choparp

• Copyright (c) 1997 Takamichi Tateoka (tree@mma.club.uec.ac.jp)
• Copyright (c) 2002-2015 Thomas Quinot (thomas@cuivre.fr.eu.org)

Changes and original English man page from the FreeBSD port by Jun-ichiro itojun Hagino itojun@freebsd.org.

With contributions from:

• Darrin B. Jewell dbj@netbsd.org.
• Renato Botelho garga@FreeBSD.org

Introduction

choparp is a proxy ARP daemon. It listens for ARP requests on a network interface, and sends ARP replies with a specified MAC addresses when the requested IP addresses matches a user-provided list.

Build instructions

Requires libpcap.

gcc -o choparp choparp.c -lpcap

Usage example

For example, assume following VLSM subnet. R1 and H1 must have routing entry for subnet B (172.21.139.32/28).

  +----+                            +----+
  | R1 |                            | H1 |
  +-+--+                            +----+
    | 172.21.139.1                    | 172.21.139.96
    |                                 |
  --+--------+------------------------+--------------- subnet A
             |                                   172.21.139.0/24
             | 172.21.139.2
           +----+
           | R2 |
           +----+
             | 172.21.139.33
             |
       ------+-----------------+---------------------- subnet B
                               |                 172.21.139.32/28
                               | 172.21.139.33
                             +----+
                             | H2 |
                             +----+

If you can not set such routing entry, R1 and H1 treat hosts on the subnet B as on the subnet A. In this case, H1 broadcast ARP request for H2 to send a message for H2. This request will fail since this request can not reach to subnet B (and H2), thus H1 cannot tail with H2.

choparp running on R2 replies for ARP request, which is looking for MAC address of H2, as R2 is H2 on subnet A. Hosts on subnet A send packets for H2 to R2 (because R2 replies H2 is R2), and R2 can forward the packets to R2 with ordinary way. As a result, H1 and H2 can talk each other. You can chop subnet B from subnet A without any routing modification on subnet A.

This is a same way as proxy ARP described in RFC1027.

For above example, you should run choparp on R2, set if_name parameter as the interface on 172.21.139.2, network parameter as 172.21.139.32/255.255.255.240 (or 172.21.139.32/28). You should not use 172.21.139.32 and 172.21.139.47 as IP address for hosts because they are used as network address and broadcast address for subnet B.

Enjoy!