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telnet sessions disconnect

asked 2020-06-30 15:19:59 +0000

themis gravatar image

updated 2020-06-30 15:27:02 +0000

Hello, While in telnet session to a remote ADSL Cisco router, the session freezes and then disconnects. I see that the client side sends a [RST, ACK] after a lot of retransmissions. What is the cause of this behavior? The connection on each side is not saturated.

You can find the pcap file here:

Thank you very much.

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Looks like after 30 seconds of no response from the router the client gives up and closes the session.
Can you make a capture closer to the router to see if it is not responding or if it is, are the packets not making it to the client.

Chuckc gravatar imageChuckc ( 2020-06-30 16:38:19 +0000 )edit

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answered 2020-06-30 18:24:51 +0000

SYN-bit gravatar image

All packets up till frame 2076 are sent to the gateway with mac-address d4:21:22:71:fb:20 (OUI: Sercomm), then starting with frame 2077, packets are sent to a different Sercomm mac address (94:4a:0c:4a:4b:10).

The funny part is that the TCP/RST in frame 2086 is sent to the original mac-address again. The ACK in frame 2087 has ack value 761 (acking frame 2073), proving the data that was sent in frame 2077-2085 never reached the server.

I would look at (at least) the ARP / VRRP / HSRP packets while you do your capture. But preferably all packets to see what is happening on your subnet during this problem.

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You are absolutely right. There was another router with the same ip add on the network. Thank you very much.

themis gravatar imagethemis ( 2020-07-01 14:56:27 +0000 )edit

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Asked: 2020-06-30 15:19:59 +0000

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Last updated: Jun 30 '20