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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


Description (partial)

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.


    internal ip:
    external ip:

Turn client 


Turn client allocated a relay session 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 and is being asked to relay the packet to
    ExpE will send this packet from the relay session, so source ip 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 is sent out from internal interface of ExpE with source

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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Bug Details Include

  • Full Description (including symptoms, conditions and workarounds)
  • Status
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