Rewrite rule

Security impact is direct. Rewrite rules can add headers such as HSTS or Content-Security-Policy, remove response headers that reveal backend technology, and preserve the correct client address for downstream security devices. They can also create risk if a rule rewrites host names, query strings, or forwarding headers without review. With WAF_v2, URL or host rewrites affect the request seen by WAF, so security testing must include post-rewrite behavior. Restrict who can edit rewrite sets, audit changes through Activity Log, and test rules against header spoofing, open redirect, cache poisoning, and backend exposure scenarios. Include these checks in deployment gates, not only in annual penetration tests.

Rewrite rules do not create a separate billing meter, but they can influence cost indirectly through gateway SKU choice, WAF usage, troubleshooting time, and backend traffic patterns. The feature requires Application Gateway v2, so teams running older or smaller edge designs may need a SKU migration before using it. A rewrite that prevents needless redirects can reduce backend chatter and user retries. A faulty rule can do the opposite by generating loops, extra logs, WAF evaluations, and support effort. FinOps review should connect rewrite changes to gateway capacity, WAF logging volume, test environments, and incident labor. That visibility keeps a small edge rule from becoming a recurring hidden operations expense.

Reliability depends on rewrite correctness. A rule that changes paths, query strings, or Location headers can fix a migration issue cleanly, but it can also cause redirect loops, broken health checks, unexpected 500 responses, or traffic going to a backend that never receives the expected URL. Application Gateway warns about several rewrite pitfalls, including reevaluating path maps in ways that create loops. Reliable operations require staging tests, ordered rules, explicit conditions, backend-health validation, and rollback to the previous rewrite set. Keep rewrite changes separate from unrelated listener and backend-pool changes whenever possible. Track the owning application team so responders know who can approve emergency removal.

Performance impact is usually indirect but real. Simple header or URL rewrites add little overhead compared with TLS termination, WAF inspection, and backend response time, but complex conditions and bad routing outcomes can hurt users. Rewrites can improve perceived performance by avoiding external redirects and keeping users on the same public URL. They can hurt performance if they trigger WAF processing differences, send traffic to a slower backend path, or create retry and redirect loops. Measure end-to-end latency, backend latency, HTTP status rates, WAF matches, and client redirect counts before calling a rewrite safe. Compare the same request path before and after the rewrite rather than averaging all gateway traffic.

Operators inspect rewrite rules by listing rule sets, rule sequence numbers, conditions, request-header actions, response-header actions, URL actions, and the routing rules that reference them. Before making changes, they export the gateway configuration and test expected paths with curl or synthetic transactions. After deployment, they check backend health, access logs, WAF logs, HTTP status trends, and redirect behavior. Runbooks should describe how to identify the active rewrite set, which headers are intentionally modified, which rule owns a path change, and how to disable or roll back a rule without deleting the gateway. Keep examples of expected before-and-after requests close to the runbook.