[Bug 1608965] Re: ssh GSSAPI rekey failure

[Launchpad Bug Tracker]

Status changed to 'Confirmed' because the bug affects multiple users.

** Changed in: openssh (Ubuntu)
       Status: New => Confirmed

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https://bugs.launchpad.net/bugs/1608965

Title:
  ssh GSSAPI rekey failure

Status in openssh package in Ubuntu:
  Confirmed

Bug description:
  If I have ssh set up using GSSAPI with rekeying enabled, then the
  connection fails on rekey, and tries to do host-based verification
  'mid-session'.

  Steps to reproduce:

  $ ssh -vvv server.example.com
  <snip...>
  debug1: Authenticating to ssh.example.com:22 as 'user'
  <snip...>
  debug2: local client KEXINIT proposal
  debug2: KEX algorithms: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-gex-sha1-A/vxljAEU54gt9a48EiANQ==,gss-group1-sha1-A/vxljAEU54gt9a48EiANQ==,gss-group14-sha1-A/vxljAEU54gt9a48EiANQ==,gss-gex-sha1-bontcUwnM6aGfWCP21alxQ==,gss-group1-sha1-bontcUwnM6aGfWCP21alxQ==,gss-group14-sha1-bontcUwnM6aGfWCP21alxQ==,gss-gex-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-group1-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-group14-sha1-eipGX3TCiQSrx573bT1o1Q==,curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,ext-info-c
  <snip...>
  debug2: peer server KEXINIT proposal
  debug2: KEX algorithms: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==,curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
  <snip...>
  Last login: Tue Aug 02 10:47:20 2016 from foo

  # Then do 'kinit' on the client to get a new ticket...

  debug1: need rekeying
  debug1: SSH2_MSG_KEXINIT sent
  debug1: rekeying in progress
  debug1: SSH2_MSG_KEXINIT received
  debug2: local client KEXINIT proposal
  debug2: KEX algorithms: curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1
  debug2: host key algorithms: ecdsa-sha2-nistp256-cert-v01 at openssh.com,ecdsa-sha2-nistp384-cert-v01 at openssh.com,ecdsa-sha2-nistp521-cert-v01 at openssh.com,ssh-ed25519-cert-v01 at openssh.com,ssh-rsa-cert-v01 at openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa,null
  [...]
  debug2: peer server KEXINIT proposal
  debug2: KEX algorithms: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==,curve25519-sha256 at libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
  [...]
  debug1: kex: algorithm: curve25519-sha256 at libssh.org
  debug1: kex: host key algorithm: ecdsa-sha2-nistp256
  debug1: kex: server->client cipher: chacha20-poly1305 at openssh.com MAC: <implicit> compression: none
  debug1: kex: client->server cipher: chacha20-poly1305 at openssh.com MAC: <implicit> compression: none
  debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
  debug1: rekeying in progress
  debug1: rekeying in progress
  debug1: Server host key: ecdsa-sha2-nistp256 SHA256:w7yxbCZNBX4d5EAgmCrFYa3XUpDjvWiDOw4/YOY9q8E
  The authenticity of host 'server.example.com (10.0.0.1)' can't be established.
  ECDSA key fingerprint is SHA256:w7yxbCZNBX4d5EAgmCrFYa3XUpDjvWiDOw4/YOY9q8E.
  Are you sure you want to continue connecting (yes/no)? 
  Host key verification failed.

  It looks like the list of KEX algorithms differs between the initial
  connection, and the rekeying.

  This behaviour seems to occur with a client running 16.04 (openssh-
  client 1:7.2p2-4ubuntu1) but not on 15.10 (openssh-client
  1:6.9p1-2ubuntu0.2).

  ssh_config is as follows:

  HashKnownHosts no
  GSSAPIAuthentication yes
  GSSAPIDelegateCredentials yes
  GSSAPIRenewalForcesRekey yes
  GSSAPITrustDNS yes
  GSSAPIKeyExchange yes
  ForwardX11 yes
  ForwardX11Trusted yes

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