Sunday, December 20, 2015

Google App Engine cron jobs run as no user

Google App Engine (GAE) is a great platform to use when developing just about any new application.  I use it for a couple personal projects, as well as for some fire department apps (dispatch and inventory management).

Today I'd like to talk about two things: users and cron jobs.


One of the really convenient things about GAE is that it has built-in support for Google authentication (no surprise there).  This means that you can let GAE take care of your sign-in system (and trust me, handling the single-sign-on 3-way handshake isn't all that fun).  With GAE, you easily go one of two routes:
  1. Certain paths in your web application are automatically required to have a logged-in user.  If someone tries to access a path without being logged in, GAE will redirect him to a sign-in window and then bring him back when he's done.
  2. Server-side, you can ask GAE for log-in and log-out URLs, and you can direct a user to these at any time to have him log in or out.
I personally prefer the second route because there's no strange redirection and all of my endpoints behave as I expect them to.  For example, if I have a JSON REST API, a user (who has not signed in yet) can access the endpoint and be given a normal 400-level error (in JSON) instead of being redirected to a Google sign-in page.  My REST clients much prefer this.

To see if a user is signed in, a Java Servlet can call the "getCurrentUser()" method of the "UserService" instance.  If the result is null, then no one is logged in.  Otherwise, you get a couple of helpful methods to tell you about the user:
  1. "getEmail()"; this returns the user's e-mail address.
  2. "getUserId()"; this returns a unique numeric ID for the user.
  3. "isUserAdmin()"; this returns whether or not the user is an administrator of the GAE application.
For my apps, user authentication is whitelist style.  I check the "@" portion of the e-mail address to see if the user is in one of the domains that I care about (I typically build apps internal to an organization that uses Google's mail system), and I check the administrator status to grant administrator powers to my admins.

If someone tries to access a sensitive API endpoint without being logged in appropriately, I'll send back a 400-level error stating that the user is not signed in with an appropriate account.

Pretty easy stuff.

UserService userService = UserServiceFactory.getUserService();
User user = userService.getCurrentUser();
if( user == null ) {
   // There is no user logged in.
} else {
   // The user is logged in.
   System.out.println( "User is logged in: " + userService.isUserLoggedIn() );
   System.out.println( "User is administrator: " + userService.isUserAdmin() );
   System.out.println( "User:" );
   System.out.println( "   Auth Domain: " + user.getAuthDomain() );
   System.out.println( "   E-mail: " + user.getEmail() );
   System.out.println( "   Federated ID: " + user.getFederatedIdentity() );
   System.out.println( "   Nickname: " + user.getNickname() );
   System.out.println( "   User ID: " + user.getUserId() );

Warning: you cannot call "userService.isUserAdmin()" if the user is not already logged in.  If you try to, then it will throw an exception.

Cron Jobs

Another thing that GAE can do is schedule cron jobs.  Basically, these are page requests that are scheduled like normal Linux "cron" jobs.  So if you need to have some task performed regularly, create an endpoint for it and schedule a job to access that endpoint.

Cron jobs act as if an administrator is making the request, so they can access all paths with "admin" requirements.  Howerver, you cannot check this using "userService.isUserAdmin()" because cron jobs do not run as any particular user.

To determine if a request is coming from the cron scheduler, you have to check for the "X-Appengine-Cron" header.  This header cannot be faked (except by admins); if you try to set this header, GAE will quietly remove it by the time that it gets to your Servlet.

To detect a cron job, you have to check for the header and make sure that its value is "true".

String cronHeader = request.getHeader("X-Appengine-Cron");
if( cronHeader != null && cronHeader.compareTo("true") == 0 ) { "Cron service is making this request." );

Ultimately, if I'm checking to see whether a user is allowed to access a particular section, I go through these steps:
  1. (Assume no access at all.)
  2. No user is logged in.
    1. Is the "X-Appengine-Cron" header set to "true"?  If so, then allow administrative access.
  3. A user is logged in.
    1. Is the user a GAE admin of the application?  If so, then allow administrative access.
    2. Is the user's e-mail domain in the whitelist of basic user access?  If so, then allow basic access.

Monday, September 14, 2015

Fixing the default Ubuntu snmpd configuration

SNMP is super helpful for performance and health monitoring of any production equipment.  It's lightweight, easy to understand, and very resilient when Bad Things happen to the network.  If you're not monitoring your production equipment with SNMP, then probably should look into that right away (we use SevOne NMS at work).

Getting "snmpd", the Linux SNMP daemon, up and running on Ubuntu is simply a matter of installing "snmpd":
sudo apt-get install snmpd;

Or is it?

Default configuration woes


By default, Ubuntu wants to log literally everything that "snmpd" does to syslog.  While I love the enthusiasm, this quickly leads to overflowing logs and the headache around them (plus it makes it impossible to find any event that's actually important).

How many times do you want to see messages like this in your logs?
Sep 11 16:48:23 your-server snmpd[19552]: Connection from UDP: []:49867->[]
Sep 11 16:48:23 snmpd[19552]: last message repeated 199 times

The logging options are specified on the "snmpd" command line, and are thus configured in "/etc/default/snmpd".

The default logging settings are:

"-L" is for the logging options.  "s" is for syslog.  "d" is for the daemon facility.

What we want are these settings:
-LS 4 d

"-L" again is for logging options.  Capital "S" is for a priority-filtered syslog, with "4" being "warning-level or higher".  Again, "d" is for the daemon facility.

Port access

By default, Ubuntu locks down SNMP access to "localhost", so it's 100% useless from a monitoring perspective.  While I respect the security-mindedness displayed here, I need my boxes to actually respond to requests.

The access options are specified in the "snmpd.conf" file, which is located here: "/etc/snmp/snmpd.conf".

At the top of the file, there is a configuration item called "agentAddress".  By default, this limits requests to those originating locally.
agentAddress udp:

There is usually a line following it that's commented out, and that's the one that we want.  Get rid of the line above and make sure that this one is enabled:
agentAddress udp:161,udp6:[::1]:161

This makes sure that any requests to port 161 (the standard SNMP port) will be allowed.


Yes, yes, we should all be using SNMPv3's great user-based access-control mechanism, but for an internal-to-the-company server that can't be reached from the Internet, we can often afford to be lax.  And hey, I'm not stopping you from setting up SNMPv3 access control.  Go nuts.

Here, we're going to allow the community string of "public" to access everything about the box (but not make any changes at all).

The default configuration allows "public" to see some basic system information, but that's not good enough:
rocommunity public default -V systemonly

Get rid of that line and replace it with one that doesn't have the "systemonly" restriction:
rocommunity public

Restart "snmpd" and you'll be ready to respond to SNMP requests from your local management station.
sudo service snmpd restart;

Sunday, September 13, 2015

Google App Engine and Google Authentication

One of the things that I love about the year 2015 is "cloud computing"; in particular, I love that I can hand Google App Engine a Java project and it will host it, handle redundancy, auto-scale it, provide me a database, and do just about everything else that I could ever want.

However, security is still a major concern, and there are some applications that I work on where "leaking" some private data onto the public Internet is bad news.

User authentication

There are lots of ways to authenticate users at this point.  Years ago, we did everything ourselves (remember those days?).  Each application had its own user database with varying degrees of security, and passwords were being stolen and sold all the time.  Now we have things like OAuth, where we can pass off user authentication to other systems (which we have to trust), so we don't have to store anything more than an e-mail address.  If the OAuth server says that that e-mail address is legit and logged in, then it's legit and logged in.  This saves us time and money, since who wants to build and maintain a user authentication layer, anyway?

Google App Engine provides a pretty easy and awesome way to lock down your application to "signed in" users.  Here, "signed in" users can be one of two things:
  1. Any user on Earth with a Google account; or
  2. Any user in your Google Apps domain.
If you have a Google Apps domain, then with a couple of tweaks to both the domain and the app, then Google will make sure that only those people in the domain can log in to the app.  Otherwise, if you'll have to check for a particular domain from your logged-in users in your REST API calls (your app is RESTful, right?).  No biggie, either way.

(In this article, I'll be talking about the Java version of the Google App Engine SDK, so any files or calls will be related to that.)

To set up your app to force everything to require a logged-in user, you just need to update "web.xml" and add a "security-constraint" (obviously, you can play a lot with this):

The "url-pattern" of "/*" means "MATCH ALL THE URLS!", and the "role-name" of "*" means "all users signed in with a Google account".

I can't log out!

I recently put together an application for a group where I don't own the Google Apps domain, so I couldn't set up the sweet domain-level restriction at the app level.  Like I said, it's no big deal, so at the top of my API class was a check for "" in the user's e-mail address (remember, Google already verified that it was a Google account, so I just needed to make sure that it was in the right domain).

I asked the group to try out the application, and half loved it; the other half couldn't see any data.  It turned out those people logged in with their personal accounts (not their organization-specific accounts), and now were stuck in an application where they had no access to any data.

What I needed was a "switch accounts" button (like all of the other Google applications), or at least a way to invalidate the user's session so that they could log in again (and use their organization account, this time).  Google App Engine's built-in authentication system does some magic, so the usual Google authentication guides don't apply.  I tried all of the stuff that people online were talking about (invalidating sessions, deleting cookies, etc.), but none of it worked, and it seemed like a hack anyway.

It turns out that Google App Engine provides a "UserService" class help deal with this kind of problem.  In particular, you may request a login URL and a logout URL.  Since my whole app requires the user to be logged in, all I needed to do was log the user out, and he would be immediately redirected to the Google login/pick-account screen again.

I added a simple API call to return some information about the current user (so I could show the "Logged in as ..." message), and that call returned an additional property for the logout URL (remember, this URL is generated by Google App Engine and isn't trivial).  When my UI loads, it makes an AJAX call for the user information, grabs the logout URL, and provides a "Logout" link for any users that need to log out.

Problem solved.

Here's how to get the logout URL:
UserService userService = UserServiceFactory.getUserService();
String logoutUrl = userService.createLogoutURL("/");

The "/" argument to "createLogoutUrl" is where the user should be redirected once he logs out.  In this case, I'm sending him right back to the main screen of the app, where he will be asked to log in again.

Additional resources

  1. For more information on "web.xml" within Google App Engine, see this document.
  2. To learn how to set up authentication with Google App Engine, see this article.

Beware the AppScale firewall

If you haven't already looked into AppScale for your company's internal application needs, then you may want to spend some time looking into it.  In short, it's an open-source implementation of Google App Engine that can run on a "private" cloud.  Why is this cool?  Well, it lets me use all of the power and convenience of Google App Engine apps without having to put my app in the public cloud (high latency and billing), instead letting me use all the resources that I want in my company data center.

tl;dr: AppScale runs "iptables" on its own, so if you want to run an additional service (such as SNMP) on a node, then you'll have to configure AppScale to allow it.

My goal: SNMP monitoring

At work, I'm setting up an AppScale cluster to serve some internal applications.  The first thing that any server needs to do, once online, is provide performance statistics (via SNMP) to our performance management tool (in our case, we use SevOne NMS).

Typically, this is the world's easiest task:
  1. Install "snmpd" (apt-get install snmpd).
  2. Allow "snmpd" to respond to remote requests (duh).
  3. Fix Ubuntu's terribly verbose SNMP logging defaults.
Unfortunately, I fought with this for an hour because, no matter what I did, "snmpd" would not respond to any requests from my management tool, and nothing in the logs said why.  For reasons not perfectly clear to me, AppScale (for Ubuntu) runs on Ubuntu 12.04, so I thought that maybe there was some ancient security measure in place that I had forgotten about over the years.

I eventually stumbled on "iptables" as a culprit (it's never first on my list, but probably should be).  I ran "iptables -L -n" to list the current "iptables" rules, and sure enough, the system had some:
Chain INPUT (policy ACCEPT)
target     prot opt source               destination        
ACCEPT     all  --            
ACCEPT     all  --              state RELATED,ESTABLISHED
ACCEPT     tcp  --              tcp dpt:22
ACCEPT     tcp  --              tcp dpt:80
ACCEPT     tcp  --              tcp dpt:443
ACCEPT     tcp  --              tcp dpt:1080
ACCEPT     tcp  --              tcp dpt:1443
ACCEPT     tcp  --              tcp dpt:2812
ACCEPT     tcp  --              tcp dpt:5222
ACCEPT     tcp  --              tcp dpt:5555
ACCEPT     tcp  --              tcp dpt:6106
ACCEPT     tcp  --              tcp dpts:8080:8100
ACCEPT     tcp  --              tcp dpts:4380:4400
ACCEPT     tcp  --              tcp dpt:17443
ACCEPT     tcp  --              tcp dpt:4343
ACCEPT     all  --          
ACCEPT     all  --          
DROP       all  --            

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination        

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

However, no amount of "iptables" magic would allow me to get the system to respond to SNMP requests.  I'd add a rule, and it might respond for a few seconds, but after that, my SNMP requests would time out again.  My rule?  Gone.

The AppScale firewall

It turns out that AppScale maintains the "iptables" setup for the box, and any change that you make will quickly be reverted by it.  This doesn't, in principle, bother me except that it's not really documented anywhere.  The only real mention of it is the Performance Tuning document, and even then, it's just a quick mention in order to get HAProxy stats from the box.

The AppScale firewall configuration lives in "appscale/firewall.conf" (the default installation guide had me put the "appscale" directory in "/root", so the file was located in "/root/appscale/firewall.conf" for me).  Once I saw what was going on, it was simply a matter of making a quick change to the file and waiting a few seconds (AppScale periodically re-reads the file and makes any changes live).

To tell AppScale to allow SNMP requests, I simply had to add the following line after the other "iptables -A" lines:
iptables -A INPUT -p udp -m udp --dport 161 -j ACCEPT # SNMP

Problem solved.

Additional resources

  1. The current default version of AppScale's "firewall.conf" can be found here.