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Networking
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TCP ZeroWindow in telnet connections

Jose M. del Rio
Frequent Advisor

TCP ZeroWindow in telnet connections

We are having intermittent problems in a server: users can't telnet to the server.
By sniffing we can see the sequence (see attached capture.pcap):
- 3-way handshaking
- client sends first telnet packet
- server acks the packet immediately at the TCP level
- many seconds, even minutes, pass without the server sending another packet (first telnet response packet)
- the client eventually sends FIN to close the connection
- server acks the packet immediately at the TCP level
- some time later the server advertises TCP ZeroWindow, FIN ACK
- client rejects (RST) these packets
- server logs "Dec 20 18:04:22 alicante telnetd[4756]: getpid: peer died: Error 0" in syslog.log.

After reading many forum threads, I wonder if raising tcp_recv_hiwater_def could help.
Nevertheless, I have some existential questions:
1) How is it possible that the socket just opened by the new telnetd process can advertise TCP ZeroWindow if only a few bytes have been received?
2) What is the server doing during those minutes?

5 REPLIES
Laurent Menase
Honored Contributor

Re: TCP ZeroWindow in telnet connections

update xport patch, it is a very old bug fixed in a very old patch
Jose M. del Rio
Frequent Advisor

Re: TCP ZeroWindow in telnet connections

Do you mean latest ARPA patch (PHNE_39386)?
We already have it.
Laurent Menase
Honored Contributor

Re: TCP ZeroWindow in telnet connections

indeed I didn't read precisely your question,
and I thought about zero window replied to syn.


looks like you have a firewall between the 2 systems.

windows is closed since peer as sent a fin
Laurent Menase
Honored Contributor

Re: TCP ZeroWindow in telnet connections

you should take a trace from the 2 side
Jose M. del Rio
Frequent Advisor

Re: TCP ZeroWindow in telnet connections

It's a reverse DNS resolution issue: telnetd is trying to resolve IP -> name for IPs it doesn't know about and that process is taking longer than the telnet client is willing to wait for, so it closes the connection.