Multiple interfaces with the same IP address

Multiple interfaces with the same IP address

[Greg Wilson-Lindberg]

At this point I'm just asking for a direction to go to get the information that I need.  While I've 
been writing low level networking software for many years, I've never had to get into routing.

Here's my problem, we have an environment where we have a machine with one ethernet connection to 
the wider network and a number of HDLC cards that connect to other embedded systems that we NAT out 
to the ethernet WAN.

                    ------------------------------
                    |       (172.16.4.9/12) hdlc2|---------\
                    |                            |         |
WAN 10.0.0.0/8 ----| eth0 (10.1.32.x/8)         |         |
                    |                            |         |
                    |       (172.16.4.9/12) hdlc1|------\  |
                    |                            |      |  |
                    |       (172.16.4.9/12) hdlc0|---\  |  |
                    ------------------------------   |  |  |
                                                     |  |  |
                                                     |  |  |
                         machine1a-------------------|  |  |
                     (172.24.127.249)                |  |  |
                     (172.16.129.9)                  |  |  |
                                                     |  |  |
                         machine2a-------------------|  |  |
                     (172.24.0.37)                   |  |  |
                     (172.16.129.17)                 |  |  |
                                                     |  |  |
                         machine3a-------------------|  |  |
                     (172.24.0.41)                   |  |  |
                     (172.16.129.25)               ...  |  |
                                                        |  |
                         machine1b----------------------|  |
                     (172.24.127.250)                   |  |
                     (172.16.129.9)                     |  |
                                                        |  |
                         machine2b----------------------|  |
                     (172.24.0.38)                      |  |
                     (172.16.129.18)                    |  |
                                                        |  |
                         machine3b----------------------|  |
                     (172.24.0.42)                      |  |
                     (172.16.129.26)                  ...  |
                                                           |
                         machine1c-------------------------|
                     (172.24.127.251)                      |
                     (172.16.129.11)                       |
                                                           |
                         machine2c-------------------------|
                     (172.24.0.39)                         |
                     (172.16.129.19)                       |
                                                           |
                         machine3c-------------------------|
                     (172.24.0.43)                         |
                     (172.16.129.27)                       |
                                                         ...

As can be seen from the above diagram, the addresses on the HDLC channels are not on separate 
subnets.  We also have some broadcast addresses that the machines on the HDLC nets will need to 
receive.  We do know all of the addresses for all of the machines on the HDLC nets, but we could end 
up with as many as 30-50 on each net.  Most of the packets coming from the ethernet WAN will go to 
just one of the HDLC nets, but the broadcast packets will need to go to all of them.  Also, the HDLC 
nets do not need to be able to communicate between themselves.

Can anybody at least point me in the proper direction on how I need to attack this problem?

Thanks in advance,

Greg Wilson-Lindberg

Re: Multiple interfaces with the same IP address

[Rick Jones]

Greg Wilson-Lindberg wrote:
> At this point I'm just asking for a direction to go to get the 
> information that I need.  While I've been writing low level networking 
> software for many years, I've never had to get into routing.
> 
> Here's my problem, we have an environment where we have a machine with 
> one ethernet connection to the wider network and a number of HDLC cards 
> that connect to other embedded systems that we NAT out to the ethernet WAN.
> 
>                    ------------------------------
>                    |       (172.16.4.9/12) hdlc2|---------\
>                    |                            |         |
> WAN 10.0.0.0/8 ----| eth0 (10.1.32.x/8)         |         |
>                    |                            |         |
>                    |       (172.16.4.9/12) hdlc1|------\  |
>                    |                            |      |  |
>                    |       (172.16.4.9/12) hdlc0|---\  |  |
>                    ------------------------------   |  |  |
>                                                     |  |  |
>                                                     |  |  |
>                         machine1a-------------------|  |  |
>                     (172.24.127.249)                |  |  |
>                     (172.16.129.9)                  |  |  |
>                                                     |  |  |
>                         machine2a-------------------|  |  |
>                     (172.24.0.37)                   |  |  |
>                     (172.16.129.17)                 |  |  |
>                                                     |  |  |
>                         machine3a-------------------|  |  |
>                     (172.24.0.41)                   |  |  |
>                     (172.16.129.25)               ...  |  |
>                                                        |  |
>                         machine1b----------------------|  |
>                     (172.24.127.250)                   |  |
>                     (172.16.129.9)                     |  |
>                                                        |  |
>                         machine2b----------------------|  |
>                     (172.24.0.38)                      |  |
>                     (172.16.129.18)                    |  |
>                                                        |  |
>                         machine3b----------------------|  |
>                     (172.24.0.42)                      |  |
>                     (172.16.129.26)                  ...  |
>                                                           |
>                         machine1c-------------------------|
>                     (172.24.127.251)                      |
>                     (172.16.129.11)                       |
>                                                           |
>                         machine2c-------------------------|
>                     (172.24.0.39)                         |
>                     (172.16.129.19)                       |
>                                                           |
>                         machine3c-------------------------|
>                     (172.24.0.43)                         |
>                     (172.16.129.27)                       |
>                                                         ...
> 
> As can be seen from the above diagram, the addresses on the HDLC 
> channels are not on separate subnets.  We also have some broadcast 
> addresses that the machines on the HDLC nets will need to receive.  We 
> do know all of the addresses for all of the machines on the HDLC nets, 
> but we could end up with as many as 30-50 on each net.  Most of the 
> packets coming from the ethernet WAN will go to just one of the HDLC 
> nets, but the broadcast packets will need to go to all of them.  Also, 
> the HDLC nets do not need to be able to communicate between themselves.
> 
> Can anybody at least point me in the proper direction on how I need to 
> attack this problem?

Is there a specific reason you cannot further subnet the 10.0.0.0/8 network for 
your HDLC channels and eschew the NAT?  Then at least ignoring the issue of 
"routing" broadcast datagrams (these are IP broadcasts and not simply ethernet 
broadcasts right?) reaching-out and touching the HDLC devices from the WAN would 
seem to be more straightforward.

rick jones
visions of many static routing table entries floating through his head

Re: Multiple interfaces with the same IP address

[Greg Wilson-Lindberg]

Rick Jones wrote:
...

> 
> Is there a specific reason you cannot further subnet the 10.0.0.0/8 network for 
> your HDLC channels and eschew the NAT?  Then at least ignoring the issue of 
> "routing" broadcast datagrams (these are IP broadcasts and not simply ethernet 
> broadcasts right?) reaching-out and touching the HDLC devices from the WAN would 
> seem to be more straightforward.
> 
> rick jones
> visions of many static routing table entries floating through his head

The 10.0.0.0/8 network is an existing network that has 100's of computers on it.  We are planning on 
  using some 10.x.0.0/16 addresses to NETMAP to the 172.16.0.0/16 addresses in some limited 
situations, but this doesn't solve the problem of routing to the correct HDLC channel to get to the 
final destination.

Greg

Re: Multiple interfaces with the same IP address

[Rick Jones]

Greg Wilson-Lindberg wrote:
> 
> Rick Jones wrote:
> ...
> 
>>
>> Is there a specific reason you cannot further subnet the 10.0.0.0/8 
>> network for your HDLC channels and eschew the NAT?  Then at least 
>> ignoring the issue of "routing" broadcast datagrams (these are IP 
>> broadcasts and not simply ethernet broadcasts right?) reaching-out and 
>> touching the HDLC devices from the WAN would seem to be more 
>> straightforward.
>>
>> rick jones
>> visions of many static routing table entries floating through his head
> 
> 
> The 10.0.0.0/8 network is an existing network that has 100's of 
> computers on it. 

Not knowing (perhaps I just missed it) "the rest of the story" (a la Paul 
Harvey), simply 100's of computers doesn't sound like all that much for a 
renumbering - should it come to that.

Are the IPs of the "HDLC machines" similarly constrained?

> We are planning on  using some 10.x.0.0/16 addresses 
> to NETMAP to the 172.16.0.0/16 addresses in some limited situations, but 
> this doesn't solve the problem of routing to the correct HDLC channel to 
> get to the final destination.

I have not fully parsed the manpage for the ip command, but it does have tidbits 
which suggest being able to specify interfaces by name when manipulating routing 
tables.  If that is correct you should be able to add specific host routes 
pointing-out specific interface names if you are indeed forced to put assign the 
same IP address to each of the HCLC interfaces on the "gateway."

Earlier you mentioned broadcasts that needed to reach through and touch the HDLC 
systems - did you actually mean IP multicast?

rick jones

Re: Multiple interfaces with the same IP address

[Oskar Berggren]

2009/10/7 Rick Jones <rick.jones2@hp.com>:
> Greg Wilson-Lindberg wrote:
>>
>> Rick Jones wrote:
>> ...
>>

>
> I have not fully parsed the manpage for the ip command, but it does have
> tidbits which suggest being able to specify interfaces by name when
> manipulating routing tables.  If that is correct you should be able to add
> specific host routes pointing-out specific interface names if you are indeed
> forced to put assign the same IP address to each of the HCLC interfaces on
> the "gateway."
>

Yes,

  ip route add x.x.x.x/x dev eth1

or similar works fine.

If you like, you can also use ip addr add x.x.x.x/32 dev eth1 to set
an ip-address without having the kernel automatically install a subnet
route. Then manage the routing tables complete by yourself.

/Oskar

Re: Multiple interfaces with the same IP address

[Greg Wilson-Lindberg]

Ok, I can use route to add the various IP addresses that are on each HDLC channel to the routing 
tables.  There are a lot of machines, but because the addresses are well known/formatted, generating 
the scripts automatically won't be to bad.  But how do I handle broadcast addresses such as 
172.16.255.255, 172.16.128.255, or 172.16.129.15 that need to go out all of the HDLC channels?

Oskar Berggren wrote:
> 2009/10/7 Rick Jones <rick.jones2@hp.com>:
>> Greg Wilson-Lindberg wrote:
>>> Rick Jones wrote:
>>> ...
>>>
> 
>> I have not fully parsed the manpage for the ip command, but it does have
>> tidbits which suggest being able to specify interfaces by name when
>> manipulating routing tables.  If that is correct you should be able to add
>> specific host routes pointing-out specific interface names if you are indeed
>> forced to put assign the same IP address to each of the HCLC interfaces on
>> the "gateway."
>>
> 
> Yes,
> 
>   ip route add x.x.x.x/x dev eth1
> 
> or similar works fine.
> 
> If you like, you can also use ip addr add x.x.x.x/32 dev eth1 to set
> an ip-address without having the kernel automatically install a subnet
> route. Then manage the routing tables complete by yourself.
> 
> /Oskar
> --
> To unsubscribe from this list: send the line "unsubscribe netfilter" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: Multiple interfaces with the same IP address

[Rick Jones]

Greg Wilson-Lindberg wrote:
> Ok, I can use route to add the various IP addresses that are on each 
> HDLC channel to the routing tables.  There are a lot of machines, but 
> because the addresses are well known/formatted, generating the scripts 
> automatically won't be to bad.  But how do I handle broadcast addresses 
> such as 172.16.255.255, 172.16.128.255, or 172.16.129.15 that need to go 
> out all of the HDLC channels?

How are those going to appear on the gateway's 10net interface in the first 
place?  I thought stuff like "directed broadcast" was verboten these days and 
essentially unsupported by any stack.  That is why I was asking if these were to 
be broadcasts or perhaps IP multicasts - where IP multicasts *can* and do get 
routed.

Please don't take this the wrong way, but on the surface, the setup you've 
described sounds terribly kludged and so very brittle, and given the domain of 
your email and a bit of reader imagination that is, well, troubling :)

rick jones

> 
> Oskar Berggren wrote:
> 
>> 2009/10/7 Rick Jones <rick.jones2@hp.com>:
>>
>>> Greg Wilson-Lindberg wrote:
>>>
>>>> Rick Jones wrote:
>>>> ...
>>>>
>>
>>> I have not fully parsed the manpage for the ip command, but it does have
>>> tidbits which suggest being able to specify interfaces by name when
>>> manipulating routing tables.  If that is correct you should be able 
>>> to add
>>> specific host routes pointing-out specific interface names if you are 
>>> indeed
>>> forced to put assign the same IP address to each of the HCLC 
>>> interfaces on
>>> the "gateway."
>>>
>>
>> Yes,
>>
>>   ip route add x.x.x.x/x dev eth1
>>
>> or similar works fine.
>>
>> If you like, you can also use ip addr add x.x.x.x/32 dev eth1 to set
>> an ip-address without having the kernel automatically install a subnet
>> route. Then manage the routing tables complete by yourself.
>>
>> /Oskar
>> -- 
>> To unsubscribe from this list: send the line "unsubscribe netfilter" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> -- 
> To unsubscribe from this list: send the line "unsubscribe netfilter" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: Multiple interfaces with the same IP address

[Greg Wilson-Lindberg]

The 'broadcasts' are just to allow us to get to a subset of the machines on the HDLC nets.  I 
suppose that they could be called IP multicasts, although the are not using the traditional 
multicast addresses.  The broadcasts addresses should never be generated outside of the 'gateway' 
(we actually call these machines proxies).  The gateway is actually used as a proxy for some 
services, hence the need to be able to broadcast packets to some subset of the devices on the HDLC 
nets (commands for a class of machines).  The traffic that goes between the HDLC nets and the 
ethernet is (at this point) all initiated from the machines on the HDLC nets, things like NFS 
mounts.  As I stated in a previous email, we have contemplated using the iptable NETMAP target to 
directly map something like 10.192 on the ethernet WAN to 172.16 on the HDLC nets.  But this will be 
for the convenience of being able to then directly telnet into the machines on the HDLC nets, not 
for sending 'broadcasts'.

Greg

Rick Jones wrote:
> Greg Wilson-Lindberg wrote:
>> Ok, I can use route to add the various IP addresses that are on each 
>> HDLC channel to the routing tables.  There are a lot of machines, but 
>> because the addresses are well known/formatted, generating the scripts 
>> automatically won't be to bad.  But how do I handle broadcast addresses 
>> such as 172.16.255.255, 172.16.128.255, or 172.16.129.15 that need to go 
>> out all of the HDLC channels?
> 
> How are those going to appear on the gateway's 10net interface in the first 
> place?  I thought stuff like "directed broadcast" was verboten these days and 
> essentially unsupported by any stack.  That is why I was asking if these were to 
> be broadcasts or perhaps IP multicasts - where IP multicasts *can* and do get 
> routed.
> 
> Please don't take this the wrong way, but on the surface, the setup you've 
> described sounds terribly kludged and so very brittle, and given the domain of 
> your email and a bit of reader imagination that is, well, troubling :)
> 
> rick jones
> 
>> Oskar Berggren wrote:
>>
>>> 2009/10/7 Rick Jones <rick.jones2@hp.com>:
>>>
>>>> Greg Wilson-Lindberg wrote:
>>>>
>>>>> Rick Jones wrote:
>>>>> ...
>>>>>
>>>> I have not fully parsed the manpage for the ip command, but it does have
>>>> tidbits which suggest being able to specify interfaces by name when
>>>> manipulating routing tables.  If that is correct you should be able 
>>>> to add
>>>> specific host routes pointing-out specific interface names if you are 
>>>> indeed
>>>> forced to put assign the same IP address to each of the HCLC 
>>>> interfaces on
>>>> the "gateway."
>>>>
>>> Yes,
>>>
>>>   ip route add x.x.x.x/x dev eth1
>>>
>>> or similar works fine.
>>>
>>> If you like, you can also use ip addr add x.x.x.x/32 dev eth1 to set
>>> an ip-address without having the kernel automatically install a subnet
>>> route. Then manage the routing tables complete by yourself.
>>>
>>> /Oskar
>>> -- 
>>> To unsubscribe from this list: send the line "unsubscribe netfilter" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> -- 
>> To unsubscribe from this list: send the line "unsubscribe netfilter" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>