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Cisco Bug: CSCvt97187 - STUN packets contain the wrong source IP address

Last Modified

Aug 27, 2020

Products (2)

  • Cisco TelePresence Video Communication Server (VCS)
  • Cisco Expressway

Known Affected Releases

X12.5.2

Description (partial)

Symptom:
Expressway sends STUN packets received on the external ethernet interface out of the internal ethernet interface using the external ethernet IP address as the source address in in IP packets.

Example:
ExpE

    internal ip: 10.10.10.10
    external ip: 20.20.20.20

Turn client 

    ip: 30.30.30.30

Turn client allocated a relay session 20.20.20.20:24000 and when the turn client is using this relay session to send a message to a device in the same subnet as the internal ip of ExpE the following happens:

    ExpE receives the request from the turn client on 20.20.20.20:3478 and is being asked to relay the packet to 10.10.10.20:50000
    ExpE will send this packet from the relay session, so source ip 20.20.20.20 and source port 24000
    Since the target ip is in the same subnet as the internal ip of the ExpE it will be sent out on the internal interface

-> packet towards 10.10.10.20:50000 is sent out from internal interface of ExpE with source 20.20.20.20:24000

Conditions:
When using Expressway-E as TURN server with Dual NIC option
+ TURN client via the Expressway TURN server sending a packet (via the external interface of Expressway TURN server) to a destination IP which is either in the subnet of the internal interface of Expressway or has a static route via the intenal gateway
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