Subject: Re: RFC: Established connections hash function

From: Eric Dumazet <[email-masked]>
Date: Thu, 22 Mar 2007 23:03:04 +0100

David Miller a écrit :
> From: "Nikolaos D. Bougalis" <[email-masked]>
> Date: Thu, 22 Mar 2007 12:44:09 -0700
> 
>>     People _have_ had problems. _I_ have had problems. And when
>> someone with a few thousand drones under his control hoses your
>> servers because he can do math and he leaves you with 20000-item
>> long chains, _you_ will have problems.
> 
> No need to further argue this point, the people that matter
> (ie. me :-) understand it, don't worry..

Yes, I recall having one big server hit two years ago by an attack on tcp hash 
function. David sent me the patch to use jhash. It's performing well :)

Welcome to the club :)

===== net/ipv4/tcp_ipv4.c 1.114 vs edited =====
--- 1.114/net/ipv4/tcp_ipv4.c    2005-03-26 15:04:35 -08:00
+++ edited/net/ipv4/tcp_ipv4.c    2005-04-05 13:39:52 -07:00
@@ -103,14 +103,15 @@
   */
  int sysctl_local_port_range[2] = { 1024, 4999 };
  int tcp_port_rover = 1024 - 1;
+static u32 tcp_v4_hash_rand;

  static __inline__ int tcp_hashfn(__u32 laddr, __u16 lport,
                   __u32 faddr, __u16 fport)
  {
-    int h = (laddr ^ lport) ^ (faddr ^ fport);
-    h ^= h >> 16;
-    h ^= h >> 8;
-    return h & (tcp_ehash_size - 1);
+    return jhash_2words(laddr ^ faddr,
+                (lport << 16) | fport,
+                tcp_v4_hash_rand) &
+        (tcp_ehash_size - 1);
  }

 >  static __inline__ int tcp_sk_hashfn(struct sock *sk)
 > @@ -2626,6 +2627,9 @@
 >          panic("Failed to create the TCP control socket.\n");
 >      tcp_socket->sk->sk_allocation   = GFP_ATOMIC;
 >      inet_sk(tcp_socket->sk)->uc_ttl = -1;
 > +
 > +    get_random_bytes(&tcp_v4_hash_rand, 4);
 > +    tcp_v4_hash_rand ^= jiffies;
 >
 >      /* Unhash it so that IP input processing does not even
 >       * see it, we do not wish this socket to see incoming
 >
 >