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Hi folks, this week I applied the first incarnation of the new passwd/group handling code to the Cygwin repository and after fixing a crash which manifested in Denis Excoffier's network, I think we're at a point which allows to push this forward. This is a pretty intrusive change, in need of some serious testing, so I'd like to ask for volunteers. The latest 2014-02-13 snapshot from http://cygwin.com/snapshots/ contains the changes, including the latest bugfix. If you're interested in helping, please read on. It's a long text, but I feel it's necessary to explain how this works, so you get a grasp of what to look out for. In the long run, this is the basis for the documentation to go into the User's Guide. ---------------------------------------------------------------------- Please note: If you have questions or comments, please do *NOT* full quote this mail! Just quote the important text snippet and ask away. Thanks for considering. ---------------------------------------------------------------------- Did I mention that this mail is *really* long? Sorry about that, but I don't see how to explain all the details otherwise. Fetch yourself some drink, preferredly some strong coffee or tea. Oh, and there be footnotes. =========================== So, what is this all about? =========================== For as long as Cygwin has existed, it has stored user and group information in /etc/passwd and /etc/group files. Under the assumption that these files would never be too large, the first process in a process tree, as well as every execing process within the tree would parse them into structures in memory. Thus every Cygwin process would contain an expanded copy of the full information from /etc/passwd and /etc/group. This approach has a few downsides. One of them is that the idea to have always small files is flawed. Another one is that reading the entire file is most of the time entirely useless, since most processes only need information on their own user and the primary group. Last but not least, the passwd and group files have to be maintained separately from the already existing Windows user databases, the local SAM and Active Directory. On the other hand, we have to have a mapping between Windows SIDs and POSIX uid/gid values (for how this works in Cygwin so far, see ), so we rely on some mechanism to convert SIDs to uid/gid values and vice versa. Microsoft's "Services for UNIX" (SFU) (which are deprecated since Windows 8/Server 2012) never used passwd/group files. Rather, SFU used a fixed, computational mapping between SIDs and POSIX uid/gid. It allows to generate uid/gid values from SIDs and vice versa. The mechanism is documented, albeit in a confusing way and spread over multiple MSDN articles. For my changes, I took the liberty to clone the mapping, with a tiny difference for backward compatibility with existing Cygwin applications. =================================== Yes, yes, but how does it work now? =================================== You *are* comfortable with the concept of SIDs and RIDs, right? If not, please read  again. I'll wait... Finished? Ok, let's start. Cygwin's new mapping between SIDs and uid/gid values works in two ways. - Read /etc/passwd and /etc/group files, like before, mainly for backward compatibility. - If no files are present, or if an entry is missing in the files, ask Windows. At least, that's the default behaviour now. It will be configurable using a file /etc/nsswitch.conf, but I'm getting ahead of myself. Let's stick to the default for now. If files are present, they will be scanned on demand as soon as a mapping from SIDs to uid/gid or user names is required. The new mechanism will never read the entire file into memory, but only scan for the requested entry and cache this one in memory. If no entry is found, or no passwd or group file was present, Cygwin will ask the OS. Note: If the first process in a Cygwin process tree determines that no passwd or group file is present, no other process in the entire process tree will try to read the files later on. This is done for self-preservation. It's kind of bad if the uid or gid of a user changes during the ride. So if we've drawn a blank reading the files, we're going to ask the OS. First thing, we ask the local machine for the SID or the user name. The OS functions (LookupAccountSid/LookupAccountName) are kind of intelligent. They have all the stuff built in to ask for any account of the local machine, the Active Directory domain of the machine, the Global Catalog of the forest of the domain, as well as any trusted domain of our forest for the information. One OS call and we're practically done. Except... ...the calls only return the mapping between SID, account name and the account's domain. We don't have a mapping to POSIX uid/gid and we're missing information on the user's home dir and login shell. Let's discuss the SID<=>uid/gid mapping first. Here's how the SID<=>uid/gid mapping works. If you're fuzzy on the existing well-known SIDs, see . - Well-known SIDs in the NT_AUTHORITY domain of the S-1-5-RID type, or aliases of the S-1-5-32-RID type are mapped to the uid/gid value RID. Examples: "SYSTEM" S-1-5-18 <=> uid/gid: 18 "Users" S-1-5-32-545 <=> uid/gid: 545 - Other well-known SIDs in the NT_AUTHORITY domain (S-1-5-X-RID): S-1-5-X-RID <=> uid/gid: 0x1000 * X + RID Example: "NTLM Authentication" S-1-5-64-10 <=> uid/gid: 0x4000A == 262154 - Other well-known SIDs: S-1-X-Y <=> uid/gid: 0x10000 + 0x100 * X + Y Example: "LOCAL" S-1-2-0 <=> uid/gid: 0x10200 == 66048 "Creator Group" S-1-3-1 <=> uid/gid: 0x10301 == 66305 - Logon SIDs: The own LogonSid is converted to the fixed uid 0xfff == 4095 and named "CurrentSession". Any other LogonSid is converted to the fixed uid 0xffe == 4094 and named "OtherSession". - Mandatory Labels: S-1-16-RID <=> uid/gid: 0x60000 + RID Example: "Medium Mandatory Level" S-1-16-8192 <=> uid/gid: 0x62000 == 401408 - Accounts from the local machine's user DB (SAM): S-1-5-21-X-Y-Z-RID <=> 0x30000 + RID Example: "Administrator" S-1-5-X-Y-Z-500 <=> uid/gid: 0x301f4 == 197108 - Accounts from the machine's primary domain: S-1-5-21-X-Y-Z-RID <=> 0x100000 + RID Example: "Domain Users" S-1-5-X-Y-Z-513 <=> 0x100201 == 1049089 - Accounts from a trusted domain of the machine's primary domain: S-1-5-21-X-Y-Z-RID <=> trustPosixOffset(domain) + RID Hang on... "trustPosixOffset"? Uh, yes. This value exists in Windows domains already since before Active Directory days. What happens is this. If you create a domain trust between two domains, a trustedDomain entry will be added to both databases. It describes how *this* domain trusts the *other* domain. One attribute of a trust is a 32 bit value called "trustPosixOffset" For each new trust, trustPosixOffset will get some automatic value. In recent AD domain implementations, the first trusted domain will get trustPosixOffset set to 0x80000000. Following domains will get lower values. Also, the domain admins are allowed to set the trustPosixOffset value for each trusted domain to some arbitrary 32 bit value, thus allowing any kind of collisions between the trustPosixOffsets of domains. Nice, isn't it? Anyway, as the user of this value, we have to *trust* the domain admins to set trustPosixOffset to sensible values, or to leave it alone. So, for the first (or only) trusted domain of your domain, the automatic offset is 0x80000000. An example for a user of that trusted domain is: S-1-5-X-Y-Z-1234 <=> uid/gid 0x800004d2 == 2147484882 There's only one problem with this approach. Assuming you're running in the context of a local user on a domain member machine. Local users don't have the right to fetch this kind of domain information from the DC, they'll get permission denied. In this case Cygwin will fake a, mostly, sensible trustPosixOffset value for this session. - Local accounts from another machine in the network: There's no SID<=>uid/gid mapping implemented for this case. The problem is, there's no way to generate a bijective mapping. There's no central place which keeps an analogue value of the trustPosixOffset, and there's the additional problem that the LookupAccountSid and LookupAccountName functions cannnot resolve the SIDs, unless they know the name of the machine this SID comes from. And even then it will probably suffer a "Permission denied" when trying to ask the machine for its local account. SFU just prints the account RID in this case, Cygwin maps the account to the fake accounts "Unknown+User"/"Unknown+Group" with uid/gid -1. Ok, now we have a semi-bijective mapping between SIDs and POSIX uid/gid values, but, given that we have potentially users and groups in different domains having the same name, how do we uniquely differ between them by name? Well, we can do that by making their names unique in a per-machine way. Dependent on the domain membership of the account, and dependent of the machine being a domain member or not, the user and group names will be generated using a domain prefix and a separator character between domain and account name. The default separator character is the plus sign, '+', as in SFU. - Well-known SIDs will have the separator character prepended: "+SYSTEM", "+LOCAL", "+Medium Mandatory Level", ... - If the machine is no domain member machine, only local accounts can be resolved into names, so for ease of use, just the account names are used as Cygwin user/group names: "corinna", "bigfoot", "None", ... - If the machine is a domain member machine, all accounts from the primary domain of the machine are mapped to Cygwin names without domain prefix: "corinna", "bigfoot", "Domain Users", ... while accounts from other domains are prepended by their domain: "DOMAIN1+corinna", "DOMAIN2+bigfoot", "DOMAIN3+Domain Users", ... - Local machine accounts of a domain member machine get a Cygwin user name the same way as accounts from another domain: The local machine name gets prepended: "MYMACHINE+corinna", "MYMACHINE+bigfoot", "MYMACHINE+None", ... - If LookupAccountSid fails, Cygwin checks the accounts against the known trusted domains. If the account is from one of the trusted domains, an artificial account name is created. It consists of the domain name, and a special name created from the account RID: "MY_DOM+User(1234)", "MY_DOM+Group(5678)" Otherwise we know nothing about this SID, so it will be mapped to the fake accounts "Unknown+User"/"Unknown+Group" with uid/gid -1. Any questions? No? Ok, let's have a short break and then, next point. [...5 minutes commercials...] ========================================== Cygwin user names, home dirs, login shells ========================================== "That's all very interesting" you think (if you're still awake, that is), "but passwd entries consist of more than just uids. What's up with that?" Obviously, if you don't maintain passwd and group files, you need to have a way to maintain the other fields of a passwd entry as well. Three things come to mind: - You want to use a Cygwin user name different from your Windows user name. - You want a home dir different from the default /home/$USER. - You want to specify a different login shell than /bin/sh. How this is done depends on your account being a domain account or a local account. Let's start with the default. Assuming your Windows account name is "bigfoot" and your domain is "MY_DOM". Your default passwd entry in absence of anything I'll describe below looks like this: bigfoot:*:<uid>:<gid>:U-MY_DOM\bigfoot,S-1-5-....:/home/bigfoot:/bin/sh or, if your account is from a different domain than the primary domain of the machine: MY_DOM+bigfoot:*:<uid>:<gid>:U-MY_DOM\bigfoot,S-1-5-....:/home/bigfoot:/bin/sh Yes, the default homedir is still /home/bigfoot. If your account is a domain account: Either create an /etc/passwd and/or /etc/group file with entries for your account and use that, just as before. Or, Cygwin will utilize the posixAccount/posixGroup attributes per RFC 2307. These attributes are by default available in Active Directory since Windows Server 2003 R2. They are "not set", unless utilized by the (deprecated since Server 2012 R2) Active Directory "Server for NIS" feature. The user attributes utilized by Cygwin are: uid If set, will be used as Cygwin user name. uidNumber See next section. gecos Content will be added to the pw_gecos field. unixHomeDirectory If set, will be used as Cygwin home directory. loginShell If set, will be used as Cygwin login shell. The group attributes utilized by Cygwin are: cn If set, will be used as Cygwin group name. gidNumber See next section. Apart from power shell scripting or inventing new CLI tools, these attributes can be changed using the "Attribute Editor" tab in the user properties dialog of the "Active Directory Users and Computers" MMC snap-in. Alternatively, if the "Server for NIS" administration feature has been installed, ther will be a "UNIX Attributes" tab which contains the required fields, except for the gecos field, which isn't really important anyway. Last resort is "ADSI Edit". The primary group of a user is always the Windows primary group set in Active Directory and can't be changed. If your machine is not a domain member machine or your account is a local account for some reason: Either create an /etc/passwd and/or /etc/group file with entries for your account and use that, just as before. Or enter the information into the "Comment" field of your local user entry. In the "Local Users and Groups" MMC snap-in it's called "Description". You can utilze this field even if you're running a "home edition" of Windows, using the command line. The "net user" command allows to set all values in the SAM, even if the GUI is crippled. A Cygwin SAM comment entry looks like this: <cygwin key="value" key="value" [...] /> The supported keys are name="value" Sets the Cygwin user name to value. home="value" Sets the Cygwin home dir to value. shell="value" Sets the Cygwin login shell to value. group="value" Sets the Cygwin primary group of the account to value, provided that the user *is* already a member of that group. This allows to override the default "None" primary group for local accounts. One nice idea here is, for instance group="Users". This is the *Windows* name of the group, not the Cygwin name, assuming they differ. unix="value" Sets the NFS/Samba uid of the user to the decimal value. See the next chapter. The <cygwin .../> string can start at any point in the comment, but you have to follow the rules: - It starts with "<cygwin " and ends with "/>". - The "cygwin" string and the key names have to be lowercase. - No spaces between key and "value", just the equal sign. - The value must be placed within double quotes and it must not contain a double quote itself. The double quotes are required for the decimal values as well! CMD example: net use corinna /comment:"<cygwin home=\"/home/foo\"/>" Bash example (use single quotes): net use corinna /comment:'<cygwin home="/home/foo"/>' For changing group comments, use the `net localgroup' command. The supported key/value pair for groups are name="value" Sets the Cygwin group name to value. unix="value" Sets the NFS/Samba gid of the group to the decimal value. See the next chapter. ============================= NFS and Samba account mapping ============================= If you're using Microsoft's NFS client to access UNIX shares, like me, you might have been bothered by each file on the share seemingly being yours. The reason for this is that Microsoft's NFS client does not map the uid/gid values on the NFS shares to SIDs. There's no such thing as a (fake) security descriptor returned to the application. Rather, via an undocumented API you can fetch RFC 1813 compatible NFSv3 stat information from the share. This is what Cygwin is using to show stat information for files on NFS shares. The problem is this. While all other information in this stat record, like timestamps, or file size etc., can be used by Cygwin, Cygwin had no way to map the values of the st_uid and st_gid members to a Windows SID. So it just faked the file owner info and claimed that it's you. However, SFU has, over time, developed multiple methods to map UNIX uid/gid values on NFS shares to Windows SIDs. You'll find the full documentation of the mapping methods in . Cygwin now utilizes the RFC 2307 mapping for this purpose. This is most of the time provided by an AD domain, but it could also be a standalone LDAP mapping server. Per RFC 2307, the uid is in the attribute uidNumber. For groups, the gid is in the gidNumber attribute. When Cygwin stat's files on an NFS share, it asks the mapping server via LDAP in two different ways, depending on the role of the mapping server. - If the server is an AD domain controller, it asks for an account with uidNumber attribute == st_uid field of the stat record returned by NFS. If an account matches, AD returns the Windows SID, so we have an immediate mapping from UNIX uid to a Windows SID, if the user account has a valid uidNumber attribute. For groups, the method is the same, just that Cygwin asks for a group with gidNumber attribute == st_gid field of the stat record. - If the server is a standalone LDAP mapping server Cygwin asks for the same uidNumber/gidNumber attributes, but it can't expect that the LDAP server knows anything about Windows SIDs. Rather, the mapping server returns the account name. Cygwin then asks the DC for an account with this name, and if that succeeds, we have a mapping between UNIX uid/gid and Windows SIDs. The mapping will be cached for the lifetime of the process, and inherited by child processes. And what about Samba? A fully set up Samba with domain integration is running winbindd to map Window SIDs to artificially created UNIX uids and gids, and this mapping is transparent within the domain, so Cygwin doesn't have to do anything special. However, setting up winbindd isn't for everybody, and it fails to map Windows accounts to already existing UNIX users or groups. In contrast to NFS, Samba returns security descriptors, but unmapped UNIX accounts get special SIDs: - A UNIX user account with uid X is mapped to the Windows SID S-1-22-1-X. - A UNIX group account with gid X is mapped to SID S-1-22-2-X. As you can see, even though we have SIDs, they just reflect the actual uid/gid values on the UNIX box in the RID value. It's only marginally different from the NFS method, so... why not just do the same rote as for NFS? That's what Cygwin will do. If it encounters a S-1-22-x-y SID, it will perform the same RFC 2307 mapping as for NFS shares. Hang on. What about home users without any Windows domain or LDAP server per RFC 2307, but with a Linux machine running Samba? Not all is lost. If you're a user on a standalone machine, you can add this information to your SAM account. Assuming the uid of your Linux user account is 505 and the gid of your primary group is, say, 100, just add the values to your SAM user and group accounts. The following example assumes you didn't already add something else to the comment field. To your user's SAM comment (remember: called "Description" in the GUI), add: <cygwin group="Users" unix="505"/> To the user's group SAM comment add: <cygwin unix="100"/> This should be sufficient to work on your Samba share and to see all files owned by your Linux user account as your files. ====================================================================== The /etc/nsswitch.conf file =========================== Last, but not least, let's talk about a way to configure how the stuff works on your machine. On Linux and some other UNIXy OSes, we have a file called /etc/nsswitch.conf. One part of it is to specify how the passwd and group entries are generated. That's what Cygwin now provides as well. The /etc/nsswitch.conf file is optional. If you don't have one, Cygwin uses sensible defaults. Note: The /etc/nsswitch.conf file is read exactly once by the first process of a Cygwin process tree. If there was no /etc/nsswitch.conf file when this first process started, then no other process in the running Cygwin process trees will try to read the file. If you create or change /etc/nsswitch.conf, make sure to stop and restart all Cygwin processes to pick up the change. So, what mischief can we perform with /etc/nsswitch.conf? To explain, lets have a look into an /etc/nsswitch.conf file set up to all default values: # /etc/nsswitch.conf passwd: files db group: files db db_prefix: auto db_separator: + db_cache: yes The first line, starting with a hash '#' is a comment. The hash sign starts a comment, just as in shell scripts. Everything up to the end of the line is ignored. So this: foo: bar # baz means, for the entry foo, do bar, ignore everything after the hash sign, so baz is only a comment. The other lines define the available settings. The first word up to a colon is a keyword. Note that the colon *must* follow immediately after the keyword. This is a valid line: foo: bar This is not valid: foo : bar Apart from this restriction, the reminder of the line can have as may spaces and TABs as you like. This is a valid line: foo: bar baz Now let's have a look at the available keywords and settings. The two lines starting with the keywords "passwd" and "group" define where Cygwin gets its passwd and group information from. "files" means, fetch the information from the corresponding file in the /etc directory. "db" means, fetch the information from the Windows account databases, the SAM for local accounts, Active Directory for domain account. Examples: passwd: files Read passwd entries only from /etc/passwd. group: db Read group entries only from SAM/AD. group: files # db Read group entries only from /etc/group ("db" is ignored due to the preceding hash sign). passwd: files db Read passwd entries from /etc/passwd. If a user account isn't found, try to find it in SAM or AD. This is the default for both, passwd and group information. group: db files This is a valid entry, but the order will be ignored by Cygwin. If both, files and db are specified, Cygwin will always try the files first, then the db. The remaining entries define certain aspects of the Windows account database search. "db_prefix" determines how the Cygwin user or group name is created: db_prefix: auto This is the default. If your account is from the primary domain of your machine, or if your machine is a standalone machine (not a domain member), your Cygwin account name is just the Windows account name. If your account is from another domain, or if you're logged in as local user on a domain machine, the Cygwin username will be the combination of Windows domainname and username, with the separator char in between: MY_DOM+username (foreign domain) MACHINE+username (local account) Builtin accounts have just the separator char prepended: +LOCAL +Users Unknown accounts on NFS or Samba shares (that is, accounts which cannot be mapped to Windows user accounts via RFC 2307) get a Cygwin account name consisting of the artificial domains "UNIX_User" or "UNIX_Group" and the uid/gid value, for instance: UNIX_User+0 (root) UNIX_Group+10 (wheel) db_prefx: primary Like "auto", but primary domain accounts will be prepended by the domainname as well. db_prefix: always All accounts, even the builtin accounts, will have the domain name prepended: BUILTIN+Users As a special case, if the Cygwin account name differs from the Windows account name, it will be prepended by the artificial domain name "Posix_User" or "Posix_Group" if db_prefix is set to "always": Posix_User+cygwin_user_name Posix_Group+cygwin_group_name "db_separator" defines the spearator char used to prepend the domain name to the user or group name. The default is '+': MY_DOM+username With "db_separator", you can define any ASCII char except space, tab, carriage return, line feed, and the colon, as separator char. Example: db_separator: \ MY_DOM\username "db_cache" allows to specify if user and group data should be cached in the process at all or not. Default is "yes". "no" is an experimental feature. The idea is this. Assuming you're working in a volatile domain environment, and you have long-running processes like sshd. You might want sshd to recognize changes in the existing user entries as soon as they occur. Caching user data in sshd *might* not be helpful in such a scenario. However, the user data which gets changed a lot is typically *not* the data required for login purposes, except for the password, and that doesn't matter in Cygwin's case. So take this feature with a grain of salt. ========== Footnotes: ==========  Not dragons. Just a test.  http://cygwin.com/cygwin-ug-net/ntsec.html  This may change. Right now the file is read in 32K chunks, but we could easily read the file in 64K chunks and, if we find the file is < 64K anyway, just cache the entire bunch, like before. Not implemented yet, but something to keep in mind.  http://msdn.microsoft.com/en-us/library/windows/desktop/aa379166%28v=vs.85%29.aspx http://msdn.microsoft.com/en-us/library/windows/desktop/aa379159%28v=vs.85%29.aspx  http://support.microsoft.com/kb/243330  This is where Cygwin differs from SFU. The reason is that we need the old uid/gid values for backward compatibility. There are Cygwin packages (cron, for instance) who rely on the fact that the uid of SYSTEM is 18. In SFU, these accounts get mapped like the other built in SIDs.  https://tools.ietf.org/html/rfc2307  https://tools.ietf.org/html/rfc1813  http://msdn.microsoft.com/en-us/library/cc980032.aspx  http://linux.die.net/man/5/nsswitch.conf -- Corinna Vinschen Please, send mails regarding Cygwin to Cygwin Maintainer cygwin AT cygwin DOT com Red Hat
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