Can't download image with NSURLSession with some link - objective-c

Question

I just can't download this image:
http://img2.imgtn.bdimg.com/it/u=3742910000,2737153630&fm=15&gp=0.jpg
I use NSURLSession downloadTask, but it's invalid, Chrome and safari can get it.
My code:
task = [_session downloadTaskWithURL:attachmentURL
completionHandler: ^(NSURL *temporaryFileLocation, NSURLResponse *response, NSError *error) {
if (error != nil) {
NSLog(#"error：%#", error.localizedDescription);
} else {
NSString *filePath = [self saveFrom:...];
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"%#", filePath);
NSData *data = [NSData dataWithContentsOfFile:filePath];
NSLog(#"data length: %lu", data.length);
// data length is error! smaller than should be
});
}
}];
[task resume];

Answer 1

All clients transmit more information than just 'get me this resource'. They also transmit data such as the user agent that's being used (i.e. what the client is). In the case of nsurlsession, it's set the user agent to something that does not look like a web browser, and in all odds the server is replying with a 403 error in that case (try to use curl on the URL and you'll get a 403 error).
Download normally:
$ curl "http://img2.imgtn.bdimg.com/it/u=3742910000,2737153630&fm=15&gp=0.jpg" -o img.jpg
$ file img.jpg
img.jpg: XML 1.0 document text, ASCII text
When I alter the user agent string to look like firefox:
$ curl -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:66.0) Gecko/20100101 Firefox/66.0' "http://img2.imgtn.bdimg.com/it/u=3742910000,2737153630&fm=15&gp=0.jpg" -o img.jpg
$ file img.jpg
img.jpg: JPEG image data, JFIF standard 1.01, resolution (DPI), density 72x72, segment length 16, baseline, precision 8, 500x333, frames 3
So we need to do the same, or similar for your nsurlsession. As we have the _session variable, we can do something like (this is only example code - please make your own):
NSMutableDictionary *md = [_session.configuration.httpAdditionalHeaders mutableCopy];
if (md == nil) {
md = [[NSMutableDictionary alloc] init];
}
md[#"User-Agent"] = #"Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:66.0) Gecko/20100101 Firefox/66.0";
_session.configuration.HTTPAdditionalHeaders = md;
Although, if you're creating the session with some NSURlSessionConfiguration already, it might make sense to add the headers there.