libpcap inject or sendpacket function error "send: message too large" or "send: bad address"

[Kroptokin]

Hi everybody.
I am using libpcap with Python 3 for a school projct.
I am trying to redirect the traffic in order to achieve a MITM attack after poisoning the ARP tables of a server and a client.
The problem is that the libpcap function sendpacket and inject give me the given errors for the given inputs:

ERROR - sending a packet with sendPacketPcapGlobal. ERROR - DEBUG - sendPacketPcapGlobal - str(''.join(pkt_data)).replace("0x","").replace(" ","") = 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 ERROR - DEBUG - sendPacketPcapGlobal - func_sizePacketDataInt = 442 ERROR - DEBUG - sendPacketPcapGlobal - resInt = -1 ERROR - Error Description: b'send: Bad address'

ERROR - sending a packet with sendPacketPcapGlobal. ERROR - DEBUG - sendPacketPcapGlobal - str(''.join(pkt_data)).replace("0x","").replace(" ","") = 0242c0a878500242c0a8784608004508febc448040004006fe1bc0a87846c0a8783c8bcc9e786e68335f71b0bd2b801801fe70830000010 IMAGINE MORE HEX VALUES HERE - Because GITHUB SAYS: "Comment is too long (maximum is 65536 characters)" ERROR - DEBUG - sendPacketPcapGlobal - func_sizePacketDataInt = 65226 ERROR - DEBUG - sendPacketPcapGlobal - resInt = -1 ERROR - Error Description: b'send: Message too large'

I set the snap length to more than 65226 bytes before activating the pcap_t object. Some code that could be helpful is the function used to inject/send the packet, which is the following:
(Sorry for the bad formatting, I could not get it to work properly...)
"CODE START"
`
def sendPacketPcapGlobal(pkt_data):
"""
sendPacketPcapGlobal sends the data contained in the call variable pkt_data using some global variables, which should be set before calling this function.
The global variables used by this function are:
gl_ifName (str) --> Name of the interface which will be used in order to send the packet
gl_ifNetworkMask (int) --> Network Mask of the interface which will be used in order to send the packet.

"""
global gl_ifNameStr, gl_ifNetworkMaskInt

func_ifName = gl_ifNameStr
func_ifNetworkMask = gl_ifNetworkMaskInt

#Initialize a pcap_t struct
#print("DEBUG - Starting myPcapLoop! \n dev = ", dev_Func, "\n idStr = ", idStr, "\n buf_Func = ", str(buf_Func))

#1. Create a pcap_t 
#pcap_t *pcap_create(const char *source, char *errbuf);
func_dev = ctypes.c_char_p(func_ifName.encode(gl_pcap_encode))
func_errBuff = ctypes.create_string_buffer(libpcap.PCAP_ERRBUF_SIZE)
func_pcap_t = libpcap.create(func_dev, func_errBuff)

if not func_pcap_t:
	print("ERROR - myPcapLoop could not create a pcap_t object for the device: ", dev_Func)
	return 1

#1.2 Set SnapLength
#func_pcap_snaplen = 65535
func_pcap_snaplen = 65535*2
pcap_snaplen = ctypes.c_int(func_pcap_snaplen)

#int pcap_set_snaplen(pcap_t *p, int snaplen);
intRes = libpcap.set_snaplen(func_pcap_t, pcap_snaplen)
if intRes !=0:
	print("ERROR - Setting snaplen for capturing data")
	



#2. Activate a pcap_t in order to be able to capture packets with it
#int pcap_activate(pcap_t *p);
activateInt = libpcap.activate(func_pcap_t)

#Activate returns 0 for succes:
if activateInt != 0:
	print("ERROR - Could NOT activate the pcap_t struct for the device = ", dev_Func)
	
	if activateInt == libpcap.PCAP_WARNING_PROMISC_NOTSUP:
		print("ERROR - PCAP_WARNING_PROMISC_NOTSUP - Promiscuous mode was requested, but the capture source doesn't support promiscuous mode. " )
	
	elif activateInt == libpcap.PCAP_WARNING_TSTAMP_TYPE_NOTSUP:
		print("ERROR - PCAP_WARNING_TSTAMP_TYPE_NOTSUP - The time stamp type specified in a previous pcap_set_tstamp_type(3PCAP) call isn't supported by the capture source (the time stamp type is left as the default),  ")
		
	elif activateInt == libpcap.PCAP_WARNING:
		print("ERROR - PCAP_WARNING - Another warnign occurred.")
		func_error =  libpcap.geterr(func_pcap_t)
		print("ERROR - ",str(func_error))
		
	elif activateInt == libpcap.PCAP_ERROR_ACTIVATED:
		print("ERROR - PCAP_ERROR_ACTIVATED - The handle has already been activated." )
		
	elif activateInt == libpcap.PCAP_ERROR_NO_SUCH_DEVICE:
		print("ERROR - PCAP_ERROR_NO_SUCH_DEVICE - The capture source specified when the handle was created doesn't exist. ")
		
	elif activateInt == libpcap.PCAP_ERROR_PERM_DENIED:
		print("ERROR - PCAP_ERROR_PERM_DENIED - The process doesn't have permission to open the capture source. ")
		
	elif activateInt == libpcap.PCAP_ERROR_PROMISC_PERM_DENIED:
		print("ERROR - PCAP_ERROR_PROMISC_PERM_DENIED - The process has permission to open the capture source but doesn't have permission to put it into promiscuous mode. " )
		
	elif activateInt == libpcap.PCAP_ERROR_RFMON_NOTSUP:
		print("ERROR - PCAP_ERROR_RFMON_NOTSUP - Monitor mode was specified but the capture source doesn't support monitor mode. " )
		
	elif activateInt == libpcap.PCAP_ERROR_IFACE_NOT_UP:
		print("ERROR - PCAP_ERROR_IFACE_NOT_UP - The capture source device is not up. ")
		
	elif activateInt == libpcap.PCAP_ERROR:
		print("ERROR - PCAP_ERROR - Another error occurred:")
		func_error =  libpcap.geterr(func_pcap_t)
		print("ERROR - ",str(func_error))
		
	else:
		print("ERROR - Unknow error for libpcap.activate() function")





#Fix the pkt_data if needed
func_pkt_data = str(''.join(pkt_data)).replace("0x","").replace(" ","")
#print("DEBUG - func_pkt_data = ", func_pkt_data)
func_sizePacketDataInt = math.ceil(len(func_pkt_data)/2)


#Store the packet data in a suitable variable to use the library libpcap
#Commented HERE - Possible error
#buf_pcap = (ctypes.c_ubyte*(func_sizePacketDataInt))(*bytes.fromhex(func_pkt_data.replace("0x","")))

#Pointer to packet data:
func_packetPointer = ctypes.cast(func_pkt_data, ctypes.POINTER(ctypes.c_ubyte))


#packetDataHex =  
#0242c0a878500242c0a8783c080045000083934740004006355ac0a8783cc0a87846d12200153e89599d41abd7ea801801f5724900000101080a89b2ac700d7890534d44544d2067616c6178792d6e61747572652d6165737468657469632d6261636b67726f756e642d7374617272792d736b792d6d6f756e7461696e2d72656d697865642d6d656469612e6a70670d0a

#0242c0a878500242c0a8784608004500006f7b044000400633a0fc0a87846c0a8783c0015930c14a79c94ddfd0cec801801fe723500000101080a0d7c552e89b671473232302057656c636f6d6520746f206d792046545020736572766963652e204e6f7420796f75727321204d792074726561737572652e2e2e2e0d0a


#0242c0a878500242c0a87846080045000069f959400040063b85ec0a87846c0a8783c0015d12241abd7fe3e8959f2801801fe722f00000101080a0d78905489b2ac7032323720456e746572696e672050617373697665204d6f646520283139322c3136382c3132302c37302c3131322c313136292e0d0a

#Send the packet:
#OLD - Because of too large messages
#resInt = libpcap.sendpacket(func_pcap_t, func_packetPointer, int(func_sizePacketDataInt))
#NEW - With inject function to try to avoid bug
resInt = libpcap.inject(func_pcap_t, func_packetPointer, int(func_sizePacketDataInt))

#Error checking for libpcap.inject function
if resInt != int(func_sizePacketDataInt):
	print("ERROR -  sending a packet with sendPacketPcapGlobal.")
	print("""ERROR - DEBUG - sendPacketPcapGlobal - str(''.join(pkt_data)).replace("0x","").replace(" ","") = """, str(''.join(pkt_data)).replace("0x","").replace(" ",""))
	print("ERROR - DEBUG - sendPacketPcapGlobal - func_sizePacketDataInt = ", func_sizePacketDataInt)
	print("ERROR - DEBUG - sendPacketPcapGlobal - resInt = ", resInt)
	
	if resInt ==  libpcap.PCAP_ERROR_NOT_ACTIVATED:
		print("ERROR - pcap_t not activated")
		
	elif resInt == libpcap.PCAP_ERROR:
		#char *pcap_geterr(pcap_t *p);
		errorBuff = libpcap.geterr(func_pcap_t)
		print("ERROR - Error Description: ", errorBuff)
	else:
		print("ERROR - UNKNOWN - resInt = ", resInt)

else:
	#print("SUCCESS!!! - Packet send using the function sendPacketPcapGlobal !!!")
	pass

"""
#OLD - Because of too large messages
#resInt = libpcap.sendpacket(func_pcap_t, func_packetPointer, int(func_sizePacketDataInt))
#Error checking for sendpacket function
if resInt != 0:
	print("ERROR -  sending a packet with sendPacketPcapGlobal.")
	print("ERROR - DEBUG - sendPacketPcapGlobal - str(''.join(pkt_data)).replace("0x","").replace(" ","") = ", str(''.join(pkt_data)).replace("0x","").replace(" ",""))
	print("ERROR - DEBUG - sendPacketPcapGlobal - func_sizePacketDataInt = ", func_sizePacketDataInt)
	
	if resInt ==  libpcap.PCAP_ERROR_NOT_ACTIVATED:
		print("ERROR - pcap_t not activated")
		
	elif resInt == libpcap.PCAP_ERROR:
		#char *pcap_geterr(pcap_t *p);
		errorBuff = libpcap.geterr(func_pcap_t)
		print("ERROR - Error Description: ", errorBuff)
	else:
		print("ERROR - UNKNOWN")

"""
return 0

`
"CODE END"

EDIT: I forgot some possibly important info about the system:

/home # pip list
Package Version
appdirs 1.4.4
contextlib2 21.6.0
importlib-metadata 6.8.0
importlib-resources 6.1.0
libpcap 1.11.0b7
more-itertools 8.13.0
numpy 1.26.1
ordered-set 4.0.2
packaging 21.3
pep517 0.12.0
pip 22.1.1
pkg-about 1.0.8
pyparsing 2.4.7
retrying 1.3.3
setuptools 68.2.2
six 1.16.0
tomli 2.0.1
zipp 3.17.0
/home # pip install libpcap
Requirement already satisfied: libpcap in /usr/lib/python3.10/site-packages (1.11.0b7)
Requirement already satisfied: setuptools>=63.2.0 in /usr/lib/python3.10/site-packages (from libpcap) (68.2.2)
Requirement already satisfied: pkg-about>=1.0.7 in /usr/lib/python3.10/site-packages (from libpcap) (1.0.8)
Requirement already satisfied: importlib-resources>=5.7.1 in /usr/lib/python3.10/site-packages (from pkg-about>=1.0.7->libpcap) (6.1.0)
Requirement already satisfied: packaging>=21.3.0 in /usr/lib/python3.10/site-packages (from pkg-about>=1.0.7->libpcap) (21.3)
Requirement already satisfied: tomli>=2.0.1 in /usr/lib/python3.10/site-packages (from pkg-about>=1.0.7->libpcap) (2.0.1)
Requirement already satisfied: importlib-metadata>=4.12.0 in /usr/lib/python3.10/site-packages (from pkg-about>=1.0.7->libpcap) (6.8.0)
Requirement already satisfied: zipp>=0.5 in /usr/lib/python3.10/site-packages (from importlib-metadata>=4.12.0->pkg-about>=1.0.7->libpcap) (3.17.0)

I was wondering how to avoid both problems, or if it was a bug of the library libpcap in C or in Python. Any help would be really welcome. Thank you for your time.

[guyharris]

"send: Message too large" can be avoided by not sending packets that are larger than the maximum packet size for the link layer on which the packet is being sent. For example, an Ethernet has a maximum packet size of 1518 bytes, if the packet being sent includes the 4-byte CRC at the end.

"send: Bad address" means that a pointer value and packet length are being passed to send() where the range of addresses starting with the pointer value and ending at the pointer value plus the pointer length minus 1 are not all in the process's address space. This is probably because the value in func_packetPointer is not a valid address or does not correspond to the beginning of a range of valid addresses of size func_sizePacketDataInt.