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Thread: Internet outage disrupting LAN

  1. #1
    asiinc NNTP User

    Default Internet outage disrupting LAN

    I have recently experienced a strange problem. A client of our had his DSL go down and it caused the LAN to slow to a craw. They are running Quickbooks 2008 with the database on the server and it got so slow that they could not access Quickbooks, other files were still accessible albeit slowly. When the DSL service was restored thing started working normally again. This appears to be repeatable.

    Their setup is as follows:

    Server with two NICS (LAN and WAN) WAN connected to a DSL modem running in Bridge mode.
    Suse 9.3 OS
    SAMBA
    Squid Proxy
    Shorewall Firewall
    Dansguardian Internet Content Filter
    Quickbooks 2008 in Alternate configuration (See this link)

    The process went like this:

    Client calls complaining no internet and LAN slow and Quickbooks not working.

    We move Quickbooks to a Windows box temporally to get them up and running

    We begin troubleshooting and discover no WAN IP being delivered.

    We call ISP and they determine modem to be defective and will send new modem.

    Next day ISP tech arrive and says modem is not bad, DSL had been down and is back up now.

    We move Quickbooks back to server and everything is working fine.

    We inform client it appears his LAN was disrupted because his WAN was down. Weird huh?

    Three days later Quickbooks stops working again and client does not bother to call us because he knows the DSL is down (not even a dial tone on that line)

    After DSL comes back up Quickbooks also starts working again. (NO THEY ARE NOT RUNNING QUICKBOOKS ONLINE EDITION )

    Client calls letting us know what happened and wondering what we can do to prevent this in the future.

    So...
    The second outage appears to confirm that when the internet goes down it breaks Quickbooks and causes slow performance on the LAN! Anybody have any clue as to what might be going on here? Be glad to provide log files to help troubleshoot if needed.

  2. #2
    Join Date
    Jun 2008
    Location
    Tampa, FL
    Posts
    27

    Default Re: Internet outage disrupting LAN

    What do the two network interface configurations look like (you can exclude the publicly routable IPs).

    Also, can you show the contents of the following files:

    /etc/nsswitch.conf
    /etc/resolv.conf

    and the output to the command:

    /sbin/route -n

    Also, how do the client systems contact the server? Are they using a DNS name? If so, what is serving up DNS for the client systems.

  3. #3
    Tilman Schmidt NNTP User

    Default Re: Internet outage disrupting LAN

    asiinc schrieb:
    > I have recently experienced a strange problem. A client of our had his
    > DSL go down and it caused the LAN to slow to a craw. [...]
    > When the DSL service was restored thing started working
    > normally again. This appears to be repeatable.


    This sort of trouble is typically caused by DNS queries timing out.
    Set up a local DNS server resolving all local host names/addresses
    both forward and reverse, and check the search paths to make sure
    they only contain domains served locally.

    HTH
    T.

  4. #4

    Default Re: Internet outage disrupting LAN

    Their DNS in should be set to the squid proxy server/linux box. DNS caching should be enabled on the linux box. You should not have your clients use the DNS of the upstream router/modem. If the dsl modem goes dark and the lan drops dns, then you are distributing the dsl dns ip's to the clients- or you don't have dns caching enabled on the linux box.

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