Interface gets pinged even when cable not connected



  • We are usinng soekris net 6501-70 boards and just today i stumbled upon this strange phenomena. I disconnected the cable from one of the interfaces and pinged it randomly and was surprised to get a reply. The cable was disconnected no doubt about that. Should this happen? If not whats wrong ?


  • LAYER 8 Global Moderator

    Yes the packets magically flow through the air to the interface.

    You either had a different path to get to the ip than you thought, or your were pinging something else then what you thought you were pinging.



  • you got a rogue device on the network that is using the same ip



  • you got a rogue device on the network that is using the same ip

    But if that were the case, he would be seeing all kinds of network issues.


  • LAYER 8 Global Moderator

    Another possible issue could be that some sort of vip/hsrp/carp setup where another interface picks up that IP you were pinging.

    Maybe he was having issues with his network, which is why he disconnected the wire with the IP the thought was having a problem and it still pings the other device using that IP, etc.

    We have no details on the configuration, no details of any kind to be honest.  But what I can tell you for FACT is you have this

    deviceA –-- switch --- deviceB and unplug either of those then its not going to be possible to ping.. So either there is another path that you didn't know you were using, or your not pinging the device you thought you were pinging, etc..

    Unless the packets are magically spanning the air gap that was created ;)



  • @johnpoz:

    Yes the packets magically flow through the air to the interface.

    You either had a different path to get to the ip than you thought, or your were pinging something else then what you thought you were pinging.

    there is absolutely no path possible. I pulled a device d  :-\ :-\ :-\from the rack and connected to my PC via one interface, all other interfaces were disconnected  :-\


  • Banned

    Yeah, call ghostbusters. Or stop smoking that stuff, it's too strong.


  • LAYER 8 Netgate

    Do it again, ping, look at the ARP table, get the MAC address for the IP you're magically pinging and track it down.

    Is there Wi-Fi involved anywhere?


  • LAYER 8 Global Moderator

    Think about it for all of 2 seconds dude…  If you have something connected with a wire with no other paths to it..  And you can still ping that IP.. you either have another path to that IP.. Or your pinging something else that answers to that IP..

    Or by some crazy odd weird magic your packets are jumping the air gap and flowing through the air??

    What is more logical your over looking a path or pinging something else with that same ip???  My vote is your over looking something... Come on... packets don't magically flow over the air gap...

    Just think about it for 2 freaking seconds...  That you even posted this thread points to your high on something or not thinking straight or just some lame attempt at trolling.



  • Since nobody believes me yet, i'm posting a diagram what I am doing now to clarify things.
    Eth0 ip: 192.168.1.1/24
    PC1 ip: 192.168.1.10/24
    Eth1 ip: 192.168.2.1/24
    PC2 ip: 192.168.2.10/24

    Disconnected the cable between Eth0 & PC1
    Ping Eth0 from PC2 and it gets pinged



  • LAYER 8 Netgate

    pfSense does not take interface IP addresses down when ethernet carrier is lost like, say, a Cisco does.

    In fact, the interface addresses actually route to loopback interfaces internally.

    What you're seeing is completely normal.  You can ping a local interface address even though the interface has nothing plugged into it.  Your initial description had everyone thinking you were saying you could still ping 192.168.1.1 from 192.168.1.10 even though it was unplugged.

    Amazing how a diagram clears things up immediately.


  • Banned

    HAHAHAHAHAHAHAHAH

    You gotta be kidding :D

    Your pinging the interface INSIDE the Soekris box. You need to ping 192.168.1.10 to see the result of you pulling the cable. Try to disable the interface in pfsense and your ping will be gone ;)

    :D


  • LAYER 8 Netgate

    I don't think it's that funny.  A Cisco takes the local interface IP address down if it loses carrier, at least by default.



  • No this II funny :)


  • Banned


  • LAYER 8 Netgate

    Difference of opinion on how a router should actually behave I guess.  Neither behavior is wrong.  But I would bet the pfSense guys would welcome more Cisco-like behavior if FreeBSD were to implement it.  Optional, of course.

    Anyway, OP, the reason these guys are bagging on you is easily answered by looking at your diagram.  See the ping source, then the ping response, then the connection break FURTHER down the path?

    There is really no reason for the router not to respond.  The connection break is the next hop.

    (Cisco routers do respond in this situation BTW, with an ICMP Destination Unreachable)

    
    R2-2620#ping 192.168.1.1
    
    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
    U.U.U
    
    


  • Interface IPs going down can mess with services on the firewall box making is more likely for a service to not rebind to the IP when the interface comes back up. Many services get around this by restarting when a new interface comes up. Many sandbox setups will drop privileges to bind interfaces once the software initializes, meaning the software can't bind to a new IP.

    There's many fewer corner cases if you just let the IP stay on the interface even when it's not plugged in. Use a heart beat if you need to check uplink status.



  • Maybe if one does ifconfig down on the interface after you pull the cable, you get the desired response.


  • LAYER 8 Netgate

    Interface IPs going down can mess with services on the firewall box making is more likely for a service to not rebind to the IP when the interface comes back up.

    I can certainly see that being a problem.


  • LAYER 8 Global Moderator

    since when would these IPs go down even in a cisco switch.. Normally your ip would be on loopback or svi, not actually tied to any physical interface..  Can not remember putting an IP on the actual interface before in cisco..  If you were running services on a cisco switch like dhcp or routing even that would be over a SVI, not actually tied to a physical interface being up or down..



  • I have had switch/router devices before that would stop responding to ping on an interface if the interface was physically down - from memory it was Cabletron/Enterasys gear. Once you knew the behaviour, it meant you knew when the cable had fallen out at a remote place.


Log in to reply