If I check the "content-length" header, is it 100% accurate?

Question

If not, how accurate is it?

I want to know the size of the image before I download it.

score 7 · Answer 1 · answered Jan 07 '12 at 00:30

7

Can the HTTP Content-length header be malformed? Yes.
Should you trust it to be a fair representation of the size of the message body? Yes.

answered Jan 07 '12 at 00:30

FrozenFire

963
1
6
15

score 3 · Answer 2 · answered Jan 07 '12 at 00:35

It should be, and usually is, accurate. However it is entirely possible for a web server to report a incorrect content length although this obviously doesn't happen often (I recall old versions of apache retuning nonsensical content lengths on files > 2GB).

It is also not mandatory to provide a Content-Length header

score 1 · Accepted Answer · answered Jan 07 '12 at 00:29

1

It had better be - otherwise why have it at all?

If it can't be reliably determined in advanced, it shouldn't be served by the server at all. (When dealing with dynamically generated text, for example, something like chunked transfer encoding may be used - which doesn't require the final length to be known when the HTTP header is written at the beginning of the stream.)

answered Jan 07 '12 at 00:29

ziesemer

27,712
8
86
94

Because users expect a progress bar and the content length is not always known in advance. Browsers generally error when the length specified is smaller than the content delivered but accept gracefully when length is over estimated. Great article here http://bit.ly/1yjXciq – AaronM Jun 18 '14 at 19:44

score 0 · Answer 4 · answered Apr 28 '22 at 01:54

A sharing for what I've discovered recently.

My case was using NodeJS to post http request to another server, which I'm able to set the content-length value by my self into the header (the value is lower than the actual size).

When the other server received my http request, it only process the request param/body size up to the point that the content-length told the server.

E.g. my request actual length is 100, but content-length mentioned 80, the server who received the request will only process 1-80 and the last 20 wasn't being processed and caused some error like "Invalid parameter".

Nowadays sever side will automatically insert the content-length for you, unless there are needs and you know what you are doing, else you don't want to change it as it may cause you trouble.

score 0 · Answer 5 · answered Jan 07 '12 at 00:34

0

Content-Length can be sent by the server code (or) by the apache layer itself. When the code is not sending apache will send it.

There are known client-crashes when the client connects and closes the socket when the content-length is sent smaller.

Since the images are usually not generated by code in run-time, you can rely on it.

answered Jan 07 '12 at 00:34

nizam.sp

4,002
5
39
63

Apache adds Content-Length only for small response bodies. Source comments from (http://stackoverflow.com/a/9728553/468921). – Antti Rytsölä Apr 12 '12 at 09:45

score 0 · Answer 6 · edited May 23 '17 at 11:44

0

Browsers can be unforgiving if the content-length is incorrect.

I was having a problem here, where the server was sometimes returning a content-length that was too low. The browsers just wouldn't handle it.

So yes, you can assume that the server is setting the content-length correctly, based on the knowledge that browser clients work on the same assumption.

edited May 23 '17 at 11:44

Community

1
1

answered Jan 07 '12 at 00:34

Andrew Shepherd

44,254
30
139
205

If I check the "content-length" header, is it 100% accurate?

6 Answers6

Linked