title: Connection closed by UNKNOWN port 65535 when attempting SSH in Corporate Proxy
body: <p>I've been trying to set up a SSH jump server in order to connect to my servers when I'm at work, which is employing a really annoying (and slow) corporate proxy and blocking everything going to the outside besides port 443 and 80.</p>
  <p>I've set up a test server, which I will use as a jump server if this works, to accept SSH connection on port 443.</p>
  <p>This is how I'm trying to connect</p>
  <pre><code>ssh -vvv ubuntu@jump-ssh-server -p 443 -o ProxyCommand=&quot;ncat --proxy proxy_server:3128 --proxy-auth proxy_user:proxy_password -C %h %p&quot;
  </code></pre>
  <p>If I try to run only the <code>ProxyCommand</code> in the terminal it works. However, when I try to run it with the ssh connection I get an error</p>
  <pre><code>OpenSSH_8.6p1, LibreSSL 3.3.5
  debug1: Reading configuration data /Users/user/.ssh/config
  debug1: Reading configuration data /etc/ssh/ssh_config
  debug1: /etc/ssh/ssh_config line 21: include /etc/ssh/ssh_config.d/* matched no files
  debug1: /etc/ssh/ssh_config line 54: Applying options for *
  debug2: resolve_canonicalize: hostname jump_server is address
  debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' -&gt; '/Users/user/.ssh/known_hosts'
  debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' -&gt; '/Users/user/.ssh/known_hosts2'
  debug1: Authenticator provider $SSH_SK_PROVIDER did not resolve; disabling
  debug1: Executing proxy command: exec ncat --proxy proxy_server:3128 --proxy-auth user:password -C jump_server 443
  debug1: identity file /Users/user/.ssh/id_rsa type 0
  debug1: identity file /Users/user/.ssh/id_rsa-cert type -1
  debug1: identity file /Users/user/.ssh/id_dsa type -1
  debug1: identity file /Users/user/.ssh/id_dsa-cert type -1
  debug1: identity file /Users/user/.ssh/id_ecdsa type -1
  debug1: identity file /Users/user/.ssh/id_ecdsa-cert type -1
  debug1: identity file /Users/user/.ssh/id_ecdsa_sk type -1
  debug1: identity file /Users/user/.ssh/id_ecdsa_sk-cert type -1
  debug1: identity file /Users/user/.ssh/id_ed25519 type -1
  debug1: identity file /Users/user/.ssh/id_ed25519-cert type -1
  debug1: identity file /Users/user/.ssh/id_ed25519_sk type -1
  debug1: identity file /Users/user/.ssh/id_ed25519_sk-cert type -1
  debug1: identity file /Users/user/.ssh/id_xmss type -1
  debug1: identity file /Users/user/.ssh/id_xmss-cert type -1
  debug1: Local version string SSH-2.0-OpenSSH_8.6
  debug1: Remote protocol version 2.0, remote software version OpenSSH_8.2p1 Ubuntu-4ubuntu0.5
  debug1: compat_banner: match: OpenSSH_8.2p1 Ubuntu-4ubuntu0.5 pat OpenSSH* compat 0x04000000
  debug2: fd 5 setting O_NONBLOCK
  debug2: fd 4 setting O_NONBLOCK
  debug1: Authenticating to jump_server:443 as 'ubuntu'
  debug3: put_host_port: [jump_server]:443
  debug1: load_hostkeys: fopen /Users/user/.ssh/known_hosts2: No such file or directory
  debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
  debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
  debug3: order_hostkeyalgs: no algorithms matched; accept original
  debug3: send packet: type 20
  debug1: SSH2_MSG_KEXINIT sent
  debug3: receive packet: type 20
  debug1: SSH2_MSG_KEXINIT received
  debug1: kex: algorithm: curve25519-sha256
  debug1: kex: host key algorithm: ssh-ed25519
  debug1: kex: server-&gt;client cipher: chacha20-poly1305@openssh.com MAC: &lt;implicit&gt; compression: none
  debug1: kex: client-&gt;server cipher: chacha20-poly1305@openssh.com MAC: &lt;implicit&gt; compression: none
  debug3: send packet: type 30
  debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
  debug3: receive packet: type 3
  debug1: Received SSH2_MSG_UNIMPLEMENTED for 0
  Connection closed by UNKNOWN port 65535
  </code></pre>
  <p>Is this caused by my corporate firewall trying to block the connection? Is there any way around it?</p>