Linux-Cisco-ADSL-Cisco-Linux Connection hangs....

Discussion in 'Cisco' started by Adrian de los Santos, Jul 22, 2003.

  1. Hi guys..

    Im having a strange problem with linux servers and cisco routers here
    its my setup:


    Linux Server---Cisco router---ADSL Link---Cisco router--- Linux Server


    The strange stuff its that i can ping from one linux to the other with
    no problems, but when i open a telnet session from one linux to the
    other and type any command (ls, top, ifconfig, etc) the connection just
    hangs and stays there forever.

    If i open another telnet session to the linux server it works until the
    command prompt (asks for login, password) and when i type any command
    and enter, the connection hangs..

    this its happening not just with telnet, pop3, smtp, any TCP connection
    seems to work at first but later just hangs in there.

    Can anyone share some light here..

    Thanks..
    Adrian de los Santos, Jul 22, 2003
    #1
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  2. In article <>,
    Adrian de los Santos wrote:
    > Hi guys..
    >
    > Im having a strange problem with linux servers and cisco routers here
    > its my setup:
    >
    >
    > Linux Server---Cisco router---ADSL Link---Cisco router--- Linux Server
    >
    >
    > The strange stuff its that i can ping from one linux to the other with
    > no problems, but when i open a telnet session from one linux to the
    > other and type any command (ls, top, ifconfig, etc) the connection just
    > hangs and stays there forever.
    >
    > If i open another telnet session to the linux server it works until the
    > command prompt (asks for login, password) and when i type any command
    > and enter, the connection hangs..
    >
    > this its happening not just with telnet, pop3, smtp, any TCP connection
    > seems to work at first but later just hangs in there.
    >
    > Can anyone share some light here..
    >


    Sounds like all the classic symptoms of a PMTUD black hole to me.

    Running PPPoE on that ADSL line?

    --
    David Van Cleef - Engineering Manager
    - Fusion Network Services, K.K.
    - Global OnLine Japan
    -- "We have forgotten at least two things..." --
    David Van Cleef, Jul 22, 2003
    #2
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  3. Adrian de los Santos

    mrtravel Guest

    Adrian de los Santos wrote:
    > Hi guys..
    >
    > Im having a strange problem with linux servers and cisco routers here
    > its my setup:
    >
    >
    > Linux Server---Cisco router---ADSL Link---Cisco router--- Linux Server
    >
    >
    > The strange stuff its that i can ping from one linux to the other with
    > no problems, but when i open a telnet session from one linux to the
    > other and type any command (ls, top, ifconfig, etc) the connection just
    > hangs and stays there forever.
    >
    > If i open another telnet session to the linux server it works until the
    > command prompt (asks for login, password) and when i type any command
    > and enter, the connection hangs..
    >
    > this its happening not just with telnet, pop3, smtp, any TCP connection
    > seems to work at first but later just hangs in there.
    >
    > Can anyone share some light here..
    >
    > Thanks..


    DNS..........possibly
    When you open a connection to a server, that server may do a dns query
    on the ip address to map it back to a fully qualified domain name.
    So, here's the test. On the server you are trying to connect to, do
    "nslookup x.x.x.x" where x.x.x.x is the other box's ip address.
    mrtravel, Jul 22, 2003
    #3
  4. this its happening not just with telnet, pop3,
    >> smtp, any TCP connection seems to work at first but later just hangs
    >> in there. Can anyone share some light here..

    >
    > Sounds like all the classic symptoms of a PMTUD black hole to me.
    >
    > Running PPPoE on that ADSL line?



    Yes im running PPPoE

    what its the PMTUD black hole, where i can get information about it ?

    Thanks a lot guys..
    Adrian de los Santos, Jul 22, 2003
    #4
  5. >> in there. Can anyone share some light here.. Thanks..
    >
    > DNS..........possibly
    > When you open a connection to a server, that server may do a dns query
    > on the ip address to map it back to a fully qualified domain name.
    > So, here's the test. On the server you are trying to connect to, do
    > "nslookup x.x.x.x" where x.x.x.x is the other box's ip address.



    I dont think so, because i can ping and get into the server, but when i
    type any linux/unix command (ls, who, last, etc) the connection hangs.

    Thanks..
    Adrian de los Santos, Jul 22, 2003
    #5
  6. In article <>,
    Adrian de los Santos wrote:
    > this its happening not just with telnet, pop3,
    >>> smtp, any TCP connection seems to work at first but later just hangs
    >>> in there. Can anyone share some light here..

    >>
    >> Sounds like all the classic symptoms of a PMTUD black hole to me.
    >>
    >> Running PPPoE on that ADSL line?

    >
    >
    > Yes im running PPPoE
    >
    > what its the PMTUD black hole, where i can get information about it ?
    >


    Google is your friend. "PMTUD Black Hole" brings up useful info.

    --
    David Van Cleef - Engineering Manager
    - Fusion Network Services, K.K.
    - Global OnLine Japan
    -- "We have forgotten at least two things..." --
    David Van Cleef, Jul 22, 2003
    #6
  7. In article <>,
    Adrian de los Santos wrote:
    >
    >>
    >> Sounds like all the classic symptoms of a PMTUD black hole to me.
    >>
    >> Running PPPoE on that ADSL line?
    >>

    >
    > Yes i just return from a google search and yes indeed its a PMTUD black
    > hole, any way i can fix that from the router side ? (i dont have access
    > to the root users on the linux servers)
    >
    > Thanks for all..


    on your facing interface on both sides, put a policy to unset the
    df bit.

    example:

    int f0/0
    ip policy route-map clear-df-bit
    access-list 101 permit ip any any
    route-map clear-df-bit permit 10
    match ip address 101
    set ip df 0

    --
    David Van Cleef - Engineering Manager
    - Fusion Network Services, K.K.
    - Global OnLine Japan
    -- "We have forgotten at least two things..." --
    David Van Cleef, Jul 22, 2003
    #7
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