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OpenSSH: cause of random kex_exchange_identification errors?

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When I want to connect with SSH (ssh/scp) to some machine, I sometimes
get errors, either

kex_exchange_identification: Connection closed by remote host

or

kex_exchange_identification: read: Connection reset by peer

immediately after the connection attempt. This happens randomly,
and there are some periods where this happens quite often. The
client machine doesn't seem to matter, and this issue also even
occurs from machines on the local network.

With ssh -vvv, the output ends with

debug1: Local version string SSH-2.0-OpenSSH_8.7p1 Debian-4
kex_exchange_identification: read: Connection reset by peer
Connection reset by [...] port 22

In the source, this corresponds to function kex_exchange_identification
in kex.c:

    len = atomicio(read, ssh_packet_get_connection_in(ssh),
        &c, 1);
    if (len != 1 && errno == EPIPE) {
            error_f("Connection closed by remote host");
            r = SSH_ERR_CONN_CLOSED;
            goto out;
    } else if (len != 1) {
            oerrno = errno;
            error_f("read: %.100s", strerror(errno));
            r = SSH_ERR_SYSTEM_ERROR;
            goto out;
    }

so either with EPIPE or with ECONNRESET, and this apparently occurs
before the exchange of banners.

I could reproduce the issue with telnet, which gives

[...]
Escape character is '^]'.
Connection closed by foreign host.

while one normally has

SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u2

just after the "Escape character..." line.

Note that this is different from a "Connection refused". Here, the
connection is accepted, but immediately closed.

The admin of the machine could see nothing particular in the logs.
He eventually modified the MaxStartups value, but this did not
solve the issue (but AFAIK, if this were the cause, there would
have been something about it in the logs). The machine has enough
available memory.

Any idea about the possible cause of these random errors?

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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Thread

OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-02 15:50 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? Hans <hans.ullrich@loop.de> - 2022-02-02 16:20 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-02 16:30 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? Bijan Soleymani <bijan@psq.com> - 2022-02-02 17:50 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? David Wright <deblis@lionunicorn.co.uk> - 2022-02-02 18:00 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? gene heskett <gheskett@shentel.net> - 2022-02-02 20:30 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Greg Wooledge <greg@wooledge.org> - 2022-02-02 20:30 +0100
      Reusing ssh keys on a new installation, was Re: OpenSSH: cause of  random kex_exchange_identification errors? David Wright <deblis@lionunicorn.co.uk> - 2022-02-02 22:00 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-03 04:20 +0100
  Re: OpenSSH: cause of random kex_exchange_identification errors? Henrique de Moraes Holschuh <hmh@debian.org> - 2022-02-05 22:40 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-02-08 11:20 +0100
    Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-07 18:00 +0200
      Re: OpenSSH: cause of random kex_exchange_identification errors? Tim Woodall <debianuser@woodall.me.uk> - 2022-06-07 18:20 +0200
        Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-14 13:00 +0200
          Re: OpenSSH: cause of random kex_exchange_identification errors? Tim Woodall <debianuser@woodall.me.uk> - 2022-06-14 20:20 +0200
            Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-15 03:50 +0200
              Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-15 15:20 +0200
                Re: OpenSSH: cause of random kex_exchange_identification errors? Vincent Lefevre <vincent@vinc17.net> - 2022-06-15 16:00 +0200

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