[CentOS] Routing issue

Fri Sep 28 10:52:28 UTC 2012
Steve Clark <sclark at netwolves.com>

On 09/27/2012 09:47 PM, Manish Kathuria wrote:
> On Thu, Sep 27, 2012 at 8:55 PM, Steve Clark <sclark at netwolves.com> wrote:
>> On 09/27/2012 11:01 AM, Manish Kathuria wrote:
>> On Thu, Sep 27, 2012 at 7:04 PM, Steve Clark <sclark at netwolves.com> wrote:
>> On 09/26/2012 11:57 PM, Manish Kathuria wrote:
>> On Thu, Sep 27, 2012 at 7:46 AM, Gordon Messmer <yinyang at eburg.com> wrote:
>> On 09/26/2012 09:15 AM, Steve Clark wrote:
>> The routes-x.y-z.diff is a unified patch containing different parts
>> which include support for Dead Gateway Detection as well.  However,
>> since that is limited to the first hop, it is preferable to have a
>> userspace script as you are doing. I also use a script to check the
>> accessibility of a remote popular site from each of the ISPs and based
>> upon the response the links are treated alive or dead and the default
>> gateway is changed. However, the routing problem as described by you
>> will only be solved after applying this patch (routes-x.y-z.diff).
>> As for marking the incoming packets to ensure that they go out from
>> the same interface they came from, you could do something like the
>> following:
>> Using iptables mark the incoming traffic from external interfaces
>> /sbin/iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
>> /sbin/iptables -t mangle -A OUTPUT -j CONNMARK --restore-mark
>> /sbin/iptables -t mangle -A PREROUTING -i eth1 -j MARK  --set-mark 1
>> /sbin/iptables -t mangle -A PREROUTING -i eth1 -j CONNMARK --save-mark
>> /sbin/iptables -t mangle -A PREROUTING -i eth2 -j MARK  --set-mark 2
>> /sbin/iptables -t mangle -A PREROUTING -i eth2 -j CONNMARK --save-mark
>> Add the following rules to your existing ones for policy routing
>> /sbin/ip rule add fwmark 1 table T1
>> /sbin/ip rule add fwmark 2 table T2
>> Hi Manish,
>> Thanks for the info. The one question I have is about
>> /sbin/iptables -t mangle -A OUTPUT -j CONNMARK --restore-mark
>> I thought the OUTPUT chain was only for packets originating locally. I am
>> only concerned
>> with clients behind my Linux router, do I still need this?
> Yes you are right but in case if you have any services running on the
> linux router itself (for example sshd) and  accessible from the
> internet, it would help.
Hi Manish,

The above rules appear to be for clients coming into the router from external. They
don't solve the problem for clients inside the router going out thru the load balanced

I have done much googling and testing without much luck. At this point in time I would
be satisfied with just being able to have a client inside the router do FTP over just one
of the outbound interfaces without any load balancing for FTP. I have this working but only
for active mode FTP by using the following:

/sbin/iptables -t mangle -A PREROUTING -p tcp --dport 20:21 -j MARK --set-mark 1

But it doesn't work for passive because you don't know what ports are going to be used.


Stephen Clark
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at netwolves.com