[CentOS] iptables port forwarding

[Marian Marinov]

On Monday 27 June 2011 06:50:27 muiz wrote:
> Dear Marian and all,
>   It seems don't works:
> /sbin/iptables -t nat -A PREROUTING -j DNAT -p tcp --dport 8080 --to
> a.b.c.d:8181 /sbin/iptables -t nat -A POSTROUTING -j SNAT -s
> 192.168.0.0/255.255.255.0 --to a.b.c.d echo 1 >
> /proc/sys/net/ipv4/ip_foward

Yup, its normal not to work... You got the SNAT rule wrong :)

It should be to the IP of the server that is DOING the forwarding...

so 

/sbin/iptables -t nat -A POSTROUTING -j SNAT -s 192.168.0.0/255.255.255.0 --to 
192.168.1.250

Marian

> 
> 
> I check the Fedora iptables setting:  /etc/sysconfig/iptables files:
> ...
> 
> :POSTROUTING ACCEPT [0:0]
> 
> -A PREROUTING -i eth+ -p tcp --dport 8080 -j DNAT --to-destination
> a.b.c.d:8080 ....
> 
> :OUTPUT ACCEPT [0:0]
> 
> -A FORWARD -i eth+ -m state --state NEW -m tcp -p tcp -d a.b.c.d --dport
> 8080 -j ACCEPT
> 
> 
> And more rules I add is :
> /sbin/iptables -t nat -A POSTROUTING -d  a.b.c.d -p tcp --dport 8080 -j
> MASQUERADE
> 
> 
> Then it works!  But if I don't use system-config-firewall GUI tools, then
> how?
> 
> 
> 
> 
> Thanks very much !
> 
> At 2011-06-27，"Marian Marinov" <mm at yuhu.biz> wrote:
> >On Monday 27 June 2011 00:08:08 muiz wrote:
> >> Thanks  Marian,
> >> The server only has one IP. I think I should add more iptables records,
> >> only one NAT record is not enough,isit correct?  If yes , then how?
> >
> >Huh, I'm sorry yes you need a second rule. So the rules are:
> >iptables -t nat -A PREROUTING -j DNAT -p tcp --dport 8080 --to
> >a.b.c.d:8181 iptables -t nat -A POSTROUTING -j SNAT -s local_ip/local_net
> >--to 192.168.1.250
> >echo 1 > /proc/sys/net/ipv4/ip_foward
> >
> >The Source NAT(SNAT) rule is needed, cause otherwise the packaets that
> >reach a.b.c.d will be comming from the ip of the local client not
> >192.168.1.250 and so 192.168.1.250 will never receive the replies from
> >a.b.c.d.
> >Since the packets reach the client directly from a.b.c.d, the client will
> >simply disregard them and will wait for packets comming from .1.250.
> >
> >So the SNAT rule changes the SOURCE IP of the packets to 1.250 so a.b.c.d
> >will return the answares to the right source.
> >
> >Marian
> >
> >>  2011-06-26 23:38:58，"Marian Marinov" <mm at yuhu.biz> wrote：
> >>  
> >> >On Sunday 26 June 2011 12:53:07 muiz wrote:
> >> >> Dear all,
> >> >> 
> >> >>   I would like to forward a port to an internet server, but failed.
> >> >>   can you
> >> >> 
> >> >> help me? Server:  eth0: 192.168.1.250, Port: 8080 TCP, CentOS 5.6
> >> >> Remote server:   IP: a.b.c.d  Port: 8181
> >> >> 
> >> >> 
> >> >> Forward path:  client1(192.168.1.10) -> 192.168.1.250:8080 (forward)
> >> >> -> a.b.c.d  Port: 8181 ----------------------------------------- In
> >> >> Fedora, I successfully to config the firewall using
> >> >> system-config-firewall and iptables command: 1. Run
> >> >> system-config-firewall
> >> >> 
> >> >>  1.1 open local port 8080
> >> >>  1.2 add a forward rule: local 8080 to remote a.b.c.d:8181, tcp
> >> >> 
> >> >> 2. echo 1 > /proc/sys/net/ipv4/ip_foward
> >> >> 3. add a iptables rule: /sbin/iptables -t nat -A POSTROUTING -d 
> >> >> a.b.c.d -p tcp --dport 8181 -j MASQUERADE That's all.
> >> >> 
> >> >> 
> >> >> 
> >> >> 
> >> >> Thanks !
> >> >
> >> >You have to use Destination NAT for the job:
> >> >
> >> >iptables -t nat -A PREROUTING -j DNAT -p tcp --dport 8080 --to
> >> >a.b.c.d:8181 echo 1 > /proc/sys/net/ipv4/ip_foward
> >> >
> >> >If you have more then one IPs on the local machine its a good idea to
> >> >specify the destination -d 192.168.1.250
> >> >
> >> >Marian
> >> 
> >> _______________________________________________
> >> CentOS mailing list
> >> CentOS at centos.org
> >> http://lists.centos.org/mailman/listinfo/centos

-- 
Best regards,
Marian Marinov
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