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[OT] Embedded Linux hosts sharing Ethernet broadcast domains with two IP subnets

I think this is little OT here, because the question is mostly network orie nted. However my nodes will be Linux embedded boxes and some solutions coul d be very low-level, so I think many of you could help me in some way.

I have N Ethernet hosts based on embedded Linux. Each one features an Ether net interface dedicated to the final user that could change the IP address configuration. On that interface a Web server responds. All the nodes will be configured on the same subnet and conected to a switch.

Now I need to make an "internal" communication among those hosts. With the work "internal" I mean the user should ignore the presence of this communic ation and relevant details (it should not be necessarily invisible to a tra ffic monitoring tools). For example, when the user changes the IP address of host 1, the new IP add ress shouldn't be configured in host 2 too and the internal communication b etween host 1 and 2 should continue without interruption.

I thought about three possible solution, but I'm not a netowrk guru.

The first is to add other logical IP addresses (aliases) used for internal communication only. Those IP addresses will be hardcoded and never changed by whoever. The user will be able to configure only the main IP address for each host. Is this could work with a simple switch? I think yes, because t he switch looks at the MAC address and doesn't see the IP addresses. Anyway I don't know what negative effects could be when the same broadcast domain is shared by two different subnet (internal, with hardcoded and fixe d IP addresses, and external, configurable by the user as he wants).

Another solution is to implement the internal communication staying at leve l 2, so avoiding IP addresses, but using only MAC addresses. I could invent a proprietary layer 2 protocol. This could avoid any conflict with IP addr esses customized by the user, but there are some disadvantages. How host 1 could know the MAC address of host 2 for internal communication? It could be written during delivery of the system, but replacing one host on the field could be difficult (the MAC address of replaced host should be changed on the other nodes). I could invent a proprietary broadcast ARP pr otocol (what is MAC address of **host 2**). In order to avoid the issue of knowing the MAC address of other hosts for i nternal communication, I could invent a L2 protocol made by all broadcast f rames, considering it will a low-bandwidth protocol.

The third solution is using VLANs, but I don't know if they can be useful i n my case. Indeed, I couldn't use static (port-based) VLAN, because I will have two VLANs on the same switch ports. Anyway I should have two IP addres sed on each host (as in the first solution).

Other suggestions?

Reply to
pozzugno
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[Cross-posting to news:comp.protocols.tcp-ip, as the issue at hand seems more on-topic there. Apologies for some over-quoting to preserve context.]

There're none that I know of. The only trouble to watch out for is the user (accidentally) configuring the "main" address that matches the "internal" network.

JFYI, there're already MAC-based IPv6 "link-local" addresses -- which, unless you've configured your kernel differently, are probably already available. Consider, for example:

$ ip addr list ...

2: eno1: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether D0:BA:98:76:54:32 brd ff:ff:ff:ff:ff:ff ... inet6 fe80::D2BA:98ff:fe76:5432/64 scope link valid_lft forever preferred_lft forever ... $ ping6 -n -c 3 -- fe80::D2BA:98ff:fe76:5432%eno1 ; ## that is, local host PING fe80::D2BA:98ff:fe76:5432%eno1(fe80::D2BA:98ff:fe76:5432%eno1) 56 data bytes 64 bytes from fe80::D2BA:98ff:fe76:5432%eno1: icmp_seq=1 ttl=64 time=0.020 ms ... 3 packets transmitted, 3 received, 0% packet loss, time 2042ms rtt min/avg/max/mdev = 0.020/0.026/0.030/0.006 ms $

(Note that the exact interface to use -- eno1 in this case -- needs to be specified when referring to these addresses.)

There're already a number of "service discovery" protocols available. You may want to check [1] and follow from there.

[1]
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FSF associate member #7257  58F8 0F47 53F5 2EB2 F6A5  8916 3013 B6A0 230E 334A
Reply to
Ivan Shmakov

This method works very well and is easy to set up. You can set up eth0:1 with static address in your internal between-nodes address space and eth0:0 with DHCP to provide the web UI.

Using layer 2 or VLANs is much more complicated, especially if you've got limited network experience.

One interesting application for VLANs is that if you can use a PHY chip that has integrated switch and VLAN support so that on the CPU you have only one MII with VLANs and you route different VLANs to different ports on the switch, effectively creating multiple separated ethernet interfaces for an embedded system.

--
mikko
Reply to
Mikko OH2HVJ

You might get more details in comp.os.linux.networking.

That will work fine, as long as your "internal" network does not collide with the users network addresses. Pick something in the 10.x.y.z range, rather than 192.168.x.y. Also consider just using IPv6 here.

As you note the traffic will not be invisible to monitoring tools - it is easy to hack into this traffic.

That would be theoretically possible, but a great deal of work. Don't bother.

VLANs have a lot of uses, but in a case like this you would have to make sure that the switches involved are at least VLAN aware, even if they are not configured with specific VLAN setups. Standard cheap Ethernet switches don't pass through packets with VLAN tags.

Your first suggestion is fine.

Reply to
David Brown

iented. However my nodes will be Linux embedded boxes and some solutions co uld be very low-level, so I think many of you could help me in some way.

ernet interface dedicated to the final user that could change the IP addres s configuration. On that interface a Web server responds. All the nodes wil l be configured on the same subnet and conected to a switch.

e work "internal" I mean the user should ignore the presence of this commun ication and relevant details (it should not be necessarily invisible to a t raffic monitoring tools).

ddress shouldn't be configured in host 2 too and the internal communication between host 1 and 2 should continue without interruption.

l communication only. Those IP addresses will be hardcoded and never change d by whoever. The user will be able to configure only the main IP address f or each host. Is this could work with a simple switch? I think yes, because the switch looks at the MAC address and doesn't see the IP addresses.

t domain is shared by two different subnet (internal, with hardcoded and fi xed IP addresses, and external, configurable by the user as he wants).

vel 2, so avoiding IP addresses, but using only MAC addresses. I could inve nt a proprietary layer 2 protocol. This could avoid any conflict with IP ad dresses customized by the user, but there are some disadvantages.

n? It could be written during delivery of the system, but replacing one hos t on the field could be difficult (the MAC address of replaced host should be changed on the other nodes). I could invent a proprietary broadcast ARP protocol (what is MAC address of **host 2**).

internal communication, I could invent a L2 protocol made by all broadcast frames, considering it will a low-bandwidth protocol.

in my case. Indeed, I couldn't use static (port-based) VLAN, because I wil l have two VLANs on the same switch ports. Anyway I should have two IP addr essed on each host (as in the first solution).

I think what you need is a VPN (merging two subnets), and not to reinvent t he wheel.

Reply to
edward.ming.lee

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