On 08/01/14 17:19, Joel Gerber wrote:
You're going to have to re-acquaint me with your network layout. I'm having a hard time remembering what all of your interfaces are being used for and how they fit into your network.
Oh! Sorry, Joel. I forgot this is a simple mailing list, without a cute
threaded web interface. I "helpfully" snipped out our initial discussion
to lighten the payload...
I will resend my last post within the entire reply, then you will have
everything in one place. Discard this one and the "full" version will
follow soon.
Thanks,
Brian
Joel Gerber
Network Specialist
Network Operations
Eastlink
E: Joel.Gerber@xxxxxxxxxxxxxxxx T: 519.786.1241
-----Original Message-----
From: lartc-owner@xxxxxxxxxxxxxxx [mailto:lartc-owner@xxxxxxxxxxxxxxx] On Behalf Of Brian Burch
Sent: January-08-14 11:18 AM
To: Linux Advanced Routing List
Subject: Re: How to override the default source ipv4 address on packets originating from a router
On 21/12/13 12:53, Brian Burch wrote:
On 20/12/13 18:46, Brian Burch wrote:
On 20/12/13 13:05, Joel Gerber wrote:
Your problem is one I've come up against a number of times in the
past. You are definitely on the right route using "policy routing"
(otherwise known as utilizing IP Rules), but the snag you're hitting
is that the source address of a locally generated packet isn't
chosen until a destination route has already been selected. Using
the from statement in an ip rules table is only useful when you're
routing packets from other subnets.
What a quick and helpful reply! Not only did my problem motivate you
to respond, but I am very relieved to find the answer isn't just RTFM.
Your explanation about the timing for kernel selection of the source
address is crucial and now my failed experiments all make sense. Your
suggested solution also makes sense, although it is counter-intuitive...
at least to me!
Now, just so you know, the local table should normally be left alone.
It's used for locally connected subnets only. It is given the
highest priority on purpose... don't mess with it! :)
I read somewhere (possibly the howto guide or the ip-cref) a
statement that the kernel treats this table as read-only. My system
is running ubuntu 13.04 with a custom non-pae 3.8.0 kernel, so it is
reasonably modern. When I thought more about the purpose of this
table, I decided there would be no point in trying to modify it, even
if that was possible.
There are a few methods you could try here, but I'm only going to
recommend one of them. Have your main routing table hold the routes
that your local machine will use, and then have a separate table for
all routed hosts. Then, use ip rule statements to force any traffic
coming in from a routed host into that routing table.
IE: on firewall-router (assuming eth1 is facing your static subnet
hosts):
# ip rule list
0: from all lookup local
32765: iif eth1 lookup static-hosts
32766: from all lookup main
32767: from all lookup default
Have table main include all of the routes your local firewall-router
should use, and have static-hosts contain all of the routes that
your static-subnet-hosts should use.
Great! I understand the principle you propose and will try to
implement it over the weekend. I will update this thread when I have
something to report.
Excellent news!! My router/firewall is currently collecting 200+
updated packages from the repository server!
The solution was achieved using ONLY policy routing, based on Joel's
suggested approach.
However, I hit some fairly nasty snags along the way and had to do a
lot of trial and error fiddling to make it work. I am certain my
working solution can be made more elegant, but that will take a couple
of hours pen-and-pencil time to map out the current routing table
entries in selection order and then perform some experiments.
I don't have time to do it at the moment, but will get started over
the next couple of days. If I get stuck, I will post the "current best"
configuration and ask for help to improve it.
<snip>
I tried to improve my "current best" definitions, but my efforts to remove apparent redundancy have all failed. Here is the setup:
brian@chenin:~$ ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
link/ether 00:15:8a:01:91:e3 brd ff:ff:ff:ff:ff:ff
inet 217.154.193.209/28 brd 217.154.193.223 scope global eth1
inet6 fe80::215:8aff:fe01:91e3/64 scope link
valid_lft forever preferred_lft forever
3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
link/ether 00:40:63:f5:a5:72 brd ff:ff:ff:ff:ff:ff
inet 172.16.101.2/24 brd 172.16.101.255 scope global eth0
inet6 fe80::240:63ff:fef5:a572/64 scope link
valid_lft forever preferred_lft forever
brian@chenin:~$ ip rule list
0: from all lookup local
32764: from 217.154.193.209 lookup CHENIN_TO_INTERNET
32765: from all iif eth1 lookup SUBNET_217
32766: from all lookup main
32767: from all lookup default
brian@chenin:~$ ip route list table CHENIN_TO_INTERNET
163.1.221.67 via 172.16.101.1 dev eth0
brian@chenin:~$ ip route list table SUBNET_217 default via 172.16.101.1 dev eth0
brian@chenin:~$ ip route list table main
163.1.221.67 dev eth0 scope link src 217.154.193.209
169.254.0.0/16 dev eth0 scope link metric 1000
172.16.101.0/24 dev eth0 proto kernel scope link src 172.16.101.2
217.154.193.208/28 dev eth1 proto kernel scope link src 217.154.193.209
I know Joel's suggestion did not include my CHENIN_TO_INTERNET rule, but without it chenin simply times out on its http gets.
I also tried keeping CHENIN_TO_INTERNET, and removing the explicit route in the main table, but that fails too, although I wasn't very surprised.
I have tried trimming these rules and routes to discover the minimum requirement, but this seems to be the smallest working subset.
Here is my theory to account for what is happening:
1. The SUBNET_217 default route MUST be able to handle ANY traffic
arriving on eth1. When I changed the rule to match on just
217.154.193.208/28, the forwarded traffic became very glitchy, with poor
response times and some dropped tcp sessions.
2. The route to 163.1.221.67 in the main table is required to assign the
eth1 source address 217.154.193.209 to its outbound packets. This works
because the kernel cannot find any other matching routes from chenin to
163.1.221.67 (which might encourage it to assign eth0's 172.16.101.2 as
the source address).
4. Having assigned the 217.154.193.209 source address, the packet must
be routed to the next hop. The CHENIN_TO_INTERNET rule matches the
explicit source host address at a higher priority than the SUBNET_217
rule, so 172.16.101 is selected.
5. The main table has a route to 172.16.101.1 via eth0, so the packet is
sent on its way.
That seems to be three passes through the policy routing tables. Am I
correct?
I still feel it should be possible to improve the solution, but I don't
understand what is going on well enough to see how. I would be very
grateful if anyone can enlighten me.
Thanks,
Brian
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