Getting metadata (identity) about an SMB volume's server

Alright, I figured out a way to identify the server by contacting it via https and then checking its certificate. That should be pretty reliable.

Below the code I use, though I ran into one complication: The mount URL I get for my QNAP NAS is: "QNAS._smb._tcp.local", but that is not a valid host name I can use in a NSURLRequest!

I need to transform that into the actual host name, which is "QNAS.local". Since the former is a Bonjour related name, and when I browse the Bonjour registry with BonJeff, I can find the mapping, this seems to be an overly complicated method. I wonder what the proper way is to get the basic host name from such a service name. I've googled for a while but could not find anything about it. For now, I simply remove all components from the host name that start with an underscore, but I'm not sure if that's a safe method.

#import <NetFS/NetFS.h>
#import <Security/Security.h>
#import "AppDelegate.h"

@interface AppDelegate () <NSURLSessionDelegate>
@end

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
	CFURLRef furl = CFURLCreateWithFileSystemPath(NULL, CFSTR("/Volumes/TheNAS"), kCFURLPOSIXPathStyle, true);
	NSURL *url = CFBridgingRelease(NetFSCopyURLForRemountingVolume (furl));
	NSArray *parts = [[url.host componentsSeparatedByString:@"."] filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(NSString* _Nullable part, id _Nullable bindings) {
		return ![part hasPrefix:@"_"];
	}]];
	NSString *addr = [parts componentsJoinedByString:@"."];
	NSLog(@"host: %@ -> %@", url.host, addr);

	NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
	[request setURL:[NSURL URLWithString:[NSString stringWithFormat:@"https://%@", addr]]];
	NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:nil];
	[[session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
		NSLog(@"Done.");
		[NSApp terminate:self];
	}] resume];
}

-(void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition, NSURLCredential * _Nullable))completionHandler
{
	SecTrustRef trustRef = [[challenge protectionSpace] serverTrust];
	SecCertificateRef certRef = SecTrustGetCertificateAtIndex(trustRef, 0);
	CFStringRef name = nil;
	SecCertificateCopyCommonName(certRef, &name);
	NSLog(@"name: %@", name);

	// reject the challenge because we have all we wanted
	completionHandler (NSURLSessionAuthChallengeCancelAuthenticationChallenge, nil);
}

@end

The mount URL I get for my QNAP NAS is: QNAS._smb._tcp.local

Oh, wow, that’s the name of an SRV record. I wasn’t expecting that.

I wonder what the proper way is to get the basic host name from such a service name.

I recommend that you use the DNS-SD API for this. To get started, look at the SRVResolver sample code.

IMPORTANT That sample predates DNSServiceSetDispatchQueue, which greatly simplifies this whole business. I recommend that you combine the SRV concepts it shows with some code that uses DNSServiceSetDispatchQueue, such as DNSSDObjects.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

@eskimo, With the code from SRVResolver I can indeed resolve QNAS._smb._tcp.local. Problem is: If the host name is a plain one, like syno.local, then I get neither an error nor a callback. And using a Timeout for a local-only NS resolve seems wrong to me.

How should I best handle this? Is there a way to tell when I need to use the SRV record resolver?

Is there a way to tell when I need to use the SRV record resolver?

Not as such. However, it’s easy to distinguish the two cases by looking for the ._smb._tcp. pattern. This can’t appear in an IP address or a standard DNS host name (_ is not a valid character for DNS host name).

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

@eskimo,

There's a new problem I just ran into:

If the host is a Windows server, the domain name of the mounted volume resolve to a name that I cannot use - neither directly nor with the proposed SRVResolver.

For instance, I set up a Windows server with the netbios(?) name "win-serv", and a volume named "WinServer". When I Get Info on the mounted volume, it shows as:

smb://win-serv/WinServer

When I retrieve the domain name, e.g. with statfs, the name I get is just "win-serv", but that can't be used as an IP address.

I found that I can resolve this name with the shell cmd smbutil lookup win-serv, though.

So my questions are:

1. Is there an API for doing what smbutil lookup does?
2. How do I tell that I need to use this method? I guess I could just always fall back to it when I cannot reach the server via its given name, but is there a clearer indicator that this is a WINS(?) and not a usual DNS name?

I have a related question on StackOverflow but that hasn't gotten me anywhere, either. This entire WINS/NetBIOS thing is a mystery on macOS, it seems.

1. Is there an API for doing what smbutil lookup does?

Not that I’m aware of.

2. How do I tell that I need to use this method?

I can’t think of any good options for that.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"