netfilter: Redirecting incoming udp packets to other port

netfilter: Redirecting incoming udp packets to other port

Post by murug » Tue, 12 Dec 2006 14:56:49

    I want to redirect incoming udp packets comming to router to only
go out from a praticular port. will it be possible by using iptables
command. will the command change skbuff contents and tuple. Do reply
packets  will be the routed properly.


client ------------> router -------------------->server
         <-----------           <--------------------

if receive a orignal udp packet from client from client port x to
router port y, can i use the same port y to forward to server (y acting
as source port) and then receive the reply packet to  port y as
destination and successfully forward to client port x.

will the cmd like will work
iptables -A POSTROUTING -t nat -p udp --dport y --sport y -j MASQUERADE

can any one give idea

thanks in advance


1. pb setting up iptables for redirecting incoming UDP packets to LAN (voip)

Hi there,

I tried to set up a crude configuration of iptables to be able to use
a VoIP client (Ericsson phone doubler, pc2phone) on the windows 98 of
my LAN, using a linux redhat redhat 7.3 connected to internet (with a
fixed IP number) :

    LAN                                         Internet
_________________        ______________
|               |        | redhat 7.3 |
| machine win 98| __eth__| iptables   |__ppp0__

  IP: >>         <<IP:
                                fixed IP>>

To redirect the incoming UDP packets to the LAN, i consider using
iptables (after loading ip_tables, iptable_filter, ip_conntrack,
ip_conntrack_ftp modules) and flushing the tables, with the following
rules (from iptables-save) :


-A PREROUTING -i ppp0 -p udp -j DNAT --to-destination
-A PREROUTING  -i ppp0 -p tcp -j DNAT --to-destination
:INPUT ACCEPT [923:163570]
:FORWARD ACCEPT [8592:2056629]
:OUTPUT ACCEPT [406:57892]

the VoIP client got working but after a few second the windows 98

Any idea ? Should i add some rules to test the packets ?

ps : I know, i know, but security questions are not the matter in here

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