1

Imagine a device A (192.168.32.5/24). Then two hosts, B and C, connected to the same interface (through a switch) having the same IP address (192.168.32.8/24) configured statically. No fancy tools/techniques to prevent two hosts to have the same IP address with different MAC addresses. Imagine that those B and C pinged A at slow pace (every 5 seconds) continuously.

If the device "A" were a host then such "A" host would update its ARP table based on packets received because end hosts don't normally do extra checks on arriving packets, they just look at the src MAC address and IP address and update the ARP table accordingly. I'm not interested in knowing if some ICMP echo replies get correctly to the ICMP echo requests sender, I'm just interested in, what I think happens, seeing the ARP table getting update every time a new echo request comes in.

On the contrary, if the device "A" were a firewall what I know is that the first ICMP echo request will update the FW's ARP table whereas the second will do nothing, meaning that the FW will keep the first entry. Only upon expiration of the first entry the FW would be ready to accept a ap packet whose src IP and MAC addresses were different.

Is what I described in the second example the default behaviour of firewalls? If yes where can I find documentation? I have always seen firewalls behaving like (Cisco, Checkpoints for instance), I was always told that they do so because they are "firewalls" but I could not find a single statement about it, not even from Security docs that may describe firewall roles. Now Fortinet don't seem to behave that way. I would like to know if there is a "default" expected behaviour for firewalls.

TIA

1 Answer 1

3

You have some misconceptions.

No fancy tools/techniques to prevent two hosts to have the same IP address with different MAC addresses.

That is an OS requirement for your hosts, nothing special or fancy. See RFC 5227, IPv4 Address Conflict Detection (note what I highlighted):

2.1. Probing an Address

Before beginning to use an IPv4 address (whether received from manual configuration, DHCP, or some other means), a host implementing this specification MUST test to see if the address is already in use, by broadcasting ARP Probe packets. This also applies when a network interface transitions from an inactive to an active state, when a computer awakes from sleep, when a link-state change signals that an Ethernet cable has been connected, when an 802.11 wireless interface associates with a new base station, or when any other change in connectivity occurs where a host becomes actively connected to a logical link.


If the device "A" were a host then such "A" host would update its ARP table based on packets received because end hosts don't normally do extra checks on arriving packets, they just look at the src MAC address and IP address and update the ARP table accordingly.

No, they do not. Only ARP packets update ARP tables. That is a requirement in the RFC, and a frame with an EtherType for IPv4 (0x0800) or IPv6 (0x86DD) is sent to the corresponding IP process, not the ARP process. Only frames with the ARP EtherType (0x0806) are sent to the ARP process. Even reception of unsolicited ARP packets will not create an entry in the ARP table; that will only update an existing entry in the ARP table (see the pseudocode below).


On the contrary, if the device "A" were a firewall what I know is that the first ICMP echo request will update the FW's ARP table whereas the second will do nothing, meaning that the FW will keep the first entry.

Not exactly, but closer. Receiving an ICMP echo request will not update the ARP table, but sending an ICMP echo reply may if the ARP table does not already have the destination and needs to update it with an ARP request.

This answer is related.


RFC 826, An Ethernet Address Resolution Protocol has the pseudocode for updating the ARP table:

When an address resolution packet is received, the receiving Ethernet module gives the packet to the Address Resolution module which goes through an algorithm similar to the following. Negative conditionals indicate an end of processing and a discarding of the packet.

?Do I have the hardware type in ar$hrd?
Yes: (almost definitely)
  [optionally check the hardware length ar$hln]
  ?Do I speak the protocol in ar$pro?
  Yes:
    [optionally check the protocol length ar$pln]
    Merge_flag := false
    If the pair <protocol type, sender protocol address> is
        already in my translation table, update the sender
        hardware address field of the entry with the new
        information in the packet and set Merge_flag to true.
    ?Am I the target protocol address?
    Yes:
      If Merge_flag is false, add the triplet <protocol type,
          sender protocol address, sender hardware address> to
          the translation table.
      ?Is the opcode ares_op$REQUEST?  (NOW look at the opcode!!)
      Yes:
        Swap hardware and protocol fields, putting the local
            hardware and protocol addresses in the sender fields.
        Set the ar$op field to ares_op$REPLY
        Send the packet to the (new) target hardware address on
            the same hardware on which the request was received.

Notice that the <protocol type, sender protocol address, sender hardware address> triplet is merged into the table before the opcode is looked at. This is on the assumption that communcation is bidirectional; if A has some reason to talk to B, then B will probably have some reason to talk to A. Notice also that if an entry already exists for the <protocol type, sender protocol address> pair, then the new hardware address supersedes the old one. Related Issues gives some motivation for this.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.