DNS over TLS in FreeBSD 12

With the arrival of OpenSSL 1.1.1, an upgraded Unbound, and some changes to the setup and init scripts, FreeBSD 12, currently in beta, now supports DNS over TLS out of the box. We show how to set it up and discuss its advantages and disadvantages.

With the arrival of OpenSSL 1.1.1, an upgraded Unbound, and some changes to the setup and init scripts, FreeBSD 12.0, currently in beta, now supports DNS over TLS out of the box.

DNS over TLS is just what it sounds like: DNS over TCP, but wrapped in a TLS session. It encrypts your requests and the server’s replies, and optionally allows you to verify the identity of the server. The advantages are protection against eavesdropping and manipulation of your DNS traffic; the drawbacks are a slight performance degradation and potential firewall traversal issues, as it runs over a non-standard port (TCP port 853) which may be blocked on some networks. Let’s take a look at how to set it up.

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Twenty years

Yesterday was the twentieth anniversary of my FreeBSD commit bit, and tomorrow will be the twentieth anniversary of my first commit. I figured I’d split the difference and write a few words about it today.

My level of engagement with the FreeBSD project has varied greatly over the twenty years I’ve been a committer. There have been times when I worked on it full-time, and times when I did not touch it for months. The last few years, health issues and life events have consumed my time and sapped my energy, and my contributions have come in bursts. Commit statistics do not tell the whole story, though: even when not working on FreeBSD directly, I have worked on side projects which, like OpenPAM, may one day find their way into FreeBSD.

My contributions have not been limited to code. I was the project’s first Bugmeister; I’ve served on the Security Team for a long time, and have been both Security Officer and Deputy Security Officer; I managed the last four Core Team elections and am doing so again this year.

In return, the project has taught me much about programming and software engineering. It taught me code hygiene and the importance of clarity over cleverness; it taught me the ins and outs of revision control; it taught me the importance of good documentation, and how to write it; and it taught me good release engineering practices.

Last but not least, it has provided me with the opportunity to work with some of the best people in the field. I have the privilege today to count several of them among my friends.

For better or worse, the FreeBSD project has shaped my career and my life. It set me on the path to information security in general and IAA in particular, and opened many a door for me. I would not be where I am now without it.

I won’t pretend to be able to tell the future. I don’t know how long I will remain active in the FreeBSD project and community. It could be another twenty years; or it could be ten, or five, or less. All I know is that FreeBSD and I still have things to teach each other, and I don’t intend to call it quits any time soon.

Previously

Not up to our usual standards

For a few years now, I’ve been working on and off on a set of libraries which collect cryptography- and security-related code I’ve written for other projects as well as functionality which is not already available under a permissive license, or where existing implementations do not meet my expectations of cleanliness, readability, portability and embeddability.

(Aside: the reasons why this has taken years, when I initially expected to publish the first release in the spring or summer of 2014, are too complex to explain here; I may write about them at a later date. Keywords are health, family and world events.)

Two of the major features of that collection are the OATH Authentication Methods (which includes the algorithm used by Google Authenticator and a number of commercial one-time code fobs) and the Common Platform Enumeration, part of the Security Content Automation Protocol. I implemented the former years ago for my employer, and it has languished in the OpenPAM repository since 2012. The latter, however, has proven particularly elusive and frustrating, to the point where it has existed for two years as merely a header file and a set of mostly empty functions, just to sketch out the API. I decided to have another go at it yesterday, and actually made quite a bit of progress, only to hit the wall again. And this morning, I realized why. Continue reading “Not up to our usual standards” »

FreeBSD and CVE-2015-7547

As you have probably heard by now, a buffer overflow was recently discovered in GNU libc’s resolver code which can allow a malicious DNS server to inject code into a vulnerable client. This was announced yesterday as CVE-2015-7547. The best sources of information on the bug are currently Google’s Online Security Blog and Carlos O’Donnell’s in-depth analysis.

Naturally, people have started asking whether FreeBSD is affected. The FreeBSD Security Officer has not yet released an official statement, but in the meantime, here is a brief look at the issue as far as FreeBSD is concerned.

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OpenSSH, PAM and user names

FreeBSD just published a security advisory for, amongst other issues, a piece of code in OpenSSH’s PAM integration which could allow an attacker to use one user’s credentials to impersonate another (CVE 2015-6563, original patch). I would like to clarify two things, one that is already mentioned in the advisory and one that isn’t.

The first is that in order to exploit this, the attacker must not only have valid credentials but also first compromise the unprivileged pre-authentication child process through a bug in OpenSSH itself or in a PAM service module.

The second is that this behavior, which is universally referred to in advisories and the trade press as a bug or flaw, is intentional and required by the PAM spec (such as it is). There are multiple legitimate use cases for this, such as:

  • Letting PAM, rather than the application, prompt for a user name; the spec allows passing NULL instead of a user name to pam_start(3), in which case it is the service module’s responsibility (in pam_sm_authenticate(3)) to prompt for a user name using pam_get_user(3). Note that OpenSSH does not support this.

  • Mapping multiple users with different identities and credentials in the authentication backend to a single “template” user when the application they need to access does not need to distinguish between them, or when this determination is made through other means (e.g. environment variable, which service modules are allowed to set).

  • Mapping Windows user names (which can contain spaces and non-ASCII characters that would trip up most Unix applications) to Unix user names.

That being said, I do not object to the patch, only to its characterization. Regarding the first issue, it is absolutely correct to consider the unprivileged child as possibly hostile; this is, after all, the entire point of privilege separation. Regarding the second issue, there are other (and probably better) ways to achieve the same result—performing the translation in the identity service, i.e. nsswitch, comes to mind—and the percentage of users affected by the change lies somewhere between zero and negligible.

One could argue that instead of silently ignoring the user name set by PAM, OpenSSH should compare it to the original user name and either emit a warning or drop the connection if it does not match, but that is a design choice which is entirely up to the OpenSSH developers.

Update 2015-08-27 NIST rates exploitability as “medium” rather than “low” because an attacker who is able to impersonate the UID used by the unprivileged child can use a debugger or other similar method to modify the username that the child passes back to the parent. In other words, an attacker can leverage elevated privileges into other elevated privileges. I disagree with the rating, but have never had any luck getting NIST to correct even blatantly false information in the past.