port rerouting with iptables not working

Question

I want to reroute all traffic from a port e.g. 4445 (from localhost) to another port on another machine in the LAN e.g. 3305. I expected this should do the deal:

iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 4445 -j DNAT --to 192.168.72.1:3305

I am listening on the target machine with

nc -k -l 3305

and a direct connection to that ip:port works and nc displays the transmitted messages.

telnet 192.168.72.1 3305
Trying 192.168.72.1...
Connected to 192.168.72.1.
Escape character is '^]'.
test
^]

But as soon as I want to make use of my iptables rule suddenly the connection is refused:

$>telnet localhost 4445
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused

I read multiple times that ip forwarding has to be activated:

$>cat /proc/sys/net/ipv4/ip_forward
1

thus my IP-forwarding should be active and I did restart the machine after the change from 0 to 1.

I also checked whether iptables lists the rule:

    $>iptables -L -t nat
    Chain PREROUTING (policy ACCEPT)
    target     prot opt source               destination         
    DNAT       tcp  --  anywhere             anywhere             tcp dpt:4445 to:192.168.72.1:3305

    Chain INPUT (policy ACCEPT)
    target     prot opt source               destination         

    Chain OUTPUT (policy ACCEPT)
    target     prot opt source               destination         

    Chain POSTROUTING (policy ACCEPT)
    target     prot opt source               destination

--

$>iptables -L -n -v
Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
 4745  586K ufw-before-logging-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
 4745  586K ufw-before-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
  121  8712 ufw-after-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-after-logging-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-reject-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-track-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:4445

Chain FORWARD (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ufw-before-logging-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-before-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-after-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-after-logging-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-reject-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ufw-track-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
 3344  655K ufw-before-logging-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
 3344  655K ufw-before-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
   29  3082 ufw-after-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
   29  3082 ufw-after-logging-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
   29  3082 ufw-reject-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
   29  3082 ufw-track-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-after-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-after-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:137
    0     0 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:138
    0     0 ufw-skip-to-policy-input  tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:139
    0     0 ufw-skip-to-policy-input  tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:445
    0     0 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:67
    0     0 ufw-skip-to-policy-input  udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:68
  121  8712 ufw-skip-to-policy-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type BROADCAST

Chain ufw-after-logging-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW BLOCK] "

Chain ufw-after-logging-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW BLOCK] "

Chain ufw-after-logging-output (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-after-output (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-before-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 3
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 4
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 11
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 12
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 8
    0     0 ufw-user-forward  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-before-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         
  146  223K ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0           
 4471  353K ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ufw-logging-deny  all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 3
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 4
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 11
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 12
    0     0 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0            icmptype 8
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp spt:67 dpt:68
  128  9132 ufw-not-local  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            224.0.0.251          udp dpt:5353
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            239.255.255.250      udp dpt:1900
  128  9132 ufw-user-input  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-before-logging-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-before-logging-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-before-logging-output (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-before-output (1 references)
 pkts bytes target     prot opt in     out     source               destination         
  146  223K ACCEPT     all  --  *      lo      0.0.0.0/0            0.0.0.0/0           
 3169  428K ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
   29  3082 ufw-user-output  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-logging-allow (0 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW ALLOW] "

Chain ufw-logging-deny (2 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID limit: avg 3/min burst 10
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10 LOG flags 0 level 4 prefix "[UFW BLOCK] "

Chain ufw-not-local (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    7   420 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type LOCAL
    0     0 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type MULTICAST
  121  8712 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ADDRTYPE match dst-type BROADCAST
    0     0 ufw-logging-deny  all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 10
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-reject-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-reject-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-reject-output (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-skip-to-policy-forward (0 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-skip-to-policy-input (7 references)
 pkts bytes target     prot opt in     out     source               destination         
  121  8712 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-skip-to-policy-output (0 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-track-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-track-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-track-output (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    3   180 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate NEW
   26  2902 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate NEW

Chain ufw-user-forward (1 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-user-input (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    1    60 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:22
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:22
    6   360 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:80
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:80
    0     0 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:443
    0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0            udp dpt:443

Chain ufw-user-limit (0 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0            limit: avg 3/min burst 5 LOG flags 0 level 4 prefix "[UFW LIMIT BLOCK] "
    0     0 REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            reject-with icmp-port-unreachable

Chain ufw-user-limit-accept (0 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Chain ufw-user-logging-forward (0 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-user-logging-input (0 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-user-logging-output (0 references)
 pkts bytes target     prot opt in     out     source               destination         

Chain ufw-user-output (1 references)
 pkts bytes target     prot opt in     out     source               destination

Why is this not working as expected/What is my error?

score 3 · Answer 1 · answered Mar 01 '16 at 13:38

3

This will not work for at least two reasons:

You added a rule with DNAT target in PREROUTING chain and this will not be hit when connecting locally. Intead, you need to add it to your OUTPUT chain in nat table.
You have specified input interface -i eth0 and your test traffic will not come from this interface. It will traverse through loop-back interface lo.

You can try by changing your rule to read:

iptables -A OUTPUT -t nat -p tcp --dport 4445 -j DNAT --to 192.168.72.1:3305

Of course, you can keep both rules at the same time (for incoming traffic and for locally generated traffic).

answered Mar 01 '16 at 13:38

Khaled

35,688
8
69
98

now i do not get a connection refused anymore but a connection is still not established. It keeps displaying `Trying 127.0.0.1...` until it times out – Sim Mar 01 '16 at 13:44
Did you allow incoming and outgoing traffic in `filter` table? Please, post the output of `iptables -L -n -v`. – Khaled Mar 01 '16 at 13:59
see update of my post – Sim Mar 01 '16 at 14:04

score 0 · Answer 2 · answered Mar 01 '16 at 13:58

0

Along with @Khaled suggestion as an answer...dont forget to fire this: sudo iptables -A INPUT -p tcp --dport 4445 -j ACCEPT too?

answered Mar 01 '16 at 13:58

Faron

107
6

this rule does not have any effect as the the counter of the packages does not increase while running my experiment. The traffic is still lost/dropped somewhere. – Sim Mar 02 '16 at 13:13

port rerouting with iptables not working

2 Answers2