Ok to let all ICMP traffic through firewall?

Discussion in 'Computer Security' started by Franklin, Sep 22, 2005.

  1. Franklin

    Mike Scott Guest

    But a decent firewall will be stateful - so eg outbound ping will enable
    the reply to be received. No-one 'out there' has any business pinging
    me so they don't get to do it.

    I am well aware it's against the rules, but I block all unsolicited
    inbound icmp - never noticed any problems. I'm afraid the rfc's were
    drawn up in a less dangerous internet age :-(
    Agreed. A real pain for some smtp servers in particular. My firewall
    just sends a reset.
    Mike Scott, Sep 23, 2005
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  2. Franklin

    Leythos Guest


    So, you're saying that it doesn't break any functionality that we use to
    block it, so we should allow it because the designers of it are almost
    positive that there is no exploit for it, but, since it's not going to
    hurt anything that even though I don't need it, I should allow it, even
    though I don't need it......

    If I don't need it I don't allow it - it's a very simple matter of
    security - never expose anything that you don't need to expose.
    Leythos, Sep 23, 2005
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  3. Franklin

    Mike Guest

    Be sure to deny Echo Request that is sent to the broadcast address for
    your subnet (.255 and .0 for /24 subnets). If a malicious person
    sends several hundred of those per second, you'll wind up with a lot
    of ICMP traffic on your subnet as each host tries to send back the
    Mike, Sep 23, 2005
  4. Franklin

    Peter Guest

    That's your local policy, but not mine. I allow some remote sites to
    ping me as part of mutual reachability testing.
    You block Destination Unreachable as well?
    Peter, Sep 23, 2005
  5. Leythos sez:
    Your 100 networks are not, strictly speaking, a part of the Internet
    since they don't comply with the Internet standards.

    Dimitri Maziuk, Sep 23, 2005
  6. Franklin

    Mike Scott Guest

    Sounds like you're allowing them proper access. Fine by me :)
    I believe (may be wrong though) that ipf is pretty clever about what it
    lets through or not. Dest ureachable must match existing outbound
    packets before it's useful, and I believe ipf will let appropriate (ie
    implicitly "solicited") ones through. No doubt someone will correct me
    if I'm wrong!
    Mike Scott, Sep 23, 2005
  7. Franklin

    Leythos Guest

    The there are many users/companies that are not part of the Internet as
    many companies block many of the services provided for in the RFC's.
    Blocking Ping is very common, as is blocking inbound 135~139, 445, FTP,

    The net is more than your narrow definition, there is a Use side, a
    Provide side, and a shared side.

    While you may think we're not part of the internet, we are part of a
    Narrow segment that provides ICMP only between our partners and block
    the rest.

    As a matter of fact, we offer web services at many locations, but we
    have block lists that block most IP's outside our country, does that
    means we're not part of the Internet, NO, it means we believe in

    No where in the RFC's does is say that it's mandated that I must offer
    services in order to use the Internet networks.
    Leythos, Sep 23, 2005
  8. that is indeed a logical reason to block ping. One wouldn't expect An
    error in the ICMP protocol. But, ping of death, is probably an error
    in the software handling ICMP, rather than the ICMP protocol itself.
    jameshanley39, Sep 23, 2005
  9. and - as you said - if you did want ICMP responses, you could rsetrict
    ICMP responses to hosts of your choosing.

    but what if an ISP or non ISP telephone computer tech is diagnosing a
    non technical home user. The user doesn't have the ability to block
    ICMP on only certain hosts. The homse user isn't running any services
    either(may be behind a NAT device). Ping is ideal in this instance.
    what other option is there to see that he is online,. as a first step
    in diagnosing the problem?
    jameshanley39, Sep 23, 2005
  10. Franklin

    Leythos Guest

    Sorry, that's not a good reason. The ISP can see if the modem is on-
    line, and the ISP can see if there is a connection between the modem and
    the NAT device or PC at the hardware level. You don't have to allow ping
    for any testing/reason, there are always ways around it.
    Leythos, Sep 23, 2005
  11. Leythos sez:
    Which part of "standard" do you not understand? Here's hint: it
    does not mean "flag" in this context.

    Dimitri Maziuk, Sep 23, 2005
  12. Franklin

    Art Guest

    I'm curious .... how does the ISP know?

    In that vein, I noticed Sygate alerting on the kernel (I think it was)
    calling out. Using the traffic log I found that the attempts were to
    my ISP. Blocking the attempts has no effect on my internet activity,
    as near as I can tell. I wonder what the purpose of this attempted
    outbound might be. I don't use any software supplied by my ISP, so
    it's not spyware (which some ISPs do use).


    Art, Sep 23, 2005
  13. Franklin

    Leythos Guest

    As with most ISP provided devices, you get a Cable or DSP modem when you
    get service from them - or a router if a T1, but not many home users
    have T1's.

    The modem device has ports, it's easy to see if the port is active once
    you connect to the device using the ISP's passwords and such. The ISP
    can tell if you have a device (or more if your device has multiple
    ports), how many bytes you've sent, how many you've received, when the
    device was last power-cycled, and other status indicators (signal

    While ping is a simple test, it does not clearly indicate the presence
    of any device on the other end.
    Leythos, Sep 23, 2005
  14. Franklin

    Leythos Guest

    I completely understand the standards, and I understand the idea of
    networking, the idea of security, and that most of the RFC's didn't for
    see many of the uses of the internet that we have today.

    So, you've failed to show why I must allow ANY form of ICPM, other than
    you whining about the RFC's - my network designs do not require any
    public exposure of ICPM, don't break anything that our partners or our
    network needs, and provide one less exposure (actually many less, ICMP
    is just one example)....

    So, show me where our decision to not allow ICMP hurts our ability to
    provide the services we do, impacts our ability to use Internet
    services, or our ability to share information with our business
    partners, or stuff it.

    Here is the RFC's introduction to the ICMP - and it even includes
    statements that indicate that it's not foolproof, some datagrams may
    still be lost, and that other protocols may not use it, that
    communications can be unreliable.....

    The internet protocol does not provide a reliable communication
    facility. There are no acknowledgments either end-to-end or
    hop-by-hop. There is no error control for data, only a header
    checksum. There are no retransmissions. There is no flow control.

    Errors detected may be reported via the Internet Control Message
    Protocol (ICMP) [3] which is implemented in the internet protocol

    From another section:

    The Internet Protocol is not designed to be absolutely reliable. The
    purpose of these control messages is to provide feedback about
    problems in the communication environment, not to make IP reliable.
    There are still no guarantees that a datagram will be delivered or a
    control message will be returned. Some datagrams may still be
    undelivered without any report of their loss. The higher level
    protocols that use IP must implement their own reliability procedures
    if reliable communication is required.
    Leythos, Sep 23, 2005
  15. Franklin

    Bob Eager Guest

    You are confusing two different layers. Blocking ICMP is one thing, but
    not supporting an application protocol is quite another. It worries me
    that you don't appear to understand the difference.
    ICMP isn't a service, but part of the underlying protocol stack; a fact
    which you ignore because you apparently don't know any better.
    Bob Eager, Sep 23, 2005
  16. Franklin

    Dave Dowson Guest

    How do you handle PMTU discovery - or do you prevent segments with the
    DF bit set leaving your network, or do you mangle the headers and
    remove the DF flag, or do you just accept that some sites on that
    Internet may not be reachable from nodes on your network, or do you
    rely on Windows rather inefficent "PMTU Blackhole discovery" feature
    working ?

    If you don't allow *any* inbound ICMP and don't implement effective
    work arounds then you (or your network users) would have some problems
    with all of my locally hosted servers - but then you don't have
    access anyway, so you maybe you can live with the fact that your
    implementation is broken ;-)

    PS - You are not alone in your screwed up thinking - the company I
    used to work for adopted a similar policy, and it effectively
    caused all my VPN connections from work to home to fail. Easy
    to 'fix' since I controlled the 'home' end of the VPN, but not
    necessarily quite so easy to fix for an arbitary site on the
    Dave Dowson, Sep 23, 2005
  17. Franklin

    Steve Welsh Guest

    Are you telling me they can read your router's ARP table then?

    Steve Welsh, Sep 23, 2005
  18. Franklin

    Steve Welsh Guest

    I guess that you and your company would be quite happy then if your ISP,
    and other up-line carriers decided not to route any traffic from a
    network that was not RFC compliant?

    Think not ;)

    Steve Welsh, Sep 23, 2005
  19. Franklin

    Mike Civil Guest

    The passages you refer to are talking about _IP_ and the use of ICMP
    packets to report errors situations in IP. The words not foolproof etc
    refer to IP not ICMP.

    Mike Civil, Sep 24, 2005
  20. Franklin

    Leythos Guest

    Sorry to have confused you with other things I block. You said that I
    was breaking things by not allowing ICMP, I said that many security
    types block most things, not just ICMP and also indicated some things I

    Nothing in the RFC indicates I have to permit ICMP of any type - please
    show where it's mandated if you want to continue this, oh, and don't
    quote the RFC since I've already read it, years ago, and it's not
    mandated that I permit any ICMP inbound to my network.
    Leythos, Sep 24, 2005
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