C99 Subdomain Finder Alternative — Free and Up-to-Date
C99.nl was, for years, the default free subdomain API for bug bounty hunters who needed a quick JSON response from a shell script. That era is largely over: the free tier has been pared back, queries require purchased credits, and community reports increasingly flag stale data. If you arrived here looking for a C99 alternative that is still actually free, still fresh, and does not require pasting a curl command into a terminal, SubDomainsFinder.com is the closest replacement for browser-based use — with IP, port, ASN, and CDN context built into a single result view.
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TL;DR — when to use which
- Use SubDomainsFinder when you want a free, browser-based subdomain lookup with fresh data and full context (IP, ports, ASN, CDN) without buying credits or running a script.
- Use C99 when you specifically need its API and you are willing to pay for credits and tolerate occasional data staleness as a tradeoff for source variety.
- Use Subfinder as the free, open-source CLI replacement for what C99's API used to offer — better data freshness, no credits, scriptable.
What is the C99 Subdomain Finder?
C99.nl is a small, long-running site that hosts a collection of roughly thirty miscellaneous OSINT and network tools — a subdomain finder, an IP-to-domain reverse lookup, a port scanner, ping and traceroute utilities, WHOIS lookups, and various encoding and conversion helpers. The subdomain finder is the most widely cited tool on the site. It exposes both a minimal web form and an HTTP API endpoint that returns a plain list of discovered subdomains for a given root domain. The API design is what made it popular: a single GET request with a query parameter, a clean response, and almost no ceremony.
From around 2018 to 2020, C99's subdomain endpoint was effectively free for casual use and appeared in a large fraction of the bug bounty recon pipelines published during that period. It sat alongside Sublist3r, Amass, and crt.sh as a default source in many shell scripts. The site itself remained minimal — a single-page directory of tools with little visual polish — which was part of its appeal. It was fast, it was free, and it did one thing well.
The current product is different. The free tier has been progressively constrained, and most practical use now requires purchasing API credits sold in bundles starting at roughly $2 per 1000 queries. The web form often returns truncated or sample-only results without a paid key. Community discussions on Reddit and bug bounty forums over the past two years have repeatedly raised concerns that the subdomain database is not being refreshed at the same cadence as competitors — entries can be months out of date on actively changing domains. None of this makes C99 a bad tool; it makes it a different tool than the one referenced in legacy tutorials. For someone who lands on c99.nl today expecting the 2019 experience, the gap is the source of most frustration.
Feature comparison
| Feature | SubDomainsFinder | C99 |
|---|---|---|
| No payment requiredC99 sells API credits | ||
| Browser-based UIC99 is API-first; web form is minimal | ||
| Subdomain discovery | ||
| IP addresses per subdomain | ||
| Open ports detectionC99 has a separate port scan tool | ||
| ASN & hosting provider | ||
| API access | ||
| Free tierC99's free tier is essentially gone | ||
| Data freshnessC99 data quality has declined per community | ||
| Scriptable / batch queries | ||
| Polished UI | ||
| Multi-tool suiteC99 hosts 30+ misc OSINT tools |
Yes No Partial / limited
Where C99 excels
- API-first design. C99's subdomain endpoint is genuinely easy to consume from a shell script or a recon framework. A single GET request returns a clean, parseable response — no authentication dance, no nested JSON, no pagination headaches. For engineers who want to fold subdomain enumeration into an automated pipeline, that simplicity is the tool's most enduring strength, even at the current price point.
- A few unique passive sources. C99's backend appears to aggregate sources that do not fully overlap with the standard Sublist3r/Amass/Subfinder stack. Practitioners running it alongside other tools occasionally report subdomains that show up only on C99. The hit rate on those uniques is low, but for exhaustive enumeration on a high-value target, adding C99 as a tertiary source can sometimes earn its keep.
- Cheap per-query pricing for small automation. The pay-as-you-go credit model — roughly $2 per 1000 queries — is inexpensive enough that a hobbyist running a recon script across a small set of targets can budget a few dollars a month and not think about it. That is a different value proposition from Pentest-Tools' $85/month floor, and for the right user it is a fair price.
- Adjacent OSINT utilities under one domain. The site bundles a port scanner, IP-to-domain, WHOIS, encoders, and various other small tools. If you live in C99 for other lookups, having the subdomain finder share the same UI and (paid) API key is convenient. It is the same workflow logic that drives users to suites like Pentest-Tools, scaled down to a hobbyist budget.
- Longevity. C99 has been running for a decade and is unlikely to disappear overnight. For workflows that depend on a stable third-party endpoint, that track record is worth something even if it is hard to quantify.
Where SubDomainsFinder has the edge
- Actually free, with no credits to buy. SubDomainsFinder has no paid tier, no credit balance to top up, and no truncation logic that hides results until you pay. If your search began with the phrase 'free subdomain finder,' the answer is closer to what you were looking for than C99 currently is.
- A polished web UI instead of a curl command. C99's subdomain page is functional but spartan, and most of its value lives behind an API. SubDomainsFinder is built as a web product first: a clean input, a result table, IPs and ports rendered inline, ASN and CDN identified automatically, and the option to copy or export the list when you are done. For users who do not want to write a script to use a tool, that difference is the entire user experience.
- IP, ports, ASN, and CDN in one view. C99 has separate tools for port scanning and IP lookups; you query them individually and stitch the data together. SubDomainsFinder consolidates that context next to each subdomain in the default result set. For prioritizing which discovered hosts deserve a closer look, having it all in one place saves real time.
- More current data. Community concerns about C99 data freshness have been consistent enough that we treat it as the most important practical difference. SubDomainsFinder pulls from current Certificate Transparency logs and active passive DNS feeds, which on most public-facing targets surfaces a more complete and more recent picture. We are not going to claim perfection — every passive subdomain source has blind spots — but the freshness gap on actively changing infrastructure is real.
- No signup, no payment flow, no account. Even if C99's credit model is cheap, it still involves a payment step, an account, and a key to manage. SubDomainsFinder has none of that. For one-off lookups, the friction delta is the entire difference between using the tool and bouncing.
Which tool is right for you?
Pentesters & bug bounty
For interactive triage on a new target, SubDomainsFinder is faster and gives you more context per result. For automated, scripted recon across many targets, neither tool is the modern best answer — Subfinder has effectively taken that slot. C99 still earns a place as a tertiary source for hunters who want to squeeze out a few extra uniques on a high-value program and do not mind paying for credits.
Blue teams & defenders
SubDomainsFinder fits cleanly into ad-hoc external attack surface checks: a domain in scope, a quick lookup, IPs and ports surfaced for triage. C99 is less appropriate for defenders because of the data freshness questions — when you are looking for unexpected exposure, stale results can lead to false negatives. If you need recurring monitoring, pair SubDomainsFinder for spot-checks with a proper attack surface management product for continuous tracking.
Sysadmins & IT teams
If the question is 'what subdomains do we have on this domain,' SubDomainsFinder answers it in a browser with no setup, no script, and no credit balance to manage. C99 is rarely the right choice for this audience — its strengths are aimed at recon engineers comfortable with HTTP APIs, not infrastructure teams doing periodic inventory checks.
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