linux-kernel - Re: routing bug?

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Date:	Fri, 18 Nov 2011 14:33:27 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Pozsár Balázs <pozsy@...linux.hu>
Cc:	Sven-Haegar Koch <haegar@...net.de>,
	Linux-Kernel-Mailinglist <linux-kernel@...r.kernel.org>,
	Tamási János <janusz@...linux.hu>,
	netdev@...r.kernel.org
Subject: Re: routing bug?

Le vendredi 18 novembre 2011 à 14:23 +0100, Pozsár Balázs a écrit :
> On 2011-11-18 14:09, Eric Dumazet wrote:
> > Le vendredi 18 novembre 2011 à 13:48 +0100, Sven-Haegar Koch a écrit :
> >    
> >> Added netdev list to CC:, there you should have a higher chance of a
> >> usefull answer.
> >>
> >> On Fri, 18 Nov 2011, Pozsár Balázs 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@...sybook:~# 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@...sybook:~# 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@...sybook:~# 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!

We are working on it (see threads in netdev)

You can in the meantime 

echo 0 >/proc/sys/net/ipv4/conf/eth0/accept_redirects



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