(Cross post of my answer on the LARTC mailing list.)
On 12/12/06 04:29, Matt wrote:
I got the above working on our test bed, where users can get to the internal
server 192.168.0.6 via either Internet connection. The problem is getting from
our Office Network to 200.200.64.139:56100
*nod* This is a weird routing issue. (See below.)
What appears to be happening is this:
1. Packet is sent from internal router, arrives at 100.100.251.220, is routed
through 100.100.251.217 to the Internet.
2. Packet arrives at 200.200.64.139, DNAT'd to 192.168.0.6.
3. Internal Server replies, sends it to it's default gateway (192.168.0.254)
4. Linux server sees 100.100.251.220 as destination, sends to 100.100.251.218
instead of back out of 200.200.64.139. (This is not expected as I'm marking
incoming connections at the linux router using CONNMARK/MARK, and connections
go in and out of the correct interface when the destination is outside the
100.100.251.216/29 network)
Presuming that you are not doing any custom routing with IPRoute2, this is as I would
expect. What is happening is your "Linux Multihomed Router" has a direct route
back to your office's internet router. Per standard routing mechanisms, your router will
choose a directly connected route, or any other (non default) route that it knows of over
your default route. So, really your Linux router is doing what it should be doing.
Unfortunately what it should be doing is not what you want it to be doing.
(Note: I don't know if the returning connections are SNAT'd back to
200.200.64.139)
A simple TCPDump will tell you if this is the case or not. However, I suspect
that the packets are being SNATed to 100.100.251.218.
Is there a way around this? i.e. so that the multihoming still works?
Yes, multiple.
One is to make your office router know that it can reach the 200.200.64.139
host via the 100.100.251.218 router. However, this is probably not what you
really want to do. I say this is probably not want you want to do b/c I'm
willing to bet that you are wanting to be able to test things across the
internet from your office, which would be circumvented with this routing.
It seems that normal routing to the 100.100.251.216/29 network takes precedence
over my connection marked rule, that would instruct the packet to be sent out
over the correct interface (and maybe therefore SNAT'd correctly too).
Yes (see above). This is because IPTables usually does not interact with any
routing decisions. (Usually b/c IPTables can be configured to do exactly
that.) IPTables usually acts on packets before and after routing decisions
have been made.
Not sure what's going on. Can anyone point me in the correct direction?
A different and probably my recommended solution (presuming that you want the traffic to
cross the internet) would be to use a custom routing table for traffic that is to use the
200.200.64.139 interface. This custom routing table would not include any knowledge of
the 100.100.251.218 network. Thus any traffic back to the office at 100.100.251.220
would be routed through the default gateway and out across the internet back to the
office. Presuming that you have MARK and CONNMARK working correctly you could use an
"ip rule" to look for the firewall mark to instruct the Linux kernel to use the
alternant routing table.
Grant. . . .
|