Pathology 101


Pathod is a pathological HTTP daemon designed to let you craft almost any conceivable HTTP response, including ones that creatively violate the standards. HTTP responses are specified using a small, terse language which pathod shares with its evil twin pathoc. To start playing with pathod, fire up the daemon:

>>> pathod

By default, the service listens on port 9999 of localhost, and the default crafting anchor point is the path /p/. Anything after this URL prefix is treated as a response specifier. So, hitting the following URL will generate an HTTP 200 response with 100 bytes of random data:

See the language documentation to get (much) fancier. The pathod daemon also takes a range of configuration options. To view those, use the command-line help:

>>> pathod --help

Mimicing a proxy

Pathod automatically responds to both straight HTTP and proxy requests. For proxy requests, the upstream host is ignored, and the path portion of the URL is used to match anchors. This lets you test software that supports a proxy configuration by spoofing responses from upstream servers.

By default, we treat all proxy CONNECT requests as HTTPS traffic, serving the response using either pathod’s built-in certificates, or the cert/key pair specified by the user. You can over-ride this behaviour if you’re testing a client that makes a non-SSL CONNECT request using the -C command-line option.


Anchors provide an alternative to specifying the response in the URL. Instead, you attach a response to a pre-configured anchor point, specified with a regex. When a URL matching the regex is requested, the specified response is served.

>>> pathod -a "/foo=200"

Here, “/foo” is the regex specifying the anchor path, and the part after the “=” is a response specifier.

File Access

There are two operators in the language that load contents from file - the + operator to load an entire request specification from file, and the > value specifier. In pathod, both of these operators are restricted to a directory specified at startup, or disabled if no directory is specified:

>>> pathod -d ~/staticdir"

Internal Error Responses

Pathod uses the non-standard 800 response code to indicate internal errors, to distinguish them from crafted responses. For example, a request to:

... will return an 800 response because “foo” is not a valid page specifier.


Pathoc is a perverse HTTP daemon designed to let you craft almost any conceivable HTTP request, including ones that creatively violate the standards. HTTP requests are specified using a small, terse language, which pathod shares with its server-side twin pathod. To view pathoc’s complete range of options, use the command-line help:

>>> pathoc --help

Getting Started

The basic pattern for pathoc commands is as follows:

pathoc hostname request [request ...]

That is, we specify the hostname to connect to, followed by one or more requests. Lets start with a simple example:

> pathoc get:/
07-06-16 12:13:43: >> 'GET':/
<< 302 Found: 261 bytes

Here, we make a GET request to the path / on port 80 of Pathoc’s output tells us that the server responded with a 302 redirection. We can tell pathoc to connect using SSL, in which case the default port is changed to 443 (you can over-ride the default port with the -p command-line option):

> pathoc -s get:/
07-06-16 12:14:56: >> 'GET':/
<< 302 Found: 262 bytes

Multiple Requests

There are two ways to tell pathoc to issue multiple requests. The first is to specify them on the command-line, like so:

> pathoc get:/ get:/
07-06-16 12:21:04: >> 'GET':/
<< 302 Found: 261 bytes
07-06-16 12:21:04: >> 'GET':/
<< 302 Found: 261 bytes

In this case, pathoc issues the specified requests over the same TCP connection - so in the above example only one connection is made to

The other way to issue multiple requests is to use the -n flag:

> pathoc -n 2 get:/
07-06-16 12:21:04: >> 'GET':/
<< 302 Found: 261 bytes
07-06-16 12:21:04: >> 'GET':/
<< 302 Found: 261 bytes

The output is identical, but two separate TCP connections are made to the upstream server. These two specification styles can be combined:

pathoc -n 2 get:/ get:/

Here, two distinct TCP connections are made, with two requests issued over each.

Basic Fuzzing

The combination of pathoc’s powerful request specification language and a few of its command-line options makes for quite a powerful basic fuzzer. Here’s an example:

pathoc -e -I 200 -t 2 -n 1000 localhost get:/:b@10:ir,@1

The request specified here is a valid GET with a body consisting of 10 random bytes, but with 1 random byte inserted in a random place. This could be in the headers, in the initial request line, or in the body itself. There are a few things to note here:

  • Corrupting the request in this way will often make the server enter a state where it’s awaiting more input from the client. This is where the -t option comes in, which sets a timeout that causes pathoc to disconnect after two seconds.
  • The -n option tells pathoc to repeat the request 1000 times.
  • The -I option tells pathoc to ignore HTTP 200 response codes. You can use this to fine-tune what pathoc considers to be an exceptional condition, and therefore log-worthy.
  • The -e option tells pathoc to print an explanation of each logged request, in the form of an expanded pathoc specification with all random portions and automatic header additions resolved. This lets you precisely replay a request that triggered an error.

Interacting with Proxies

Pathoc has a reasonably sophisticated suite of features for interacting with proxies. The proxy request syntax very closely mirrors that of straight HTTP, which means that it is possible to make proxy-style requests using pathoc without any additional syntax, by simply specifying a full URL instead of a simple path:

>>> pathoc -p 8080 localhost "get:''"

Another common use case is to use an HTTP CONNECT request to probe remote servers via a proxy. This is done with the -c command-line option, which allows you to specify a remote host and port pair:

>>> pathoc -c -p 8080 localhost get:/

Note that pathoc does not negotiate SSL without being explictly instructed to do so. If you’re making a CONNECT request to an SSL-protected resource, you must also pass the -s flag:

>>> pathoc -sc -p 8080 localhost get:/

Embedded response specification

One interesting feature of the Request specification language is that you can embed a response specification in it, which is then added to the request path. Here’s an example:

>>> pathoc localhost:9999 "get:/p/:s'401:ir,@1'"

This crafts a request that connects to the pathod server, and which then crafts a response that generates a 401, with one random byte embedded at a random point. The response specification is parsed and expanded by pathoc, so you see syntax errors immediately. This really becomes handy when combined with the -e flag to show the expanded request:

07-06-16 12:32:01: >> 'GET':/p/:s'401:i35,\x27\\x1b\x27:h\x27Content-Length\x27=\x270\x27:h\x27Content-Length\x27=\x270\x27':h'Host'='localhost'
<< 401 Unauthorized: 0 bytes

Note that the embedded response has been resolved before being sent to the server, so that “ir,@1” (embed a random byte at a random location) has become “i15,’o’” (embed the character “o” at offset 15). You now have a pathoc request specification that is precisely reproducible, even with random components. This feature comes in terribly handy when testing a proxy, since you can now drive the server response completely from the client, and have a complete log of reproducible requests to analyze afterwards.

Request Examples

get:/ Get path /
get:/:b@100 100 random bytes as the body
get:/:h”Etag”=”&;drop table browsers;” Add a header
get:/:u”&;drop table browsers;” Add a User-Agent header
get:/:b@100:dr Drop the connection randomly
get:/:b@100,ascii:ir,@1 100 ASCII bytes as the body, and randomly inject a random byte
ws:/ Initiate a websocket handshake.

Response Examples

200 A basic HTTP 200 response.
200:r A basic HTTP 200 response with no Content-Length header. This will hang.
200:da Server-side disconnect after all content has been sent.
200:b@100 100 random bytes as the body. A Content-Length header is added, so the disconnect is no longer needed.
200:b@100:h”Etag”=”’;drop table servers;” Add a Server header
200:b@100:dr Drop the connection randomly
200:b@100,ascii:ir,@1 100 ASCII bytes as the body, and randomly inject a random byte
200:b@1k:c”text/json” 1k of random bytes, with a text/json content type
200:b@1k:p50,120 1k of random bytes, pause for 120 seconds after 50 bytes
200:b@1k:pr,f 1k of random bytes, but hang forever at a random location
200:b@100:h@1k,ascii_letters=’foo’ 100 ASCII bytes as the body, randomly generated 100k header name, with the value ‘foo’.