continuwuity/docs/advanced/dns.mdx

# DNS Tuning (recommended)

For federation, Matrix homeservers conduct an enormous amount of DNS requests, sometimes up to thousands of queries per minute. Normal DNS resolvers are simply not designed for this load, and running Continuwuity with them will likely result in various [DNS and federation errors](../troubleshooting#dns-issues).

To solve this issue, it is strongly recommended to self-host a high-quality, external caching DNS resolver for Continuwuity. This guide will use [Unbound][unbound] as the recommended example, but the general principle applies to any resolver.

[unbound]: https://wiki.archlinux.org/title/Unbound

## Overview

For generic deployments, install your resolver of choice and configure `/etc/resolv.conf` to point to it. The resolver should ideally reside on the same host as Continuwuity.

```txt title="/etc/resolv.conf"
nameserver 127.0.0.1
```

**Avoid using `systemd-resolved`** as it does **not** perform very well under high load, and we have identified its DNS caching to not be very effective.

### For Docker users

Docker bridge networks uses a non-performant resolver to intercept and respond to container hostnames, and **this should also be avoided**. Instead, mount a custom `/etc/resolv.conf` file into the container, and hardcode a resolver address to bypass Docker's.

It is recommended to run a dedicated resolver container for Continuwuity, as to separate from the host's resolver setup. To do this, create a custom bridge network and IP range, and explicitly define an IP address for the resolver container.

<details>
<summary>Example Docker deployment with unbound</summary>

```yaml title="docker-compose.yml"
networks:
  matrix_net:
    ipam:
      driver: default
      config:
        - subnet: "10.10.10.0/24"

services:
    homeserver:
        # ...
        volume:
            - ./continuwuity-resolv.conf:/etc/resolv.conf:ro

    unbound:
        # ...
        networks:
            matrix_net:
                ipv4_address: 10.10.10.20
```

```txt title="continuwuity-resolv.conf"
nameserver 10.10.10.20
```

</details>

### For IPv4-only users

If you don't have IPv6 connectivity, changing `ip_lookup_strategy` to only resolve for IPv4 will reduce unnecessary AAAA queries.

```toml title="continuwuity.toml"
[global]
# 1 - Ipv4Only (Only query for A records, no AAAA/IPv6)
ip_lookup_strategy = 1
```

## Unbound

[Unbound][unbound] is the recommended resolver to run with Continuwuity. For Docker users, the `docker.io/madnuttah/unbound` image ([Github repo][madnuttah-unbound-repo]) can be used.

After installation, you can tune `/etc/unbound/unbound.conf` values according to your needs. While Continuwuity cannot recommend a "works-for-everyone" Unbound DNS setup guide, the official [Unbound tuning guide][unbound-tuning-guide] and the [Unbound Arch Linux wiki page][unbound-arch-linux] may be of interest.

Some values that should be tuned include:

- Increase `rrset-cache-size` and `msg-cache-size` to something much higher than the default `4M`, such as `64M`.

- Increase `discard-timeout` to something like `8000` to wait longer for upstream resolvers, as recursion can take a long time to respond to some domains. Continuwuity default to `dns_timeout = 10` seconds, so dropping requests sooner would lead to unnecessary retries and/or failures.

- If you want to use forwarders instead of Unbound's default recursion module, configure them as following:

    ```
    # use public resolvers as upstreams, since
    # they're usually faster than recursion
    forward-zone:
        name: "."
        forward-addr: 1.0.0.1@53
        forward-addr: 1.1.1.1@53
        # Also use IPv6 ones if you're dual-stack
        forward-addr: 2606:4700:4700::1001@53
        forward-addr: 2606:4700:4700::1111@53

    # alternatively, use DNS-over-TLS for forwarders.
    # this generally takes as much time as recursion
    # due to TLS overhead, but allows on-wire encryption
    # forward-zone:
        # name: "."
        # forward-tls-upstream: yes
        # forward-addr: 1.0.0.1@853#cloudflare-dns.com
        # forward-addr: 1.1.1.1@853#cloudflare-dns.com
        # forward-addr: 2606:4700:4700::1001@853#cloudflare-dns.com
        # forward-addr: 2606:4700:4700::1111@853#cloudflare-dns.com
    ```

[madnuttah-unbound-repo]: https://github.com/madnuttah/unbound-docker/
[unbound-tuning-guide]: https://unbound.docs.nlnetlabs.nl/en/latest/topics/core/performance.html
[unbound-arch-linux]: https://wiki.archlinux.org/title/Unbound

## Other resolvers

### dnsproxy

[Dnsproxy][dnsproxy] and its sister product [AdGuard Home][adguard-home] are known to work with Continuwuity and has an official Docker image. They have support for DNS-over-HTTPS as well as DNS-over-QUIC, but not recursion.

To best utilise dnsproxy, you should enable proper caching with `--cache` and set `--cache-size` to something bigger, like `64M`.

[dnsproxy]: https://github.com/AdguardTeam/dnsproxy
[adguard-home]: https://github.com/AdguardTeam/AdGuardHome

### dnsmasq

[dnsmasq][arch-linux-dnsmasq] can possibly work with Continuwuity, though it only support forwarding rather than recursion. Increase the `cache-size` to something like `10000` for better caching performance.

However, `dnsmasq` does not support TCP fallback which can be problematic when receiving large DNS responses such as from large SRV records. If you still want to use dnsmasq, make sure you disable `dns_tcp_fallback` in Continuwuity config.

[arch-linux-dnsmasq]: https://wiki.archlinux.org/title/Dnsmasq

### Technitium

[Technitium][technitium] supports recursion as well as a myriad of forwarding protocols, allows saving cache to disk natively, and does work well with Continuwuity. Its default configurations however ratelimits single-IP requests by a lot, and hence must be changed. You may consult this [community guide][technitium-continuwuity] for more details on setting up a dedicated Technitium for Continuwuity.

[technitium]: https://github.com/TechnitiumSoftware/DnsServer
[technitium-continuwuity]: https://muoi.me/~stratself/articles/technitium-continuwuity/

### None

If you can't install a local DNS caching resolver for some reason, you may still configure your machine to talk directly to public resolvers:

```txt title="/etc/resolv.conf"
nameserver 1.0.0.1
nameserver 1.1.1.1

# (optional) use DNS-over-TCP
# options use-vc
```

Cloudflare DNS are recommended over Quad9 and Google, as they don't tend to ratelimit queries.

In this case, you may want to increase `dns_cache_entries` to a higher value. However, this is an imperfect solution, as the internal cache isn't as robust and reliable as an external one.

## Testing

As a rough stress test, you can issue `!admin query resolver flush-cache -a` or `!admin server clear-caches` to trigger a netburst of DNS queries. If your resolver can handle these loads without problem, then it should be ready for regular Continuwuity activity.

To test connectivity against a specific server, use `!admin debug ping <SERVER_NAME>` and `!admin debug resolve-true-destination <SERVER_NAME>`.

Note that it is expected that not all servers will be resolved, as some of them may be temporarily offline, has broken DNS and/or discovery configuration, or have been decommissioned.

## Further steps

- (Recommended) Consider setting **`dns_cache_entries = 0`**, as to fully rely on the more performant external resolver.

- Consider employing **persistent cache to disk**, so your resolver can still run without hassle after a restart. For Unbound, this can be done by pairing it with a Redis database using the [Cache DB module][unbound-cachedb].

- Consider [enabling **Serve Stale**][unbound-serve-stale] functionality to serve expired data beyond DNS TTLs, as Matrix homeservers are generally static IPs that doesn't change. Also consider [enabling **prefetching**][unbound-prefetching] to always update DNS hot cache.

- If you still experience DNS performance issues, another step could be to **disable DNSSEC** (which is computationally expensive) at a cost of slightly decreased security. On Unbound this is done by commenting out `trust-anchors` config options and removing the `validator` module.

- For all resolvers except dnsmasq, some users have reported that setting `query_over_tcp_only = true` in Continuwuity has improved DNS reliability at a slight performance cost due to TCP overhead. Generally this is not needed if your resolver and homeserver is on the same machine.

[unbound-cachedb]: https://unbound.docs.nlnetlabs.nl/en/latest/manpages/unbound.conf.html#cache-db-module-options
[unbound-serve-stale]: https://wiki.archlinux.org/title/Unbound#Serving_expired_records
[unbound-prefetching]: https://wiki.archlinux.org/title/Unbound#Keeping_DNS_cache_always_up_to_date