SSH jump host fails for leaf cluster in separate port mode

[atburke]

Expected behavior:
ssh -J <leaf-proxy-addr> <leaf-node> successfully connects to the leaf node.

Current behavior:
ssh -J <leaf-proxy-addr> <leaf-node> fails with error channel 0: bad ext data. Leaf node reports that it cannot get the remote site for the root cluster.

tsh ssh -J <leaf-proxy-addr> <leaf-node> in separate port mode and tsh proxy ssh -J <leaf-proxy-addr> <leaf-node> in multiplex mode both work correctly.

Bug details:

• Teleport version
• Recreation steps
  • Create a root and leaf cluster, both with auth_service.proxy_listener_mode: separate
  • Log in to the leaf cluster via the root cluster in tsh
  • Attempt to ssh into a leaf node using the leaf proxy as a jump host
• Debug logs

2024-05-30T16:21:04-07:00 DEBU [PROXY]     Successfully authenticated fingerprint:ssh-rsa-cert-v01@openssh.com SHA256:D+kXyt99DFGCI66SR6vdPITUAWr5CbrUca1LephbMDs local:127.0.0.1:4023 remote:127.0.0.1:42268 user:andrew srv/authhandlers.go:433
2024-05-30T16:21:04-07:00 DEBU [SSH:PROXY] Incoming connection 127.0.0.1:42268 -> 127.0.0.1:4023 version: SSH-2.0-OpenSSH_8.9p1 Ubuntu-3ubuntu0.7, certtype: "user" sshutils/server.go:548
2024-05-30T16:21:04-07:00 DEBU             Mapped remote roles [editor access] to local roles [editor access] and traits map[aws_role_arns:[] azure_identities:[] db_names:[] db_roles:[] db_users:[] gcp_service_accounts:[] host_user_gid:[] host_user_uid:[] kubernetes_groups:[] kubernetes_users:[] logins:[andrew root -teleport-internal-join] windows_logins:[]]. services/access_checker.go:1175
2024-05-30T16:21:04-07:00 DEBU             Mapped remote roles [editor access] to local roles [editor access] and traits map[aws_role_arns:[] azure_identities:[] db_names:[] db_roles:[] db_users:[] gcp_service_accounts:[] host_user_gid:[] host_user_uid:[] kubernetes_groups:[] kubernetes_users:[] logins:[andrew root -teleport-internal-join] windows_logins:[]]. services/access_checker.go:1175
2024-05-30T16:21:04-07:00 DEBU [NODE]      Proxy subsystem: routing user "andrew" to cluster "main" based on the route to cluster extension. regular/proxy.go:169
2024-05-30T16:21:04-07:00 ERRO             Unable instantiate proxy subsystem: invalid format for proxyrequest: unknown cluster "main". regular/sshserver.go:2222

full debug logs from leaf auth/proxy/node