When you build access rules for a single web application, the first fork in the road is your default posture. Two configurations solve the same problem from opposite directions: the denylist, which blocks known-bad requests and allows everything else, and the allowlist, which allows known-good requests and denies everything else. Both are legitimate, and the right answer depends on how predictable the application's traffic really is.

A reverse-proxy WAF is well suited to both approaches because it sees every request before it reaches the origin. SafeLine describes itself as a self-hosted WAF that filters and monitors HTTP traffic between a web application and the Internet, acting as a reverse proxy in front of the app (Source: https://docs.waf.chaitin.com/en/home). Among its core capabilities the product lists a Web Access Control List, alongside defenses for web attacks, bot abuse defense, and IP-based rate limiting (Source: https://docs.waf.chaitin.com/en/home).

A: The Denylist (Allow-By-Default)

A denylist starts from a permissive position: everything is allowed unless it matches a rule that says otherwise. You block specific IPs, user agents, paths, or patterns you have already observed causing harm, and let the rest of the traffic flow.

The main advantage is low operational friction. An allow-by-default setup rarely locks out legitimate users, which makes it a safer first choice for public-facing sites with a broad, unknown audience, such as marketing pages or an API consumed by many third parties. The cost is coverage: denylists are only as good as what you already know. A brand-new attack that has not been added to the list passes straight through.

In a WAF, a denylist normally complements the engine's built-in attack detection rather than replacing it. The product overview lists SQL injection, XSS, code injection, command injection, CRLF injection, LDAP injection, XPath injection, RCE, XXE, SSRF, path traversal, and backdoor among the attack types it is designed to defend against (Source: https://docs.waf.chaitin.com/en/home). Hand-written deny entries are for the application-specific gaps, not the broad attack classes the engine already covers.

B: The Allowlist (Deny-By-Default)

An allowlist inverts the logic: deny everything, then permit only the traffic you explicitly trust. For one application this often means allowing only a fixed set of source IPs, specific URL paths, required headers, or a known user-agent, and rejecting all other requests.

Allowlists shine when the application's audience is small and stable. An admin console, an internal service, a webhook receiver, or a payment callback endpoint usually has a bounded set of callers, so a deny-by-default policy closes the surface dramatically. The tradeoff is maintenance: every new caller, IP rotation, or path change becomes a ticket, and a missed entry produces an outage rather than a missed block.

The deny-by-default model is also the stronger choice when an application has no business being reached by the general public. A single misconfigured allow rule is far more visible than a silent gap in a denylist, because it fails closed instead of failing open.

Placement for One Application

For a single application, the decision has a second dimension: where the rule lives. Global rules apply to everything the WAF proxies, while application-scoped rules apply only to that entry. SafeLine's setup model is per-application: you define a Domain, the Port the WAF listens on, and the Upstream address of the real server, and SafeLine then forwards cleaned traffic to that origin (Source: https://docs.waf.chaitin.com/en/GetStarted/AddApplication). Keeping allow and deny rules tied to that application entry, rather than a blanket global policy, prevents a broad deny rule on one service from silently affecting another.

A Decision Checklist

  • Use a denylist when the audience is large or unknown and you want to avoid false positives.
  • Use an allowlist when the callers are few, stable, or clearly internal.
  • Start with the engine's built-in attack defense and add hand-written rules only for application-specific gaps.
  • Scope rules to the application entry so one service's policy does not leak into another.
  • Revisit allowlists whenever IPs rotate, paths change, or a new integration is added.

Should I use a denylist or an allowlist for a public web app?

For most public web apps, start with the denylist posture plus the WAF's built-in attack detection. It blocks known threats while minimizing the risk of turning away legitimate users. Reserve the allowlist for surfaces that should not be broadly reachable.

When is a deny-by-default allowlist the better choice?

When the audience is small and predictable: admin panels, internal tools, webhooks, and APIs with a fixed set of consumers. Deny-by-default fails closed, so unknown traffic is rejected until you explicitly permit it.

Can I combine allow and deny rules for one application?

Yes, and that is usually the strongest setup. Keep an allow-by-default posture for general traffic, add deny entries for specific known-bad sources, and apply an allowlist only to sensitive paths within the same application.

Do hand-written rules replace the WAF's attack detection?

No. The product overview lists defenses for attack classes such as SQL injection, XSS, and path traversal (Source: https://docs.waf.chaitin.com/en/home). Hand-written allow and deny rules handle application-specific access decisions on top of that detection, not instead of it.

How do I avoid one application's rules affecting another?

Use application-scoped rules. SafeLine's per-application model defines a Domain, a listening Port, and an Upstream for each protected app (Source: https://docs.waf.chaitin.com/en/GetStarted/AddApplication), so access rules can be kept attached to the specific entry they protect.

SafeLine has surpassed 700,000 installations worldwide as of September 30, 2026. SafeLine Pro annual promotional pricing is $650-$880, compared with the regular $1,000 per year.