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What Does Your User Agent Say About You?

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A user agent is a computer program representing a person, for example, a browser in a Web context.

Besides a browser, a user agent could be a bot scraping webpages, a download manager, or another app accessing the Web. Along with each request they make to the server, browsers include a self-identifying User-Agent HTTP header called a user agent (UA) string. This string often identifies the browser, its version number, and its host operating system.

Spam bots, download managers, and some browsers often send a fake UA string to announce themselves as a different client. This is known as user agent spoofing.

The user agent string can be accessed with JavaScript on the client side using the navigator.userAgent property.

A typical user agent string looks like this: "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0".

(Source: Mozilla.org)

User Agent String

Browser Data

User Agent Rfc

author
Ava Flores
• Thursday, 05 November, 2020
• 31 min read

The Chrome (or Chromium/Blink-based engines) user agent string is similar to Firefox’s. For compatibility, it adds strings like HTML, like Gecko and Safari.

(Source: www.slideshare.net)

Contents

The Opera browser is also based on the Blink engine, which is why it almost looks the same, but adds “Or/”. In this example, the user agent string is mobile Safari’s version.

It’s included with every HTTP request and can potentially be very long and unique to a user in certain uncommon situations. RFC 7540-HTTP/2 is concerned w/ the HTTP message itself and not the headers where UA lives, so 7231 applies there too.

It’s interesting to note that, even in this RFC, it already warns of anomalies like some proxies will append data to the UA which makes interpretation ambiguous, and some clients don’t follow the product token syntax. So, in theory, you could shove emoji or a base64 essay into the UA, but all bets are off on how the server decides if and how to handle it.

What’s notable is the addition of very specific prohibitions against putting non-essential, fine-grained information or advertising in the UA. Overly long UAS are specifically stated to potentially fingerprint users (we’ll get to this later).

This is most likely what prompted the latest RFCs to specifically say using another product’s string (i.e. Mozilla) is discouraged. What you’ll also notice is that every browser is fairly idiosyncratic with the sequence and types of product tokens it has.

layer application rfc
(Source: www.slideshare.net)

With enough study you can fairly reliably know what device and browser a request is coming from. IE11 uses the comment Trident/7.0 to denote its render engine and “RV:11.0” to denote the IE version, Chrome (and Edge which switched to being chromium on the back end) are full product-version tokens, but will also claim Safari compatibility because they both use Apple WebKit.

Internet Explorer 6 was perhaps the most egregious example of where site designers needed to know the browser version because of how poorly it handled modern web standards. Fun side note, IE6 was a HUGE problem in the late 2000s/early 2010s due to its horrible standards support requiring lots of custom hacks from developers but stubbornly high usage numbers.

Using JavaScript to pull browser info was still in its infancy, so User -Agents were the most informative thing that you had. From this, you could see what percentage of your users were using IE6 versus Firefox (and whether you should stop using certain HTML features or not).

In practice, this method only really works on the tiny subset of people who have installed enough browser extensions or poorly spoofed their UA enough to stand out from the crowd of default strings. But even so, the UA still provides bits of entropy in a larger user fingerprinting framework that involves the use of JavaScript and HTML5 methods.

With many advanced browser features being detected directly via JavaScript these days, this is probably the primary modern use of the UA string, figuring out what device/platform a client is running. While UAS used to tell you what percent of your user base was on IE, Firefox or Chrome on a Windows machine or Mac, we suddenly started seeing exciting new strings like iPad, iPhone, Android.

common network services rfc
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Running (up to) a thousand regex searches on a single string is obviously very resource-intensive, but this is the only way to make sure to identify a UA string to the fullest extent possible short of reading it manually. In practice, you can speed up things massively with hashing and caching by relying on the fact that most users will have the same general default setup, so you’d only have to run regex on novel strings.

Often this is thinking from a mix of outdated thinking from a decade ago when people often only had 1 internet-connected computer, or they’ve heard about browser fingerprinting but aren’t aware of how many data points it takes to fingerprint a browser. I’ve personally had to use UAS to put very rough bounds on the minimum number of browsers used to view a site, with the raw hit count being the upper limit.

When people hear that UA scan be spoofed easily, they often worry that they’re being lied to and their metrics are going to be off. The primary reason is that the vast majority of users have no motivation to bother messing with their UA string.

The only people who want to manipulate their UA are usually a microscopic number of users writing bots/crawlers of some sort. Out of hundreds, thousands, possibly millions of humans using your site every day, they’re ultimately insignificant because they represent such a fractional percent of traffic.

So long as they’re generally well-behaved and low volume, they won’t skew your metrics much so you can ignore it like the above. There are often signs of problems (like your systems overloading like it’s a DDoS attack) when you’re facing these sorts of bots.

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(Source: hellocoding.de)

It’s pretty rare to have to worry about spoofing unless you’re running something that robot-makers have an incentive to abuse (and UA is not likely to be the solution if that’s your problem). I suppose there are some edge cases: like if you get practically no traffic, so the bots will dominate, but you also have little reason to do a lot of analytics at that point.

After these limited methods, the whole idea starts wandering into the realm of bot and fraud detection tech, which is a bit out of scope for here. This section defines the syntax and semantics of all standard HTTP/1.1 header fields.

For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity. Each media-range MAY be followed by one or more accept-params, beginning with the “q” parameter for indicating a relative quality factor.

Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the value scale from 0 to 1 (section 3.9). If more than one media range applies to a given type, the most specific reference has precedence.

The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response. If an Accept-Encoding field is present in a request, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the 406 (Not Acceptable) status code.

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(Source: www.slideserve.com)

If no Accept-Encoding field is present in a request, the server MAY assume that the client will accept any content coding. The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request.

Each language-range MAY be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. If no language- range in the field matches the tag, the language quality factor assigned is 0.

If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable. The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain.

The directives specify behavior intended to prevent caches from adversely interfering with the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol might apply these directives to header fields not defined in HTTP/1.1.

Note: This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content. No-cache If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server.

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However, the specified field-name(s) MUST NOT be sent in the response to a subsequent request without successful revalidation with the origin server. No-store The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example, on backup tapes).

“MUST NOT store” in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. History buffers MAY store such responses as part of their normal operation.

The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.

When the max-age cache-control directive is present in a cached response, the response is stale if its current age is greater than the age value given (in seconds) at the time of a new request for that resource. This might be useful if certain HTTP/1.0 caches improperly calculate ages or expiration times, perhaps due to desynchronized clocks.

Note: An origin server might wish to use a relatively new HTTP cache control feature, such as the “private” directive, on a network including older caches that do not understand that feature. The manage directive also implies the semantics of the proxy-revalidate directive (see section 14.9.4), i.e., that the shared cache must not use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server.

layer application
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Note that older caches, not compliant with this specification, do not implement any cache-control directives. Other directives allow an user agent to modify the basic expiration mechanism.

Max-age Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Min-fresh Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds.

Max-stale Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds.

End-to-end reload The request includes a “no-cache” cache-control directive or, for compatibility with HTTP/1.0 clients, “Drama: no-cache”. Specific end-to-end revalidation The request includes a “max-age=0” cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server.

The initial request includes a cache-validating conditional with the client's current validator. Unspecified end-to-end revalidation The request includes “max-age=0” cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server.

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(Source: www.slideshare.net)

The initial request does not include a cache-validating conditional; the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional with its current validator. In this case, the cache MAY use either validator in making its own request without affecting semantic transparency.

If the server replies with 304 (Not Modified), then the cache can return its now validated copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate cache can compare the returned validator with the one provided in the client's request, using the strong comparison function.

If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Only-if-cached In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses that it currently has stored, and not to reload or revalidate with the origin server.

Must-revalidate Because a cache MAY be configured to ignore a server's specified expiration time, and because a client request MAY include a max- stale directive (which has a similar effect), the protocol also includes a mechanism for the origin server to require revalidation of a cache entry on any subsequent use. Servers SHOULD send the must-revalidate directive if and only if failure to revalidate a request on the entity could result in incorrect operation, such as a silently executed financial transaction.

Recipients MUST NOT take any automated action that violates this directive, and MUST NOT automatically provide an invalidated copy of the entity if revalidation fails. Although this is not recommended, user agents operating under severe connectivity constraints MAY violate this directive but, if so, MUST explicitly warn the user that an invalidated response has been provided.

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(Source: www.cisco.com)

The warning MUST be provided on each invalidated access, and SHOULD require explicit user confirmation. No-transform Implements of intermediate caches (proxies) have found it useful to convert the media type of certain entity bodies.

A non- transparent proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. Serious operational problems occur, however, when these transformations are applied to entity bodies intended for certain kinds of applications.

For example, applications for medical imaging, scientific data analysis and those using end-to-end authentication, all depend on receiving an entity body that is bitten for a bit identical to the original entity-body. This implies that the cache or proxy MUST NOT change any aspect of the entity-body that is specified by these headers, including the value of the entity-body itself.

The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional assigned value. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.

In this way, extensions to the cache-control directives can be made without requiring changes to the base protocol. This extension mechanism depends on an HTTP cache obeying all the cache-control directives defined for its native HTTP-version, obeying certain extensions, and ignoring all directives that it does not understand.

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An origin server wishing to allow the UCI community to use an otherwise private response in their shared cache(s) could do so by including In either the request or the response header fields indicates that the connection SHOULD NOT be considered `persistent' (section 8.1) after the current request/response is complete.

This protects against mistaken forwarding of such header fields by pretty/1.1 proxies. When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field.

Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type. Typically, the entity-body is stored with this encoding and is only decoded before rendering or analogous usage.

However, a non-transparent proxy MAY modify the content-coding if the new coding is known to be acceptable to the recipient, unless the “no-transform” cache-control directive is present in the message. Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification.

An example would be a beginner's language primer, such as “A First Lesson in Latin,” which is clearly intended to be used by an English-literate audience. Content-Language MAY be applied to any media type -- it is not limited to textual documents.

rfc layer application message header
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Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4.4. In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred, unless this is prohibited by the rules in section 4.4.

The Content-Location entity-header field MAY be used to supply the resource location for the entity enclosed in the message when that entity is accessible from a location separate from the requested resource's URI. However, the Content- Location can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section 13.6.

The meaning of the Content-Location header in PUT or POST requests is undefined; servers are free to ignore it in those cases. The Content-MD5 entity-header field, as defined in RFC 1864 , is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body.

(Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.) The Content-MD5 header field MAY be generated by an origin server or client to function as an integrity check of the entity-body.

Any recipient of the entity- body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received. This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no transfer-encoding were being applied.

rfc message formats applications ppt powerpoint presentation fields header transport related
(Source: www.slideserve.com)

Conversion of all line breaks to Calf MUST NOT be done before computing or checking the digest: the line break convention used in the text actually transmitted MUST be left unaltered when computing the digest. The asterisk “*” character means that the instance-length is unknown at the time when the response was generated.

A server sending a response with status code 416 (Requested range not falsifiable) SHOULD include a Content-Range field with a byte-range- resp-spec of “*”. A response with status code 206 (Partial Content) MUST NOT include a Content-Range field with a byte-range- resp-spec of “*”.

When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart message.

A response to a request for a single range MUST NOT be sent using the multipart/byte ranges media type. A client that cannot decode a multipart/byte ranges message MUST NOT ask for multiple byte-ranges in a single request.

If the server receives a request (other than one including an If- Range request-header field) with an unsatisfiable Range request- header field (that is, all of whose byte-range-spec values have a first-byte-pos value greater than the current length of the selected resource), it SHOULD return a response code of 416 (Requested range not falsifiable) (section 10.4.17). Further discussion of methods for identifying the media type of entity is provided in section 7.2.1.

(Source: present5.com)

A received message that does not have a Date header field MUST be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a Date. Clients SHOULD only send a Date header field in messages that include an entity-body, as in the case of the PUT and POST requests, and even then it is optional.

A client without a clock MUST NOT send a Date header field in a request. In practice, the date can be generated at any time during the message origination without affecting its semantic value.

An origin server without a clock MUST NOT assign Expires or Last- Modified values to a response, unless these values were associated with the resource by a system or user with a reliable clock. The Tag response-header field provides the current value of the entity tag for the requested variant.

The Expect request-header field is used to indicate that particular server behaviors are required by the client. The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time.

HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value “0”, as in the past (i.e., “already expired”). HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.

(Source: www.slideshare.net)

The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field (section 14.9). The Form request-header field, if given, SHOULD contain an Internet e-mail address for the human user who controls the requesting user agent.

This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.

A client MUST include a Host header field in all HTTP/1.1 request messages. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value “*” matches any current entity of the resource.

A server MUST use the strong comparison function (see section 13.3.3) to compare the entity tags in If-Match. The meaning of “If-Match: *” is that the method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.44) exists, and MUST NOT be performed if the representation does not exist.

(Source: www.slideserve.com)

A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge.

The If-Modified-Since request-header field is used with a method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. As a special case, the value “*” matches any current entity of the resource.

Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache- related header fields (particularly Tag) of one of the entities that matched. For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).

If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.

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(Source: blogs.sap.com)

The meaning of “If-None-Match: *” is that the method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.44) exists, and SHOULD be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.

However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body. The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified.

For 3xx responses, the location SHOULD indicate the server's preferred URI for automatic redirection to the resource. The Max-Forwards request-header field provides a mechanism with the TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the number of proxies or gateways that can forward the request to the next inbound server.

This can be useful when the client is attempting to trace a request chain which appears to be failing or looping in mid-chain. The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded.

The Drama general-header field is used to include implementation- specific directives that might apply to any recipient along the request/response chain. This drama directive has the same semantics as the no-cache cache-directive (see section 14.9) and is defined here for backward compatibility with HTTP/1.0.

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Clients SHOULD include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. The Proxy-Authenticate response-header field MUST be included as part of a 407 (Proxy Authentication Required) response.

The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the proxy for this Request-URI. Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the current connection and SHOULD NOT be passed on to downstream clients.

The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires authentication. The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and/or realm of the resource being requested.

Proxy-Authorization header field is consumed by the first outbound proxy that was expecting to receive credentials. By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity.

If the byte-range-set is unsatisfiable, the server SHOULD return a response with a status of 416 (Requested range not falsifiable). Otherwise, the server SHOULD return a response with a status of 206 (Partial Content) containing the falsifiable ranges of the entity-body.

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The Referee request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained (the “referrer”, although the header field is misspelled.) The Referee request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc.

The Referee field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard. The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response.

The field can contain multiple product tokens (section 3.8) and comments identifying the server and any significant subproducts. The product tokens are listed in order of their significance for identifying the application.

Its value may consist of the keyword “trailers” and/or a comma-separated list of extension transfer-coding names with optional accept parameters (as described in section 3.6). Therefore, the keyword MUST be supplied within a Connection header field (section 14.10) whenever TE is present in an HTTP/1.1 message.

Doing so allows the recipient to know which header fields to expect in the trailer. The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient.

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(Source: www.slideserve.com)

Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification. The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol.

The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional.

The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. This specification only defines the protocol name “HTTP” for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of section 3.1 and future updates to this specification.

The field can contain multiple product tokens (section 3.8) and comments identifying the agent and any subproducts which form a significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the application.

The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation. For unreachable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation.

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A cache MAY assume that the same selection will be made for future requests with the same values for the listed field names, for the duration of time for which the response is fresh. A Vary field value of “*” signals that unspecified parameters not limited to the request-headers (e.g., the network address of the client), play a role in the selection of the response representation.

It is analogous to the “Received” field of RFC 822 and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain. The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain.

The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients. The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message.

However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. Multiple Via field values represents each proxy or gateway that has forwarded the message.

Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message.

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For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms.

This information is typically used to warn about a possible lack of semantic transparency from caching operations or transformations applied to the entity body of the message. The warn-text SHOULD be in a natural language and character set that is most likely to be intelligible to the human user receiving the response.

It MUST then add any Warning headers received in the validating response. In other words, Warning headers are those that would be attached to the most recent relevant response.

If it is not possible to inform the user of all the warnings, the user agent SHOULD follow these heuristics: Systems that generate multiple Warning headers SHOULD order them with this user agent behavior in mind.

Requirements for the behavior of caches with respect to Warnings are stated in section 13.1.2. 111 Revalidation failed MUST be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability to reach the server.

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113 Heuristic expiration MUST be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours. If an implementation sends a message with one or more Warning headers whose version is HTTP/1.0 or lower, then the sender MUST include in each warning-value a warn-date that matches the date in the response.

(This prevents bad consequences of naive caching of Warning header fields.) The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI.

HttpClient client = new HttpClient(); HttpRequestMessage request = new HttpRequestMessage(HttpMethod. Get, http://msdn.microsoft.com/ “); request. Headers. Date = Daytime. Now. Subtract(new Time Span(10,0, 0)); request. Headers. UserAgent. ParseAdd(“New User Agent Value”); HttpResponseMessage response = await client. SendAsync(request); string resulted = response. StatusCode. ToString(); But this property is not available for Metro Style App in Win8 Consumer Preview.

HttpWebRequest is a complete implementation of a HTTP request using raw TCP in .NET. Robin MSN Community Support | Feedback to us Get or Request Code Sample from Microsoft Please remember to mark the replies as answers if they help and unmask them if they provide no help.

“An exception to type 'System. FormatException' occurred in System. Net. Http.dll but was not handled in user code Additional information: The format of value '”not submitting” to {RFC2616}' is invalid.” When a software agent operates in a network protocol, it often identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer.

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For example, if a user's product were called WikiBrowser, their user agent string might be WikiBrowser/1.0 Gecko/1.0. During the first browser war, many web servers were configured to send web pages that required advanced features, including frames, to clients that were identified as some version of Mozilla only.

Automated web crawling tools can use a simplified form, where an important field is contact information in case of problems. Automated agents are expected to follow rules in a special file called robots.txt “.

The popularity of various Web browser products has varied throughout the Web's history, and this has influenced the design of websites in such a way that websites are sometimes designed to work well only with particular browsers, rather than according to uniform standards by the World Wide Web Consortium (W3C) or the Internet Engineering Task Force (IETF). Websites often include code to detect browser version to adjust the page design sent according to the user agent string received.

Thus, various browsers have a feature to cloak or spoof their identification to force certain server-side content. For example, the Android browser identifies itself as Safari (among other things) in order to aid compatibility.

User agent sniffing is the practice of websites showing different or adjusted content when viewed with certain user agents. An example of this is Microsoft Exchange Server 2003's Outlook Web Access feature.

port mail submission defines difference transfer message between
(Source: www.my-tiny.net)

When viewed with Internet Explorer 6 or newer, more functionality is displayed compared to the same page in any other browsers. Web browsers created in the United States, such as Netscape Navigator and Internet Explorer, previously used the letters U, I, and N to specify the encryption strength in the user agent string.

Until 1996, when the United States government disallowed encryption with keys longer than 40 bits to be exported, vendors shipped various browser versions with different encryption strengths. Following the lifting of export restrictions, most vendors supported 256-bit encryption.

Browser Versions Carry 10.5 Bits of Identifying Information on Average “, Electronic Frontier Foundation, 27 January 2010. I've been rejected until I come back with Netscape” ^ “Android Browser Reports Itself as Apple Safari”.

^ User Agent String explained: Android WebKit Browser”. Mozilla/5.0 (Linux; U; Android 2.2; ends; HTC_DesireHD_A9191 Build/FRF91) Apple WebKit/533.1 (HTML, like Gecko) Version/4.0 Mobile Safari/533.1 ^ Emberton, Stephen.

Here at the Electronic Frontier Foundation, we have a guiding motto: “I Fight For the Users.” We didn't pick that one by accident (nor merely because we dig the 1982 classic film “Tron”), but because it provides such a clear moral compass when we sit down to work every day.

(Source: pfelix.wordpress.com)

The RFC lists several ways that end-users can be involved in technical architecture decisions, and ponders the strengths and drawbacks of each: the difficulty of discussing esoteric technology with users who lack the background to understand it; the imperfection of relying on government representatives to represent the interests of their citizens (and the conflicts between those governments and the governments of other states). As the go-to group to represent users' interests with both technical depth and a genuine ethical posture.

Alas, as the RFC points out, the latest wave of Internet of Things devices have all but abandoned the idea of serving as user -agents. Instead, these sensor-studded, actuator-connected gadgets act as outposts for the corporations that sold them, sneaking around behind our backs to spy on us, corralling us into arranging our affairs to suit the manufacturer's shareholders' interests at the expense of our own.

The IETF is an Internet original, a 34-year-old institution that does the hard, unglamorous work of setting and updating standards. The “rough consensus and running code” ethic it defined gave birth to the Internet as it once was, and as it has become.

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Sources
1 passivetech.com - https://passivetech.com/2020/04/28/how-to-change-your-browsers-user-agent-and-trick-websites/
2 www.howtogeek.com - https://www.howtogeek.com/113439/how-to-change-your-browsers-user-agent-without-installing-any-extensions/
3 www.ubergizmo.com - https://www.ubergizmo.com/how-to/change-browser-user-agent/
4 www.ctrl.blog - https://www.ctrl.blog/entry/brave-user-agent-detection.html
5 helpdeskgeek.com - https://helpdeskgeek.com/how-to/how-to-change-your-browsers-user-agent-without-installing-an-extension/
6 github.com - https://github.com/brave/brave-browser/issues/1052
7 www.reddit.com - https://www.reddit.com/r/brave_browser/comments/kjbg1x/useragent_issue/
8 www.maketecheasier.com - https://www.maketecheasier.com/change-user-agents-chrome-firefox-edge/
9 stackoverflow.com - https://stackoverflow.com/questions/36523448/how-do-i-tell-if-a-user-is-using-brave-as-their-browser