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Asked: 2026-06-11T21:43:29+00:00

I’m trying to learn how to directly (no libraries) send DHCP request from python

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I’m trying to learn how to directly (no libraries) send DHCP request from python on multi-homed machine (multiple interfaces).
I’ve looked at pydhcplib, but still do not get it.

This code send DHCP packet on specific interface (eth3 in my case – no IP assigned), but it sends with eth0 IP address. How to change my src IP to 0.0.0.0?
dhcp-message is truncated in this example

LOCAL_PORT=68
SERVER_PORT=67
LOCAL_IP="0.0.0.0"
BCAST_IP="255.255.255.255"
LISTEN_DEV="eth3"
MSG_SIZE=2048
Conn=socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
Conn.settimeout(5)
Conn.setsockopt(socket.SOL_SOCKET,IN.SO_BINDTODEVICE,LISTEN_DEV+'\0')
Conn.setsockopt(socket.SOL_SOCKET,socket.SO_BROADCAST,1)
Conn.bind((LOCAL_IP, LOCAL_PORT))
# Create DHCP-Discovery
msg="010106003f7d1664......"
Conn.sendto(msg.decode("hex"),(BCAST_IP,SERVER_PORT))
received = Conn.recv(MSG_SIZE)
Conn.close()
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1 Answer

  1. Editorial Team

    I assume you already know about the Advanced Interprocess Communication Tutorial.

    Spoiler Alert: If you want to jump straight to the bottom line, have a look at the DHCP Query recipe.

    Edit:

    The special value INADDR_ANY (0.0.0.0, or the empty string ” in a python socket) is not an IP address.

    “When an address is specified as INADDR_ANY (a manifest constant
    defined in < netinet/in.h >), the system interprets the address as ‘any
    valid address’.”

    From RFC 2131:

    In the case of a client using DHCP for initial configuration
    (before the client’s TCP/IP software has been completely
    configured), DHCP requires creative use of the client’s TCP/IP
    software and liberal interpretation of RFC 1122. The TCP/IP
    software SHOULD accept and forward to the IP layer any IP packets
    delivered to the client’s hardware address before the IP address is
    configured; DHCP servers and BOOTP relay agents may not be able to
    deliver DHCP messages to clients that cannot accept hardware
    unicast datagrams before the TCP/IP software is configured.

    Presumably you’re running this program on a system where the ethernet interfaces have already been configured and have valid IP addresses. I’m not sure why you’d want the source IP address to be 0.0.0.0, but perhaps you could set the interface IP to 0.0.0.0 with ifconfig to get the effect you want.

    Or you could use a RAW socket and build the IP and UDP headers yourself to contain anything.

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