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I'm trying to create a simple iptable rule using the command sample below. But the routing does not work. Any inputs on what is missing as I'm not familiar with iptables.

sudo iptables -t nat -A PREROUTING -p tcp -d 10.10.20.10 --dport 8321 -j DNAT --to-destination 192.168.56.101:8321

The ip 10.10.20.10 is not assigned to any interface.

The iptables rules are as follows:

# Generated by iptables-save v1.6.1 on Tue Mar  5 14:21:30 2019
*nat
:PREROUTING ACCEPT [5:2009]
:INPUT ACCEPT [5:2009]
:OUTPUT ACCEPT [141:9332]
:POSTROUTING ACCEPT [141:9332]
:DOCKER - [0:0]
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A PREROUTING -d 10.10.20.10/32 -p tcp -m tcp --dport 8321 -j DNAT --to-destination 192.168.56.101:8321
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE
-A DOCKER -i docker0 -j RETURN
COMMIT
# Completed on Tue Mar  5 14:21:30 2019
# Generated by iptables-save v1.6.1 on Tue Mar  5 14:21:30 2019
*filter
:INPUT ACCEPT [923:68802]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [810:87756]
:DOCKER - [0:0]
:DOCKER-ISOLATION-STAGE-1 - [0:0]
:DOCKER-ISOLATION-STAGE-2 - [0:0]
:DOCKER-USER - [0:0]
-A FORWARD -j DOCKER-USER
-A FORWARD -j DOCKER-ISOLATION-STAGE-1
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -o docker0 -j DOCKER
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
-A DOCKER-ISOLATION-STAGE-1 -i docker0 ! -o docker0 -j DOCKER-ISOLATION-STAGE-2
-A DOCKER-ISOLATION-STAGE-1 -j RETURN
-A DOCKER-ISOLATION-STAGE-2 -o docker0 -j DROP
-A DOCKER-ISOLATION-STAGE-2 -j RETURN
-A DOCKER-USER -j RETURN
COMMIT

The ip addr ouput is

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:46:d2:d7 brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic enp0s3
       valid_lft 85059sec preferred_lft 85059sec
    inet6 fe80::a00:27ff:fe46:d2d7/64 scope link
       valid_lft forever preferred_lft forever
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:0e:42:40 brd ff:ff:ff:ff:ff:ff
    inet6 fd0c:6493:12bf:2942::ac18:1164/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe0e:4240/64 scope link
       valid_lft forever preferred_lft forever
4: enp0s9: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:bf:83:a2 brd ff:ff:ff:ff:ff:ff
    inet 192.168.56.101/24 brd 192.168.56.255 scope global dynamic enp0s9
       valid_lft 908sec preferred_lft 908sec
    inet6 fe80::a00:27ff:febf:83a2/64 scope link
       valid_lft forever preferred_lft forever
5: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
    link/ether 02:42:8a:d2:57:bd brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever

ip route output is:

10.0.2.0/24 dev enp0s3 proto kernel scope link src 10.0.2.15
10.0.2.2 dev enp0s3 proto dhcp scope link src 10.0.2.15 metric 100
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown
192.168.56.0/24 dev enp0s9 proto kernel scope link src 192.168.56.101
  • How are you testing this? Are you trying to access 10.10.20.10 from another host, or the same host? – wurtel Mar 5 at 15:01
  • Testing is from the same host. – Joshua S N Mar 6 at 2:43
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Traffic from the same host as where the nat PREROUTING DNAT rule is set does not traverse that nat PREROUTING chain, which is why you are not seeing it being applied.

Instead you need to use the nat OUTPUT chain for locally-generated packets:

sudo iptables -t nat -A OUTPUT -p tcp -d 10.10.20.10 --dport 8321 -j DNAT --to-destination 192.168.56.101:8321

You can find iptables processing flowcharts by searching images with those keywords in google, which makes it clear how iptables work.

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