CanUSeeMe

A self-hosted auto-responder for remote self-testing local endpoints/websites.

When setting up self-hosted programs or developing web services, or troubleshooting firewall or proxy/reverse-proxy things, sometimes its nice to be able to look outside your network and query it from beyond, like how someone else would see it. You can do this now with a webproxy, but that only works for HTTP/S. Even better is if we could do so programmatically - like if our program listens on a port, or at a particular hostname have an external agent try reaching us at that hostname:port but then report back so we know we're reachable -if not we can bail or log warnings appropriately.

All CanUSeeMe does (for now) is expose a single endpoint /lb (for lookback) which given a URL, breaks it down and tells you:

• what external IP you're talking from
• what hostname.domain you're asking me to check
• what IP that actually resolves to
• can I ping that ip?
• can I ping that hostname?
• if you gave me a port, can I reach it? if not, can I reach 80 or 443 (HTTPS)? if its a common port like FTP or SSH or SMTP do I get an expected response?
• if its HTTP/S, what do I get when I curl it?
• do I get valid HTTPS certificate?

Basically, enough information for a running network service to be able to determine itself if its exposed to, and reachable from the internet. CanUSeeMe was written as a goofy little service for FOSS platform maintainers to run so that user instances have somewhere to phone home to; one to help check that you've set your DNS and firewall up so your thing is accessible if you're self-hosting. Future work might be to report instance creation and "up"ness to a "master" instance... or to load share in a P2P manner.

Sample Output

I have a sample instance running and used it to query my self-hosted blog:

https://canuseeme.tezoatlipoca.com/lb/?url=https://awadwatt.com/tezoatlipoca

which results in:

{
  "host": "awadwatt.com",
  "port": "443",
  "path": "/",
  "url": "https://awadwatt.com",
  "http": false,
  "https": true,
  "portType": "HTTPS",
  "pingResponse": "Ping reply: Success",
  "portResponse": "Sent command: GET / HTTP/1.1\r\nHost: awadwatt.com\r\nConnection: closeReceived response: HTTP/1.1 302 FoundHTTP 302 Found. Redirect to: /?landing=1",
  "curlResponse": "querying url: GET https://awadwatt.com\nresponseString: \u003C!DOCTYPE HTML\u003E\n\u003Chtml\u003E\n\t\u003Chead\u003E\n\t\t\n\u003Ctitle\u003Eawadwatt.com\u003C/title\u003E\n\n\u003Cstyle type=\"text/css\"\u003E\nh2 {\n\tfont-weight: normal;\n}\n#pricing.content-container div.form-container #payment-form {\n\tdisplay: block !impor",
  "certDebug": "Certificate validation passed.\nCertificate Subject: CN=awadwatt.com\nCertificate Issuer: CN=E6, O=Let's Encrypt, C=US\nCertificate Effective Date: 2/28/2025 11:17:30 PM\nCertificate Expiration Date: 5/30/2025 12:17:29 AM\nChain Element Subject: CN=awadwatt.com\nChain Element Issuer: CN=E6, O=Let's Encrypt, C=US\nChain Element Subject: CN=E6, O=Let's Encrypt, C=US\nChain Element Issuer: CN=ISRG Root X1, O=Internet Security Research Group, C=US\nChain Element Subject: CN=ISRG Root X1, O=Internet Security Research Group, C=US\nChain Element Issuer: CN=ISRG Root X1, O=Internet Security Research Group, C=US"
}

.. which isn't pretty but its functional. This is output that has been pretty-printed by my browser, so that may be adding some unintended relish. If you want nice HTML formatted output add &html to the end of the web request to canuseeme... so that last example would be:

https://canuseeme.tezoatlipoca.com/lb/?url=https://awadwatt.com/tezoatlipoca&html

which gives... well you can see here.

Command Line

Usage: ./canuseeme(.exe) -- [options]
Options:
--port=PORT			  Port to listen on. Default is 5000
--bind=IP			    IP address to bind to. Default is *       
--hostname=URL		URL to use in links. Default is http://localhost 
--runlevel=LEVEL	Log level. Default is Information (Trace -> Debug -> Information -> Warn -> Fatal)
--sitecss=URL			PATH to the site stylesheet. Default is null       
--sitepng=URL			PATH to the site favicon.ico (jpg and png). Default is null 
--help            This

Using:

The /lb endpoint only returns JSON (maybe we'll add an HTML mode later) - Once you have canuseeme running and the port is exposed if you try and hit the /lb endpoint with no querystring you'll get

"No URL provided."

If you give it a valid url parameter it will do its best to query that url for the info above. If you provide the parameter html (e.g. lb?url=<url>&html) the results will be encapsulated in some nice HTML for readability in a browser.. instead of the default which is json. If you specify portType it will try and use a little intelligence when it queries the port (if one is given as part of the URL), but only so far as what language to speak to that port with, like using EHLO to talk to an SMTP service. If you try and talk FTP to an IMAP service you're going to get weird results (but canuseeme will still try). Here are the ports and what canuseeme tries to interrogate it with:

                                case "HTTP": // HTTP
                                case "HTTPS": // HTTPS
                                    command = $"GET / HTTP/1.1\r\nHost: {rsd.host}\r\nConnection: close\r\n\r\n";
                                    break;
                                case "SMTP": // SMTP 25
                                    command = $"EHLO {rsd.host}\r\n";
                                    //expectedResponse = "250";
                                    break;
                                case "FTP": // FTP 21
                                    command = "USER anonymous\r\n";
                                    //expectedResponse = "331";
                                    break;
                                case "POP3": // POP3 110
                                    command = "USER test\r\n";
                                    //expectedResponse = "+OK";
                                    break;
                                case "IMAP": // IMAP 143
                                    command = "a001 LOGIN test test\r\n";
                                    //expectedResponse = "a001 OK";
                                    break;
                                case "TELNET": // Telnet 23
                                               // No specific command, just establish a connection
                                               //expectedResponse = "Connected to";
                                    break;
                                case "SSH": // SSH 22
                                            // No specific command, just establish a connection
                                            //expectedResponse = "SSH-2.0";
                                    break;
                                case "MySQL": // MySQL 3306
                                              // No specific command, just establish a connection
                                              //expectedResponse = "\x00\x00\x00\x0a";
                                    break;
                                case "Redis": // Redis - 6379
                                    command = "PING\r\n";
                                    //expectedResponse = "+PONG";
                                    break;

Note that canuseeme doesn't evaluate the response from that port, it just passes it along in the results and closes the connection. (unless its HTTP/s then it does a webget) The default ports above (e.g. TELNET - 23) were for my reference - canuseeme makes no assumption about what port anything is running on - so if you setup your SSH service running on port 6978, you'd use
/lb?url=yourdomain:6978&portType=SSH Consider the non-HTTP/S port interrogation to be experimental for now. Here's how I query my email server:

https://canuseeme.tezoatlipoca.com/lb?url=smtp.awadwatt.com:25&portType=SMTP

gets me

{
  "host": "smtp.awadwatt.com",
  "port": "25",
  "path": "/",
  "url": "smtp.awadwatt.com:25",
  "http": false,
  "https": false,
  "portType": "SMTP",
  "pingResponse": "Ping reply: Success",
  "portResponse": "Sent command: EHLO smtp.awadwatt.comReceived response: 554 5.5.0 Error: SMTP protocol synchronization",
  "curlResponse": "querying url: GET smtp.awadwatt.com:25\nException: The URI prefix is not recognized. \n\nInnerException:  \n",
  "certDebug": null
}

I get AN SMTP error just not the one I was expecting (more work is needed.)

Future Work

• improve non HTTP port interrogation (thats still experimental)
• for non HTTP ports don't bother doing a webget/curl or the cert.
• rate limit requests by IP address to prevent abuse/spam flooding or use as a ddos relay
• add the ability to require some form of user credentials or some secret key to prevent public use
• HTML form output so the request could be made by a browser, or stuffed into an iframe.
• webadmin pages?
• /about page?
• have endpoints to facilitate master/sibling relationships

Try it before you buy it

https://canuseeme.tezoatlipoca.com/lb?url=&portType=[HTTP|HTTPS|SMTP|FTP|SSH|.. etc]