Re: routing bug?

From: PozsÃr BalÃzs
Date: Fri Nov 18 2011 - 08:23:26 EST

On 2011-11-18 14:09, Eric Dumazet wrote:
Le vendredi 18 novembre 2011 Ã 13:48 +0100, Sven-Haegar Koch a Ãcrit :
Added netdev list to CC:, there you should have a higher chance of a
usefull answer.

On Fri, 18 Nov 2011, PozsÃr BalÃzs wrote:

Hi all,

I have been struggling with this not easily reproducible issue since a while.
I am using linux kernel v3.1.0, and sometimes routing to a few IP addresses
does not work. What seems to happen is that instead of sending the packet to
the gateway, the kernel treats the destination address as local, and tries to
gets its MAC address via ARP.

For example, now my current IP address is 172.16.1.104/24, the gateway is
172.16.1.254:

|# ifconfig eth0 eth0 Link encap:Ethernet HWaddr 00:1B:63:97:FC:DC
inet addr:172.16.1.104 Bcast:172.16.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:230772 errors:0 dropped:0 overruns:0 frame:0
TX packets:171013 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:191879370 (182.9 Mb) TX bytes:47173253 (44.9 Mb)
Interrupt:17

# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 172.16.1.254 0.0.0.0 UG 0 0 0 eth0
172.16.1.0 0.0.0.0 255.255.255.0 U 1 0 0 eth0
|

I can ping a few addresses, but not 172.16.0.59:

|# ping -c1 172.16.1.254
PING 172.16.1.254 (172.16.1.254) 56(84) bytes of data.
64 bytes from 172.16.1.254: icmp_seq=1 ttl=64 time=0.383 ms

--- 172.16.1.254 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.383/0.383/0.383/0.000 ms
root@pozsybook:~# ping -c1 172.16.0.1
PING 172.16.0.1 (172.16.0.1) 56(84) bytes of data.
64 bytes from 172.16.0.1: icmp_seq=1 ttl=63 time=5.54 ms

--- 172.16.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 5.545/5.545/5.545/0.000 ms
root@pozsybook:~# ping -c1 172.16.0.2
PING 172.16.0.2 (172.16.0.2) 56(84) bytes of data.
64 bytes from 172.16.0.2: icmp_seq=1 ttl=62 time=7.92 ms

--- 172.16.0.2 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 7.925/7.925/7.925/0.000 ms
root@pozsybook:~# ping -c1 172.16.0.59
PING 172.16.0.59 (172.16.0.59) 56(84) bytes of data.
From 172.16.1.104 icmp_seq=1 Destination Host Unreachable

--- 172.16.0.59 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms
|

When trying to ping 172.16.0.59, I can see in tcpdump that an ARP req was
sent:

|# tcpdump -n -i eth0|grep ARP
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
15:25:16.671217 ARP, Request who-has 172.16.0.59 tell 172.16.1.104, length 28
|

and /proc/net/arp has an incomplete entry for 172.16.0.59:

|# grep 172.16.0.59 /proc/net/arp

172.16.0.59 0x1 0x0 00:00:00:00:00:00 * eth0
|

Please note, that 172.16.0.59 /is/ accessible from this LAN from other
computers.


Does anyone have any idea of what's going on? Thanks,


Balazs Pozsar

ps: I think it is related to this one: https://lkml.org/lkml/2011/11/16/292

--
Could you send us result of :

ip route get 172.16.0.59
ip route list cache match 172.16.0.59

I did not tell you in my first mail, that some times different hosts are reachable and unreachable. I will try to not confuse you :)
As of now, 172.16.0.59 is OK, and 172.16.0.37 is NOT OK.
Also, 172.16.0.64 is OK now, and 172.16.0.42 is NOT OK now.

The two commands you have requested give the following output for these IP addresses:

These are OK:

# ip route get 172.16.0.64
172.16.0.64 via 172.16.1.254 dev eth0 src 172.16.1.22
cache
# ip route get 172.16.0.59
172.16.0.59 via 172.16.1.254 dev eth0 src 172.16.1.22
cache

These are NOT OK:

# ip route get 172.16.0.37
172.16.0.37 dev eth0 src 172.16.1.22
cache <redirected> ipid 0x97a4
# ip route get 172.16.0.42
172.16.0.42 dev eth0 src 172.16.1.22
cache <redirected> ipid 0x0d21

These are OK:

# ip route list cache match 172.16.0.59
172.16.0.59 via 172.16.1.254 dev eth0 src 172.16.1.22
cache
# ip route list cache match 172.16.0.64
172.16.0.64 via 172.16.1.254 dev eth0 src 172.16.1.22
cache

These are NOT OK:

# ip route list cache match 172.16.0.37
172.16.0.37 dev eth0 src 172.16.1.22
cache <redirected> ipid 0x97a4
172.16.0.37 from 172.16.1.22 dev eth0
cache <redirected> ipid 0x97a4
172.16.0.37 from 172.16.1.22 dev eth0
cache <redirected> ipid 0x97a4
# ip route list cache match 172.16.0.42
172.16.0.42 dev eth0 src 172.16.1.22
cache <redirected> ipid 0x0d21
172.16.0.42 from 172.16.1.22 dev eth0
cache <redirected> ipid 0x0d21


How can I fix this?

Thanks!
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