Subclass of NSPortNameServer that uses distributed notifications (in a sandbox-friendly way) to implement a fast name server for socket ports. Decent demonstration of NSDistributedNotificationCenter, stupid run loop tricks, and SecTransform.

TsDistributedNotificationPortNameServer.h

//
//  TsDistributedNotificationPortNameServer.h
//  Typesetter
//
//  Created by Brent Royal-Gordon on 4/10/13.
//  Copyright (c) 2013 Groundbreaking Software. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface TsDistributedNotificationPortNameServer : NSPortNameServer

+ (TsDistributedNotificationPortNameServer*)sharedServer;

- (NSSocketPort *)portForName:(NSString *)name;
- (NSSocketPort *)portForName:(NSString *)name host:(NSString *)host;

- (BOOL)registerPort:(NSSocketPort *)port name:(NSString *)name;

- (BOOL)removePortForName:(NSString *)name;

@end

TsDistributedNotificationPortNameServer.m

//
//  TsDistributedNotificationPortNameServer.m
//  Typesetter
//
//  Created by Brent Royal-Gordon on 4/10/13.
//  Copyright (c) 2013 Groundbreaking Software. All rights reserved.
//

#import "TsDistributedNotificationPortNameServer.h"

static NSString * const TsDistributedNotificationPortNameServerRequestNotificationPrefix = @"com.groundbreakingsoftware.typesetter.TsDistributedNotificationPortNameServerRequest ";
static NSString * const TsDistributedNotificationPortNameServerResponseNotificationPrefix = @"com.groundbreakingsoftware.typesetter.TsDistributedNotificationPortNameServerResponse ";

static CFStringRef const kTsDistributedNotificationPortNameServerWaitingForResponseMode = CFSTR("TsDistributedNotificationPortNameServerWaitingForResponseMode");

@interface TsDistributedNotificationPortNameServer ()

@property (readonly,strong) NSMapTable * registeredPorts;
@property (readonly,strong) NSMutableDictionary * responsePorts;
@property (readonly,strong) NSMutableDictionary * responseRunLoops;

@end

@implementation TsDistributedNotificationPortNameServer

+ (void)initialize {
}

- (id)init {
    if(!_registeredPorts && (self = [super init])) {
        _registeredPorts = [NSMapTable strongToWeakObjectsMapTable];
        _responsePorts = [NSMutableDictionary new];
        _responseRunLoops = [NSMutableDictionary new];
        
        // NSDistributedNotificationCenter only receives notifications when the run loop is in a common mode.
        CFRunLoopAddCommonMode(CFRunLoopGetMain(), kTsDistributedNotificationPortNameServerWaitingForResponseMode);
    }
    return self;
}

+ (TsDistributedNotificationPortNameServer *)sharedServer {
    static TsDistributedNotificationPortNameServer * singleton;
    static dispatch_once_t once;
    
    dispatch_once(&once, ^{
        singleton = [TsDistributedNotificationPortNameServer new];
    });
    
    return singleton;

}

- (BOOL)registerPort:(NSSocketPort *)port name:(NSString *)name {
    if([self portForName:name]) {
        return NO;
    }
    
    // XXX There are race conditions possible. These *probably* shouldn't matter for my purposes. I hope.
    [self.registeredPorts setObject:port forKey:name];
    [NSDistributedNotificationCenter.defaultCenter addObserver:self selector:@selector(requested:) name:[TsDistributedNotificationPortNameServerRequestNotificationPrefix stringByAppendingString:name] object:nil];
    
    return YES;
}

- (BOOL)removePortForName:(NSString *)name {
    [NSDistributedNotificationCenter.defaultCenter removeObserver:self name:[TsDistributedNotificationPortNameServerRequestNotificationPrefix stringByAppendingString:name] object:nil];
    [self.registeredPorts removeObjectForKey:name];
    return YES;
}

- (NSSocketPort *)portForName:(NSString *)name {
    NSString * uuid = [NSUUID new].UUIDString;
    [NSDistributedNotificationCenter.defaultCenter addObserver:self selector:@selector(responded:) name:[TsDistributedNotificationPortNameServerResponseNotificationPrefix stringByAppendingString:uuid] object:nil];
    
    [NSDistributedNotificationCenter.defaultCenter postNotificationName:[TsDistributedNotificationPortNameServerRequestNotificationPrefix stringByAppendingString:name] object:uuid userInfo:nil deliverImmediately:YES];
    
    self.responseRunLoops[uuid] = (__bridge id)CFRunLoopGetCurrent();
    CFRunLoopRunInMode(kTsDistributedNotificationPortNameServerWaitingForResponseMode, 1, false);
    [self.responseRunLoops removeObjectForKey:uuid];
    
    [NSDistributedNotificationCenter.defaultCenter removeObserver:self name:[TsDistributedNotificationPortNameServerResponseNotificationPrefix stringByAppendingString:uuid] object:nil];
    
    if(!self.responsePorts[uuid]) {
        return nil;
    }
    
    NSSocketPort * port = self.responsePorts[uuid];
    [self.responsePorts removeObjectForKey:uuid];
    return port;
}

- (NSSocketPort *)portForName:(NSString *)name host:(NSString *)host {
    NSParameterAssert(host == nil || host.length == 0);
    return [self portForName:name];
}

- (void)requested:(NSNotification*)note {
    NSString * name = [note.name stringByReplacingCharactersInRange:NSMakeRange(0, TsDistributedNotificationPortNameServerRequestNotificationPrefix.length) withString:@""];
    NSSocketPort * port = [self.registeredPorts objectForKey:name];
    
    if(!port) {
        return;
    }
    
    SecTransformRef encoder = SecEncodeTransformCreate(kSecBase64Encoding, NULL);
    if (!encoder) {
        return;
    }
    if(!SecTransformSetAttribute(encoder, kSecTransformInputAttributeName, (__bridge CFDataRef)port.address, NULL)) {
        CFRelease(encoder);
        return;
    }
    NSData * encodedAddressData = (__bridge_transfer NSData*)SecTransformExecute(encoder, NULL);
    CFRelease(encoder);
    if (!encodedAddressData) {
        return;
    }
    NSString * encodedAddress = [[NSString alloc] initWithData:encodedAddressData encoding:NSASCIIStringEncoding];
    
    NSDictionary * dict = @{ @"protocolFamily": @(port.protocolFamily), @"socketType": @(port.socketType), @"protocol": @(port.protocol), @"address": encodedAddress };
    NSData * jsonData = [NSJSONSerialization dataWithJSONObject:dict options:0 error:NULL];
    if(!jsonData) {
        return;
    }
    NSString * json = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
    
    [NSDistributedNotificationCenter.defaultCenter postNotificationName:[TsDistributedNotificationPortNameServerResponseNotificationPrefix stringByAppendingString:note.object] object:json userInfo:nil deliverImmediately:YES];
}

- (void)responded:(NSNotification*)note {
    NSString * requestID = [note.name stringByReplacingCharactersInRange:NSMakeRange(0, TsDistributedNotificationPortNameServerResponseNotificationPrefix.length) withString:@""];
    
    NSData * jsonData = [note.object dataUsingEncoding:NSUTF8StringEncoding];
    NSDictionary * json = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:NULL];
    
    if(!json) {
        return;
    }
    
    CFRunLoopRef runLoop = (__bridge CFRunLoopRef)self.responseRunLoops[requestID];
    
    if(!runLoop || ![(__bridge_transfer NSString*)CFRunLoopCopyCurrentMode(runLoop) isEqualToString:(__bridge NSString*)kTsDistributedNotificationPortNameServerWaitingForResponseMode]) {
        return;
    }
        
    SecTransformRef decoder = SecDecodeTransformCreate(kSecBase64Encoding, NULL);
    if (!decoder) {
        return;
    }
    if(!SecTransformSetAttribute(decoder, kSecTransformInputAttributeName, (__bridge CFDataRef)[json[@"address"] dataUsingEncoding:NSASCIIStringEncoding], NULL)) {
        CFRelease(decoder);
        return;
    }
    NSData * decodedAddressData = (__bridge_transfer NSData*)SecTransformExecute(decoder, NULL);
    CFRelease(decoder);
    if (!decodedAddressData) {
        return;
    }
    
    NSSocketPort * port = [[NSSocketPort alloc] initRemoteWithProtocolFamily:[json[@"protocolFamily"] intValue] socketType:[json[@"socketType"] intValue] protocol:[json[@"protocol"] intValue] address:decodedAddressData];
    
    self.responsePorts[requestID] = port;
    
    CFRunLoopStop(runLoop);
}

@end