How risky is it to have a web server on the internal LAN?

Discussion in 'Broadband' started by Peter, Nov 29, 2011.

  1. Peter

    Peter Guest

    Take the case of a typical ADSL modem+router, doing NAT for the
    internal LAN, and you get a little extra subnet of IPs from the ISP
    and bring that subnet through the router to the internal LAN, where a
    web server is set up to respond to one of the extra IPs.

    There is no DMZ - this is all done with just one router e.g. one of
    the better Drayteks.

    So the internal LAN has some machines on 192.168.1.x, and the server
    will be on a public IP of say

    The vulnerability I see in mixing stuff like that is that *all*
    packets arriving on are presented to *all* machines on
    the internal LAN.

    In theory, if a machine is not responding to that IP, all should be
    well, but there have been loads of attacks involving malformed IP

    How much protection does a normal ethernet controller provide? I
    though the controller itself will ignore packets addressed to IPs
    other than its own one - or is this an O/S function?

    The router won't provide any protection to the stuff
    because that bypasses NAT, and AFAICS also bypasses the "DOS attack
    protection" which the Drayteks offer.

    It would be better if the 192.168.1.x internal LAN was behind another
    NAT router, but you lose a lot of that protection if that 2nd router
    has any open ports, which it will have if you need to support e.g. RDP
    over VPN from the outside. I suppose one could terminate the VPN in
    the *2nd* router, but then you have to open ports in the 1st router...

    **Realistically** what is the risk in doing all this with a single

    I would have thought that after all these years, WinXP will have been
    well patched against the obvious network attacks using malformed
    packets which are *not* addressed to the machine in question.

    And the web server has a unix firewall anyway. Even if you put it
    behind a NAT router, you still have to open port 80, and a few others.

    We have seen loads of dictionary attacks over the years, against port
    443 usually. The routers do not have external admin enabled :)
    Peter, Nov 29, 2011
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  2. Peter

    Java Jive Guest

    Don't most routers have a DMZ setting that allows external packets
    through to the IP you give it?

    Please always reply to ng as the email in this post's
    header does not exist. Or use a contact address at:
    Java Jive, Nov 29, 2011
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  3. ARP is your protection here. The router (in fact any machine sending
    an IP packet) will look up the ethernet address that corresponds to the
    IP address, and put that in the headers. Other machines won't even see
    the packet as it won't have their ethernet address, which either comes
    from the network card or else is chosen by the wireless stack.

    Only local machines can do ethernet broadcasts that would be seen by
    all machines on your network.
    In practical terms, only the risk that the router may get pwned, plus the
    risk to some target computer that can receive packets, where "receive"
    includes responses to its outgoing requests.
    That's an application level attack which is quite different.

    Nick Leverton, Nov 29, 2011
  4. Peter

    Graham J Guest


    The more modern Drayteks also support VLANS. So one specific port on
    the LAN switch component could be allocated to the public subnet, and
    connected to the web server. The other ports could be allocated to the
    NATted internal LAN.
    Graham J, Nov 29, 2011
  5. The usual method is to use a server with dual ports - one faces the internal
    LAN and the other the web.
    R. Mark Clayton, Nov 30, 2011
  6. No they are not. Not unless you have a hub. Does anyone MAKE a hub anymore?

    Malformed IP packets dont traverse the internet very well.

    Look even with a hub, the packet HAS to have a well formed target
    address or it wont be there. Nothing will respond to that except
    something that has its interae iund to that address..OK you might try a
    flood ping omna network address..but even then its inlikel;ty to do much.

    I guess if its running a windows stack on a 286 it might be at risk..

    With a switch the switch will know which IP address corresponds to which
    MAC address and will route on that. Even a flood wont work because teh
    actual networ adress is different to teh internal machines so a network
    bridacts will ONLY go to those mahines known to be on that network.

    Its more that an ethernet packet has a MAC address. And the switch knows

    It has to discover it initially using ARP - which is an ethernet
    broadcast requesting an IP address to announce its MAC address.

    But after that unless it cant 'get through' it will cache the arp for at
    least a time.

    A switch IS an Ethernet level router. its designed to be that way to
    increase bandwidth so that a matrix of four machines communicating 2 x 2
    don't flood each others cables needlessly.. For example.

    So once you have a switch, it learns what machines are down what bit of

    You dint propaget MAC addresses across the internet, so unless your
    router is dioing NAT yiu cant talk to 192,168 machines directly.

    Well thats cos you were dumb enough not to put the server on the 192 net
    and redirect the porst to it, where its decently firewalled.

    I've run a web server under NAT for years .. just get the router to
    redirect port 80 traffic to or whatever the server is
    actually on. No need for public IP addresses beyond the one for the router.
    That way you have full firewalling except on that port..but that's what
    you want anyway - full access to port 80 from anywhere and nothing to
    any other ports from the big bad internet.

    I wouldnt take any bets based on WINXP security, but its simply
    irrelevant. It will never see any packets.

    uneccessary if you NAT it.

    I only run a firewall on a server if its otherwise wide open. In this
    case it wont be. You should NAT it like everything else.

    443 is https isn't it?
    The Natural Philosopher, Nov 30, 2011
  7. DMZ less good for a single server than direct port pass through.
    The Natural Philosopher, Nov 30, 2011
  8. No it isn't.

    The USUAL method is a static IP address and simply pass what server
    ports are needful through to whatever server you have on the LAN..

    Leave the server on a 'local' IP address and let the router sort out the
    NAT - that's what it does best.

    No need for dual porting or firewalls beyond what's on the router at all.

    If you want e.g. remote admin access to the server, open up its port and
    then firewall that port on the router except from 'trusted' admin source

    I've got a networked printer that is 'accessible' from the internet
    here. BUT only from ONE external IP address - where I have a remote
    virtual server that has a print queue sending back to my address here.

    I've got a web server, but that is globally accessible. Except I got fed
    up with people busting my bandwidth so its password protected and the
    high traffic stuff is now on the hosted virtual server.

    Been running that for years with no security problems Just bandwith
    The Natural Philosopher, Nov 30, 2011
  9. Peter

    Phil W Lee Guest

    You don't need to get any additional IPs - just make sure your IP is
    fixed rather than dynamic.
    Then you set your router up to forward port 80 traffic to 192.168.n.n
    - being the internal address of the webserver, port 25 traffic to the
    address of your mailserver, and so on for any other servers you have.

    Most routers support port forwarding, and most give a choice between
    doing it port by port (good, because any unassigned traffic gets
    blocked) or by setting a "default forwarding address" for any incoming
    traffic (which is less sensible, as it exposes all ports on the
    nominated server to the internet).

    More risky than completely hiding the servers from the internal lan in
    their own dmz, but less risky than giving them public IP addresses and
    completely exposing all their ports on the internet.
    I certainly wouldn't daisychain NAT. NATted Nat is an abomination.
    I'd use a router and a physically separate firewall, with the firewall
    being built on a PC with 3 ethernet cards - one for lan, one for
    connection to the router (with the router passing the public IP
    address through to it), and the third a dmz for servers which may be
    reached from either lan or internet. You can configure the dmz using
    local addresses (on a different subnet to the lan) and use port
    forwarding in the firewall to send everything to the right place.
    Or you can have a /29 subnet which gives you 6 IPs to play with, 5 of
    which could be for use on the DMZ (you need one for the external
    address of the firewall).
    You only really need more than one IP address if you want to run more
    than one server on the same port, and there aren't many IP addresses
    to go around, so I'd recommend the private IP through NAT route.
    You may need to run your own DNS server to provide local resolution
    for the real addresses of servers in the dmz, or you could do that
    with hosts files - it depends on how many PCs you have to look after.
    On anything that can be exposed to that kind of attack, I run 3
    strikes and out account locking (the number of strikes can be varied
    depending on the security level you want/need and how likely it is for
    a legitimate user to be clobbered by a lockout). You can still get in
    as root/admin from the local console even if something has locked the
    account by trying to break in over the network.
    Of course, on a router that may mean breaking out the serial cable and
    doing battle with the CLI :)
    Phil W Lee, Nov 30, 2011
  10. Peter

    Andy Burns Guest

    The router will send and ARP request for the public IP address, which
    only the machine configured with will respond to with its MAC address,
    then the switch (either internal to the router, or any additional ones
    you have) will deliver the packets *only* to that MC address.

    So unless you set (or add) the public IP address on a machine, the
    outside packets won't reach it.

    Without a DMZ there is still some risk, if the web server is
    compromised, an attacker then has easier access to other servers on your
    Andy Burns, Nov 30, 2011
  11. Peter

    Peter Guest

    OK :) I omitted some vital facts :)

    Straight NAT, with port forwarding of port 80 etc, is what we had

    And that remains the case i.e. we use NAT to protect the internal
    network - just like most people do.

    The reason the public IP subnet is brought straight through is because
    I wanted to be able to run an SSL VPN and access the internal network
    machines with it, AND at the same time we want to run an online shop
    on the web server which will also use port 443.

    And you cannot do that with just a single public IP.

    And we don't want to run the SSL VPN on a port other than 443 because
    the whole point of having a SSL VPN for remote access is to get a
    method which every GPRS/3G network has to support. Previously I used a
    PPTP VPN, which worked fine when it worked, but there are an awful lot
    of mobile networks, and loads of WIFI networks, which don't support
    the PPTP protocol.

    AND I wanted to do all this with just ONE box (plus the ADSL modem,
    plus the ethernet switch on the inside) for ease of admin, and due to
    my rather finite understanding of how this stuff works.

    AND I need to be able to fully understand the config myself (it's my
    own business).

    If one had an SSL VPN router on which one could config which of the
    external IPs the SSL VPN will respond on, and on which one could
    config which of the external IPs have port forwarding of port 443,
    that would be a fine solution because one could just buy a little
    subnet from the ISP (2 usable IPs would do), NAT the lot, port forward
    ports 80, etc, port forward 443 *from* a specific external IP to the
    inside LAN, etc. But I never found such a box. I spent way too much
    trying to config a piece of shit called a Sonicwall TZ100 on which I
    never even got DNS working (well not with a PPPoE-connected ADSL
    modem), I never got anything support-wise out of Fortigate (except a
    sales call weeks later).

    Sure a router with two separate ethernet interfaces on the internal
    LAN would possibly also solve this...

    On the Drayteks there is no choice which of the external IPs the VPN
    terminator responds on. If you buy a subnet of say 8 IPs (5/6 usable)
    you can bring them in using the WAN IP ALIAS, but then the VPN will
    respond on them all and you cannot run an HTTPS server anywhere. And
    any IP which is not brought in using the WAN IP ALIAS is wasted; all
    you can do is bring it in using the 2nd SUBNET feature, which is what
    we did.

    I am sure there are Cisco boxes which can do it "all" but, as old
    followers of my postings here will know ;) I have not been exactly
    successful in finding anybody who can set them up and I would never
    understand them. I used to run some Cisco 803s whose IOS config was
    rarely more than just about legible to me.

    The unix server runs under FreeBSD and has a kernel level firewall.
    This should be fine except it cannot be used to block e.g. dictionary
    attacks from the same IP because, at that level, it cannot tell the
    higher level activity.

    Ultimately, it seems to me, the vulnerability of this setup hangs
    wholly on the existence of back doors, and as always, DOS. If you port
    forward e.g. an external IP of to an internal LAN IP
    of, and on that *physical* ethernet LAN (implemented with
    a 16 port Netgear gigabit unmanaged switch) are some winXP PCs with
    IPs of .30,31,etc, plus the server, then there is no way for anybody
    to do anything other than DOS attacks (quite feasible given the ADSL
    downlink is ~5mbits/sec but the uplink is only 800k, so if you send in
    packets which require a response you can bring the system to a halt,
    but the Draytek has some bandwidth limiting feature IIRC) or make use
    of some back doors.
    Peter, Nov 30, 2011
  12. Peter

    Peter Guest

    I looked at that but I don't think this is in any way usable unless
    you use a managed switch afterwards. I also don't see any config in
    the router for actually using that feature selectively as necessary.
    Peter, Nov 30, 2011
  13. NAT is *not* protection. It even doesn't prevent incoming connections —
    if there is malicious code on your "internal" machines which wants to
    accept an incoming connection rather than doing the normal thing of
    connecting *out* to a server somewhere, there are *plenty* of ways that
    it can do so. Including uPnP and various other protocol-specific tricks
    where NAT boxes have to snoop on the traffic and open incoming ports in
    order to work around the fundamental brokenness of what they're doing.

    I think you'd be much better off abiding by the KISS principle. Just
    have a single network of public IP addresses. Firewall the insecure
    boxes if you think it's necessary, but don't mess around with NAT. Any
    of the security benefits you *think* you get from NAT, are provided just
    as well by a simple connection-tracking firewall.

    Note that if you're using a VPN you want to make sure it runs over UDP
    rather than only over TCP. TCP over TCP is not particularly efficient on
    a slow and lossy link.
    David Woodhouse, Nov 30, 2011
  14. Peter

    Graham J Guest

    That's probably fair comment. For simplicity you would probably have a
    totally separate switch to handle the VLAN for the web server (if you
    needed more than one port), but naturally a managed switch that supports
    VLANs gives better flexibility.

    Which Vigor do you have?

    In practise it may be better to run the web server at a hosting site, so
    you get the benefit of higher relability and faster internet
    connectivity, power backup, on-site tech support, and the like. But
    there may be some specific reason why you want it at your location -
    linked to a production database for factory? Web cameras of your
    garden? Do tell ....
    Graham J, Nov 30, 2011
  15. Peter

    Peter Guest


    It's a very good box, which does some really nice stuff, has really
    nice SSL VPN features like RDP emulation *in the router* (so the
    client can be just any web browser, albeit with a bit of a performance
    hit), and even I can understand how to set it up :) Not cheap at about

    In a small business you need to be able to get to the office machines
    when you are on holiday etc. I have used PPTP for years but too often
    it doesn't work, and is never reliable.
    It is partly historical.

    We used to run an email server, which ran the TMDA challenge based
    antispam, but as the years went on this got overwhelmed by up to 10k
    spam emails per day, so we went to Messagelabs and now just get a
    filtered email feed (which is obviously tightly firewalled in the

    We also run the firm's web server on it; it is low traffic (under 1GB
    per month) so runs fine over ADSL. Soon we will have an online shop on
    it too, with some fairly minimal integration to our accounts software.

    And we have a backup of the web and email servers on a machine at
    another location (my home), and Messagelabs (£400/year) will
    automatically send email to the other site if the first is dead. It
    works well for us. Home is on ZEN and work is on A&A; both very good
    ISPs, except you don't want to run Iplayer under the A&A pricing
    structure ;)

    I do run another website which is ~ 14GB/month and that one is hosted
    on a proper server somewhere. A friend did it for free, and I help to
    keep it down by putting big files in my home ISP's 1GB free filespace
    ;) which unfortunately has a 50MB filesize limit. That site is also
    backed up on the home machine and I can change it over in an instant,
    which I have had to do a few times.

    I know it would be more logical to host it all somewhere, but I like
    the redundancy, the ease of switching over the web server IPs in the
    registrar's DNS control panel, and it enables me to host a load of
    websites for e.g. the kids and for some friends, at zero cost.
    Peter, Nov 30, 2011
  16. I am not sure that is true actually.

    You may have to be 'interesting' in your choice of VPN ports however.

    Ah.. So you need two distinct services running off port 443, hmm.

    I HOPE you mean ADSL ROUTER.

    FULLY understanding anything is probably overly optimistic, but yes, you
    need to understand ENOUGH to get it working with at least the most
    glaring security holes covered.

    I'll skip that because it don't (as you surmise) exist.
    skip that as well.

    It seems to me that the simple solution given what you need to achieve,
    is more or less what you have arrived at./

    Stick aa single card in the unix, and bind the external address to it,
    and firewall it as appropriate.

    It wont be able to talk to the internal network unless the ROUTER is
    able to route between two internal interfaces, in which case all packets
    going between the server and the internal network will go via the router
    as everyone's default route.

    I have not seen any consistency in consumer routes when doing this. Some
    will allow you to contact a local 'web server' via the Nated or
    externalised address, some wont..

    You can solve that by binding a SECOND internal address to the SAME card
    .. then the server has two addresses on the same interface and will
    respond to either.

    Make sure there is no default route on the internal interface if you
    don't want the server using NATted sessions on traffic outbound to the

    All that will WORK as I think you have realised, but the issue is now
    one of security.

    dont respond to pings is a good start.

    They can flood you, but it wont take out the uplink.

    I am more concerned about someone getting access to the server via the VPN.

    The moment you put any kind of login passthrough on a public IP
    address that you want to be generally available, you are increasing
    risks hugely. Sure its ssl so you cant sniff the passwords - but who
    bothers? a trojan on the users PC recording keystrokes... pure
    dictionary attack...

    I think you need to assume, even if its a remote chance, that you will
    get someone logged in some day who should not be..

    And gear your security to limit where else they can get before they get

    I am sanguine about running web servers - the code is good and access to
    anything but what you serve them is limited - but admin logins or
    general purpose pass throughs? That's a different matter.

    For what its worth I don't think your original problem is a real problem
    at all. There are far worse ones - see above. I.e it doesn't matter
    where physically you put the box. Its the routing firewalling and
    general management that is at issue.

    And my experience of VPNs is not adequate to be of much further help.
    The Natural Philosopher, Nov 30, 2011
  17. Peter

    Andy Burns Guest

    Thereby negating one of the main reasons for choosing an SSL based VPN,
    that every man and his dog allows what looks like HTTPS traffic out of
    their network to the internet, so you can use your VPN most places where
    you have web access (whether or not via a proxy server).
    Andy Burns, Nov 30, 2011
  18. Well I have bitten the bullet and moved all of those to a virtual
    private server.

    Think it was around 15 quid a month.

    That's rysnced back to here, for backup.

    It allows me to make this network once more 'private' and indeed, that
    might be an easy way out for you - move the websites offsite (but give
    our IP addresses very direct access to it) and simply solve the VPN
    problem separately.

    My problem was mostly bandwidth, and the fact I needed my own code
    running on the server doing specialised stuff. So a pure hosted solution
    was out.

    Not lot of disk space, but then its not actually needed.

    I am thinking of moving email there, but that involves some real hassle
    to deal with spam and relaying that I will have to tackle. And I know
    how fast logfiles can grow if a mail server gets compromised.yuk!
    The Natural Philosopher, Nov 30, 2011
  19. It does. In te true sense of incoming connections.

    Putting it simply, if a SYN packet comes into the router from the
    internet, which machine must it forward it to? It cannot know UNLESS
    you have opened up a translation that port 80 destination packets go
    to THAT machine.

    Not really, no.

    Including uPnP and various other protocol-specific tricks
    So don't have IGDP turned on, on the router in the first place.

    If you want to play multiplayer doom or whatever, then you are by
    definition not a serious network that takes security seriously.

    Nat - pure NAT is almost all th firewall you will ever need.

    It protects the network behind form ALL attacks initiated from outside.

    If you download malware mind you, all bets are off.

    Indeed, but KISS in most peoples cases is NAT everything and a
    passthrough for port 80 to a web server.

    UDP is less secure at various levels though.
    The Natural Philosopher, Nov 30, 2011
  20. Peter

    Peter Guest

    Also, I don't see I have a choice if I want to run an SSL VPN.

    That is quite an important requirement, and it leads to having to get
    extra IPs, etc, etc.
    Peter, Nov 30, 2011
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