what does pam_unix : bad username mean?

Question

Can anybody please tell me what does this error mean.And when will this appear. I tried googling, but with no result. Please point me to the correct document to understand this error.

Answer 1

This error is triggered by the following code in the pam_unix module (source: Linux-PAM-0.99.5):

if (name == NULL || !isalnum(*name))

the name == NULL would be triggered by a programmatic error in the use of the PAM protocol where the username variable is not being set as part of the pam conversation.

The second reason, and probably the most interesting is that the first character of the username is required to be an alpha-num i.e the character must be one of A-Z, a-z or 0-9. Accented characters are not accepted.

A newer version of Linux-PAM (as seen from the fedorahosted source of pam_unix.c) says:

if (name == NULL || name[0] == '-' || name[0] == '+')

Which means that it only rejects the - and + characters - i.e. it is less strict than the older source.