From Fedora Project Wiki

Default Local DNS Resolver

Summary

Plain DNS protocol is insecure and therefore vulnerable from various attacks (e.g. cache poisoning). A client can never be sure that there is no man-in-the-middle, if it does not do the DNSSEC validation locally.

We want to have Unbound server installed and running on localhost by default on Fedora systems. Where necessary, have also dnssec-trigger installed and running by default. Unbound and dnssec-trigger will be properly integrated with the default network configuration manager (e.g. NetworkManager for Fedora Server and Workstation) and with the graphical user interface (especially GNOME). The localhost address will be the only record in /etc/resolv.conf and no other software except dnssec-trigger will be allowed to change its content.

Owner

  • Release notes owner:

Current status

Detailed Description

Plain DNS protocol is insecure and therefore vulnerable from various attacks (e.g. cache poisoning). DNSSEC is a DNS extension which enabled the client to verify the DNS query response and make sure there is no attacker to spoof some records. A user connected to network usually receives a set of resolvers from DHCP, which should be used for name resolution. These resolvers may also do the DNSSEC validation. However a client can never be sure that there is no man-in-the-middle, if it does not do the DNSSEC validation locally. Purpose of this Fedora change is to have a validating DNS resolver installed on Fedora systems by default. This includes necessary discussions, coordination and integration with other components installed on Fedora by default.

There are growing instances of discussions and debates about the need for a trusted local validating DNS resolver. There are multiple reasons for having such a resolver, most importantly security and usability. Security and protection of user's privacy becomes paramount with the backdrop of the increasingly snooping governments and service providers world wide.

People use Fedora on portable/mobile devices which are connected to diverse networks as and when required. The automatic DNS configurations provided by these networks are never trustworthy for DNSSEC validation, as currently there is no way to establish such trust.

Apart from trust, these name servers are often known to be flaky and unreliable which only adds to the overall bad and at times even frustrating user experience. In such a situation, having a trusted local validating DNS resolver not only makes sense but is, in fact, badly needed. It has become a need of the hour. (See: [1], [2], [3])

All DNS literature strongly recommends it and amongst all discussions and debates about the issues involved in establishing such trust, it is unanimously agreed upon and accepted that having a trusted local DNS resolver is the best solution possible. It will simplify and facilitate a lot of other design decisions and application development in the future. (See: [1], [2], [3])

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[1] https://www.ietf.org/mail-archive/web/dane/current/msg06469.html
[2] https://www.ietf.org/mail-archive/web/dane/current/msg06658.html
[3] https://lists.fedoraproject.org/pipermail/devel/2014-April/197755.html

Decisions made based on discussion between change owners, Gnome and NetworkManager developers

  1. dnssec-trigger will not do any Hot-Spot detection
  2. dnssec-trigger panel will not be installed by default on Fedora Workstation
  3. if dnssec-trigger determines that the connection provided resolvers are not usable for DNSSEC validation, it is not able to do the full recursion and it is not able to tunnel the communication over SSL, then it automatically switches to the "non-validating" mode. This is not to break user experience with name resolution and rather fall back to the state, which is basically the actual state in default installation of Fedora 23 or older -> no DNSSEC validation. This will be the default behavior - but configurable. The insecure mode will be implemented using one of these approaches:
    1. resolv.conf's content will be modified to contain connection provided resolvers
    2. Unbound will not do the validation for the necessary time. In this mode Unbound will NOT set the AD bit in the DNS response.
  4. Hot-Spot detection is done only by NetworkManager
  5. NetworkManager should be able to notify services (dnssec-trigger) via dispatcher or some other means, on any connectivity state change (This part is yet to be sorted out with NM developers)
  6. Hot-Spot login is handled by Gnome
  7. Gnome uses the connection provided DNS resolvers instead of the ones in resolv.conf for Hot-Spot login purposes.

Benefit to Fedora

What is the benefit to the platform?

Fedora distribution has been the industry leader when it comes to shipping the best and latest software to its users. In many cases Fedora has been the trend setter in accepting new technologies like Systemd. With having a trusted local validating DNS resolver, Fedora will not only provide greater security and usability to its users, but will also take an important step towards the secure by default computing future, wherein users do not have to worry about privacy, security and trust.

Specific benefits:

  • Improved security when using Internet services, thanks to local DNSSEC validation
  • User and applications will be able to trust the AD bit set in the DNS answer.
  • Existence of system-wide DNS cache (in Unbound server) will speed up the resolution of frequent domain names and in general will save network traffic.
  • Have a real split-DNS configuration - e.g. forward only specific DNS queries to the VPN-provided resolvers.

Scope

  • Proposal owners: Proposal owners shall have to
    • define the syntax and semantics for new configuration parameters/files.
    • properly document how to test and configure the new default setup
    • persuade and coordinate with the other package owners to incorporate new changes/workflow in their applications.
    • discuss with WGs in which products the change makes sense and what are the expectations of WGs for different Fedora products
    • resolve interoperability issues for Docker and other containers use-cases
  • Other developers: (especially NetworkManager and the likes)
    • NetworkManager has to implement notifications on connectivity state changes
    • Gnome Shell has to use the connection provided resolvers (fetched directly from NM) for Hot-Spot login purposes
    • Ideally other developers and user should test their software and application in this setup and verify that it is working as expected
  • Release engineering:
    • Make sure that the necessary packages (dnssec-trigger, unbound) are part of the composes for the appropriate Fedora Products.
    • Add services needed for the setup into the default presets (dnssec-triggerd.service)
  • Policies and guidelines:
    • Any software, including NetworkManager, will have to be configured to not tamper with the content of '/etc/resolv.conf' by default. The connection-provided resolver entries should be stored in a separate configuration file or in memory and accessible via some API.

Upgrade/compatibility impact

  • Any packages directly modifying the content of /etc/resolv.conf will not be able to modify the content any more. The software should notify about the changes the default network configuration management software (e.g. NetworkManager), rather than do the changes directly.
  • Any packages that use the system name resolution libraries should continue to work without any problems, as the name resolution library usually use the resolvers listed in '/etc/resolv.conf'.

Search domains in resolv.conf

Domain search list supplied by DHCP can not be trusted and therefore it is ignored. If you need some search domains to be set in /etc/resolv.conf, you have to use the search option in /etc/dnssec-trigger/dnssec-trigger.conf.

The reasoning is that any DHCP server can send you any search domains list (and this can be even intercepted by an attacker, because DHCP is not secure) and it would make your traffic to go elsewhere for domain names that are not FQDN.

This means that users will not see the DHCP provided search domains in the /etc/resolv.conf as they did before.

Docker

Docker and other users of network namespaces, like systemd-nspawn, would break. When docker runs, it picks up the current '/etc/resolv.conf' and puts it in the container. Currently Docker would work with the local DNS resolver, but would not use it. When /etc/resolv.conf contains the localhost address, Docker ignores it and rather puts the Google's DNS resolvers (8.8.8.8 and 8.8.4.4) into the containers's resolv.conf.

Docker and containers could be able to use host's DNSSEC resolver via a dedicated interface and iptables(8) rules, though such configurations need to be investigated for their feasibility. Please see:

Docker & iptables(8)

Docker server creates a virtual bridge interface - docker0 - as a link between containers and the host. When docker starts a container, it creates two network interfaces each bound to the docker0 bridge. At one end of the 'docker0' bridge is the container's eth0 interface and at the other end is veth* interface on the host. Ie. all network traffic from containers passes through the docker0 bridge. -> https://docs.docker.com/articles/networking/

For container applications to take advantage of the DNSSEC resolver on the host, we need to divert DNS traffic from docker0 interface to the local lo interface on the host. This can be accomplished by following these steps:

  • Enable local lo routing via docker0 bridge interface. (it is off by default)
 $ sysctl -w net.ipv4.conf.docker0.route_localnet=1
  • Enable unbound(8) server to accept requests from 172.17.0.0/16 docker sub-network.
 $ vi /etc/unbound/unbound.conf -> access-control: 172.17.0.0/16 allow
  • Use iptables(8) destination nat(DNAT) feature to divert DNS traffic from docker0 to lo interface
 $ iptables -t nat -I PREROUTING -p UDP -s 172.17.0.0/16 --dport 53 -i docker0 -j DNAT --to-destination 127.0.0.1:53

Hot-Spot detection

Based on previous discussion with GNOME and NetworkManager developers we agreed, that NetworkManager will be the only one doing the Hot-Spot detection and that the Hot-Spot detection functionality in dnssec-trigger will be turned off by default (until network connectivity state change notifications are implemented in NM, only in Fedora Workstation).

Hot-Spot login

Based on previous discussion with GNOME developers, on Fedora Workstation, the Hot-Spot login is handled only by Gnome Shell and the Hot-Spot login functionality in dnssec-trigger is disabled.

dnssec-trigger panel

dnssec-trigger panel is not installed by default any more. For spins / products that don't have their own way of handling the Hot-Spot login, the dnssec-trigger panel should be added to the kickstart / compose.

How To Test

Expectations

Ideally everything should work as usual, out of the box. Responses for DNSSEC-protected domains will be validated automatically. In case the validation fails for DNSSEC-protected domain (e.g. due to misconfiguration or some ongoing attack) it will stop working.

You can use http://dnsviz.net/ service to finding out if there is any problem with the domain. Site http://www.dnssec-failed.org/ has intentionally invalid DNSSEC signatures to simulate an attack. The site should not be reachable if your configuration is correct.

How to get Unbound and dnssec-trigger running?

  • Install dnssec-trigger daemon:
$ sudo dnf install dnssec-trigger
  • If you use NetworkManager, configure it to use unbound. Add the following line into /etc/NetworkManager/NetworkManager.conf
dns=unbound
  • Enable and start dnssec-trigger
$ sudo systemctl enable dnssec-triggerd.service



$ # disable and stop any existing DNS service, e.g., dnsmasq
$ sudo systemctl start dnssec-triggerd.service
  • Validation failures should be recorded to the system log:
journalctl -f -l -u unbound.service
unbound[1062]: [1062:0] info: validation failure <www.dnssec-failed.org. A IN
  >: no keys have a DS with algorithm RSASHA1 from 192.168.2.1 for key dnssec-failed.org. while building chain of trust

User Experience

The change is something the user should ideally not even notice, however the experience may change for misconfigured domains.

Note: There's a DNSSEC/TLSA validator for Firefox (and some other browsers) developed by CZ.NIC Labs.

Note: We need to describe how automatic split DNS and DNSSEC affects the user when accessing local resources, resources not secured by DNSSEC and spoofed resources.

Dependencies

  • Changes in Docker
  • Changes in NetworkManager
  • Changes in Gnome Shell
  • Changes in default presets and products kickstarts

Contingency Plan

  • Contingency mechanism: (What to do? Who will do it?)
    • Release Engineers will have to reverse all the changes done to default presets and kickstart files, so that Unbound and dnssec-trigger won't be installed and enabled by default in Fedora Products.
  • Contingency deadline: Beta freeze.
  • Blocks release? - No
  • Blocks product? product - No

Documentation

For more information on name resolution, DNS and DNSSEC, please check:

Release Notes

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