196.48) from 192.168.1.250 eth1: 56(84) bytes of
data.
From 192.168.1.250 icmp_seq=1 Destination Host Unreachable
From 192.168.1.250 icmp_seq=2 Destination Host Unreachable
From 192.168.1.250 icmp_seq=3 Destination Host Unreachable
Running tcpdump on eth0, I see no packet traffic related to this.
Running tcpdump on eth1, this is what I see when the ping happens:
12:35:45.263693 arp who-has linux.org tell 192.168.1.250
12:35:46.263407 arp who-has linux.org tell 192.168.1.250
12:35:47.263247 arp who-has linux.org tell 192.168.1.250
I can''t imagine how my kernel thinks that linux.org (198.182.196.48)
is directly connected to the network on eth1 and that arp will find it!
How is this possible? What is going on here? I am so confused.
The above problem happens for any Internet address I care to try.
I added back my usual default route to my default table, and that
didn''t
change anything (I didn''t expect that it would):
----- Table default ----------------------------------
default via 192.168.9.254 dev eth0 proto static src 192.168.9.250
In frustration, I started trying random things and found this work-around:
If I add a second default route out eth1, the ping starts working:
----- Table default ----------------------------------
default via 192.168.9.254 dev eth0 proto static src 192.168.9.250 metric 1
default via 192.168.1.1 dev eth1 proto static src 192.168.1.250 metric 2
With the second default, the ping now works and all the other commands
continue to work correctly. (The metric 2 appears to make the first
default the only choice for outgoing traffic, so that nothing I originate
goes out eth1 unless I force it to go there, which is what I want.)
I have no idea why this second default route works or fixes ping.
As I understand it, the second default should be completely ignored
since the "ping -I eth1" should be operating using my table set up for
eth1 addresses. Why doesn''t ping work the same way as nc and mtr?
Help?
(Aside: The traceroute command is completely unable to function in my
configuration without any default routes defined, even using the -i and
-s options to set the source address to either interface:
# traceroute -i eth1 linux.org
traceroute to linux.org (198.182.196.48), 30 hops max, 38 byte packets
traceroute: sendto: Network is unreachable
1 traceroute: wrote linux.org 38 chars, ret=-1
# traceroute -i eth0 linux.org
traceroute to linux.org (198.182.196.48), 30 hops max, 38 byte packets
traceroute: sendto: Network is unreachable
1 traceroute: wrote linux.org 38 chars, ret=-1
Again, I don''t know why traceroute isn''t operating using my
table set
up for eth0 or eth1 addresses. How can nc and mtr both work so nicely
without a general default route yet the venerable traceroute behaves
so badly? Is this my LRTC misconfiguration or a problem with traceroute?
)
Detailed config follows:
----- IP Rules ----------------------------------
0: from all lookup local
10: from 192.168.9.0/24 lookup 1 # this is for eth0
20: from 192.168.1.0/24 lookup 2 # this is for eth1
32766: from all lookup main
32767: from all lookup default
----- Table local ----------------------------------
broadcast 192.168.1.0 dev eth1 proto kernel scope link src 192.168.1.250
broadcast 127.255.255.255 dev lo proto kernel scope link src 127.0.0.1
broadcast 192.168.9.0 dev eth0 proto kernel scope link src 192.168.9.250
broadcast 192.168.1.255 dev eth1 proto kernel scope link src 192.168.1.250
broadcast 192.168.9.255 dev eth0 proto kernel scope link src 192.168.9.250
local 192.168.1.250 dev eth1 proto kernel scope host src 192.168.1.250
broadcast 127.0.0.0 dev lo proto kernel scope link src 127.0.0.1
local 192.168.9.250 dev eth0 proto kernel scope host src 192.168.9.250
local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1
local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
----- Table 1 ----------------------------------
default via 192.168.9.254 dev eth0 proto static src 192.168.9.250
----- Table 2 ----------------------------------
default via 192.168.1.1 dev eth1 proto static src 192.168.1.250
----- Table main ----------------------------------
192.168.1.0/24 dev eth1 proto kernel scope link src 192.168.1.250
192.168.9.0/24 dev eth0 proto kernel scope link src 192.168.9.250
127.0.0.0/8 dev lo scope link
----- Table default ----------------------------------
default via 192.168.9.254 dev eth0 proto static src 192.168.9.250 metric 1
default via 192.168.1.1 dev eth1 proto static src 192.168.1.250 metric 2
--
-IAN! Ian! D. Allen Ottawa, Ontario, Canada
EMail: idallen@idallen.ca WWW: http://www.idallen.com/
College professor (Linux) via: http://teaching.idallen.com/
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