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Server 1 and Server 2 (Centos 7) both host the same application, which listens on UDP port 1514. Server 1 receives traffic on this port. The goal is for the application on Server 2 to receive a copy of this traffic.

Server 1 uses iptables to duplicate traffic on UDP 1514 and send it to Server 2:

iptables -t mangle -A PREROUTING -i ens160 -p udp --dport 1514 -j TEE --gateway 10.88.72.40 [server 2]

A tcpdump on Server 2 reveals all of the traffic coming in; HOWEVER, the destination IP is still 10.88.72.41, so the listening application does not detect the traffic. Please help me find a solution so that the application on the second server can detect this traffic. On Server 2, I have tried this policy to rewrite the destination IP of the incoming packets:

iptables -t nat -A PREROUTING -p udp -d 10.88.72.41 --dport 1514 -j NETMAP --10.88.72.40

On Server 1 I have tried similar commands to rewrite (or DNAT) the destination IP of the outgoing packets. I have been working on this for days - if you can solve it I will be eternally grateful!

Brandt Tullis
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    Add the ip from server one as an alias on the loop back device on server two. –  May 08 '15 at 03:05

2 Answers2

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You sire, are a scholar, a gentleman, and a saint! On my second server, I added a virtual loopback device with the IP of the first server, and IT'S ALIVE! Here is how to add a virtual loopback device in Centos 7:

cd /etc/sysconfig/network-scripts
cp ifcfg-lo ifcgf-lo:1
vi ifcfg-lo:1

and put in that file:

DEVICE=lo:1
IPADDR=10.88.72.41
NETMASK=255.255.255.255
NETWORK=10.88.72.41
BROADCAST=10.88.72.41
ONBOOT=yes
NAME=loopback:1
NM_CONTROLLED="no"

then do:

ifup lo:1
ifconfig  # you should see lo:1 in there

On Server 1, add a static arp entry for Server 2:

arp -s <server2 IP> <server 2 MAC>
Brandt Tullis
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And if you need to do the same thing on a Windows host, you can follow the instructions found in this post: https://www.pingzic.net/how-to-enable-loopback-adapter-in-windows-10/

Which are essentially:

  1. Open Device Manager
  2. Click on a category in the left-hand navigation of DevMgmt, then select the Action menu and choose "Add legacy hardware"
  3. Choose "Install the hardware that I manually select from a list (Advanced)" then click Next
  4. Choose "Network adapters" and click Next
  5. Choose "Microsoft" in the left-hand (Manufacturer) list, then select the Microsoft KM-TEST Loopback Adapter in the right-hand list, then click Next until finished

After you have added the adapter, you can select the adapter in the typical network adapter control panel and apply a static IP of your choice to make the IP a live, active IP on the server, but not actually respond to traffic at that IP on a live network outside of this server.

David J. Pryke
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