[kwlug-disc] Web references on issues of router behind router?

unsolicited unsolicited at swiz.ca
Sat May 29 01:21:15 EDT 2010

It looks like you're right - it's only private addresses that don't 
get passed on the WAN.

Without other change, I set the WAN ip address to, gateway, and connected it via USB ethernet adapter to a computer
at A wi-fi computer on the LAN side was able to ping the
computer on the WAN side.

Looks like a WAN private address space address is not being passed by
the router. I'm astonished - I never credited any router with having
that much intelligence.

Especially as I have seen traffic wherein an internal host tries to
register itself in DNS using a private address space IP. [The
wikipedia article notes the private address space reverse dns problem.]

I don't understand why such a router would refuse to pass the WAN
private address space packets, yet allow DNS registrations with
private address space IPs to go out.

Thank you, mystery solved, and my observations explained - that even
with a 'malformed' translation I didn't even at least see the echo
request come in to a computer.

Unfortunately, googling did not reveal any useful information or proof
of this - I must be using the wrong search terms.

Thank you again.

bbierman wrote, On 05/28/2010 11:33 AM:
> I haven't tried this so tell me if it works.
> Most cheap routers don't block bogon addresses. Use a bogon address
> space (I suggest 5.X)for the outer lan and the inner lan cheap
> router should pass the traffic through to the inner lan as if it is
> on the internet.
> This should work for any protocol on the inner lan. The reason for
> the bogon is that 10.X, 172.16-31.X, and 192.168. should never go
> out the WAN port of an internet connected router, hence cheap
> routers assume that the WAN port is internet connected.
> -----Original Message----- From: unsolicited <unsolicited at swiz.ca> 
> Date: Fri, 28 May 2010 06:06:17 To: KWLUG
> discussion<kwlug-disc at kwlug.org> Subject: Re: [kwlug-disc] Web
> references on issues of router behind router?
> OK, let's pursue this further, it is illustrative to the list.
> The situation is probably not atypical, you buy a new router, and
> want to play with it on your home net / have confidence in it
> before replacing your main gateway.
> The scenario is keeping NAT, on both, for the moment.
> [ISP(Rogers)] ^ | v [Router1(PIX)] <-> [Rest of outer LAN
>] ^ | v [Router2(TrendNET)] <-> [Rest of inner LAN
> Configuring both routers as though the other weren't there, i.e. as
>  though a typical home setup. Inner LAN gets out to internet just
> fine.
> The outer LAN is not successful in talking (initiating) to the
> inner LAN, which makes sense as Router2 has built no translation
> from outside to inside (by default).
> Realistic scenario: Providing a public wi-fi facility, say, to your
>  neighbours, at a LAN party, a meeting, or whatever. However, in
> that case, the inner and outer LANs not being able to talk to each
> other is a good thing.
> If the inner LAN tries to talk to the outer LAN, Router2 building a
>  translation in the process, is not successful in talking to the
> outer LAN. (See John's excellent explanation.) Note: This is a
> theoretical exercise, arguably, one would either not use the WAN
> port on Router2, or turn off NAT on Router2. So this is a
> networking theoretical exercise.
> John Van Ostrand wrote, On 05/27/2010 2:50 PM:
>> ----- "unsolicited" <unsolicited at swiz.ca> wrote:
>>> John Van Ostrand wrote, On 05/26/2010 10:05 PM:
>>>> 1. Make sure the subnets are different. e.g. use 192.168.1.x
>>>> on one router and 192.168.2.x on the other.
>>> Yes, that's taken care of.
>>>> 2. If #1 is good, then you should be able to have the inner 
>>>> subnet access the outer subnet's systems.
>>> Not happening, which is baffling me. It's a Cisco PIX off
>>> Roger's modem, and debug packet inside <subnet>/24 sees
>>> nothing. Wireshark on
>>> another machine never sees incoming packets from the WAN IP.
> John, given your explanation below, this machine should have seen
> the incoming ping (ICMP) and replied. (Not suggesting the reply
> would get where desired.) It did not see the packet. [PEBKAC is
> possible, but I don't think so.]
>>> Darned strange. I expect the inner WAN to MAC find the outer
>>> one. Otherwise it should go to the gateway and be routed back 
>>> internally. [Seems to me coming in and going back out the in is
>>>  verboten, but I also seem to remember that had to do with 
>>> routing protocols.] If nothing else, after the first 'go here' 
>>> the remaining packets would go directly.
>> Routing back is generally not done. It's also called hairpinning,
>>  because of the hairpin one would have to draw to show packet 
>> travel. I've done it on routers here at the office for
>> convenience, now I use split DNS.
> Thank you, 'split DNS' was the verboten term I couldn't remember.
> Seems 'DNS' in this context is a bit of a misnomer, as we're
> talking IP addresses and/or MAC addresses, at this point.
>> Hair-pinning doesn't work because of this:
>> 1. Device "A" sends packet to device "B"
>> (an address on your firewall). 2. Your firewall receives the
>> packet, looks to it's NAT table and realizes this packet is for
>> an inside system, say It rewrites the destination
>> address from to and passes the packet
>> along. 3. Device 'B', the destination, gets the packet and sends
>> a reply. The source IP for the packet is and the
>> destination is The packet is sent directly back to
>> device 'A' instead of the firewall since they share a LAN. 4.
>> When Device 'A' receives the reply packet it looks to see what
>> connection it relates to. It looks in it's table and finds a
>> connection to, but that can't be it since this packet is
>> from It assumes the packet is erroneous and drops
>> it.
> Summarizing: The outgoing packet from Router2 built a translation
> to Router1, but the incoming packet is from a local (outer LAN)
> machine, and Router2 has no such translation, so drops the packet.
> [Interesting / makes sense.]
> Actually: if the outer LAN machine were in the ARP table, such as
> by a ping from Router2, I would have guessed the ping from the
> Inner LAN to that same Outer LAN machine would get a direct
> translation built. The MAC redirect from Router1 should have had
> Router2 update its ARP table, certainly in time for the next ping /
> packet.
> Or, even, subsequent pings from inner to outer would start to
> succeed. Perhaps the translation would have to expire first, which
> may have a timeout longer than the ARP table timeout.
> Hold on - isn't this a 'bug'? You would think a router would
> recognize the packet is destined for the WAN net, do an arp whois
> broadcast, and _then_ build the translation entry. Perhaps another
> case of chicken and egg - I've seen where some expected
> functionality isn't possible due to the order of processing. Last
> time I remember when this really hit was wanting to do both source
> and address ip and port translations, it could only do one at a
> time, and by the time the one was done, the second couldn't be. Or
> something. (By the time the 'if it came from here' rule kicked in,
> it was no longer 'from here' - half had been translated.) [Was
> doing a site to site VPN at the time, and neither side wanted the
> other to know anything at all about the other - not IP's, not
> ports, not nothing.]
>> One way to get hair-pinning to work is to perform SNAT on the 
>> packet on the firewall. That way Device 'B' will send the packet 
>> back to the firewall and it will be de-SNATed.
> Sadly, a capability beyond most residential (read: 'cheap')
> routers.
>> The idea that your inner router (PIX) will perform ARP on it's
>> WAN to route to your outer LAN.
> Unfortunately, my devices are reversed. PIX is to the ISP. I don't 
> expect that to change any time soon - it's a much more capable / 
> functional device than any home router. Some day I'll replace it
> with mythbuntu or something, but not any time soon.
>> Like this:
>> [Inner LAN] ---- [PIX] ---- [Outer LAN] ---- [Linksys?] ---- 
>> [Internet]
>> Structurally the Inner LAN to Outer LAN is identical to a typical
>>  home Internet connection, just change the labels. 192.168
>> addresses are called non-routable and some routers are configured
>> so as not to route those addresses.
> Sadly, from what I've seen, very few. Best practices seems to
> indicate one should block them oneself. Let alone bogons, or
> martians (which should be a superset of both.)
> Note: From the 2 or 3 Rogers internet shutdown of it's clients I've
>  seen, it appears they were shutdown because of malware scanning
> out to non-existent addresses, starting at Infuriatingly,
> the shut down didn't happen until a week later, when virus software
> had long killed the malware, so there was nothing to find to
> determine what had caused Rogers to shut things down. When I
> ultimately got to a deep enough level of Rogers tech to have them
> say that, he indicated one can associate an e-mail address with
> them to receive logs when they do shut people down - but the e-mail
> address must be in place before the situation occurs. I've yet to
> see such a log, probably because I have yet to see anyone else shut
> down since. Never been shut down, myself. Note this is residential.
> Paul's experience, Roger's business, is that he can get zero out of
> Rogers to explain his frequent shutdowns.
> Readers - See: -
> http://en.wikipedia.org/wiki/IP_address#IPv4_private_addresses -
> http://en.wikipedia.org/wiki/Bogon_filtering -
> http://en.wikipedia.org/wiki/Martian_packet -
> http://www.team-cymru.org/Services/Bogons/ -
> http://www.team-cymru.org/Services/Bogons/http.html (Lists)
>> It's possible that the PIX has some setting for that.
> Probably, but, sadly, not the TrendNet.
>>> Problem is I don't have a dual NIC <something> around to put 
>>> transparently between the WAN and the rest of the network to
>>> see if it's actually sending something out or not. Or the
>>> router is just blocking everything to it's WAN subnet. The
>>> router can ping everything, but not something behind the
>>> router. And, of course, the outer can't ping the inner - no
>>> outgoing nat translation is present.
>> How about a linksys with OpenWRT? I bet you could set up port 
>> mirroring, or use the openwrt packet inspector.
> Sadly don't have one of those, either. Thus my comment about being 
> reminded of Richard's comment of OpenWRT being something to stuff
> in the kit bag. Some day, perhaps.
>>>> 3. VPNs like openvpn should work fine. IPSec can work as
>>>> well, but it's trickier.
>>> I'm actually thinking coming in from outside, not the reverse. 
>>> But it's reassuring to read your point.
>> So you have to orchestrate some firewall rules. In the process it
>>  can be confusing, certainly at first, to keep IP addresses
>> correct.
> Situation normal to track this stuff, for me in my network work.
> It's just been a while.
>>>> 4. PPTP (is anyone still using this?) can have issues with 
>>>> multiple VPNS sharing the same IP address.
>>> Why would you have multiple VPNs? Assuming machine or net to
>>> net connections. After that, it should all just be routing.
>> Multiple PCs going out to the same VPN server. I see it 
>> occasionally with customers.
> Yes, as have I. An exercise in education when that happens, and 
> usually frustration. I can see how it happens though - someone
> finds something useful, then others like it too. And when the
> situation happens, to go back and correct it with a site to site
> VPN would mean an investment of some $$$, and there's no interest.
> Let alone the security kerfluffle, at both ends - even when you
> explain you're already doing it, just via software instead of
> hardware.
> Sadly, how this all started: Only the WAN port gets ntp,
> apparently, on today's residential routers. Certainly on my two.
> Log times with the wrong timestamps are really annoying. So,
> although one might normally not use the WAN port, one has to to get
> the time. Without the WAN port you have a simple (wi-fi) bridge /
> AP. With it ... complexity. The Netgear even outsmarts you - if you
> set both LAN and WAN on the same subnet, it automatically shifts
> the LAN subnet. Argh.
> Thanks for the post John. Hope people find the information here
> useful.
> Still expected the inner and outer lans to see each other. I
> actually expected to have to put in blocks to prevent it. Curious,
> although the explanation makes sense.

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