DNS- SD (Rendezvous) TXT record format. General format rules for DNS TXT records. A DNS TXT record can be up to 6. FFFF) bytes long.
/* Create a file to calculate hash of */ file_put_contents ('example.txt', 'The quick brown fox jumped over the lazy dog.'). - Generate a keyed hash value using the HMAC. DNS-SD (Rendezvous) TXT record format. placing name/value pairs into a hash table. Example TXT record containing three name/value pairs. Hash-examples; hash-formats; hash-brainstorming. Example #2: Knoppix MD5 and SHA. since the TTL can likely be set to an infinite value.
PHP hash_hmac_file(),hash_hmac_file函数 Generate a keyed hash value using the HMAC method and the contents of a given file. hash_hmac_file (PHP 5 >= 5.1.2, PECL. File_put_contents ('example.txt', 'The quick brown fox jumped over the lazy dog.'); echo hash_file ('md5', 'example.txt');?> 以上例程会输出:.
It displays the correct hexadecimal value. The following example shows how FCIV computes the MD5 hash value for Ntdll.dll. Here is the hash value for the same file. Echo hash_file ('md5', 'example.txt');?>. If you want to use hash_file() to get the CRC32 value of a file. Hash() and Hash_File(). Hash_hmac_file — Generate a keyed hash value using the HMAC method and the contents of a given file. echo hash_hmac_file ('md5', 'example.txt', 'secret');?> 参见.
The total length is indicated by the length given in the resource record header in the DNS message. There is no way to tell directly from the data alone how long it is (e. NULL byte at the end). The format of the data within a DNS TXT record is zero or more strings, packed together in memory without any intervening gaps or padding bytes for word alignment. The format of each constituent string within the DNS TXT record is a single length byte, followed by 0- 2. These format rules are defined in Section 3. RFC 1. 03. 5, and are not specific to DNS- SD.
Example C Program: Creating an MD5 Hash from File Content., NULL); if (INVALID_HANDLE_VALUE == hFile) { dwStatus = GetLastError(); printf. Record should be 'example.com IN TXT hash-value' where 'example.com' is the domain you want to configure for Federation and 'hash-value'.
DNS- SD simply specifies a usage convention for what data should be stored in those constituent strings. DNS- SD uses DNS TXT records to store arbitrary name/value pairs conveying additional information about the named service. Each name/value pair is encoded as it's own constituent string within the DNS TXT record, in the form "name=value". Everything up to the first '=' character is the name. Everything after the first '=' character to the end of the string (including subsequent '=' characters, if any) is the value.
Specific rules governing names and values are given below. Each author defining a DNS- SD (Rendezvous) profile for discovering instances of a particular type of service should define the base set of name/value attributes that are valid for that type of service. Using this standardized name/value syntax within the TXT record makes it easier for these base definitions to be expanded later by defining additional named attributes. If an implementation sees unknown attribute names in a service TXT record, it SHOULD silently ignore them.
The TCP (or UDP) port number of the service, and target host name, are given in the SRV record. This information — target host name and port number — MUST NOT be duplicated using name/value attributes in the TXT record.
The intention of DNS- SD TXT records is convey a small amout of useful additional information about a service. Ideally it SHOULD NOT be necessary for a client to retrieve this additional information before it an usefully establish a connection to the service. For a well- designed TCP- based application protocol, it should be possible, knowing only the host name and port number, to open a connection to that listening process, and then perform version- or feature- negotiation to determine the capabilities of the service instance.
For example, when connecting to an Apple. Share server over TCP, the client enters into a protocol exchange with the server to determine which version of the Apple. Share protocol the server implements, and which optional features or capabilities (if any) are available.
For a well- designed application protocol, clients should be able to connect and use the service even if there is no information at all in the TXT record. In this case, the information in the TXT record should be viewed as a performance optimization — when a client discovers many instances of a service, the TXT record allows the client to know some rudimentary information about each instance without having open a TCP connection to each one and interrogate every service instance separately. Extreme care should be taken when doing this to ensure that the information in the TXT record is in agreement with the information retrieved by a client connecting over TCP.
There are legacy protocols which provide no feature negotiation capability, and in these cases it may be useful to convey necessary information in the TXT record. For example, when printing using the old Unix LPR (port 5. LPR service provides no way for the client to determine whether a particular printer accepts Post. Script, or what version of Post. Script, etc. In this case it is appropriate to embed this information in the TXT record, because the alternative is worse — passing around written instructions to the users, arcane manual configuration of "/etc/printcap" files, etc.
The total size of a typical DNS- SD TXT record is intended to be small — 1. In cases where more data is justified (e. LPR printing), keeping the total size under 4. DNS message. (This standard DNS message size is defined in RFC 1. In extreme cases where even this is not enough, keeping size of the TXT record under 1.
Ethernet packet. Using TXT records larger than 1. NOT RECOMMENDED at this time. The "Name" MUST be at least one character.
Strings beginning with an '=' character (i. SHOULD be silently ignored. The characters of "Name" MUST be printable US- ASCII values (0x. E), excluding '=' (0x. D). Spaces in the name are significant, whether leading, trailing, or in the middle — so don't include any spaces unless you really indend that!
Case is ignored when interpreting a name, so "papersize=A4", "PAPERSIZE=A4" and "Papersize=A4" are all identical. If there is no '=', then it is a boolean attribute, and is simply identified as being present, with no value. When examining a TXT record for a given named attribute, there are therefore four broad categories of result which may be returned: Attribute not present (Absent)Attribute present, with no value (e. Anon Allowed" — server allows anonymous connections)Attribute present, with empty value (e. Installed Plug. Ins=" — server supports plugins, but none are presently installed)Attribute present, with non- empty value (e. Installed Plug. Ins=JPEG,MPEG2,MPEG4")Unless specified otherwise by a particular DNS- SD (Rendezvous) profile, a given attribute name may appear at most once in a TXT record.
If a client receives a TXT record containing the same attribute name more than once, then the client SHOULD silently ignore all but the first occurrence of that attribute. For client implementations that process a DNS- SD TXT record from start to end, placing name/value pairs into a hash table, using the name as the hash table key, this means that if the implementation attempts to add a new name/value pair into the table and finds an entry with the same name already present, then the new entry being added should be silently discarded instead.
For client implementations that retrieve name/value pairs by searching the TXT record for the requested name, they should search the TXT record from the start, and simply return the first matching name they find. Each author defining a DNS- SD (Rendezvous) profile for discovering instances of a particular type of service should define the interpretation of these different kinds of result. For example, for some keys, there may be a natural boolean interpretation: Absent implies 'false'Present with no value implies 'true'For other keys it may be sensible to define other semantics, such as: Present with value implies that value.
E. g. "Color=4" for a four- color ink- jet printer, or "Color=6" for a six- color ink- jet printer. Present with emply value implies 'false'. E. g. Not a color printer. Absent implies 'Unknown'. E. g. A print server connected to some unknown printer where the print server doesn't actually know if the printer does color or not — which gives a very bad user experience and should be avoided wherever possible.(Note that this is a hypothetical example, not an example of real name/value keys for printing.)As a general rule, names that contain no dots are defined as part of the open- standard definition written by the person or group defining the DNS- SD (Rendezvous) profile for discovering that particular service type.
Vendor- specific extensions should be given names of the form "keyname. This reduces the risk of accidental conflict if different organizations each define their own vendor- specific keys.
If there is an '=', then everything after the first '=' to the end of the string is the value. The value can contain any eight- bit values including '='. Leading or trailing spaces are part of the value, so don't put them there unless you intend them to be there. Any quotation marks around the value are part of the value, so don't put them there unless you intend them to be part of the value. The value is opaque binary data.
Often the value for a particular attribute will be US- ASCII (or UTF- 8) text, but it is legal for a value to be any binary data. For example, if the value of a key is an IPv. ASCII string giving the address represented in textual dotted decimal notation. Generic debugging tools should generally display all attribute values as if they were UTF- 8 text, except for attributes where the debugging tool has embedded knowledge that the value is some other kind of data.
Authors defining DNS- SD (Rendezvous) profiles SHOULD NOT convert binary attribute data types into printable text (e. Base. 64 or UU encoding) merely for the sake of making the data be printable text when seen in a generic debugging tool. Doing this simply bloats the size of the TXT record, without truly making the data any more understandable to someone looking at it in a generic debugging tool.- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- . A | name=value | 0x. A4 | 0x. 12 | Rendezvous Is Cool |.
It is recommended that authors defining DNS- SD (Rendezvous) profiles include an attribute of the form "version=xxx" in their definition, and require it to be the first name/value pair in the TXT record. This information in the TXT record can be useful help clients maintain backwards compatibility with older implementations if becomes necessary to change or update the specification over time. Even if the profile author doesn't anticipate the need for any future incompatible changes, having a version number in the specification provides useful insurance should incompatible changes become unavoidable. Clients should ignore TXT records with a version number higher (or lower) than the version(s) they know how to interpret. Legacy Implementations. Do not assume that all current Apple products using Rendezvous present a model example of how to structure Rendezvous TXT records.