Re: Routing the DNS Traffic via specific interface.

[gapsf@yandex.ru]

No. You should use "Policy routing" with MARK target in iptables.

Mark outgoing DNS packets with iptables in mangle PREOROUTING for example.
# iptables -t mangle -A PREROUTING -p udp --dport 53 -j MARK --set-mark 0x4

Create additional routing table with different routing rules.
Add new entry in /etc/iproute2/rt_tables
=========================================
#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1     inr.ruhep
1      isp2 # <- new entry
=========================================

Then execute
# ip route flush table isp2
and add defalt route into newly created table
# route add default via <ip_of_your_eth2_gateway> dev eth2 table isp2

Add new policy in RPDB.
# ip rule add from all fwmark 0x4 table isp2

Check RPDB
# ip rule show
You should view somthing like this:
0:      from all lookup local
32763:  from all fwmark 0x4 lookup isp2
32766:  from all lookup main
32767:  from all lookup default

In result: all traffic routed with main routing table, except marked DNS traffic routed
via "isp2" routing table via its default route and iface.
View picture http://postimage.org/image/nn9owf5x7/ for example.

NG> Hi ..

NG> I have 2 interfaces eth0 and eth1 on the system connected to different subnets. I need to route all the outgoing DNS traffic of the system via eth1 interface. Pls let me know if below IPTABLES rules is proper way ?

NG> Block the output DNS traffic on eth0 interface.

NG> iptables -A FORWARD -p udp -o eth0 --dport 53 -j DROP

NG> Forward output DNS traffic from eth1 interface

NG> iptables -A FORWARD -p udp -o eth1 --dport 53 -j ACCEPT


NG> Thanks
NG> Ganesh


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