Reference

Rate limits and concurrency

Reference for the proxy's protection layers against overload, abuse, and resource exhaustion. Every limit has a default and an environment-variable override.

Auth rate limiter

Per-IP brute-force protection for SigV4 authentication and admin login endpoints.

Setting	Default	Env var
Max failures before lockout	100	`DGP_RATE_LIMIT_MAX_ATTEMPTS`
Rolling window	300 s (5 min)	`DGP_RATE_LIMIT_WINDOW_SECS`
Lockout duration	600 s (10 min)	`DGP_RATE_LIMIT_LOCKOUT_SECS`

After a lockout expires, the failure counter resets and the IP can authenticate again. Lockout responses are 429 SlowDown.

Progressive delay

Failed auth attempts add an artificial delay to responses before the lockout threshold is reached:

Failures	Delay
1–10	none
11	200 ms
12	400 ms
13	800 ms
14	1.6 s
15	3.2 s
16+	5 s (cap)

IP extraction

Rate limiting requires a client IP. The proxy reads X-Forwarded-For or X-Real-IP headers only when DGP_TRUST_PROXY_HEADERS=true; the default is false, so direct-to-internet deployments are protected against IP spoofing out of the box. DGP_TRUST_PROXY_HEADERS=true is appropriate only behind a trusted reverse proxy (nginx, Caddy, ALB) that injects these headers.

For direct-to-internet deployments without trusted headers, the rate limiter receives no IP and is effectively a no-op for those requests; SigV4 signature verification and the replay cache still apply. The admission chain's source_ip_list predicates use axum ConnectInfo (wired at startup) and continue to work in the direct case; the rate limiter does not consume ConnectInfo.

Codec semaphore

Limits concurrent xdelta3 encode/decode subprocesses. Delta reconstruction (decode) is CPU-fast but I/O-bound (fetching reference + delta from storage), so the default is generous.

Setting	Default	Env var
Max concurrent xdelta3 processes	`num_cpus * 4` (min 16)	`DGP_CODEC_CONCURRENCY`

Behavior differs by operation:

GET (decode): waits up to 60 seconds for a codec slot, then returns 503 SlowDown.
PUT (encode): fails immediately with 503 SlowDown when no slot is available, so queued uploads do not hold large request bodies in memory while waiting.

HTTP concurrency limit

Caps total in-flight HTTP requests across the server. Requests beyond the limit queue until a slot opens or the request timeout fires.

Setting	Default	Env var
Max concurrent requests	1024	`DGP_MAX_CONCURRENT_REQUESTS`

Request timeout

Per-request deadline applied to all S3 API requests; returns HTTP 504 Gateway Timeout when exceeded. Large delta reconstructions over slow storage links count toward this deadline.

Setting	Default	Env var
Request timeout	300 s	`DGP_REQUEST_TIMEOUT_SECS`

Multipart upload limit

Caps concurrent in-progress multipart uploads. Each upload holds part data in memory until completion; the limit bounds memory consumption from abandoned or excessive uploads. Returns 503 SlowDown when exceeded.

Setting	Default	Env var
Max concurrent uploads	1000	`DGP_MAX_MULTIPART_UPLOADS`

Replay detection cache

Caches SigV4 signatures and rejects duplicates within the replay window. This is independent of DGP_CLOCK_SKEW_SECONDS, which governs how far a request timestamp may drift from the server clock during SigV4 verification — a different check.

Setting	Default	Env var
Replay window	2 s	`DGP_REPLAY_WINDOW_SECS`
Clock skew tolerance	300 s	`DGP_CLOCK_SKEW_SECONDS`
Max cache entries	500,000	—

A duplicate of a mutating request (PUT/POST/DELETE) within the window is rejected with 400. A duplicate of an idempotent read (GET/HEAD) is tolerated and served — boto3 emits byte-identical signatures for the same request within one signing second, and replaying a read re-reads the same bytes. Replay rejections are not counted toward the auth-failure lockout. DGP_REPLAY_WINDOW_SECS=0 disables replay rejection entirely. When the cache exceeds 500K entries, expired signatures are evicted first.

S3 backend HEAD concurrency

During LIST operations that require per-object metadata, the proxy issues HEAD requests to the upstream S3 backend. These are limited to avoid triggering the backend's own throttling.

Setting	Default	Configurable
Max concurrent HEADs	50	No

Summary of all env vars

Env var	Default	Description
`DGP_RATE_LIMIT_MAX_ATTEMPTS`	100	Auth failures before lockout
`DGP_RATE_LIMIT_WINDOW_SECS`	300	Rolling window for failure counting
`DGP_RATE_LIMIT_LOCKOUT_SECS`	600	Lockout duration after max failures
`DGP_TRUST_PROXY_HEADERS`	false	Trust `X-Forwarded-For` / `X-Real-IP` for IP extraction (only behind a reverse proxy)
`DGP_CODEC_CONCURRENCY`	cpus*4 (min 16)	Max concurrent xdelta3 processes
`DGP_MAX_CONCURRENT_REQUESTS`	1024	Max in-flight HTTP requests
`DGP_REQUEST_TIMEOUT_SECS`	300	Per-request timeout
`DGP_MAX_MULTIPART_UPLOADS`	1000	Max concurrent multipart uploads
`DGP_CLOCK_SKEW_SECONDS`	300	SigV4 request-timestamp drift tolerance
`DGP_REPLAY_WINDOW_SECS`	2	SigV4 replay detection window (0 disables)

Authentication and access — SigV4 verification and replay-detection semantics
Configuration — full env-var registry