Debug1

Debug1 is a command-line utility used primarily in Unix-like operating systems to facilitate debugging of network-related protocols, particularly those operating over TCP/IP. It is part of the sftp-server and sshd packages and is commonly employed to monitor and analyze network connections, especially during troubleshooting sessions.

Originally developed as a tool for debugging SSH (Secure Shell) and SFTP (SSH File Transfer Protocol) sessions, Debug1 provides detailed logging of protocol interactions. When enabled, it outputs verbose information about the communication between client and server, including handshake processes, authentication attempts, and data transfers. This can be particularly useful for identifying issues such as connection failures, authentication errors, or protocol mismatches.

Debug1 is invoked automatically by certain services, such as OpenSSH, when the debugging flag is set in configuration files. For example, in the SSH daemon configuration file (`sshd_config`), the `LogLevel` directive can be adjusted to include `VERBOSE` or `DEBUG1` to enable detailed logging. Similarly, SFTP sessions may generate Debug1 output when the server is run with debugging enabled.

The output from Debug1 is typically directed to system logs or a specified file, making it accessible for review. While useful for administrators, the verbose nature of Debug1 can generate large log files, which may require careful management to avoid filling up system storage. Additionally, exposing Debug1 output in production environments should be done cautiously, as it may inadvertently reveal sensitive information.

Debug1 is not limited to SSH and SFTP; it can also be used in other network debugging scenarios where protocol-level details are necessary. However, for broader network debugging, tools like Wireshark or tcpdump may be more appropriate. Debug1 remains a valuable resource for administrators seeking to diagnose issues at the protocol level.