Solved 3CX IOS client doesn't answer after SYN ACK recieved on port 5001?

Discussion in '3CX Clients' started by angeraer, Dec 4, 2017.

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  1. angeraer

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    Hey,

    Fairly new and doing a POC on the 3CX PABX software I noticed a strange thing. I have a phone in a subnet different from the subnet the PABX is in. The client on the phone is cycling between 'registering' and 'timeout'.
    Doing a packet capture in my network the closest to my phone (port mirror on outgoing interface of firewall) I observe the following:

    192.168.2.208 = IOS phone
    192.168.88.249 = PABX

    1. Client sends SYN for TCP 5001
    22:07:45.449914 IP 192.168.2.208.49720 > 192.168.88.249.5001: Flags [S.], seq 2561195304, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 336657334 ecr 0,sackOK,eol], length 0

    2. PABX confirms and sends SYN ACK for TCP 5001
    22:07:45.449970 IP 192.168.88.249.5001 > 192.168.2.208.49720: Flags [S.], seq 1064058999, ack 2561195305, win 28960, options [mss 1460,sackOK,TS val 21360150 ecr 336657334,nop,wscale 7], length 0

    2. PABX didn't recieve ACK so resends SYN ACK for TCP 5001 and on and on...
    22:07:46.479507 IP 192.168.88.249.5001 > 192.168.2.208.49720: Flags [S.], seq 1064058999, ack 2561195305, win 28960, options [mss 1460,sackOK,TS val 21360408 ecr 336657334,nop,wscale 7], length 0

    The phone is receiving the SYN ACK to create the TCP session, but isn't responding after that?
    I can make calls from with the 3CX client, but my presence just isn't working. If I disconnect and connect via 4G, it works. I don't have any NAT between these 2 networks. Just simple routing.

    I also have my PC with a 3CX client in exactly the same network. This isn't having any issues:

    example data part:
    22:13:50.568282 IP 192.168.88.249.5001 > 192.168.2.233.63651: Flags [P.], seq 200:275, ack 1, win 245, length 75
    22:13:50.623689 IP 192.168.2.233.63651 > 192.168.88.249.5001: Flags [.], ack 275, win 252, length 0

    Any ideas why this is happening? :)

    Andy.
     
  2. Vali_3CX

    Vali_3CX Well-Known Member
    Staff Member 3CX Support

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    As long as you can access from iPhone's Safari the PBX on 5001, the client should work too; if not - not too.
     
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  3. angeraer

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    Thanks, when you said the native http access needs to work I realized it might be a case of assymetric routing causing the issue. The reply from the client passed a different firewall dropping it because it didn't see the full TCP stream.

    My bad :)

    Thanks for the reply!

    Andy.
     
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