I'm writing a tiny HTTP server using C++ (just for fun).
When receiving request from a client, should I worry about charset of HTTP headers? Is it guaranteed that all of them constist only of one-byte ASCII characters?
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2 Answers
Is it guaranteed that all of them constist only of one-byte ASCII characters?
No. HTTP uses TCP, so octets >= 128 can be transferred.
Does HTTP allow non-ASCII characters?
Yes. See the ABNF for field-content (RFC 2616, Section 4.2) and quoted-string (RFC 2616, Section 2.2).
Does HTTP define the encoding?
More or less, by stating that non-ISO-8859-1 characters require an additional layer of encoding (again, from 2.2):
The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message parser. Words of *TEXT MAY contain characters from character sets other than ISO-8859-1 [22] only when encoded according to the rules of RFC 2047 [14].
Is this used in practice?
Yes. For instance, in Content-Disposition.
Is this a good idea?
No, because many recipients and intermediates get this wrong.
8 Comments
Maksim Tyutmanov
I don't get it. According to this doc rfc822 (BNF on the page 9, where CHAR is ASCII character). Do you mean custom headers?
Julian Reschke
Comrade - How is RFC 822 relevant?
Bob Kuhar
In section 4.2 it calls out RFC 822: w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2 sounds to me like the standard is being ignored.
Bob Kuhar
Methinks some reference is in order here. Where have you seen non-ascii chars in headers and what spec are they following such that we can write some code around this.
Julian Reschke
Bob: "follow the same generic format" doesn't mean they are the same. RFC 2616 itself defines the format, and it definitely allows code points >= 128. I'm not saying it is a good idea, but it's what the spec says. Re: "Where have you seen non-ascii chars in headers and what spec are they following such that we can write some code around this." - the spec is RFC 2616. Example header fields where I have seen non-ASCII are Content-Disposition (in the Filename parameter) and WWW-Authenticate (in the Realm parameter).
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That's a great question and I don't know but would like to. I believe you will find the answer here: http://www.w3.org/Protocols/rfc2616/rfc2616.html
That doc says that Headers follow RFC822 (http://www.ietf.org/rfc/rfc0822.txt) and that one says ASCII. I'm thinking that you can rely upon the ASCIIness of it all.
2 Comments
Maksim Tyutmanov
It seems that you're right: "Each header field can be viewed as a single, logical line of ASCII characters, comprising a field-name and a field-body." Thanks a lot!
Julian Reschke
The answer above is incorrect. RFC 2616 mentions RFC 822, but this is not a normative statement, just a reference to a prior, similar format. The ABNF in RFC 2616 makes it clear that you can have octets >= 128.