pycapng 0.4

Python bindings for libpcapng — read/write pcapng files and live packet capture

pycapng

Python library for reading, writing, and capturing network traffic in the pcapng file format.

pip install pycapng

• Read pcapng files packet by packet
• Write pcapng files with hand-crafted Ethernet/IP/TCP/UDP/ICMP/DNS/DHCP/NTP/TLS packets
• Capture live traffic with a Wireshark-style display filter (Linux / macOS)
• Reassemble fragmented IP packets
• Script packet flows with the built-in pcapsh engine

---

Reading a pcapng file

import pycapng

p = pycapng.PcapNG()
p.OpenFile("capture.pcapng", "r")

def on_packet(data: bytes, ts: int) -> None:
    print(f"packet {len(data)} bytes  ts={ts}")

p.ForeachPacket(on_packet)
p.CloseFile()

---

Writing packets

Raw bytes

import pycapng

p = pycapng.PcapNG()
p.OpenFile("out.pcapng", "w")

raw = bytes([0x45, 0x00, 0x00, 0x28])   # any bytes
p.WritePacket(raw)

p.CloseFile()

TCP

p.WriteTcpPacket(
    src_ip="192.168.1.10", dst_ip="93.184.216.34",
    src_port=54321,        dst_port=80,
    payload=b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n",
    flags=0x02,   # SYN
)

Build a packet without writing it

syn = p.BuildTcpPacket(
    src_ip="10.0.0.1", dst_ip="10.0.0.2",
    src_port=1234, dst_port=443,
    payload=b"", flags=0x02,
)
# syn is bytes — inspect it, modify it, write it later
p.WritePacket(syn)

UDP / ICMP

pkt = p.BuildUdpPacket(
    src_ip="10.0.0.1", dst_ip="8.8.8.8",
    src_port=12345, dst_port=53,
    payload=b"\x00\x01...",   # your DNS query bytes
)

icmp = p.BuildIcmpPacket(
    src_ip="10.0.0.1", dst_ip="10.0.0.2",
    icmp_type=8, icmp_code=0,   # Echo Request
    payload=b"hello",
)

Application-layer helpers

# DNS
query    = p.BuildDnsQuery("example.com")
response = p.BuildDNSResponse("example.com", "93.184.216.34")

# DHCP
discover = p.BuildDhcpDiscover()
offer    = p.BuildDhcpOffer(client_mac="aa:bb:cc:dd:ee:ff",
                             offered_ip="192.168.1.100")

# NTP
req  = p.BuildNtpRequest()
resp = p.BuildNtpReply()

# TLS handshake (produces realistic-looking bytes for testing)
ch = p.BuildTlsClientHello()
sh = p.BuildTlsServerHello()
ct = p.BuildTlsCertificate()
fin = p.BuildTlsFinished()
app = p.BuildTlsApplicationData(b"GET / HTTP/1.1\r\n")

Write with a timestamp

import time

p.WritePacketTime(raw_bytes, int(time.time() * 1e9))   # nanoseconds

---

Live capture

Requires root or CAP_NET_RAW on Linux, root or the network entitlement on macOS.

Simple callback loop

import pycapng

with pycapng.Capture("eth0") as cap:
    cap.set_filter("tcp.dstport == 443")
    cap.loop(0, lambda pkt: print(f"{pkt.captured_len} bytes"))

Full example

import pycapng, signal

cap = pycapng.Capture(pycapng.capture_default_device())
cap.set_snaplen(65535)
cap.set_promisc(True)
cap.set_timeout(100)                        # ms between ring deliveries
cap.set_filter("ip and not arp")

signal.signal(signal.SIGINT, lambda *_: cap.break_loop())

def on_packet(pkt: pycapng.PacketInfo) -> None:
    ts = pkt.timestamp_ns // 1_000_000_000
    print(f"[{ts}] {pkt.direction}  {pkt.captured_len}/{pkt.original_len} bytes")

cap.loop(0, on_packet)

stats = cap.get_stats()
print(f"received={stats.received}  dropped={stats.dropped}")
cap.close()

Display filter syntax

The filter language is a subset of Wireshark's display filter syntax.

Example	Meaning
tcp	any TCP packet
tcp.dstport == 443	TCP to port 443
ip.src == 192.168.0.0/16	source in subnet
tcp.flags.syn and not tcp.flags.ack	SYN-only
udp.port == 53	DNS (src or dst)
ip.addr == 10.0.0.1	any direction to/from IP
tcp and not (tcp.dstport == 80 or tcp.dstport == 443)	non-web TCP
eth.addr == aa:bb:cc:dd:ee:ff	by MAC (src or dst)

Built-in fields: eth.{src,dst,type,addr} · ip.{src,dst,proto,ttl,len,addr} · ip6.{src,dst} · tcp.{srcport,dstport,flags,flags.syn/ack/rst/fin,port} · udp.{srcport,dstport,port} · icmp.{type,code}

Capture directly to a pcapng file

# one-liner: capture 500 TLS packets and save to a file
pycapng.capture_to_file("eth0", "tls.pcapng",
                         filter="tcp.dstport == 443", count=500)

List available interfaces

for dev in pycapng.capture_list_devices():
    print(dev["name"], "(loopback)" if dev["loopback"] else "")

iface = pycapng.capture_default_device()

---

IP fragment reassembly

import pycapng

r = pycapng.Reassembler()
p = pycapng.PcapNG()
p.OpenFile("fragments.pcapng", "r")

def on_packet(data: bytes, ts: int) -> None:
    complete = r.add(data)
    if complete:
        print(f"reassembled datagram: {len(complete)} bytes")

p.ForeachPacket(on_packet)

---

pcapsh script engine

The pycapng.pcapsh submodule exposes a scripting engine for building and replaying packet flows declaratively.

from pycapng import pcapsh

engine = pcapsh.PcapSH()
engine.RunScript("my_flow.pcapsh", "out.pcapng")

TLS record builders are also available as standalone functions:

from pycapng.pcapsh import (
    tls_client_hello, tls_server_hello,
    tls_certificate, tls_finished,
    tls_application_data,
)

---

C library

pycapng is the Python interface to libpcapng, a C library that also ships a standalone command-line tool (pcapsh) and a full C API. See the repository for C usage, build instructions, and the capture API reference.