[olug] Ping Is Broken

Tue Oct 13 00:50:03 UTC 2009

</delurk>

On Mon, 12 Oct 2009, Rob Townley wrote:

> On Mon, Oct 12, 2009 at 3:09 AM, Aric Aasgaard <aric at omahax.com> wrote:
>> The interface isn't on the network your trying to ping.
>
> Maybe i don't understand what you are saying, but eth0's ip address is
> 4.3.2.8 and when 4.3.2.8 is passed as the interface, it works.  When
> eth0 is passed as the interface name, ping fails.  So it isn't a
> problem with the network or routing rules or routing tables, it is a
> problem in being able to address a device alias.

Actually - not completely. The main problem is that when you call ping 
with the -I eth0 then you also are invoking your rule 32762 because when 
the ping command goes to use the interface it then runs through the rule 
set and as 32762 is the lowest rule it is invoked and your ping packet 
goes out through eth0 to the default gateway of table nic0:

/sbin/ip route show table nic0
default via 4.3.2.1 dev eth0

And I suspect that cannot get to the device you want to ping.

On the other hand when you specify -I 4.x.x.x to the ping command then the 
packet goes out from lo0 using that source address and is routed by the 
default rule "from all lookup main" which means it gets routed out the 
eth1 interface but with the 4.x address.

Do you see the difference?

What gives this away is the following:

1: The ping utility is from iputils (Alexey's utility set) and thus 
behaves according to the policy structure in the kernel which _does_ 
differentiate between IP address assignments and mere network interfaces. 
Remember - the IP address DOES NOT belong to an interface and does not 
even need to be bound to such in order to function in a Linux kernel.

2: When you said "All exhibit the issue that ethX can't be used as an 
interface name when the default gateway isn't reached thru ethX."

That told me that you were slightly confusing the way in which -I works 
in Alexey's ping utility. It is confusing considering that you must 
understand the difference between interface sourcing and ip address 
sourcing.

In summary -

-I <interface> will use the primary IP address of the named 
interface AND act as though it had been sourced from that interface which 
triggers any relevant rules, routes, etal.

-I <IP addr> merely sets the Source address of the packet but then sources 
the packet from lo0 and this would invoke the default routing

Sense?

Let me know. Thanks!!

P.S. I am considering starting work on a second edition as I now am back 
working in the IT field...

mgm

>> How are you attempting to get from one NIC to the other? Not via the same
>> switch I hope.
>
> Both of these nics in all examples are in the same machine.  Some have
> ip_forward = 1 and some don't.  All exhibit the issue that ethX can't
> be used as an interface name when the default gateway isn't reached
> thru ethX.  Even machines with two nics behind the same switch and
> therefore have the same default gateway on each interface exhibit a
> problem referencing the interface by ethX -- however by IP address
> works just fine.  So some are behind the same switch and some use
> totally different paths to the internet - either way it is broken.
>
>>
>> May I ask what you are trying to do?  There might be an easier way.
>
> i have a bunch of machines with 2 or more nics (HP and Dell Server
> Class) that i will use for many types of projects.  The easiest way is
> for ping and tracert to be fixed.  it is the first tool in network
> assurance and debugging and it is very broken.  i suspect this would
> cause problems for users of VPNs, wireless, and other multi nic
> scenarios.
>
>>
>> As far a multi NIC BSD systems, yeah I got a few pfsense boxes, it is BSD.
>> I started using pfsense when I couldn't get multi WAN Linux to work.
>
> i wish i would have known to give up 2-3 years ago - i knew i wasn't
> that stupid as i have only been using ping for 20 years.  i also had
> problems with ping on dd-wrt and as this busybox mailing list shows, i
> am not the only one.   Note, these busybox guys didn't test this too
> well because their examples never ping an ip on the other side of a
> wire, but it is the exact same problem.
> http://lists.busybox.net/pipermail/busybox/2009-July/069972.html
>
>>
>>
>>
>> -----Original Message-----
>> From: olug-bounces at olug.org [mailto:olug-bounces at olug.org] On Behalf Of Rob
>> Townley
>> Sent: Sunday, October 11, 2009 12:32 PM
>> To: Omaha Linux User Group
>> Subject: Re: [olug] Ping Is Broken
>>
>> On Sat, Oct 10, 2009 at 8:17 PM, Aric Aasgaard <aric at omahax.com> wrote:
>>> Does
>>> traceroute -i eth0 -I 208.67.222.222
>>> using ICMP
>>
>> eth0 = 4.3.2.8, the default gateway is thru eth1.
>> tracert -i eth0 -I 208.67.222.222        FAILS
>> tracert -s 4.3.2.8 -I 208.67.222.222   WORKS
>> tracert -i eth0 208.67.222.222           FAILS
>> tracert -s 4.3.2.8 208.67.222.222      WORKS
>>
>> There is a still a great deal of heated argument on the netdev list on
>> how to make network device naming better.  Some want to name
>> interfaces in the order of MAC address, others in the name of bus
>> enumeration, some think symbolic links will make things more
>> complicated, others want it.  Please test using the following script.
>> Anybody have a multinic BSD system up that would care to test this.
>> OSX?
>>
>> #!/bin/bash
>> #The following is a script to test source routing thru the network
>> interface alias vs ip address.
>> #Put in your own IP addresses here.
>> ip0=4.3.2.8
>> ip1=192.168.168.155
>> for interface in eth0 $ip0 eth1 $ip1; do echo testing $interface; ping
>> -I $interface 208.67.222.222; done;
>>
>>>
>>> or using UDP
>>> traceroute -i eth0 208.67.222.222
>>>
>>> Work?
>>>
>>> -----Original Message-----
>>> From: olug-bounces at olug.org [mailto:olug-bounces at olug.org] On Behalf Of
>> Rob
>>> Townley
>>> Sent: Friday, October 09, 2009 11:45 AM
>>> To: netdev at vger.kernel.org
>>> Cc: CentOS mailing list; Omaha Linux User Group
>>> Subject: Re: [olug] Ping Is Broken
>>>
>>> ping -I is broken
>>>
>>> The following deals with bug in ping that made it very difficult to set up
>> a
>>> system with two gateways.
>>>
>>> Demonstration that *ping -I is broken*. When specifying the source
>>> interface using -I with an *ethX* alias and that interface is not the
>>> default gateway
>>> interface, then ping fails. When specifying the interface as an ip
>> address,
>>> ping works. Search for "Destination Host Unreachable" to find the bug.
>>>
>>>
>>> eth*0* = 4.3.2.8 and the default gateway is accessed through a different
>>> interface eth*1*.
>>> eth*1* = 192.168.168.155 is used as the device to get to the default
>>> gateway.
>>> *FAILS *: ping *-I eth0* 208.67.222.222
>>> *WORKS*: ping *-I 4.3.2.8* 208.67.222.222
>>> *WORKS*: ping *-I eth1* 208.67.222.222
>>> *WORKS*: ping *-I 192.168.168.155* 208.67.222.222
>>>
>>> The following are actual results which can be reproduced from an
>> up-to-date
>>> Fedora 11 or CentOS 5.3 box. Caused a very very long episode of
>> frustration
>>> when setting up multi gatewayed systems.
>>>
>>>
>>> * ping using eth0 *:
>>>
>>> ping -c 2 -B -I  eth0 208.67.222.222
>>> PING 208.67.222.222 (208.67.222.222) from 4.3.2.8 eth0: 56(84) bytes of
>>> data.
>>> From 4.3.2.8 icmp_seq=1 Destination Host Unreachable
>>> From 4.3.2.8 icmp_seq=2 Destination Host Unreachable
>>>
>>> --- 208.67.222.222 ping statistics ---
>>> 2 packets transmitted, 0 received, +2 errors, 100% packet loss, time 999ms
>>> , pipe 2
>>>
>>> --------------------------------------
>>> The Following all WORK:
>>> * ping using 4.3.2.8 *:
>>>
>>> ping -c 2 -B -I  4.3.2.8 208.67.222.222
>>> PING 208.67.222.222 (208.67.222.222) from 4.3.2.8 : 56(84) bytes of data.
>>> 64 bytes from 208.67.222.222: icmp_seq=1 ttl=55 time=562 ms
>>> 64 bytes from 208.67.222.222: icmp_seq=2 ttl=55 time=642 ms
>>>
>>> --- 208.67.222.222 ping statistics ---
>>> 2 packets transmitted, 2 received, 0% packet loss, time 999ms
>>> rtt min/avg/max/mdev = 562.546/602.400/642.255/39.862 ms
>>>
>>>
>>> * ping using eth1 *:
>>>
>>> ping -c 2 -B -I  eth1 208.67.222.222
>>> PING 208.67.222.222 (208.67.222.222) from 192.168.168.155 eth1: 56(84)
>>> bytes of data.
>>> 64 bytes from 208.67.222.222: icmp_seq=1 ttl=54 time=270 ms
>>> 64 bytes from 208.67.222.222: icmp_seq=2 ttl=54 time=629 ms
>>>
>>> --- 208.67.222.222 ping statistics ---
>>> 2 packets transmitted, 2 received, 0% packet loss, time 1000ms
>>> rtt min/avg/max/mdev = 270.128/449.766/629.405/179.639 ms
>>>
>>>
>>> * ping using 192.168.168.155 *:
>>>
>>> ping -c 2 -B -I  192.168.168.155 208.67.222.222
>>> PING 208.67.222.222 (208.67.222.222) from 192.168.168.155 : 56(84)
>>> bytes of data.
>>> 64 bytes from 208.67.222.222: icmp_seq=1 ttl=54 time=585 ms
>>> 64 bytes from 208.67.222.222: icmp_seq=2 ttl=54 time=554 ms
>>>
>>> --- 208.67.222.222 ping statistics ---
>>> 2 packets transmitted, 2 received, 0% packet loss, time 999ms
>>> rtt min/avg/max/mdev = 554.098/569.655/585.212/15.557 ms
>>>
>>> My source route policy rules:
>>>
>>> /sbin/ip rule show
>>> 0:      from all lookup 255
>>> 32762:  from 4.3.2.8 lookup nic0
>>> 32763:  from 192.168.168.155 lookup nic1
>>> 32764:  from 192.168.168.155 lookup nic1
>>> 32765:  from 4.3.2.8 lookup nic0
>>> 32766:  from all lookup main
>>> 32767:  from all lookup default
>>>
>>>
>>>
>>> Print out routing tables using /sbin/ip route show table TABLENAME:
>>> routing table  nic0 :
>>> /sbin/ip route show table nic0
>>> default via 4.3.2.1 dev eth0
>>>
>>> routing table  nic1 :
>>> /sbin/ip route show table nic1
>>> default via 192.168.168.1 dev eth1
>>>
>>> routing table  main :
>>> /sbin/ip route show table main
>>> 4.3.2.1/27 dev eth0  proto kernel  scope link  src 4.3.2.8
>>> 192.168.168.0/24 dev eth1  proto kernel  scope link  src 192.168.168.155
>>> 169.254.0.0/16 dev eth1  scope link
>>> default via 192.168.168.1 dev eth1
>>>
>>> routing table  default :
>>> /sbin/ip route show table default
>>>
>>>
>>>
>>>
>>> NOTES: cat /etc/iproute2/rt_tables to get your own table names.
>>>
>>> ping Maintainer YOSHIFUJI Hideaki / USAGI/WIDE Project
>>>  http://www.skbuff.net/iputils/
>>> Mailing List netdev at vger.kernel.org
>>>
>>> man ping:
>>>   -I interface address
>>>        Set source address to specified interface address.
>>>        Argument may be *numeric IP address or name of device*.
>>>        When  pinging  IPv6  link-local  address  this option is required.
>>>
>>> ping -V returns the latest available on CentOS and Fedora and the
>>> maintainers website:
>>> ping utility, iputils-ss020927

--------------------------------------------------
Matthew G. Marsh
Special Email Addr for OLUG ;-}
Phone: (402) 932-7250
Email: olug4mgm at paktronix.com
WWW:  http://www.paksecured.org
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