![]() At step 5, the client sent the Server Name Indication (SNI). The following diagram illustrates the process sequence to open a website in a browser. When the traffic doesn’t have SNI, there is also no server_name extension in the ClientHello packet as seen below. Reason: SNI TLS extension was missing".Īzure Firewall uses SNI TLS headers to filter HTTPS and MSSQL traffic In these cases, when we view the logs, we will see "Action: Deny. Application rule of Azure Firewall evaluate the HTTPS traffic based on SNI resulting in traffic without SNI being blocked. When we use IP address like “ address” to access a website, it doesn’t use SNI. We need to consider the situation because these clients can’t access services like FrontDoor, AppService, etc, without supporting SNI. If we use devices which don’t support SNI, the client does not indicate the FQDN even though the URL includes the FQDN. The latest PC and mobile devices generally have no problems using SNI, but sometimes very old devices, such as filp phones are not supported so we need to care them. After the TLS handshake is established, the HTTP Request is sent from the client, and the client can view the web page in their browser when they get the response. On the web server side, it validates the FQDN in the certificate on the server based on the SNI and then proceeds to the TLS handshake. After the TCP 3-way handshake (blue), the Client Hello was sent from the client (red), which includes "Server Name : ” as the Server Name Indicate extension in the packet (green). The TLS handshake is similar to a TCP 3-way handshake but while the TCP handshake establishes a TCP connection, the TLS handshake starts after the TCP connection so TLS is in an upper layer if the OSI model. We need to confirm SNI in a the packet capture as we can’t find it in the browser.īelow is a packet capture on the client when I accessed a Microsoft document. SNI is another of the TLS extensions, defined in RFC 6066, and it indicates the FQDN from the client in a TLS handshake. I will refer HTTPS later ni this document. In the case of web services, the platform checks the FQDN set on the PaaS service to see whether the host header of the HTTP request is the same or not in order to identify user requests. Many PaaS services provided by Azure are multitenant so we share same platform with other users. This is an example of host header when I accessed a Microsoft document. We can find the header in the network tab of the developer tool in a web browser. The host header is in RFC 7230, and is used to define the hostname of the HTTP request. In HTTP headers, there are various headers such as Location, User-Agent, connection, etc. Is one of the HTTP headers and in the form of "Host: xxx". Host headers and Server Name Indication (SNI) These technologies must be used when we build a website, but as these technologies are a little complicated so I'll explain them in this article. In Azure, these are used by Application Gateway, FrontDoor, AppService, etc. ![]() In this blog, I'll write FQDN and HTTP host headers used to access to websites, and Server Name Indication (SNI) which is one of the TLS extensions. I think we should properly understand these technologies if we bulid a website in a public cloud. curl -I google.Do you know the difference between SNI and HTTP host headers ? It is sometimes very confusing. Use -I option to get the response header values. You can also use curl to fetch the response header values only. Use -versbose or -v option with the curl command to fetch the request header and response header values as following: curl -verbose cURL – get the request header and response header values 2. It is a quick tool for developers to view the request header and response header values of a website. If you’re interested in learning more about Request and Response Headers, a good place to start is by reading up on the HTTP protocol.ĬURL is a command line utility used to transmit data over different-2 protocols. Request and Response Headers are essential for web developers as they provide important information for debugging and troubleshooting. Together, the Request and Response Headers help to ensure that data is sent securely and accurately between the browser and the server. ![]() The Response Header is sent by the server in response to the request, and it contains information such as the status code of the response, the content type of the page, and any authentication credentials. The Request Header is sent by the browser as part of an HTTP request, and it contains information such as the type of request, the URL of the requested page, and any authentication credentials. Request Header and Response Header are both a part of the HTTP protocol, which is the standard used for communication between web browsers and web servers. ![]()
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