Justin Robertson wrote:
Chuck Swiger wrote:
On Mar 7, 2007, at 1:47 PM, Justin Robertson wrote:
Perhaps trying:
sysctl net.inet.tcp.sack.enable=0
...will do what you are looking for?
No (this only works in 6.x, btw) - setting sack.enable=0 simply
tells the system not to send selective acks itself, this doesn't
stop a host from sending selective acks inbound, and processing them
still causes the system to bog and die.
That sysctl is present in 5.x, at least somewhere around 5.4/5.5.
Nothing (short of a firewall) is going to prevent the other side from
sending SACKs inbound, however, if you don't enable SACKs on your
side, you won't reply with that option, and the remote host should
not continue to generate them for the rest of the TCP session.
If you're looking at a deliberate DoS attack using SACKs, well, you'd
want to block the initial SYNs entirely rather than worry about
processing the option after receiving the packets. I would not
expect that the system would bog down processing SACKs if the sysctl
disables them, but I'll take your word for it that turning off the
sysctl does not prevent the extra work from being done.
What I'm looking for here, is a patch to ipfw to allow one to set a
flag to strip the tcpoption sack from syn packets.
Is there something wrong with:
ipfw add deny tcp from any tcpoptions sack to any
...? Sure, you're going to force hosts which default to SACK being
enabled to retransmit their SYNs without that option, but
RFC-793-compliant stacks will do so without much extra delay. I'm
not sure this is a good solution, but it's not exactly clear to me
which problem you are trying to solve....
---Chuck
The issue here is that no windows PC is compliant, and continues to
try and send SYN SACK packets until giving up entirely on the
connection - I've already tried this. I can't tell you why the bsd
stack doesn't have an issue with bare SYNs, but does on SYNs with SACK
set in the tcpoptions, but it apparently does.
Example - windows PC trying to connect to POP mailserver, sacks blocked
(being viewed from firewall)
14:31:54.632444 client.52985 > server.110: S [tcp sum ok]
3734795545:3734795545(0) win 8192 <mss 1460,nop,wscale 8,nop,nop,sackOK>
(DF) (ttl 115, id 12030, len 52)
14:31:57.629151 client.52985 > server.110: S [tcp sum ok]
3734795545:3734795545(0) win 8192 <mss 1460,nop,wscale 8,nop,nop,sackOK>
(DF) (ttl 115, id 12034, len 52)
14:32:03.635511 client.52985 > server.110: S [tcp sum ok]
3734795545:3734795545(0) win 8192 <mss 1460,nop,nop,sackOK> (DF) (ttl
115, id 12046, len 48)
You get three sack attempted retransmissions, then failure. It's not
sending selective acks, it's trying to establish that selective acks are
allowed for the rest of the communication between client/server. I just
want to strip the sack options from the packet and pass it on. The
"issue" is that BSD can't cope with a flood of syn/sack permitted
packets. All IP lags, and on 6.x stops all-together.
--
Justin
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