September 2019

There were 3 posts published in September 2019 (this is page 1 of 1).

AD/LDAP authentication on Linux hosts

I’ve been working with the Lightweight Directory Access Protocol (LDAP) for 18 years now. Then Microsoft embraced and extended LDAP with Active Directory. Nowadays most companies base all of their authentication and authorization on Active Directory and for good reason. In a Windows-only world it works great. For a mixed-platform environment, it’s a bit more difficult to make work.

I recently worked out how to make Linux systems authenticate against Active Directory using only the LDAP protocol and wanted to share it here for any fellow DevOps/sysaedmins who might want to try it themselves. The goals were to do it with minimum fuss and using the native tools – no third-party apps. I also want to do it solely with LDAP and not have to worry about pointlessly “joining” a Linux host to a domain.

The modern way that Red Hat likes to connect Linux hosts to AD like to do this is to use the SSSD suite of packages, join the host to the Active Directory tree, and talk to AD directly. This seems like a lot of bloat to me when all you need is authentication. Fortunately, you can use the “legacy” means and do it all with LDAP libraries.

Bridging Active Directory and Linux hosts

One way to integrate Linux/UNIX hosts into AD is to add Microsoft Windows Services for UNIX (SFU) schema extensions. This means every AD entry would be defined with common Unix attributes like uid (user id) and gid (group id). These could sometimes get out of sync with the AD attributes and at any rate would require constant updating of the AD records.

Ideally, we won’t depend on Services for UNIX additions in AD and the complexity it brings. Instead, we’ll identify standard AD attributes and map them to Linux/UNIX equivalents. The nss-pam-ldapd package allows us to do this in the /etc/nslcd.conf file, which we’ll see in a minute.

Differences between CentOS 6/AWS and CentOS 7 hosts

One stumbling block has been that Amazon Linux (amzn) uses old, old libraries, based on CentOS 6 packages. The nss-pam-ldapd package which ships with this version of Amazon Linux is version 0.7.5; a version too old to include the mapping functionality we need to avoid using Services for UNIX.

Fortunately, we can remove the amzn version and add an updated one. I have tested one I have found at this link which updates any amzn hosts to the 0.9.8 version of nss-pam-ldapd.

The version of nss-pam-ldapd that ships with CentOS 7 is 0.8.3 and works fine with attribute mapping.

Obtaining the domain’s ObjectSID

The goal of using a directory is consistency. If a user appears in AD, that user will be available to Linux hosts. Also, that user will be treated the same on every directory-equipped server as that user will ideally have the same uid/gid. Without adding Services for UNIX, we need some way to ensure a uid on one host is consistent with the uid on another host. This is done by nss-pam-ldapd by mapping Linux uid/gids to their equivalents in AD, called ObjectSIDs. You need to obtain your AD server’s domain ObjectSID.
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My sledding souvenir

The start of the fateful sledding run

I spent this past week at the Veterans Administration’s War-Related Illnesses and Injuries Center (WRIISC), getting examined to figure out the strange health issues I’ve had since leaving the Navy (more on that later).

One issue I discussed with them has bothered me for the past few years.I’ve had a numbness that has developed along my right quadricep. It’s icy-cold sensation can wake me from a deep sleep and is quite aggravating. They asked me if I could recall any injury I may have had to my lower back.

At the time I could think of none. but when pondering it this morning the answer came to me and it is decidedly not war-related. Instead, it’s the long-delayed consequences from an injury I received from snow sledding with the family.
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The Book of Prince | The New Yorker

On January 29, 2016, Prince summoned me to his home, Paisley Park, to tell me about a book he wanted to write. He was looking for a collaborator. Paisley Park is in Chanhassen, Minnesota, about forty minutes southwest of Minneapolis. Prince treasured the privacy it afforded him. He once said, in an interview with Oprah Winfrey, that Minnesota is “so cold it keeps the bad people out.” Sure enough, when I landed, there was an entrenched layer of snow on the ground, and hardly anyone in sight.

Prince’s driver, Kim Pratt, picked me up at the airport in a black Cadillac Escalade. She was wearing a plastic diamond the size of a Ring Pop on her finger. “Sometimes you gotta femme it up,” she said. She dropped me off at the Country Inn & Suites, an unremarkable chain hotel in Chanhassen that served as a de-facto substation for Paisley. I was “on call” until further notice. A member of Prince’s team later told me that, over the years, Prince had paid for enough rooms there to have bought the place four times over.

My agent had put me up for the job but hadn’t refrained from telling me the obvious: at twenty-nine, I was extremely unlikely to get it. In my hotel room, I turned the television on. I turned the television off. I had a mint tea. I felt that I was joining a long and august line of people who’d been made to wait by Prince, people who had sat in rooms in this same hotel, maybe in this very room, quietly freaking out just as I was quietly freaking out.

Source: The Book of Prince | The New Yorker