NAT router on Linux


This is a method of configuring a NAT router on Linux and NATing and communicating with one of the addresses of two VLANs. The content is for verification purposes and does not take security into consideration. Please be careful during production operation.

Assumed scenario

Suppose you have a VLAN operated by the following two organizations.


Now suppose you want to access on VLAN2 from on VLAN1. The VLAN1 side organization assumes that the address is being used for other purposes, and the VLAN1 side makes the address visible to On the VLAN2 side, on the other hand, can be accessed without address translation.

Setup steps

  1. Install CentOS 8 and connect to VLAN1 and VLAN2. Let VLAN1 be eth1 and VLAN2 be eth2.
  2. Disable SELinux because it is troublesome. Just set `SELINUX = disabled``` in `/ etc / selinux / config```.
  3. Also, disable firewalld. `` `systemctl disable firewalld```
  4. Enable the routing function. Add `net.ipv4.ip_forward = 1``` to `/etc/sysctl.conf```.
  5. Reboot for activation.
  6. Set NAT with iptables.
# iptables -t nat -A POSTROUTING -d -j SNAT --to-source
# iptables -t nat -A PREROUTING  -d -j DNAT --to-destination

The first line translates the source address to when a packet with a destination of is received. The second line translates the destination address to when the destination receives a packet of Specify PREROUTING so that the destination address is translated and then routed. This setting disappears when you restart, so please execute it every time you start. (I didn't know how to persist)

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