fanout

fanout

About This Fork

This is a hardened fork of networkservicemesh/fanout.

Why this fork exists:

1. Supply-chain security — The original project at github.com/networkservicemesh/fanout pulls in transitive dependencies from the github.com/networkservicemesh/* ecosystem. This fork eliminates those dependencies, reducing the attack surface to the minimum required set: CoreDNS, miekg/dns, and quic-go.
2. Robustness — Additional hardening such as connection pooling with liveness checks, retry-on-stream-failure, strict TLS version enforcement, and comprehensive race-detector-enabled tests.
3. Modern encrypted DNS protocols — Full support for six DNS transport protocols, including DoH (HTTP/2), DoH3 (HTTP/3 over QUIC), and DoQ (DNS-over-QUIC, RFC 9250), in addition to the plain UDP, TCP, and DoT transports from the original.

Name

fanout — parallel proxying DNS messages to upstream resolvers.

Description

Each incoming DNS query that hits the CoreDNS fanout plugin will be replicated in parallel to each listed upstream. The first non-negative response from any of the queried DNS servers will be forwarded as a response to the application's DNS request.

Supported Protocols

Upstream addresses are distinguished by their URL prefix:

fanout . <plain-host>[:port]        # UDP/TCP  (default)
fanout . tls://<host>[:port]        # DoT
fanout . https://<host>/<path>      # DoH  (HTTP/2)
fanout . h3://<host>/<path>         # DoH3 (HTTP/3 over QUIC)
fanout . quic://<host>[:port]       # DoQ  (RFC 9250, default port 853)

You can mix protocols freely in a single fanout block. The first successful response wins, regardless of which transport delivered it.

Syntax

fanout FROM TO... {
    tls CERT KEY CA
    tls_servername NAME
    network PROTOCOL
    worker-count COUNT
    policy POLICY
    weighted-random-server-count COUNT
    weighted-random-load-factor FACTOR...
    except DOMAIN...
    except-file FILE
    attempt-count COUNT
    timeout DURATION
    bootstrap IP...
    ecs [CIDR]
    debug
    race
    race-continue-on-error
}

• tls CERT KEY CA — define the TLS properties for TLS connections. From 0 to 3 arguments can be provided with the meaning as described below:
  • tls — no client authentication is used, and the system CAs are used to verify the server certificate
  • tls CA — no client authentication is used, and the file CA is used to verify the server certificate
  • tls CERT KEY — client authentication is used with the specified cert/key pair. The server certificate is verified with the system CAs
  • tls CERT KEY CA — client authentication is used with the specified cert/key pair. The server certificate is verified using the specified CA file
• tls_servername NAME — allows you to set a server name in the TLS configuration; for instance 9.9.9.9 needs this to be set to dns.quad9.net. Multiple upstreams are still allowed in this scenario, but they have to use the same tls_servername. The legacy alias tls-server is also accepted. E.g. mixing 9.9.9.9 (Quad9) with 1.1.1.1 (Cloudflare) will not work.
• worker-count — the number of parallel queries per request. By default equals the count of the upstream list. Use this only to reduce parallel queries per request.
• policy — specifies the policy for DNS server selection. The default is sequential.
  • sequential — select DNS servers one-by-one based on their order
  • weighted-random — select DNS servers randomly based on weighted-random-server-count and weighted-random-load-factor
• weighted-random-server-count — the number of DNS servers to be queried. Equals the number of specified upstreams by default. Used only with the weighted-random policy.
• weighted-random-load-factor — the probability of selecting a server (1–100). Specified in the order of the upstream list. Default is 100 for all servers. Used only with the weighted-random policy.
• network — specific network protocol for plain upstreams: tcp, udp (default), or tcp-tls.
• except — a space-separated list of domains to exclude from proxying.
• except-file — path to a file with line-separated domains to exclude from proxying.
• attempt-count — the number of failed attempts before considering an upstream to be down. If 0, the upstream will never be marked as down and the request will run until timeout. Default is 3.
• timeout — the maximum time for the entire request. Default is 30s.
• bootstrap IP... — one or more IP addresses (with optional :port, default port 53) of plain-DNS servers used to resolve hostnames in upstream URLs. This is required when DoH, DoH3, or DoQ upstreams use hostname-based endpoints (e.g. dns.nextdns.io) and the system's default DNS resolver might point back at this very CoreDNS instance, creating a circular dependency. The bootstrap resolver sends plain-UDP DNS queries to the specified IPs, bypassing the system resolver entirely. Only the hostname resolution of upstream endpoints uses the bootstrap servers; regular client queries still go through the configured upstreams.
• ecs [CIDR] — enable EDNS0 Client Subnet (RFC 7871) on bootstrap DNS queries. Requires a prior bootstrap directive.

  • Without argument (ecs): the local outgoing IP is auto-detected and sent as a /24 (IPv4) or /48 (IPv6) prefix. This is the recommended default — it works on laptops, hotel Wi-Fi, and dynamically assigned addresses without manual configuration.

  • With argument (ecs 203.0.113.0/24): the given CIDR is used verbatim.

  • When to use: When the bootstrap resolvers are not on your local network. Without ECS, a distant bootstrap resolver (e.g. 9.9.9.11 in Frankfurt) asks the authoritative server for dns.nextdns.io and receives the IP closest to Frankfurt, not to you. With ECS enabled, your subnet is forwarded so the authoritative server returns the endpoint closest to you.

  • Privacy note: ECS reveals a prefix of your IP to the bootstrap resolver and, transitively, to authoritative name servers. If this is undesirable, simply omit ecs — the bootstrap query will then be a plain DNS lookup without subnet information.

  • Which bootstrap IPs support ECS? Not all resolvers forward ECS to authoritative servers. You must use an ECS-enabled resolver for this feature to have any effect:

    Provider	Standard (no ECS)	ECS-enabled
    Quad9	9.9.9.9, 149.112.112.112	9.9.9.11, 149.112.112.11
    Google	—	8.8.8.8, 8.8.4.4
    Cloudflare	1.1.1.1, 1.0.0.1	(ECS generally not forwarded)

  • Why not for forwarded queries? ECS on bootstrap queries only resolves the upstream hostname (e.g. dns.nextdns.io → optimal anycast IP). For the actual forwarded DNS queries, the upstream sees the IP of the fanout instance, not the original downstream client. Propagating per-client ECS through this plugin would therefore require explicit client-subnet handling in the request path and — in combination with the CoreDNS cache plugin — would still be potentially harmful: the cache keys on (qname, qtype, qclass) without considering ECS subnets, so different clients could receive cached answers optimized for someone else's location.

• debug — emit per-upstream intermediate request failures through the fanout logger so defective upstream attempts remain visible even when another upstream still answers successfully. Expected local cancellations caused by fanout shutting down losing attempts are excluded from these warning lines.
• race — gives priority to the first result, whether it is negative or not, as long as it is a valid DNS response.
• race-continue-on-error — When enabled together with race, fanout does not early-return on erroneous DNS responses such as SERVFAIL, but still treats NOERROR and NXDOMAIN as terminal answers that can end the race immediately. The default is false.

Metrics

If monitoring is enabled (via the prometheus plugin) then the following metrics are exported:

Per-upstream metrics

These count individual request attempts sent to each upstream. Because fanout queries several upstreams in parallel per incoming query, the sum of these counters is larger than the number of downstream queries.

• coredns_fanout_request_count_total{to} — request attempt count per upstream, including attempts that fail before a DNS response is received.
• coredns_fanout_request_error_count_total{to, error} — request attempt count per upstream and bounded upstream error class.
• coredns_fanout_request_cancel_count_total{to} — request attempt count per upstream that fanout canceled locally before a final upstream outcome was received.
• coredns_fanout_request_success_count_total{to} — request attempt count per upstream that completed with a valid DNS response (transport-level success). This includes responses with any RCODE, e.g. SERVFAIL or NXDOMAIN, because the upstream did respond with a well-formed DNS packet.
• coredns_fanout_response_win_count_total{to} — number of times an upstream's response was the one fanout selected and returned downstream. Because fanout queries multiple upstreams in parallel, several may succeed, but only one response is written to the client per incoming query. A win is counted for any selected response, including non-success RCODEs when no better response was available.
• coredns_fanout_response_rcode_count_total{to, rcode} — count of returned RCODEs per upstream.
• coredns_fanout_request_duration_seconds{to} — duration of request attempts that completed with a valid DNS response.

Downstream (query-level) metrics

These count whole incoming queries, regardless of how many upstreams each one fanned out to. They answer "did fanout serve the client?", which the per-upstream counters alone cannot — especially under the weighted-random policy, where not every upstream is queried per request.

• coredns_fanout_query_count_total — number of incoming queries fanout handled (queries matching the configured FROM zone; queries passed to the next plugin are not counted).
• coredns_fanout_query_failure_count_total{reason} — number of incoming queries fanout answered with a failure instead of a valid upstream response, grouped by bounded reason:
  • no_response — no upstream produced a result before the deadline (timeout or cancellation); fanout returns SERVFAIL.
  • upstream_error — every upstream attempt ended with an error; fanout returns SERVFAIL.
  • format_error — the selected upstream response did not match the question; fanout returns FORMERR.
  • write_failed — writing the selected response back to the client failed.

Where to is one of the upstream servers (TO from the config), rcode is the returned RCODE from the upstream, and error is one of the bounded classes used by fanout (for example connect_failed, request_send_failed, response_read_failed, or response_decode_failed).

The counters are designed to follow a simple accounting model per upstream:

• request_count_total = request_error_count_total (summed over error labels) + request_cancel_count_total + request_success_count_total
• request_success_count_total = response_rcode_count_total summed over all rcode labels
• response_win_count_total <= request_success_count_total (because multiple upstreams may succeed per query but only one is selected)

At the query level:

• query_count_total = (queries served downstream) + query_failure_count_total summed over all reason labels
• sum(response_win_count_total) over all upstreams = query_count_total - query_failure_count_total{reason="no_response"} - query_failure_count_total{reason="upstream_error"} (every served or FORMERR/write_failed query selected exactly one upstream response)

Practical interpretation

Use these metrics to identify the best upstream in your environment:

• An upstream with a high connect_failed error rate and zero successes is likely blocked by a firewall or misconfigured.
• An upstream with high success but low win rate works fine but is slower than competing upstreams.
• An upstream with high SERVFAIL rcode count responds, but cannot resolve queries — check the server itself.
• Compare win rates across upstreams to find the fastest and most reliable one.
• Track rate(coredns_fanout_query_failure_count_total[5m]) / rate(coredns_fanout_query_count_total[5m]) as the user-facing failure ratio — a sustained no_response/upstream_error rate means all upstreams are failing together (upstream outage, local network, or too-tight timeout), not just one slow server.

Debug logging

When debug logging is enabled, fanout logs only defective upstream attempts and includes the normalized error_class field so warning lines can be correlated directly with request_error_count_total. Expected local cancellations (i.e. losing race participants) are suppressed to avoid noise. When a request times out (context deadline exceeded), the log line includes a context_error field to distinguish timeouts from genuine network errors. Without debug enabled, final request failures are still logged at ERROR level by the CoreDNS errors plugin, including the upstream endpoint in the error message.

Examples

Plain DNS (UDP)

Proxy all requests within example.org. to four nameservers. The first positive response wins.

example.org {
    fanout . 127.0.0.1:9005 127.0.0.1:9006 127.0.0.1:9007 127.0.0.1:9008
}

Plain DNS (TCP)

Send parallel requests to three resolvers via TCP.

. {
    fanout . 10.0.0.10:53 10.0.0.11:1053 [2003::1]:53 {
        network TCP
    }
}

DNS-over-TLS (DoT)

Proxy all requests to Quad9 using DNS-over-TLS (RFC 7858). The tls_servername is mandatory because 9.9.9.9 can't be used in TLS negotiation.

. {
    fanout . tls://9.9.9.9 {
        tls_servername dns.quad9.net
    }
}

DNS-over-HTTPS (DoH)

Proxy all requests to Cloudflare via DNS-over-HTTPS (RFC 8484, HTTP/2).

. {
    fanout . https://cloudflare-dns.com/dns-query
}

DNS-over-HTTPS/3 (DoH3)

Proxy all requests to Cloudflare via DNS-over-HTTPS over HTTP/3 (QUIC). Use the h3:// prefix — it is internally converted to an HTTPS URL.

. {
    fanout . h3://cloudflare-dns.com/dns-query
}

DNS-over-QUIC (DoQ)

Proxy all requests to AdGuard DNS via DNS-over-QUIC (RFC 9250). The default port is 853/UDP. TLS 1.3 with ALPN token doq is enforced automatically.

. {
    fanout . quic://dns.adguard-dns.com
}

Mixed Protocols

Fan out to multiple upstreams across different transports simultaneously. The first successful response from any transport wins.

. {
    fanout . 1.1.1.1 https://cloudflare-dns.com/dns-query h3://cloudflare-dns.com/dns-query quic://dns.adguard-dns.com:853
}

Bootstrap Resolver

When running inside a container whose system DNS points back at CoreDNS itself, hostname-based upstreams like dns.nextdns.io cannot be resolved. The bootstrap directive breaks this circular dependency by resolving upstream hostnames through the specified plain-DNS servers.

. {
    fanout . https://dns.nextdns.io/abc123 h3://dns.nextdns.io/abc123 quic://DoQ-abc123.dns.nextdns.io:853 {
        bootstrap 9.9.9.11 149.112.112.11
    }
}

Bootstrap with ECS (Auto-Detect)

When the bootstrap resolvers are in a different geographic region, EDNS0 Client Subnet ensures the authoritative server returns the upstream's anycast endpoint closest to you — not to the bootstrap resolver. With ecs (no argument), the local IP is auto-detected.

. {
    fanout . https://dns.nextdns.io/abc123 h3://dns.nextdns.io/abc123 quic://DoQ-abc123.dns.nextdns.io:853 {
        bootstrap 9.9.9.11 149.112.112.11
        ecs
        race
        race-continue-on-error
    }
}

Bootstrap with Explicit ECS Subnet

If you need precise control over which subnet is announced (e.g. in a data centre with fixed IP ranges), pass a CIDR explicitly.

. {
    fanout . https://dns.nextdns.io/abc123 {
        bootstrap 8.8.8.8
        ecs 203.0.113.0/24
    }
}

Excluding Domains

Proxy everything except requests to example.org.

. {
    fanout . 10.0.0.10:1234 {
        except example.org
    }
}

Limiting Workers

Send parallel requests to five resolvers but limit to two concurrent workers.

. {
    fanout . 10.0.0.10:53 10.0.0.11:53 10.0.0.12:53 10.0.0.13:1053 10.0.0.14:1053 {
        worker-count 2
    }
}

Race Mode

Multiple upstreams are configured but one of them is down. With race enabled, the first result is returned immediately instead of waiting for timeouts.

. {
    fanout . 10.0.0.10:53 10.0.0.11:53 10.0.0.12:53 {
        race
    }
}

By default race will return the first DNS response that arrives as long as it is a valid DNS result — this may be a non-success RCODE such as SERVFAIL or NXDOMAIN.

If you prefer to keep the latency benefits of race but avoid early-returning on upstream error responses, enable race-continue-on-error. When both race and race-continue-on-error are set, fanout will still end the race immediately for NOERROR and NXDOMAIN, but it will keep waiting when a fast upstream returns an error such as SERVFAIL.

Example: A fast upstream returns SERVFAIL and a slow upstream returns NOERROR. With race alone the user receives SERVFAIL; with race and race-continue-on-error set the user receives a successful domain name resolution (if it arrives before the request timeout).

. {
    fanout . 10.0.0.10:53 10.0.0.11:53 10.0.0.12:53 {
        race
        race-continue-on-error
    }
}

Weighted Random Selection

Send parallel requests to two randomly selected resolvers. 127.0.0.1:9007 is selected most frequently due to its highest load factor.

example.org {
    fanout . 127.0.0.1:9005 127.0.0.1:9006 127.0.0.1:9007 {
        policy weighted-random
        weighted-random-server-count 2
        weighted-random-load-factor 50 70 100
    }
}

Building CoreDNS with fanout

See coredns/README.md for instructions on building a CoreDNS binary or Docker image with this plugin.