Check SNI requirements and Subject Alternative Names
Results
About SNI Checker
Server Name Indication (SNI) is a TLS extension that allows a client to specify the hostname it is trying to reach during the handshake. This enables servers to host multiple SSL certificates on the same IP address. Our SNI Checker tests whether a server requires SNI to serve the correct certificate, verifies SAN coverage, and helps identify configuration issues with multi-tenant hosting setups.
How to Use
Enter the domain name you want to test.
Click "Check" to perform the SNI analysis.
Review the results showing whether SNI is required, what certificate is served, and the list of covered domains.
Use the findings to troubleshoot multi-domain hosting or certificate issues.
Use Cases
•Verify SNI configuration on shared hosting and CDN setups
•Debug certificate mismatch errors caused by missing SNI
•Test compatibility with legacy clients that do not support SNI
•Audit SAN coverage across all domains hosted on a single IP
Frequently Asked Questions
Server Name Indication (SNI) is an extension to the TLS protocol that lets a client specify the hostname it wants to connect to during the handshake. This allows a server to present the correct SSL certificate when multiple domains are hosted on the same IP address.
Almost all modern browsers support SNI, including Chrome, Firefox, Safari, and Edge. However, some older browsers (notably Internet Explorer on Windows XP and some Android browsers) do not support SNI, which can cause certificate errors on shared hosting environments.
If a server requires SNI but the client does not send it, the server typically serves a default certificate that may not match the requested domain. This results in a certificate name mismatch warning or error in the client.
Compatibility, mainly, but the security consequence is real once you notice it. SNI itself is sent in the clear during the handshake, which is why the protocol has never hidden it. The risk is that a server whose certificate is selected by SNI can be made to serve the wrong one: an attacker who blocks the connection and then reconnects without SNI gets the default certificate, which may not be the one that certificate was issued for. On a shared host that default is often another customer's, which is an information leak as well as a failure. Hosting a private address name that an attacker can guess is the classic case.
Because it has no way to know which of its many virtual hosts you meant. SNI carries the hostname during the handshake so the server can select the matching certificate; without it the server falls back to whatever it considers its default, which is usually the first configured virtual host. On a dedicated server that default is often the same certificate, so nothing changes. On shared hosting or a CDN edge it is frequently a completely different site, which is exactly the configuration problem this test exposes.
Nothing to disable here, but the reverse question is worth answering: clients old enough not to send SNI are also old enough to have other problems. Browsers have supported SNI since around 2010 and every current one sends it, so the population of clients that would be affected is small and shrinking. What matters more is whether anything in your own estate omits it — a monitoring agent, a legacy integration, an old language runtime, or a script using a raw TLS library. If something fails only from inside your network, this is a likely reason.
Exactly one label beneath the domain. An entry of *.example.com matches api.example.com and mail.example.com, and does not match the bare example.com, which must be listed separately, nor a.b.example.com, which is two labels down. That limitation is the one most often misunderstood, and it is worth reading the name list rather than trusting the wildcard's presence, because a certificate carrying *.example.com with no example.com entry covers none of your actual apex requests. The tool lists every entry it finds so this is visible rather than assumed.
No, and this matters when you are debugging. What is listed here comes from the single certificate the server presents, which is normally the leaf for the hostname you asked about. The intermediates needed to build a full chain are separate documents that some servers send and others do not. So a name that appears here tells you this certificate covers it; a name that is absent might still be covered by a different certificate the server would present for that hostname. Use the SSL Certificate Chain Checker when the question is whether the path builds, and this one when it is what a particular hostname resolves to.
The tool reports that the requirement could not be determined rather than guessing either way. A connection that succeeds with SNI and then resets without it is a real and unusual server behaviour — some configurations treat a handshake without SNI as malformed and drop it, which is stricter than serving a default certificate but produces the same practical result for any client that omits the extension. The SAN information from the first connection is still reported, because that part of the result is sound.
It does not have anything to add. SNI carries a hostname, so a client connecting to a bare IP address has no name to send and the server selects its default certificate — which is why a certificate meant for an IP address has to carry an IP entry in its SAN list rather than a DNS name, and why clients still refuse to match a hostname against an IP entry. If you are serving several services by IP on one address, SNI cannot help; those need distinct addresses or a layer in front that routes by port or path instead.
On the server side, make sure the certificate is selected by the name in the handshake rather than by connection order, and confirm your virtual host definitions name the hostnames they expect. If something in your own infrastructure is the problem, pass the hostname explicitly to your TLS library — most clients take a server name argument that populates SNI, and omitting it is the usual cause of a client that mysteriously gets the wrong certificate. Test again after the change, because a configuration that appears correct can still be overridden by an earlier entry in the file.
How the Check Works
Whether a server needs SNI is not something you can read from a configuration file or infer from a certificate. It is only answerable by connecting twice and seeing what comes back, which is what this tool does.
1Connect once sending SNI, as every modern client does. The certificate returned is the one a visitor actually receives, and it is parsed as X.509.
2Read the Subject Alternative Name list from it, counting the entries and noting whether any is a wildcard, which covers a whole level of subdomains at once.
3Connect a second time without SNI. The extension is simply not transmitted, which is what an old client or a misconfigured script looks like from the server's point of view.
4Compare the two certificates. Identical means a client that omits SNI still reaches the right certificate. Different means it does not, and that difference is the answer.
If the second connection fails outright, the requirement is reported as undetermined rather than assumed — a reset on the second handshake is a finding about the server, not a verdict.
Tips & Best Practices
•Enable SNI to support multiple SSL certificates on one IP address
•Test SNI functionality from different client configurations
•Monitor SNI usage for virtual hosting and multi-tenant environments
•Configure proper fallback for clients that do not support SNI
Technical Details
Extension: Server Name Indication (SNI) support detection Virtual Hosting: Multiple certificate support on single IP Client Support: SNI support detection for different clients Output: SNI support status with configuration recommendations
