loki.write component should set a "Content-Encoding: snappy" header

[a-abella]

Component(s)

loki.write

What's wrong?

loki.write properly compresses Loki write API payloads with Snappy but does not set the Content-Encoding header;

alloy/internal/component/common/loki/client/batch.go

Lines 139 to 149 in e6d2a12

	// encode the batch as snappy-compressed push request, and returns
	// the encoded bytes and the number of encoded entries
	func (b *batch) encode() ([]byte, int, error) {
	req, entriesCount := b.createPushRequest()
	buf, err := proto.Marshal(req)
	if err != nil {
	return nil, 0, err
	}
	buf = snappy.Encode(nil, buf)
	return buf, entriesCount, nil
	}

alloy/internal/component/common/loki/client/shards.go

Lines 468 to 484 in ff508c0

	func (s *shards) send(ctx context.Context, tenantID string, buf []byte) (int, error) {
	ctx, cancel := context.WithTimeout(ctx, s.cfg.Timeout)
	defer cancel()
	req, err := http.NewRequestWithContext(ctx, "POST", s.cfg.URL.String(), bytes.NewReader(buf))
	if err != nil {
	return -1, err
	}
	const contentType = "application/x-protobuf"
	req.Header.Set("Content-Type", contentType)
	req.Header.Set("User-Agent", userAgent)
	// If the tenant ID is not empty alloy is running in multi-tenant mode, so
	// we should send it to Loki
	if tenantID != "" {
	req.Header.Set("X-Scope-OrgID", tenantID)
	}

Sending compressed data without the proper encoding header is a violation of RFC 9110 Section 8.4 Content-Encoding

Attempting to inject the header via the http_headers field of the endpoint block results in an error:

Jan 23 11:24:02 alma9 alloy[59987]: Error: remote:115:9: Failed to build component: decoding configuration: setting header "Content-Encoding" is not allowed

I've tested injecting the header upstream in a proxy and Loki ingest works fine both with and without the header set so I assume Loki detects the encoding with byte identifiers instead of header values, but any intermediate HTTP services might not be as flexible.

Steps to reproduce

• Send any log data with loki.write
• Observe request headers either via tcpdump if plain http, a mitm proxy, request debug logging on the remote server, etc

System information

No response

Software version

Grafana Alloy v1.12.1

Configuration

Sourced from the `grafana/k8s-monitoring` helm chart,


loki.write "logsservice" {
  endpoint {
    url = "http://loki.meta-monitor.svc.cluster.local:3100/loki/api/v1/push"
    retry_on_http_429 = true
    tls_config {
      insecure_skip_verify = false
    }
    min_backoff_period = "500ms"
    max_backoff_period = "5m"
    max_backoff_retries = "10"
  }
  external_labels = {
    "cluster" = "monitoring",
    "k8s_cluster_name" = "monitoring",
  }
}

Logs

Not traditional logs, but observing `tcpdump` byte stream at the remote endpoint shows the absence of the required header.



07:53:13.100928 eth0  In  IP 172-31-108-191.envoy-eg-d8c59e83.envoy-gateway-system.svc.cluster.local.45518 > redacted-vector-0.redacted-vector-headless.vector.svc.cluster.local.3100: Flags [P.], seq 2124:3245, ack 151, win 491, options [nop,nop,TS val 4054487890 ecr 3668709224], length 1121
E....h@.}..}..l...l~....4.PY........*......
...R...hPOST / HTTP/1.1
host: loki-write.monitor.redacted.com
user-agent: Alloy/v1.12.1 (linux; helm)
content-length: 592
authorization: Basic redacted
content-type: application/x-protobuf
x-agent-id: 93a3fb7c-5b52-4b4e-9636-e361a864f9c4
x-alloy-id: 93a3fb7c-5b52-4b4e-9636-e361a864f9c4
x-scope-orgid: redacted
x-forwarded-for: redacted
x-forwarded-proto: https
x-envoy-external-address: redacted
x-request-id: 3dc11957-06cb-41b9-9dbd-86eace53c65a

...F
..
..{app="konnectivity-agent", cluster="redacted", container=V<.@job="kube-system/Rb..k8s_cl.f._name=Nk....space2Q.$", service.:Ra..#.JD..}...
..............XI0123 07:53:11.796345  ..d1 client.go:557] "remote c-@.on EOF"..HID=472876.*
.pod.#k.,9lP-7b5785b7c4-7lhkk.Y
...0instance_id.B=R...H.J$..[.n[....)y..%y.......6y..2.340637%w. .y..024.........-....j...y......>...2.687861.y..3069.y..y..y..y.:y....
.M..........2l..2.84692Nz.p449] "error dialing backend" ...="..\ tcp 10.3.64.212:19100: ...	...refused".QTID=5932207892241112149.8.io...279./$Address="1>l..".......j.........
07:53:13.101085 eth0  Out IP redacted-vector-0.redacted-vector-headless.vector.svc.cluster.local.3100 > 172-31-108-191.envoy-eg-d8c59e83.envoy-gateway-system.svc.cluster.local.45518: Flags [P.], seq 151:226, ack 3245, win 1702, options [nop,nop,TS val 3668710724 ecr 4054487890], length 75
E....K@.......l~..l.........4.T.....1......
...D...RHTTP/1.1 200 OK
content-length: 0
date: Fri, 23 Jan 2026 07:53:12 GMT

Tip

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[kalleep]

Hey, yes you are correct in that we do not set this header but we should 👍

I've tested injecting the header upstream in a proxy and Loki ingest works fine both with and without the header set so I assume Loki detects the encoding with byte identifiers instead of header values, but any intermediate HTTP services might not be as flexible.

It's even simpler than that. When loki gets logs with Content-Type: application/x-protobuf it assumes snappy encoding.