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jedisct1/edgedns

262

jedisct1 / edgedns

Rust

A high performance DNS cache designed for Content Delivery Networks


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EdgeDNS

A high performance DNS cache designed for Content Delivery Networks, with built-in security mechanisms to protect origins, clients and itself.

Precompiled binaries (recommended)

Installation from source

EdgeDNS requires rust-nightly. Compile and install the server with:

git clone -b 0.3.0 https://github.com/jedisct1/edgedns
cd edgedns
cargo install

Quickstart

  1. Edit a copy of the edgedns.toml configuration file
  2. Run edgedns -c /path/to/edgedns.toml

On Linux, you may use that sample systemd service to start it.

EdgeDNS can protect authoritative servers ("virtual DNS" mode), or act as a local cache for responses received from a recursive server.

EdgeDNS as a cache for authoritative servers ("virtual DNS" mode)

For this mode, set the type property in the [upstream] section to authoritative:

[upstream]
# Type of upstream servers: "resolver" or "authoritative"
type = "authoritative"

EdgeDNS will act as a "secondary DNS server" for the zones served by one or more "primary DNS servers". The external IP address edgedns is listening to can be configured as a public authoritative server for the zone.

This will reduce the load on the "primary server", mitigate common attacks, and ensure a continuity of service even if the primary server has temporary outages.

Binding as many ports as possible (udp_ports in the [network] section) is recommended in this mode of operation. This may require some adjustments to your system configuration. See below.

EdgeDNS as a local DNS cache

For this mode, set the type property in the [upstream] section to resolver:

[upstream]
# Type of upstream servers: "resolver" or "authoritative"
type = "resolver"

And configure the local host to use 127.0.0.1 as a resolver. EdgeDNS will cache responses, balance the load across the resolvers set, and improve your experience by making DNS more reliable.

Operation

EdgeDNS has two modes of operation:

  • In "virtual DNS" mode, it acts as an authoritative server, and sits between authoritative DNS servers and resolvers. It ensures that only correct queries are sent to upstream servers, balances the load across multiple upstreams, reduces the load by caching recent and frequent queries, mitigates DDoS attacks and does its best to respond to critical queries even when upstream servers are slow or unresponsive.
  • In "resolver" mode, EdgeDNS can act as a simple, non-recursive DNS cache, sitting between a recursive resolver and stub resolvers.

"virtual DNS" is the default mode, but this can be changed with the type property in the [upstream] section.

By default, the load is distributed using jump consistent hashing, ensuring that the upstream servers get a similar share, but queries for a given zone always favor the same upstream server.

As an alternative, servers can be tried sequentially, and queries are eventually sent to the first responsive server.

That behavior is controlled by the strategy property in the [upstream] section. uniform enables consistent hashing, fallback selects servers in sequence and minload uses the power-of-two-choices algorithm.

A unique feature of EdgeDNS is that it uses a fixed number of UDP sockets. Sockets designed to receive responses from upstream servers are all open at once, and are then kept open forever. Matching responses with queries is entirely done by the application instead of the kernel.

As a result, the server will not start if the maximum number of UDP ports to use is larger than the maximum number of file descriptors allowed.

Tweaking /etc/security/limits.conf or using ulimit -n might thus be required prior to starting the server.

By default, only 8 UDP ports are open. This is acceptable for a local cache ("resolver mode") sending queries to a local resolver on a trusted network. In all other scenarios, raising this number as much as possible (up to 64511) using the udp_ports property is highly recommended.

Live metrics

If the enabled property is set to true in the [webservice] section, EdgeDNS will serve live metrics that can be collected by the Prometheus time-series database.

The default URL to access these metrics is http://0.0.0.0:9090/metrics.

Note

This software is still a work in progress. More features are planned, with a focus on automatic DDoS mitigation.

Features

Reliable

EdgeDNS is written in Rust, which by design prevents common security and reliability issues while remaining extremely fast.

EdgeDNS can thus be used as a protection layer for DNS resolvers and authoritative servers.

EdgeDNS has been running flawlessly on the public dnscrypt.org-fr server since January 2016.

DNSSEC support

EdgeDNS is fully compatible with DNSSEC.

Low memory usage

In virtual DNS mode, responses are cached independently instead of performing zone transfers, in order to favor caching of hot records.

With a large number of zones, and an uneven distribution of queries across records, this leads to a very low memory usage compared to secondary servers keeping entire zones.

EDNS0 support

EdgeDNS fully supports EDNS0 both to respond to clients, and to communicate with upstream servers. This minimizes the number of queries requiring TCP and reduces latency.

Minimal truncated responses

Responses that don't fit within the maximum payload size supported by a client get a truncated response, whose content cannot be used by a resolver or stub resolver.

Instead of forwarding truncated responses sent by authoritative servers, EdgeDNS directly synthesizes the shortest possible responses.

Correct support for the dns0x20 extension

In order to improve resistance against forgery, some clients support the dns0x20 extension, which randomizes casing in DNS names. This shouldn't lead to distinct cache entries. The cache normalize names, but respect the form initially sent by clients in its responses.

Coalescing of identical queries

Multiple clients can be simultaneously waiting for the same cache entry to be filled.

Similar in-flight queries are coalesced, and the first received response is dispatched among all waiting clients.

Latency guarantee

Slow authoritative servers can have a huge impact on clients, even if this is a temporary situation.

EdgeDNS does its best to guarantee a maximum latency. If a response that needs to be refreshed doesn't get a response within a given time frame, EdgeDNS can directly respond with cached records in order to avoid breaking the latency guarantee.

Short responses to ANY queries

ANY queries are commonly used to conduct DDoS attacks.

The ANY query type has been deprecated for a while, but not answering queries with the ANY type breaks legacy software, namely Qmail.

Cloudflare handles ANY queries by synthesizing responses with HINFO records. EdgeDNS implements this proposal as well. It doesn't violate the protocol, doesn't break Qmail and mitigate abuses, while not requiring any interactions with authoritative servers.

Query validation

Only valid queries should be sent to authoritative servers. This mitigates common attacks exploiting vulnerabilities in resolvers and authoritative servers.

Negative caching

The absence of data is cached. Temporary errors such as SERVFAIL responses are also cached for a short period of time, in order to avoid hammering upstream servers.

Automatic failover

Outages of upstream servers are quickly detected, and background probes are sent to bring them back to the pool as soon as they get operational again.

Consistent hashing

In order to improve cache locality, the same questions should preferably be always sent to the same upstream servers.

Consistent hashing limits the amount of reshuffling after servers are being flagged as down or back up, and is a strategy the proxy can use in order to distribute queries amount authoritative servers.

As an alternative, EdgeDNS can send data to a primary upstream servers, and fall back to a list of backup servers in case of an outage.

Resilience against temporary outages of authoritive servers

If authoritative servers cannot be reached for a given zone, and a previous version of the response is available, EdgeDNS serves that version instead of returning a server failure.

If authoritative responses are received, but with a significant delay, EdgeDNS responds with a previous version of the response, and updates its cache as soon as the update is effectively being received.

Resilience against cache pollution

An attacker could fill the cache with entries of little relevance, or invalid entries, in order to reduce the cache efficiency, or partly disrupt the service.

In order to protect against cache pollution, EdgeDNS uses the CLOCK-Pro algorithm. This algorithm separately tracks recently used and frequently used entries.

A significant amount of new entries does not have a perceptible impact on frequently asked DNS questions.

TCP slots reuse

The number of simultaneous connections coming from the same client IP address is capped, and dynamically adjusted according to the total number of free slots.

After the cap is reached, new connections recycle older connections from the same client IP. A single client opening many TCP connections doesn't affect the general service availablity.