UNIX System Information

Last modified Tuesday, 15-Jul-2025 06:56:55 UTC.

Passwords
- User information stored in the /etc/passwd file.
  - There is one entry per line, and each line has the format:
```
account:password:UID:GID:GECOS:directory:shell
               
```
    The field descriptions are:
    - account - The name of the user on the system. It should not contain capital letters.
    - password - The encrypted user password or a star.
    - UID - The numerical user ID.
    - GID - The numerical primary group ID for this user.
    - GECOS - This field is optional and only used for informational purposes. Usually, it contains the full user name.
    - directory - The user's $HOME directory.
    - shell - The program to run at login (if empty, use /bin/sh).
  - /etc/passwd should have general read permission (many utilities, like ls(1) use it to map user IDs to user names), but write access only for the superuser.
  - In the good old days there was no great problem with this general read permission. Everybody could read the encrypted passwords, but the hardware was too slow to crack a well-chosen password, and moreover, the basic assumption used to be that of a friendly user-community. These days many people run some version of the shadow password suite, where /etc/passwd has *'s instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable by the superuser only.
- Functions to manipulate /etc/passwd are in pwd.h and require sys/types.h.
  - struct passwd *getpwent(void); - Returns a pointer to a structure containing the broken out fields of a line from /etc/passwd. The first time it is called it returns the first entry; thereafter, it returns successive entries, and NULL when there are no more or an error.
```
struct passwd {
    char    *pw_name;       /* user name */
    char    *pw_passwd;     /* user password */
    uid_t   pw_uid;         /* user id */
    gid_t   pw_gid;         /* group id */
    char    *pw_gecos;      /* real name */
    char    *pw_dir;        /* home directory */
    char    *pw_shell;      /* shell program */
}; 
```
  - int setpwent(int stayopen); - Rewinds the file pointer to the beginning of the /etc/passwd file.
  - void endpwent(void); - Closes the /etc/passwd file.
  - struct passwd *fgetpwent(FILE *stream); - Returns a pointer to a structure containing the broken out fields of a line in the file stream.
  - struct passwd *getpwnam(const char * name); - Returns a pointer to a structure containing the broken out fields of a line from /etc/passwd for the entry that matches the user name.
  - struct passwd *getpwuid(uid_t uid); - Returns a pointer to a structure containing the broken out fields of a line from /etc/passwd for the entry that matches the UID.
  - int getpw(uid_t uid, char *buffer); - Constructs the password line entry for the given user uid in the buffer.
  - PasswdEntries.c - Program that retrieves passwd entries
- Password authentication functions
  - char *getpass(const char * prompt); - Displays a prompt to the standard error output, and reads in a password without echo. The calling process should zero the password as soon as possible to avoid leaving the cleartext password visible in the process's address space.
  - char *crypt(const char *key, const char *salt); - The password encryption function. key is a user's typed password. salt is a two-character string chosen from the set [a-zA-Z0-9./]. This string is used to perturb the algorithm in one of 4096 different ways. Requires linking libcrypt.a.
  - PasswdVerify.c - Program to read and encrypt a password.
System Information
- SysInfo.c - Program that extracts host information

Exercises

Exam Style Questions

What user information stored in the /etc/passwd file?

#include <stdio.h>
#include <sys/types.h>
#include <pwd.h>

int main(int argc,char *argv[]) {

    struct passwd *P;

    while ((P = getpwent()) != NULL) {
        if (P->pw_uid == geteuid()) {
            printf("%s\n",P->pw_name);
        }
    }
    return(0);
}

What system call is designed for reading a password from a user?
After a call to uname, what information is stored in the struct utsname pointed to be the parameter?
What system call can be used to get the IP address of a machine?