What differences are between ways to prevent a user from logging in, by passwd and shadow files?












1














From https://unix.stackexchange.com/a/314732/674




In contrast to user accounts, service accounts often don't have a "proper" login shell, i.e. they have /usr/sbin/nologin as login shell (or, back in the old days, /bin/false). Moreover, service accounts are typically locked, i.e. it is not possible to login (for traditional /etc/passwd and /etc/shadow this can be achieved by setting the password hash to arbitrary values such as * or x).




It seems to give several ways to prevent a user from login.




  • the shell field of /etc/passwd: nologin, /bin/false, /bin/true,/dev/null`;


  • the password field of /etc/shadow: * or x



What are the differences between their purposes? Do they all achieve the same purpose?



Thanks.






linux users







share|improve this question






















  • If the purpose is to prevent the user from logging in then yes, they all achieve the same purpose.
    – Jesse_b
    4 hours ago
















1














From https://unix.stackexchange.com/a/314732/674




In contrast to user accounts, service accounts often don't have a "proper" login shell, i.e. they have /usr/sbin/nologin as login shell (or, back in the old days, /bin/false). Moreover, service accounts are typically locked, i.e. it is not possible to login (for traditional /etc/passwd and /etc/shadow this can be achieved by setting the password hash to arbitrary values such as * or x).




It seems to give several ways to prevent a user from login.




  • the shell field of /etc/passwd: nologin, /bin/false, /bin/true,/dev/null`;


  • the password field of /etc/shadow: * or x



What are the differences between their purposes? Do they all achieve the same purpose?



Thanks.






linux users







share|improve this question






















  • If the purpose is to prevent the user from logging in then yes, they all achieve the same purpose.
    – Jesse_b
    4 hours ago














1












1








1







From https://unix.stackexchange.com/a/314732/674




In contrast to user accounts, service accounts often don't have a "proper" login shell, i.e. they have /usr/sbin/nologin as login shell (or, back in the old days, /bin/false). Moreover, service accounts are typically locked, i.e. it is not possible to login (for traditional /etc/passwd and /etc/shadow this can be achieved by setting the password hash to arbitrary values such as * or x).




It seems to give several ways to prevent a user from login.




  • the shell field of /etc/passwd: nologin, /bin/false, /bin/true,/dev/null`;


  • the password field of /etc/shadow: * or x



What are the differences between their purposes? Do they all achieve the same purpose?



Thanks.






linux users







share|improve this question













From https://unix.stackexchange.com/a/314732/674




In contrast to user accounts, service accounts often don't have a "proper" login shell, i.e. they have /usr/sbin/nologin as login shell (or, back in the old days, /bin/false). Moreover, service accounts are typically locked, i.e. it is not possible to login (for traditional /etc/passwd and /etc/shadow this can be achieved by setting the password hash to arbitrary values such as * or x).




It seems to give several ways to prevent a user from login.




  • the shell field of /etc/passwd: nologin, /bin/false, /bin/true,/dev/null`;


  • the password field of /etc/shadow: * or x



What are the differences between their purposes? Do they all achieve the same purpose?



Thanks.






linux users




linux users






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 4 hours ago









Tim

26.1k74246455




26.1k74246455












  • If the purpose is to prevent the user from logging in then yes, they all achieve the same purpose.
    – Jesse_b
    4 hours ago


















  • If the purpose is to prevent the user from logging in then yes, they all achieve the same purpose.
    – Jesse_b
    4 hours ago
















If the purpose is to prevent the user from logging in then yes, they all achieve the same purpose.
– Jesse_b
4 hours ago




If the purpose is to prevent the user from logging in then yes, they all achieve the same purpose.
– Jesse_b
4 hours ago










2 Answers
2






active

oldest

votes


















2














They are somewhat different. A user with a “bad” password hash may still log in if something allows him to log in without a password, for example with SSH and an authorized key. But that’s impossible without a valid shell.



On the other hand, there are some situations were a user may be granted some access without a shell (I’m thinking about an IMAP or SMTP server). Then, a user with a “good” password hash may use the service, even is he has no valid login shell.






share|improve this answer





























    1














    Just because there are multiple ways to do things, does not mean there was a real purpose as to why. There are some minor differences between system users and normal users.



    /usr/sbin/nologin is a simple "shell" that was designed to deny access by exiting immediately. Any program can be run as a shell (as long as it is listed in /etc/shells), and if the program simply exits right away then the user will have no access to the system. This is why something like /bin/false also works.



    Using an asterisk for a password in the shadow file is just how linux represents a locked account password. If you run passwd -l username as root, it will lock the account using this method.



    Another method is to expire the account by setting the expire date to 1. This can be done by running this:



    usermod -L -e 1 username





    share|improve this answer










    New contributor




    Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.














    • 1




      Note for any program to be used as a shell, it must be listed in /etc/shells
      – ivanivan
      3 hours ago










    • @ivanivan updated post to inlude this, thanks
      – Jeff A
      2 hours ago










    • You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
      – Stephen Harris
      2 hours ago










    • Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
      – Stephen Harris
      2 hours ago










    • @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
      – ivanivan
      1 hour ago











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    2 Answers
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    2 Answers
    2






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    active

    oldest

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    active

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    2














    They are somewhat different. A user with a “bad” password hash may still log in if something allows him to log in without a password, for example with SSH and an authorized key. But that’s impossible without a valid shell.



    On the other hand, there are some situations were a user may be granted some access without a shell (I’m thinking about an IMAP or SMTP server). Then, a user with a “good” password hash may use the service, even is he has no valid login shell.






    share|improve this answer


























      2














      They are somewhat different. A user with a “bad” password hash may still log in if something allows him to log in without a password, for example with SSH and an authorized key. But that’s impossible without a valid shell.



      On the other hand, there are some situations were a user may be granted some access without a shell (I’m thinking about an IMAP or SMTP server). Then, a user with a “good” password hash may use the service, even is he has no valid login shell.






      share|improve this answer
























        2












        2








        2






        They are somewhat different. A user with a “bad” password hash may still log in if something allows him to log in without a password, for example with SSH and an authorized key. But that’s impossible without a valid shell.



        On the other hand, there are some situations were a user may be granted some access without a shell (I’m thinking about an IMAP or SMTP server). Then, a user with a “good” password hash may use the service, even is he has no valid login shell.






        share|improve this answer












        They are somewhat different. A user with a “bad” password hash may still log in if something allows him to log in without a password, for example with SSH and an authorized key. But that’s impossible without a valid shell.



        On the other hand, there are some situations were a user may be granted some access without a shell (I’m thinking about an IMAP or SMTP server). Then, a user with a “good” password hash may use the service, even is he has no valid login shell.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 4 hours ago









        user2233709

        663310




        663310

























            1














            Just because there are multiple ways to do things, does not mean there was a real purpose as to why. There are some minor differences between system users and normal users.



            /usr/sbin/nologin is a simple "shell" that was designed to deny access by exiting immediately. Any program can be run as a shell (as long as it is listed in /etc/shells), and if the program simply exits right away then the user will have no access to the system. This is why something like /bin/false also works.



            Using an asterisk for a password in the shadow file is just how linux represents a locked account password. If you run passwd -l username as root, it will lock the account using this method.



            Another method is to expire the account by setting the expire date to 1. This can be done by running this:



            usermod -L -e 1 username





            share|improve this answer










            New contributor




            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.














            • 1




              Note for any program to be used as a shell, it must be listed in /etc/shells
              – ivanivan
              3 hours ago










            • @ivanivan updated post to inlude this, thanks
              – Jeff A
              2 hours ago










            • You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
              – Stephen Harris
              2 hours ago










            • Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
              – Stephen Harris
              2 hours ago










            • @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
              – ivanivan
              1 hour ago
















            1














            Just because there are multiple ways to do things, does not mean there was a real purpose as to why. There are some minor differences between system users and normal users.



            /usr/sbin/nologin is a simple "shell" that was designed to deny access by exiting immediately. Any program can be run as a shell (as long as it is listed in /etc/shells), and if the program simply exits right away then the user will have no access to the system. This is why something like /bin/false also works.



            Using an asterisk for a password in the shadow file is just how linux represents a locked account password. If you run passwd -l username as root, it will lock the account using this method.



            Another method is to expire the account by setting the expire date to 1. This can be done by running this:



            usermod -L -e 1 username





            share|improve this answer










            New contributor




            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.














            • 1




              Note for any program to be used as a shell, it must be listed in /etc/shells
              – ivanivan
              3 hours ago










            • @ivanivan updated post to inlude this, thanks
              – Jeff A
              2 hours ago










            • You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
              – Stephen Harris
              2 hours ago










            • Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
              – Stephen Harris
              2 hours ago










            • @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
              – ivanivan
              1 hour ago














            1












            1








            1






            Just because there are multiple ways to do things, does not mean there was a real purpose as to why. There are some minor differences between system users and normal users.



            /usr/sbin/nologin is a simple "shell" that was designed to deny access by exiting immediately. Any program can be run as a shell (as long as it is listed in /etc/shells), and if the program simply exits right away then the user will have no access to the system. This is why something like /bin/false also works.



            Using an asterisk for a password in the shadow file is just how linux represents a locked account password. If you run passwd -l username as root, it will lock the account using this method.



            Another method is to expire the account by setting the expire date to 1. This can be done by running this:



            usermod -L -e 1 username





            share|improve this answer










            New contributor




            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.









            Just because there are multiple ways to do things, does not mean there was a real purpose as to why. There are some minor differences between system users and normal users.



            /usr/sbin/nologin is a simple "shell" that was designed to deny access by exiting immediately. Any program can be run as a shell (as long as it is listed in /etc/shells), and if the program simply exits right away then the user will have no access to the system. This is why something like /bin/false also works.



            Using an asterisk for a password in the shadow file is just how linux represents a locked account password. If you run passwd -l username as root, it will lock the account using this method.



            Another method is to expire the account by setting the expire date to 1. This can be done by running this:



            usermod -L -e 1 username






            share|improve this answer










            New contributor




            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.









            share|improve this answer



            share|improve this answer








            edited 2 hours ago





















            New contributor




            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.









            answered 4 hours ago









            Jeff A

            3114




            3114




            New contributor




            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.





            New contributor





            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.






            Jeff A is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.








            • 1




              Note for any program to be used as a shell, it must be listed in /etc/shells
              – ivanivan
              3 hours ago










            • @ivanivan updated post to inlude this, thanks
              – Jeff A
              2 hours ago










            • You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
              – Stephen Harris
              2 hours ago










            • Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
              – Stephen Harris
              2 hours ago










            • @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
              – ivanivan
              1 hour ago














            • 1




              Note for any program to be used as a shell, it must be listed in /etc/shells
              – ivanivan
              3 hours ago










            • @ivanivan updated post to inlude this, thanks
              – Jeff A
              2 hours ago










            • You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
              – Stephen Harris
              2 hours ago










            • Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
              – Stephen Harris
              2 hours ago










            • @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
              – ivanivan
              1 hour ago








            1




            1




            Note for any program to be used as a shell, it must be listed in /etc/shells
            – ivanivan
            3 hours ago




            Note for any program to be used as a shell, it must be listed in /etc/shells
            – ivanivan
            3 hours ago












            @ivanivan updated post to inlude this, thanks
            – Jeff A
            2 hours ago




            @ivanivan updated post to inlude this, thanks
            – Jeff A
            2 hours ago












            You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
            – Stephen Harris
            2 hours ago




            You do not need to list a shell in /etc/shells for it to be used. Some applications may look in there and not present it as an option, but /usr/local/bin/foobarbaz can be used as a login shell without it being listed. Indeed, there are use cases where you don't want it; eg some FTP servers will only permit access if the login shell is listed, so putting nologin in /etc/shells may prevent ssh access but permit FTP access!
            – Stephen Harris
            2 hours ago












            Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
            – Stephen Harris
            2 hours ago




            Note that * in the crypt field means "no valid password". An ! means locked; from man 5 shadow... A password field which starts with a exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked
            – Stephen Harris
            2 hours ago












            @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
            – ivanivan
            1 hour ago




            @StephenHarris I guess I should've specified - shell must be in /etc/shells if you change to it using chsh. If the root user is simply editing /etc/passwd then you are correct, doesn't need to be in the shells file.
            – ivanivan
            1 hour ago


















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