How does the “su” command work internally?

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I am tying to understand how the su command work internally.



Say that I did the following:



su someUser


The following is what I think will happen:




  • su will ask me for the password of someUser.


  • su will hash the entered password.


  • su will compare the entered password with the password in the
    /etc/shadow file, and if the two passwords are the same, then su will
    launch the default shell for someUser (which exist in
    /etc/passwd), and su will set the user IDs and group IDs of the launched shell to the user IDs and group IDs of someUser.

Note that su has the setuid bit set, so su will run with root privileges.



Am I correct?




linux process su




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  • Yes, but su doesn't directly access /etc/shadow, it uses pam for this. ( /etc/pam.d/su )
    – Ipor Sircer
    Dec 6 '17 at 1:08














up vote
1
down vote

favorite












I am tying to understand how the su command work internally.



Say that I did the following:



su someUser


The following is what I think will happen:




  • su will ask me for the password of someUser.


  • su will hash the entered password.


  • su will compare the entered password with the password in the
    /etc/shadow file, and if the two passwords are the same, then su will
    launch the default shell for someUser (which exist in
    /etc/passwd), and su will set the user IDs and group IDs of the launched shell to the user IDs and group IDs of someUser.

Note that su has the setuid bit set, so su will run with root privileges.



Am I correct?




linux process su




share|improve this question






















  • Yes, but su doesn't directly access /etc/shadow, it uses pam for this. ( /etc/pam.d/su )
    – Ipor Sircer
    Dec 6 '17 at 1:08












up vote
1
down vote

favorite









up vote
1
down vote

favorite











I am tying to understand how the su command work internally.



Say that I did the following:



su someUser


The following is what I think will happen:




  • su will ask me for the password of someUser.


  • su will hash the entered password.


  • su will compare the entered password with the password in the
    /etc/shadow file, and if the two passwords are the same, then su will
    launch the default shell for someUser (which exist in
    /etc/passwd), and su will set the user IDs and group IDs of the launched shell to the user IDs and group IDs of someUser.

Note that su has the setuid bit set, so su will run with root privileges.



Am I correct?




linux process su




share|improve this question














I am tying to understand how the su command work internally.



Say that I did the following:



su someUser


The following is what I think will happen:




  • su will ask me for the password of someUser.


  • su will hash the entered password.


  • su will compare the entered password with the password in the
    /etc/shadow file, and if the two passwords are the same, then su will
    launch the default shell for someUser (which exist in
    /etc/passwd), and su will set the user IDs and group IDs of the launched shell to the user IDs and group IDs of someUser.

Note that su has the setuid bit set, so su will run with root privileges.



Am I correct?





linux process su





share|improve this question













share|improve this question




share|improve this question








edited Dec 6 '17 at 0:48









Jeff Schaller

32k848109




32k848109










asked Dec 6 '17 at 0:48









user7681202

237414




237414











  • Yes, but su doesn't directly access /etc/shadow, it uses pam for this. ( /etc/pam.d/su )
    – Ipor Sircer
    Dec 6 '17 at 1:08
















  • Yes, but su doesn't directly access /etc/shadow, it uses pam for this. ( /etc/pam.d/su )
    – Ipor Sircer
    Dec 6 '17 at 1:08















Yes, but su doesn't directly access /etc/shadow, it uses pam for this. ( /etc/pam.d/su )
– Ipor Sircer
Dec 6 '17 at 1:08




Yes, but su doesn't directly access /etc/shadow, it uses pam for this. ( /etc/pam.d/su )
– Ipor Sircer
Dec 6 '17 at 1:08










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su doesn't do any password stuff in most Linux distros. Instead it delegates authn/authz to PAM and only handles the user-changing.






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  • Yup.
    – JdeBP
    Dec 6 '17 at 7:04










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1 Answer
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1 Answer
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active

oldest

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active

oldest

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active

oldest

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up vote
2
down vote













su doesn't do any password stuff in most Linux distros. Instead it delegates authn/authz to PAM and only handles the user-changing.






share|improve this answer




















  • Yup.
    – JdeBP
    Dec 6 '17 at 7:04














up vote
2
down vote













su doesn't do any password stuff in most Linux distros. Instead it delegates authn/authz to PAM and only handles the user-changing.






share|improve this answer




















  • Yup.
    – JdeBP
    Dec 6 '17 at 7:04












up vote
2
down vote










up vote
2
down vote









su doesn't do any password stuff in most Linux distros. Instead it delegates authn/authz to PAM and only handles the user-changing.






share|improve this answer












su doesn't do any password stuff in most Linux distros. Instead it delegates authn/authz to PAM and only handles the user-changing.







share|improve this answer












share|improve this answer



share|improve this answer










answered Dec 6 '17 at 1:08









Ignacio Vazquez-Abrams

32.1k66780




32.1k66780











  • Yup.
    – JdeBP
    Dec 6 '17 at 7:04
















  • Yup.
    – JdeBP
    Dec 6 '17 at 7:04















Yup.
– JdeBP
Dec 6 '17 at 7:04




Yup.
– JdeBP
Dec 6 '17 at 7:04

















 

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