Fastly reference

This page provides detailed explanations of Fastly VCL configuration components. For installation and setup, see the main Fastly guide.

Crawler detection approaches

Both approaches route only crawler traffic to Edge Gateway. Regular users always go directly to origin. The difference is how crawlers are identified:

Approach	Description	Use when
Broad (default)	Generic keywords (bot, crawl, spider) + explicit patterns	You want to catch more crawlers including unknown ones
Conservative	Explicit list of known crawlers	You want predictable behavior with no false positives

Broad detection (default)

Catches crawlers using generic keywords plus explicit patterns for crawlers without these keywords in their name.

vcl

# Generic crawler keywords
if (req.http.User-Agent ~ "(?i)(bot|crawl|spider|slurp)") {
    set var.is_crawler = true;
}

# Crawlers without generic keywords in name
if (req.http.User-Agent ~ "(?i)(WhatsApp|Snapchat|facebookexternalhit|AMZN-User|Claude-User|Perplexity-User|ChatGPT-User)") {
    set var.is_crawler = true;
}

Conservative detection

Explicit patterns for known search engines, AI crawlers, social media, and messengers based on bot aliases.

To use this approach, replace the crawler detection block in the recv snippet:

vcl

declare local var.is_crawler BOOL;
set var.is_crawler = false;

# Search engines ($SearchBots alias)
if (req.http.User-Agent ~ "(?i)Googlebot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)bingbot") { set var.is_crawler = true; }

# AI crawlers ($AIBots alias)
if (req.http.User-Agent ~ "(?i)ChatGPT-User") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)GPTBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)OAI-SearchBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)OAI-AdsBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)PerplexityBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Perplexity-User") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)ClaudeBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Claude-User") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Claude-SearchBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Amazonbot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)AMZN-User") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Google-Agent") { set var.is_crawler = true; }

# Google Ads bots
if (req.http.User-Agent ~ "(?i)AdsBot-Google") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)AdsBot-Google-Mobile") { set var.is_crawler = true; }

# Social media ($Socials alias)
if (req.http.User-Agent ~ "(?i)facebookexternalhit") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)twitterbot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Pinterestbot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Applebot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)LinkedInBot") { set var.is_crawler = true; }

# Messengers ($Messengers alias)
if (req.http.User-Agent ~ "(?i)WhatsApp") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Telegrambot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)ViberBot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Snapchat") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Discordbot") { set var.is_crawler = true; }
if (req.http.User-Agent ~ "(?i)Slackbot") { set var.is_crawler = true; }

Static asset detection

The recv snippet skips rendering for static files to avoid unnecessary requests to Edge Gateway:

vcl

if (req.url.ext ~ "(?i)^(avif|css|eot|gif|gz|ico|jpeg|jpg|js|json|map|mp3|mp4|ogg|otf|pdf|png|svg|ttf|txt|wasm|wav|webm|webp|woff|woff2|xml|zip)$") {
    return(pass);
}

Fastly extracts the file extension from the URL into req.url.ext, making extension checks straightforward. This saves a round-trip to Edge Gateway for requests that never need rendering.

Loop prevention

When Edge Gateway renders a page, the Render Service fetches the target URL from your origin server. Without loop prevention, Fastly would detect the Render Service request as a crawler and route it back to Edge Gateway, creating an infinite loop.

EdgeComet adds an X-Edge-Render header to all its outgoing requests, both from the Render Service (Chrome fetches) and the Edge Gateway (bypass fetches, including bypass pre-cache). The recv snippet detects this header and passes the request directly to origin:

vcl

if (req.http.X-Edge-Render) {
    return(pass);
}

Error handling (fail-open)

The VCL configuration uses Fastly's restart mechanism to implement fail-open behavior. If Edge Gateway returns a server error, the request restarts and goes to origin instead:

vcl

# In vcl_backend_error or vcl_error
if (req.restarts == 0 && beresp.status >= 500 && req.backend == edge_comet) {
    restart;
}

On restart, the recv snippet detects req.restarts > 0 and skips the Edge Gateway routing:

vcl

# Top of vcl_recv
if (req.restarts > 0) {
    return(pass);
}

This ensures crawlers receive content even if Edge Gateway is temporarily unavailable. They get unrendered JavaScript content rather than an error page.

Fastly-level caching (optional)

The default configuration uses return(pass) for crawler requests, which bypasses Fastly's cache. Edge Gateway manages its own caching layer, so this works well for most setups.

For high-traffic sites, you can enable Fastly-level caching to reduce round-trips to Edge Gateway on repeat crawler visits. This requires two changes:

1. Allow caching in vcl_recv

Remove return(pass) from the crawler routing block and replace it with return(lookup) to let Fastly cache Edge Gateway responses:

vcl

if (var.is_crawler) {
    set req.backend = edge_comet;
    set req.http.X-Render-Key = "your_render_key_here";
    set req.http.X-Forwarded-Proto = if(req.is_ssl, "https", "http");
    set req.http.X-Forwarded-For = client.ip;
    set req.url = "/render?url=" + urlencode(if(req.is_ssl, "https", "http") + "://" + req.http.Host + req.url);

    return(lookup);
}

2. Add User-Agent to cache key

Without this change, Fastly could serve cached rendered HTML to regular users. Add a vcl_hash snippet that includes the User-Agent in the cache key for crawler requests:

vcl

# In vcl_hash
if (req.backend == edge_comet) {
    set req.hash += req.http.User-Agent;
}

When using Fastly-level caching, configure appropriate TTLs in vcl_backend_response to control how long rendered content stays in Fastly's cache. Edge Gateway sets Cache-Control headers on its responses that Fastly respects by default.

Alternative: configure cache key via UI

Instead of writing a vcl_hash snippet, you can modify the cache key through the Fastly management console:

Navigate to Settings > Cache settings

Settings sidebar showing Request settings and Cache settings

Create a cache setting and set Cache keys to req.url, req.http.host, req.http.User-Agent

Cache settings with cache key field including User-Agent

This achieves the same result as the vcl_hash snippet without writing VCL code.

Headers reference

Headers sent to Edge Gateway

Header	Description
`X-Render-Key`	Authentication token from host configuration.
`User-Agent`	Original client User-Agent for dimension matching. Fastly preserves this by default.
`X-Forwarded-For`	Original client IP address (`client.ip`).
`X-Forwarded-Proto`	Original request protocol (http/https).

Backend configuration reference

Setting	Value	Description
`.host`	Your EG hostname	Edge Gateway hostname (e.g., `render.example.com`).
`.port`	`"443"`	HTTPS port.
`.ssl`	`true`	Enable TLS for the backend connection.
`.ssl_sni_hostname`	Same as `.host`	Server Name Indication hostname for TLS handshake.
`.ssl_cert_hostname`	Same as `.host`	Expected hostname in the backend's TLS certificate.
`.share_key`	Your service ID	Optional. Enables connection pooling and health check sharing across multiple Fastly services using the same Edge Gateway backend.
`.connect_timeout`	`10s`	Maximum time to establish a TCP connection.
`.first_byte_timeout`	`60s`	Maximum time to receive the first byte. Must exceed EG `render.timeout`.
`.between_bytes_timeout`	`15s`	Maximum time between bytes after the first byte.
`.probe.request`	`"GET /health HTTP/1.1" ...`	Health check request sent to Edge Gateway's `/health` endpoint.
`.probe.interval`	`30s`	Time between health checks.
`.probe.timeout`	`5s`	Maximum time to wait for a probe response.
`.probe.window`	`5`	Number of most recent probes to evaluate.
`.probe.threshold`	`3`	Minimum successful probes (out of window) to consider backend healthy.
`.probe.initial`	`3`	Assumed successful probes at startup before real probes complete.

Logging

Use Fastly's real-time logging to track rendering decisions and debug issues.

Inline logging with std.log

Add temporary logging to the recv snippet:

vcl

import std;

if (var.is_crawler) {
    std.log("eg-render: routing crawler to EG, ua=" + req.http.User-Agent + " url=" + req.url);
}

View output in the Fastly management console under Observability > Logs.

Structured logging endpoint

For production monitoring, configure a logging endpoint (S3, BigQuery, Datadog, etc.) and add a log line in vcl_deliver:

vcl

# In vcl_deliver
if (resp.http.EC-Source) {
    log {"syslog "} req.service_id {" eg-logging :: "}
        {"{"}
            {""url": ""} req.url {"", "}
            {""render_source": ""} resp.http.EC-Source {"", "}
            {""cache_age": ""} resp.http.EC-Cache-Age {"", "}
            {""user_agent": ""} req.http.User-Agent {"", "}
            {""client_ip": ""} client.ip {"" "}
        {"}"};
}

Fastly setup - Installation and configuration
Diagnostic headers - Response header reference
Dimensions - Crawler detection via User-Agent matching
Caching - Cache configuration

Fastly reference ​

Crawler detection approaches ​

Broad detection (default) ​

Conservative detection ​

Static asset detection ​

Loop prevention ​

Error handling (fail-open) ​

Fastly-level caching (optional) ​

1. Allow caching in vcl_recv ​

2. Add User-Agent to cache key ​

Alternative: configure cache key via UI ​

Headers reference ​

Headers sent to Edge Gateway ​

Backend configuration reference ​

Logging ​

Inline logging with std.log ​

Structured logging endpoint ​

Related documentation ​