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This is not a specifically ipmasq problem, but more generally a NAT problem.
I am hoping someone has some ideas, because I cannot figure this out. The
problem is as follows (sorry for the length of this post; I am going to be
detailed to make sure I give a clear enough description of my setup and
problem):
I have a LAN located in my office, consisting of a number of Win98 boxes,
some HP printservers, and a Unix-based server. All of the systems are
networked via a Linksys 16 port 10/100 hub, and that uplinks into a Linksys
Cable/DSL router, a recently released piece of hardware designed for home
users to share multiple computers over a single cable-modem or DSL line, but
which works equally well in my setup, where my LAN is uplinked via this
router to a corporate LAN in my building, through which I am receiving
network connectivity. My Lan uses IP addresses in 192.168.1.x; I statically
assign the IP addresses rather than using the router's built-in DHCP
capability because of some problems with Win98 and the NIC's synchronizing
via the switch in the router.
The router itself uses 192.168.1.1 internally, and has an ip address in the
10.x.x.x network, because that is what the corporate LAN uses internally.
Obviously, the corporate LAN uses NAT to connect its internal IP assignments
to its limited external "real" IP addresses. THerefore, to reach hosts
outside of the corporate LAN, any packet originating on my LAN goes through
two layers of NAT, one at my router and one at the outer LAN's router to the
internet.
Therein lies the problem. I can establish outgoing connections from my LAN
to the outer corporate LAN, but not get any further. From my desktop, I can
ping any host on my LAN or on the outer corporate LAN, but not through the
next layer of NAT to the outside world. The same is true for any outgoing
connection. Going through the second layer of NAT causes it to fail. (NB: I
am told by the network operators that they are NOT filtering any packets at
the firewall level that would cause this problem.)
As I understand the operation of NAT, this doesn't make sense. It seems to
me that packets originating on my LAN and going through my router would
appear to the outside LAN as having appeared at my router, and since there
shouldn't be anyway to tell that they didn't originate on my router, they
should get NAT's and deNATed on the way back in, and then my router would
again deNAT them. I know that the routing between the corporate LAN works,
because hosts directly attached to the corporate LAN using 10.x.x.x
addresses work fine, so the problem seems to be in the double layer of NAT.
I read through the Linux sources that handle all of this, and saw nothing
that would indicate that double layers of NAT would fail, nor did I see
anything in the original RFC on NAT to explain this behavoir.
Unfortunately, I don't know what kind of system the NAT is being done with.
I would doubt it is Linux-based, more likely WinNT or most likely a Cisco
router.
Any thoughts?
Thanks.
Neal
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