# Evil Bit Plugin

The Evil Bit refers to the unused high-order bit of the IP fragment offset field in the IP header. It was defined in RFC3514 on the 1st of April 2003.

The Evil Bit plugin for PATHspider aims to detect breakage in the Internet due to the use of reserved bit in the IP fragment offset field.

## Usage Example

Note

The path given to the example list of web servers is taken from a Debian GNU/Linux installation and may differ on your computer. These are the same examples that can be found in the examples/ directory of the source distribution.

To use the EvilBit plugin, specify `evilbit` as the plugin to use on the command-line:

    pspdr measure -i eth0 evilbit </usr/share/doc/pathspider/examples/webtest.ndjson >results.ndjson

This will run two TCP connections for each job input, one with the evil bit not set and one with the evil bit set (indicating the packet’s malicious intent).

## Supported Connection Modes

This plugin supports the following connection modes:

> - tcp - Performs only a TCP 3WHS
> - dnsudp - Performs a DNS query using UDP

To use an alternative connection mode, add the `--connect` argument to the invocation of PATHspider:

    pspdr measure -i eth0 evilbit --connect tcp </usr/share/doc/pathspider/examples/webtest.ndjson >results.ndjson

## Output Conditions

The following conditions are generated for the evilbit plugin:

### evilbit.X.connectivity.Y

For each connection that was observed by PATHspider, a connectivity condition will be generated to indicate whether or not connectivity with the evil bit set was successful validated against a connection without the evil bit set.

Y may have the following values:

> - works - Both connections succeeded
> - broken - Baseline connection succeeded where experimental connection failed
> - offline - Both connections failed
> - transient - Baseline connection failed where experimental connection succeeded (this can be used to give an indication of transient failure rates included in the “broken” set)

### evilbit.mark.X

A condition is generated to show whether the evil bit was set on the return path for the experimental connection. X can have two values, “seen” or “not_seen”.

## Notes

- The evil bit is set using packet forging library Scapy. Due to the lack of a TCP state machine, connection types such as HTTP are not available.