how to use Alamofire with custom headers

For headers that change from request to request, you can pass them directly to the request method. From the docs:

Adding a custom HTTP header to a Request is supported directly in the global request method. This makes it easy to attach HTTP headers to a Request that can be constantly changing.

And the example given:

    let headers = [
        "Authorization": "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==",
        "Content-Type": "application/x-www-form-urlencoded"
    ]

    Alamofire.request(.GET, "https://httpbin.org/get", headers: headers)
             .responseJSON { response in
                 debugPrint(response)
             }

If, however, you wish to set headers that do not change, it is recommended that you do so on the NSURLConfiguration object, as others have mentioned here.

According to the official documentation, modifying the session configuration is not recommended:

This is not recommended for Authorization or Content-Type headers. Instead, use URLRequestConvertible and ParameterEncoding, respectively.

So an example usage of URLRequestConvertible for authorization would be:

enum Router: URLRequestConvertible {
    static let baseUrlString = "some url string"

    case Get(query: String)

    var URLRequest: NSMutableURLRequest {
        let (path: String, parameters: [String: AnyObject]?) = {
            switch self {
            case .Get(let query):
                return ("/get", ["q": query])
            }
        }()

        let URL = NSURL(string: Router.baseUrlString)!
        let URLRequest = NSMutableURLRequest(URL: URL.URLByAppendingPathComponent(path))
        // set header fields
        URLRequest.setValue("a", forHTTPHeaderField: "Authorization")

        let encoding = Alamofire.ParameterEncoding.URL        
        return encoding.encode(URLRequest, parameters: parameters).0
    }
}

and when you want to make a request:

Manager.sharedInstance.request(Router.Get(query: "test"))

More info about URLRequestConvertible: https://github.com/Alamofire/Alamofire#urlrequestconvertible

Old Answer

As of Alamofire v1.0 Pers answer no longer works. In the new version additional headers should be added to the HTTPAdditionalHeaders property of NSURLSessionConfiguration

Alamofire.Manager.sharedInstance.session.configuration.HTTPAdditionalHeaders = ["Authorization": authorizationToken]

More info here: https://github.com/Alamofire/Alamofire/issues/111

I've created a static headers method in a separate APIManager class.

import Foundation
import Alamofire

class APIManager {

    class func headers() -> HTTPHeaders {
        var headers: HTTPHeaders = [
            "Content-Type": "application/json",
            "Accept": "application/json"
        ]

        if let authToken = UserDefaults.standard.string(forKey: "auth_token") {
            headers["Authorization"] = "Token" + " " + authToken
        }

        return headers
    }
}

And I use it in requests:

Alamofire.request(urlString,
                      method: .get,
                      headers:APIManager.headers())

At this time , Swift 3.0 , Xcode 8.x, Alamofire 4.x:

You can use custom header as below:

let headers: HTTPHeaders = [
    "Authorization": "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==",
    "Accept": "application/json"
]

Alamofire.request("https://httpbin.org/headers", headers: headers).responseJSON { response in
    debugPrint(response)
}

For reference