SCMCREDENTIALS

SCM_CREDENTIALS, often referenced in discussions as SCMCREDENTIALS, is a control message type used with UNIX-domain sockets to convey the credentials of the sending process to the receiver. It serves as a means for applications to identify the peer that transmitted a message, aiding authentication, auditing, and access-control decisions.

In Linux and similar systems, credentials are carried in a control message with level SOL_SOCKET and type SCM_CREDENTIALS. The payload is typically a structure containing the sender’s process ID, user ID, and group ID (for example, a struct ucred with pid, uid, and gid). When assembling or parsing such messages, developers use the standard ancillary data macros (for example, CMSG_SPACE and CMSG_LEN) to allocate space and cmsghdr pointers to access the credential data. Some applications also retrieve peer credentials on connected sockets using SO_PEERCRED, which yields the same set of fields.

Platform variations exist. Linux defines the ucred structure and the SCM_CREDENTIALS control message, while other operating systems may implement similar functionality under different names or data layouts (for instance, BSD variants use a related mechanism with a different payload). As a result, code that relies on SCM_CREDENTIALS is not strictly portable across all platforms without adaptation.

Security considerations are important when using credential passing. Exposing or trusting peer credentials requires careful handling within the established UNIX-domain socket trust model. Applications should validate credentials before making authorization decisions and combine credential information with additional authentication mechanisms as appropriate.