CURRENT: ipfw: problems with timeouts and worse network performance

Jan Bramkamp crest at
Fri May 20 14:01:20 UTC 2016

On 20/05/16 15:51, Vladimir Zakharov wrote:
> On Fri, May 20, 2016, Jan Bramkamp wrote:
>> On 20/05/16 14:54, Vladimir Zakharov wrote:
>>> Hello
>>> On Fri, May 20, 2016, O. Hartmann wrote:
>>>> I reported earlier about broken pipes in ssh sessions to remote hosts,
>>>> which occur on an erratic basis. i'm investigating this problem now and
>>>> it seems that it is also ipfw-related, but I'm not sure. This problem
>>>> is present since a couple of weeks now.
>>> Maybe this could help...
>>> I've also experienced problems with broken pipes in ssh sessions some
>>> time ago. Setting in sysctl.conf
>>> net.inet.ip.fw.dyn_ack_lifetime=3600
>>> fixed problem for me. I didn't experiment with the value though. So,
>>> possibly, changing default value (300s) to 1 hour is overkill :).
>> By default the OpenSSH SSH client is configured to use TCP keepalives.
>> Those should produce enough packets at a short enough interval to keep
>> the dynamic IPFW state established.
>> Does your traffic pass through libalias?
> I guess not. How can I be sure?

Libalias is used by ipfw and the old userland natd to implement IPv4 
NAT. It requires unmodified access to all packets including their 
headers. LRO and TSO coalesce packets to reduce save CPU time but the 
process is loses some of the information required by libalias. Unless 
your ruleset uses ipfw in-kernel NAT or diverts traffic to natd you 
don't have to worry about libalias.

Use `kldstat -v | grep libalias` to check for libalias in the running 
kernel and `pgrep natd` to search for running natd instances.

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