Exception is thrown on sprintf("%TCP.flags%") with NS flag set

[Kkevsterrr]

Brief description

If packet flags are set randomly, if they are sprintf-ed to %TCP.flags%, an exception is thrown, even though every value for this field can be represented by flag letters.

Environment

• Scapy version: 3.0.0
• Python version: 3.6.3
• Operating System: OS X, Ubuntu 16.04, Kali

How to reproduce

$ scapy
>>> p = IP()/TCP()
>>> p[TCP] = 0x100
>>> p.sprintf("%TCP.flags")

Actual result

$ scapy
>>> p = IP()/TCP()
>>> p[TCP] = 0x100
>>> p.sprintf("%TCP.flags")
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/scapy/packet.py", line 979, in sprintf
    val = self.payload.sprintf("%%%s%%" % sfclsfld, relax)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/scapy/packet.py", line 977, in sprintf
    val = self.fieldtype[fld].i2repr(self,val)
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/scapy/fields.py", line 907, in i2repr
    r += self.names[i]
IndexError: string index out of range

Expected result

$ scapy
>>> p = IP()/TCP()
>>> p[TCP] = 0x100
>>> p.sprintf("%TCP.flags")
'N'

0x100 is 1000000000, which is the "NS" flag (https://tools.ietf.org/html/rfc3540). Any flag value that contains the N flag causes a 'string index out of range' error.

Related resources

Related resources: https://tools.ietf.org/html/rfc3540

[phaethon]

I assume you meant:

p[TCP].flags = 0x100
p.sprintf("%TCP.flags%")

This does not solve the issue, but necessary for the clarity of other potential readers. Looking into the issue now.

[phaethon]

It is working now in the latest github commit. PyPI package not released yet. Please, take into account renaming of the python package. See the Readme.