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SSH_OPEN_UNKNOWN_CHANNEL_TYPE

Channel FailureRFC 4254 §5.1

SSH_OPEN_UNKNOWN_CHANNEL_TYPE (3) is sent when the client requests a channel type that the server does not recognize or implement. Standard channel types include session, direct-tcpip, and forwarded-tcpip. A request for a non-standard or mistyped channel type returns this code.

Code

3

Category

Channel Failure

Standard

RFC 4254 §5.1

Description

SSH channel types are identified by string names. The standard types defined in RFC 4254 are: session (for interactive shells and commands), direct-tcpip (local port forwarding), and forwarded-tcpip (remote port forwarding). RFC 4254 also defines x11 and direct-streamlocal/forwarded-streamlocal for Unix socket forwarding.

If a client sends SSH_MSG_CHANNEL_OPEN with a channel type string the server does not recognize – due to a typo, a non-standard extension, or an implementation mismatch – the server responds with code 3.

Code 3 is distinct from code 1 (administratively prohibited): code 3 means the server does not know what this channel type is at all, while code 1 means the server knows but policy prevents it.

Common Causes

  • –Typo or incorrect channel type string in SSH client
  • –Non-standard SSH extension channel type not supported by server
  • –Client and server SSH implementation mismatch for extended channel types
  • –direct-streamlocal channel on a server that predates Unix socket forwarding support
RFC 4254 – SSH specification →