Berkeley CSUA MOTD:Entry 15395
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2025/05/25 [General] UID:1000 Activity:popular
5/25    

1999/2/10-11 [Science/Disaster, Computer/Networking] UID:15395 Activity:moderate
2/10    What is a "SYN Flood"? I keep getting a SYN Flood warning on
        /var/log/messages from this weird IP. Is it bad?
        \_ I remember an option on the Linux kernel called SYN_COOKIES
           which supposedly issues some kind of challenge and prevents
           denial of service attacks.  Don't know much about SYN floods
           myself other than someone is intentially trying to get your OS
           to panic.
           \_ I think the idea behind SYN cookies is that you send a
              random number to the computer requesting the SYN, and they
              have to send it back before the connection can continue.
              I don't think that it can directly prevent SYN flooding,
              (other than scaring off the flooder) but at least you'll
              know the real address that's doing the flooding.   - mikeym
        \_ That means that someone is trying to saturate your machine with
           connections to use up lots of resources.  It is bad.  - mikeym
        \_ The flooder creates a bunch of half-open connections till
           your data structure for holding half-open connections is full
           and you can no longer accept any new connections.  TCP
           normally has a 3-way handshake: SYN, SYN-ACK, ACK.  Client
           sends initial SYN, server responds with SYN-ACK, and client
           should send back an ACK.  In a synflood the client never
           sends the last ACK.  --sky
                \- we do a lot of syn flood detection. probably somoem's fucked
        up tcpstack, but if it is a "rando," ip address you dont do much
        business with, might be someone scanning you to see what services you
        are running. see if any other addresses on the subnet got hit. --psb
2025/05/25 [General] UID:1000 Activity:popular
5/25    

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