[Bug 252449] telnet(1) regression or incompatibility in recent 12.2-STABLE

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252449

            Bug ID: 252449
           Summary: telnet(1) regression or incompatibility in recent
                    12.2-STABLE
           Product: Base System
           Version: 12.2-STABLE
          Hardware: arm64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: zarychtam at plan-b.pwste.edu.pl

Created attachment 221307
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=221307&action=edit
Traffic dumps

I have recently upgraded a few systems running 12.2-STABLE and after upgrading
one by one I am losing the ability to connect with some Alcatel-Lucent switches
(Omniswitch 63xx and 64xx models). Connection with the host seems to be
established, but I am not able to get the login prompt:
telnet 172.x.x.x
Trying 172.x.x.x...
Connected to 172.x.x.x.
Escape character is '^]'.

^]
This state last forever and the session can be only terminated.

Telneting to other devices, from other vendors or even some different ALU
switches, still goes fine.

Since I was upgrading systems one by one and losing connectivity, I took some
tcpdumps to document this oddity.

Perhaps the culprit is TCP stack or its settings, which are the same on all
machines:

net.inet.tcp.ecn.enable=1
net.inet.tcp.nolocaltimewait=1
net.inet.tcp.mssdflt=1448
net.inet.tcp.syncache.rexmtlimit=1 
net.inet.tcp.drop_synfin=1
net.inet.tcp.rfc6675_pipe=1
net.inet.tcp.abc_l_var=44
net.inet.tcp.functions_default=rack
net.inet.tcp.cc.algorithm=htcp
net.inet.tcp.cc.htcp.rtt_scaling=1
net.inet.tcp.cc.htcp.adaptive_backoff=1
net.inet.tcp.fastopen.server_enable=1

When the system is booted from kernel.old telnet from new userland and above
TCP stack settings is able to connect again.

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