IPv6 addresses and unwanted routing entries

Wed May 13 20:46:19 UTC 2009

On Wed, May 13, 2009 at 03:11:10PM -0400, Richard Achmatowicz wrote:
> Neil
>
> Thanks for your speedy reply! Comments in-line.
>
> Neil Horman wrote:
>> On Wed, May 13, 2009 at 12:34:36PM -0400, Richard Achmatowicz wrote:
>>   
>>> Hello
>>>
>>> I'm using Fedora 8 but I have the same problem on RHEL 5. Before I   
>>> submit this issue as a bug, I wanted to check if my understanding is 
>>> not  flawed in some way.
>>>
>>> For some reason, when I create an IPv6 address (global or link-local) 
>>> on  interface eth0, three related routs are added:
>>>
>>> 3ffe:ffff:100:f101::/64                     *                         
>>>               U     256    0        0 eth0
>>> 3ffe:ffff:100:f101::/128                    *                         
>>>               U     0      0        1 lo
>>> 3ffe:ffff:100:f101::1/128                   *                         
>>>               U     0      0        1 lo
>>>
>>> The latter of these, the most specific, drives all datagrams onto lo  
>>> instead of eth0.
>>> I'm trying to find out why. I didn't ask for a route to lo to be   
>>> created, so why is it being created?
>>>
>>> This behavior is causing Sun JDK 6 to behave badly when working with  
>>> IPv6 addresses in certain contexts (Sun bug 6800096).
>>>
>>> Any ideas appreciated...A full example of what is happening is listed below.
>>>
>>> Richard
>>>
>>>     
>> This I think looks fairly normal.  Its the mask value that makes all the
>> difference.  The first entry says anything going to the 3ffe:ffff:100:f101
>> subnet (with a 64 bit netmask) should go out go out eth0.  The second and third
>> entries say that anyting going to the addresses 3ffe:ffff:100:f101:: and
>> 3ffe:ffff:100:f101::1 should go through lo.  Since those two addresses are local
>> to the system, they can be routed through the lo interface.  No other addresses
>> on that 64 bit network should match on that route however, since they're both
>> masked at 128 bits.  If anything but traffic to your local interfaces is
>> matching on those routes, its a bug, but having traffic bound for your local
>> addresses go through lo is fine.
>>
>> What exactly is the behavior that you're seeing which is leading you to think
>> that these routes are the cause?
>>   
> I think I understand now why you have these 128 length prefix rules in  
> the routing table. Thanks for the explanation. But is it really right to  
> equate messages arriving at host X on lo with messages arriving at host  
> X on eth0, which seems to be what the additional lo rules seem to  
> assume? Processes can listen on either interface...and if I am listening  
> on eth0 and messages arrive on lo, i'm not going to get them. Which  
> seems to be what is happening below.
>
I'm glad the explination helped.  To answer your subsequent question, its fine
for the routing table to do what its doing above.  You're application should be
able to receive them just fine regardless of which interface they arrive on.  If
thats not happening, that may be a bug.  I assume that you are binding your
applications sockets to INADDR_ANY, or are you selecting a specific address to
listen on?  If so, that may be your problem.
 
> My original problem occurs with the Sun JDK and the handling of IPv6  
> zone ids for link-local addresses. I'm quoting now from Sun bug #6800096:
> <quote>
> In Linux, for link-local addresses, scope id is evaluated from the ipv6  
> routing table on the proc filesystem. /proc/net/ipv6_route has the  
> entries for the ipv6 address and these entries have a mapping to the  
> respective interface through which the traffic for that ip has to be  
> routed through. Java is comparing the ipv6 address and gets its  
> respective routing device name from these entries. The corresponding  
> device index is retrieved from /proc/net/if_inet6.
> Problem here is that, on Linux, link-local address traffic is routed  
> through loopback interface and hence the routing table entry for  
> link-local address is mapped to "lo" device instead of "eth0".
> </quote>
> What happens in practice is if I have link-local address, say  
> fe80::215:58ff:fec8:81a8 defined on interface eth0, when I try to create  
> a Java socket to connect to a process listening on that address, say at  
> port 1234, something like this:
>
> InetAddress bind_addr=InetAddress.getByName("fe80::215:58ff:fec8:81a8%eth0") ;
> int port = 1234 ;
> Socket sock=new Socket();
> sock.connect(new InetSocketAddress(bind_addr, port), sock_conn_timeout);
>
> the Sun JDK seems to ignore the link-local address' scope id I pass it  
> and instead tries to pick it up from the route as described above (which  
> isn't such a good idea as as far as I understand, a single host can have  
> two interfaces with the same link-local address anyway, so using the  
> link-local address to identify a scope id shouldn't work in general). So  
Yeah, thats broken, the JDK shouldn't do that, for exactly the reason you
describe.

> my socket tries to connect via the interface lo instead of the interface  
> eth0, and it finds no process listening on lo (my server process is  
> listening on eth0). Running the program with strace, you see something  
> like this when the socket tries to connect:
>
> [pid  8997] socket(PF_INET6, SOCK_STREAM, IPPROTO_IP) = 4
> [pid  8997] connect(4, {sa_family=AF_INET6, sin6_port=htons(1234), inet_pton(AF_INET6, "fe80::215:58ff:fec8:81a8", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=if_nametoindex("lo")}, 28) = -1 EINPROGRESS (Operation now in progress)
> [pid  8997] shutdown(4, 2 /* send and receive */) = 0
> exception connecting to host: /fe80:0:0:0:215:58ff:fec8:81a8%eth0, port: 1234java.net.SocketTimeoutException: connect timed out
> java.net.SocketTimeoutException: connect timed out
>
>
> You can see that the scope id lo is getting picked up, even though I  
> passed a scoped link-local address. Sun admits this is a bug, but just  
> haven't gotten around to fixing it yet. :-(
>
> If I had a means to remove the routes
>
> 3ffe:ffff:100:f101::/128                    *              U     0      0        1 lo
> 3ffe:ffff:100:f101::1/128                   *              U     0      0        1 lo
>
> from my routing table, I could most likely work around the problem. But  
> when I try to remove them I get
> # route -A inet6 del 3ffe:ffff:100:f101::1/128 dev lo
> SIOCDELRT: No such process
>
> Is there anyway I can get rid of these two lo routes?
>
Thats odd, that should work, can you try to remove them with the iproute2
utility?  Something like this should work:
/sbin/ip -6 route del 3ffe:ffff:100:f101::/128


> Richard
>> Regards
>> Neil
>>    
>>> https://www.redhat.com/mailman/listinfo/fedora-kernel-list%eth0
>>>     
>