Slice module 206 requirement

Lucas Rolff lucas at lucasrolff.com
Sun Jul 10 09:08:18 UTC 2022


You’re truly awesome! I’ll give the patch a try tomorrow - and thanks for the other bits and pieces of information, especially regarding the expectations as well.

I wish you an awesome Sunday!

Best Regards,
Lucas Rolff

> On 10 Jul 2022, at 10:35, Maxim Dounin <mdounin at mdounin.ru> wrote:
> 
> Hello!
> 
> On Fri, Jul 08, 2022 at 07:13:33PM +0000, Lucas Rolff wrote:
> 
>> I’m having an nginx instance where I utilise the nginx slice 
>> module to slice upstream mp4 files when using proxy_cache.
>> 
>> However, I have an interesting origin where if sending a range 
>> request (which happens when the slice module is enabled), to a 
>> file that’s less than the slice range, the origin returns a 200 
>> OK, but with the range related headers such as content-range, 
>> but obviously the full file is returned since it’s within the 
>> requested range.
>> 
>> When playing the MP4s through Google Chrome and Firefox it works 
>> fine when going through the nginx proxy instance, however, it 
>> somehow breaks Safari (both on MacOS, and iOS) - I guess Safari 
>> is more strict.
>> When playing directly through the origin it works fine in all 
>> browsers.
>> 
>> The md5 of response from the origin remains the same, so it’s 
>> not that the response itself is an invalid MP4 file, and even if 
>> you compare the cache files on disk with a “working” origin and 
>> the “broken” origin (one sends a 206 Partial Content, another 
>> sends 200 OK) - the content of the cache files remain the same, 
>> except obviously the header section of the cache file.
>> 
>> The origin returns a 206 status code, only if the file exceeds 
>> the slice size, so if I configure a slice size of 5 megabyte, 
>> only files above 5 megabytes will give 206s. Anything under 5 
>> megabytes will result in a 200 OK with content-range and the 
>> correct content-length,
>> 
>> Looking in the slice module itself I see:
>> https://github.com/nginx/nginx/blob/master/src/http/modules/ngx_http_slice_filter_module.c#L116-L126
>> 
>> 
>>    if (r->headers_out.status != NGX_HTTP_PARTIAL_CONTENT) {
>>        if (r == r->main) {
>>            ngx_http_set_ctx(r, NULL, ngx_http_slice_filter_module);
>>            return ngx_http_next_header_filter(r);
>>        }
>> 
>>        ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
>>                      "unexpected status code %ui in slice response",
>>                      r->headers_out.status);
>>        return NGX_ERROR;
>>    }
>> 
>> This seems like the slice module expects a 206 status code to be 
>> returned,
> 
> For the main request, the code accepts two basic valid variants:
> 
> - 206, so the slice module will combine multiple responses to 
>  range requests as needed;
> 
> - anything else, so the slice module will give up and simply 
>  return the response to the client.
> 
> If the module sees a non-206 response to a subrequest, this is an 
> error, as the slice module expects underlying resources to be 
> immutable, and does not expect that some ranges can be requested, 
> while some other aren't.  This isn't something related to your 
> case though.
> 
>> however, later in the same function 
>> https://github.com/nginx/nginx/blob/master/src/http/modules/ngx_http_slice_filter_module.c#L200-L211
>> 
>> 
>>    if (r->headers_out.status == NGX_HTTP_PARTIAL_CONTENT) {
>>        if (ctx->start + (off_t) slcf->size <= r->headers_out.content_offset) {
>>            ctx->start = slcf->size
>>                         * (r->headers_out.content_offset / slcf->size);
>>        }
>> 
>>        ctx->end = r->headers_out.content_offset
>>                   + r->headers_out.content_length_n;
>> 
>>    } else {
>>        ctx->end = cr.complete_length;
>>    }
>> 
>> There it will do an else statement if the status code isn’t 206.
>> So would this piece of code ever be reached, since there’s the initial error?
> 
> Following the initial check, r->headers_out.status is explicitly 
> changed to NGX_HTTP_OK.  Later on the 
> ngx_http_next_header_filter() call might again change 
> r->headers_out.status as long as the client used a range request, 
> and this is what checked here.
> 
>> Additionally I don’t see in RFC7233 that 206 responses are an 
>> absolute requirement, additionally I don’t see content-range 
>> being prohibited/forbidden to be used for 200 OK responses.
>> Now, if one have a secondary proxy that modifies the response 
>> headers in between the origin returning 200 OK with the 
>> Content-Range header, and then strip out the Content-Range 
>> header, the nginx slice module seems to handle it fine, so 
>> somehow the combination of 200 OK and a Content-Range header 
>> being present seems to break the slice module from functioning.
>> 
>> I’m just curious why this happens within the slice module, and 
>> if there’s any possible solution for it (like allowing the 
>> combination of 200 OK and Content-Range, since those two would 
>> still indicate that the origin/upstream supports range requests) 
>> - obviously it would be nice to fix the upstream server but 
>> sometimes that’s sadly not possible.
> 
>> From the above explanation it is probably already clear that 
> "disabling slice when an origin returns 200 OK" is what actually 
> happens.
> 
> The issue does not appear without the slice module in your testing 
> because the Content-Range header seems to be only present in your 
> backend 200 responses when there was a Range header in the 
> request, and this is what happens only with the slice module.
> 
> I've done some limited testing with Safari and manually added 
> Content-Range header, and there seem to be at least two issues:
> 
> - Range filter in nginx does not expect the Content-Range header 
>  to be already present in 200 responses and simply adds another 
>  one.  This results in incorrect range responses with multiple 
>  Content-Range headers, and this breaks Safari.
> 
> - Safari also breaks if its test request with "Range: bytes=0-1" 
>  results in 200 with the Content-Range header.
> 
> My initial fix was to simply disable handling of 200 responses 
> with Content-Range headers in the range filter, so such responses 
> wouldn't be touched at all.  This is perfectly correct and 
> probably the most secure thing to do, but does not work with 
> Safari due to the second issue outlined above.
> 
> Another approach would be to clear pre-existing Content-Range 
> headers in the range filter.  This seems to work, at least in my 
> testing.  See below for the patch.
> 
>> I know the parts of the slice module haven’t been touched for 
>> years, so obviously it works for most people, just dipping my 
>> toes here to see if there’s a possible solution other than 
>> disabling slice when an origin returns 200 OK for files smaller 
>> than the slice size.
> 
> Note that that slice module is generally unsafe to use for 
> arbitrary upstream servers: it relies on expectations which are 
> beyond the HTTP standard requirements.  In particular:
> 
> - It requires resources to be immutable, so different range 
>  responses can be combined together.
> 
> - It does not try to handle edge cases, such as 416 returned by 
>  the upstream on empty files (which is correct per RFC, but 
>  requires complicated additional handling to convert 416 to 200, so 
>  it is better to just return 200 OK).
> 
> In general, the slice module is to be used only in your own 
> infrastructure when you control the backend and can be sure that 
> the slice module expectations are met.  As such, disabling it for 
> backends which do something unexpected might actually be a good 
> idea.  On the other hand, in this particular case the nginx 
> behaviour can be adjusted to handle things gracefully.
> 
> Below is a patch to clear pre-existing Content-Range headers 
> in the range filter.  Please test if it works for you.
> 
> # HG changeset patch
> # User Maxim Dounin <mdounin at mdounin.ru>
> # Date 1657439390 -10800
> #      Sun Jul 10 10:49:50 2022 +0300
> # Node ID 219217ea49a8d648f5cadd046f1b1294ef05693c
> # Parent  9d98d524bd02a562d9cd83f4e369c7e992c5753b
> Range filter: clearing of pre-existing Content-Range headers.
> 
> Some servers might emit Conten-Range header on 200 responses, and this
> does not seem to contradict RFC 9110: as per RFC 9110, the Content-Range
> header have no meaning for status codes other than 206 and 417.  Previously
> this resulted in duplicate Content-Range headers in nginx responses handled
> by the range filter.  Fix is to clear pre-existing headers.
> 
> diff --git a/src/http/modules/ngx_http_range_filter_module.c b/src/http/modules/ngx_http_range_filter_module.c
> --- a/src/http/modules/ngx_http_range_filter_module.c
> +++ b/src/http/modules/ngx_http_range_filter_module.c
> @@ -425,6 +425,10 @@ ngx_http_range_singlepart_header(ngx_htt
>         return NGX_ERROR;
>     }
> 
> +    if (r->headers_out.content_range) {
> +        r->headers_out.content_range->hash = 0;
> +    }
> +
>     r->headers_out.content_range = content_range;
> 
>     content_range->hash = 1;
> @@ -582,6 +586,11 @@ ngx_http_range_multipart_header(ngx_http
>         r->headers_out.content_length = NULL;
>     }
> 
> +    if (r->headers_out.content_range) {
> +        r->headers_out.content_range->hash = 0;
> +        r->headers_out.content_range = NULL;
> +    }
> +
>     return ngx_http_next_header_filter(r);
> }
> 
> @@ -598,6 +607,10 @@ ngx_http_range_not_satisfiable(ngx_http_
>         return NGX_ERROR;
>     }
> 
> +    if (r->headers_out.content_range) {
> +        r->headers_out.content_range->hash = 0;
> +    }
> +
>     r->headers_out.content_range = content_range;
> 
>     content_range->hash = 1;
> 
> -- 
> Maxim Dounin
> http://mdounin.ru/
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