[ovs-dev] [PATCH] conntrack: document NULL SNAT behavior and add a test case

Paolo Valerio pvalerio at redhat.com
Tue Mar 30 11:38:24 UTC 2021


"Eelco Chaudron" <echaudro at redhat.com> writes:

> On 22 Mar 2021, at 19:50, Paolo Valerio wrote:
>
>> Hi Eelco,
>>
>> Thanks for working on this, very useful indeed.
>> not a full review, but I have a question about a minor thing.
>>
>> Eelco Chaudron <echaudro at redhat.com> writes:
>>
>>> Currently, conntrack in the kernel has an undocumented feature 
>>> referred
>>> to as NULL SNAT. Basically, when a source port collision is detected
>>> during the commit, the source port will be translated to an ephemeral
>>> port. If there is no collision, no SNAT is performed.
>>>
>>> This patchset documents this behavior and adds a self-test to verify
>>> it's not changing.
>>>
>>> Signed-off-by: Eelco Chaudron <echaudro at redhat.com>
>>> ---
>>>  lib/ovs-actions.xml              |   10 ++++++++
>>>  tests/system-kmod-macros.at      |    7 ++++++
>>>  tests/system-traffic.at          |   45 
>>> ++++++++++++++++++++++++++++++++++++++
>>>  tests/system-userspace-macros.at |   10 ++++++++
>>>  4 files changed, 72 insertions(+)
>>>
>>> diff --git a/lib/ovs-actions.xml b/lib/ovs-actions.xml
>>> index a2778de4b..a0070e6c6 100644
>>> --- a/lib/ovs-actions.xml
>>> +++ b/lib/ovs-actions.xml
>>> @@ -1833,6 +1833,16 @@ for <var>i</var> in [1,<var>n_members</var>]:
>>>              connection, will behave the same as a bare 
>>> <code>nat</code>.
>>>            </p>
>>>
>>> +          <p>
>>> +            For SNAT, there is a special case when the 
>>> <code>src</code> IP
>>> +            address is configured as all 0's, i.e.,
>>> +            <code>nat(src=0.0.0.0)</code>. In this case, when a 
>>> source port
>>> +            collision is detected during the commit, the source port 
>>> will be
>>> +            translated to an ephemeral port. If there is no 
>>> collision, no SNAT
>>> +            is performed. Note that this is currently only 
>>> implemented in the
>>> +            Linux kernel datapath.
>>> +          </p>
>>> +
>>>            <p>
>>>              Open vSwitch 2.6 introduced <code>nat</code>.  Linux 4.6 
>>> was the
>>>              earliest upstream kernel that implemented 
>>> <code>ct</code> support for
>>> diff --git a/tests/system-kmod-macros.at 
>>> b/tests/system-kmod-macros.at
>>> index 15628a7c6..38bb1c55c 100644
>>> --- a/tests/system-kmod-macros.at
>>> +++ b/tests/system-kmod-macros.at
>>> @@ -99,6 +99,13 @@ m4_define([CHECK_CONNTRACK_FRAG_OVERLAP],
>>>  #
>>>  m4_define([CHECK_CONNTRACK_NAT])
>>>
>>> +# CHECK_CONNTRACK_NULL_SNAT()
>>> +#
>>> +# Perform requirements checks for running conntrack SNAT NULL tests.
>>> +# The kernel always supports NULL SNAT, so no check is needed.
>>> +#
>>> +m4_define([CHECK_CONNTRACK_NULL_SNAT])
>>> +
>>>  # CHECK_CONNTRACK_TIMEOUT()
>>>  #
>>>  # Perform requirements checks for running conntrack customized 
>>> timeout tests.
>>> diff --git a/tests/system-traffic.at b/tests/system-traffic.at
>>> index fb5b9a36d..1be425bb4 100644
>>> --- a/tests/system-traffic.at
>>> +++ b/tests/system-traffic.at
>>> @@ -4433,6 +4433,51 @@ 
>>> tcp,orig=(src=10.1.1.1,dst=10.1.1.2,sport=<cleared>,dport=<cleared>),reply=(src=
>>>  OVS_TRAFFIC_VSWITCHD_STOP
>>>  AT_CLEANUP
>>>
>>> +
>>> +AT_SETUP([conntrack - NULL SNAT])
>>> +AT_SKIP_IF([test $HAVE_NC = no])
>>> +CHECK_CONNTRACK()
>>> +CHECK_CONNTRACK_NULL_SNAT()
>>> +OVS_TRAFFIC_VSWITCHD_START()
>>> +
>>> +ADD_NAMESPACES(at_ns0, at_ns1)
>>> +ADD_VETH(p0, at_ns0, br0, "10.1.1.1/24")
>>> +ADD_VETH(p1, at_ns1, br0, "10.1.1.2/24")
>>> +NS_CHECK_EXEC([at_ns0], [ip route add 172.1.1.0/24 via 10.1.1.2])
>>> +
>>> +OVS_START_L7([at_ns1], [http])
>>> +
>>
>> I noticed you use nc clients, is there any specific reason you 
>> preferred
>> httpd over something like:
>>
>> NETNS_DAEMONIZE([at_ns1], [nc -l -k 80 > /dev/null], [nc0.pid])
>
> No specific reason other than the http server python script was used in 
> all the other tests above.
>

ok, thanks.

>>> +AT_DATA([flows.txt], [dnl
>>> +table=0,priority=30,ct_state=-trk,ip,action=ct(table=0)
>>> +table=0,priority=20,ip,nw_dst=10.1.1.0/24,actions=ct(commit,nat(src=0.0.0.0),table=10)

While at it, I was noticing the rule above.
This would be matched in both directions.
I think it would be better to split the logic here matching -rpl (just like the bare
nat rule below matches replies only).

WDYT?

>>> +table=0,priority=20,ip,nw_dst=172.1.1.2,actions=ct(commit,nat(dst=10.1.1.2),table=10)
>>> +table=0,priority=10,arp,action=normal
>>> +table=0,priority=1,action=drop
>>> +table=10,priority=20,ct_state=+rpl,ip,nw_dst=10.1.1.0/24 
>>> actions=ct(table=20,nat)
>>> +table=10,priority=10,ip,nw_dst=10.1.1.0/24 actions=resubmit(,20)
>>> +table=20,priority=10,ip,nw_dst=10.1.1.1,action=1
>>> +table=20,priority=10,ip,nw_dst=10.1.1.2,action=2
>>> +])
>>> +AT_CHECK([ovs-ofctl --bundle add-flows br0 flows.txt])
>>> +
>>> +dnl - Test to make sure src nat is NOT done when not needed
>>> +NS_CHECK_EXEC([at_ns0], [echo "TEST" | nc -p 30000 10.1.1.2 80 > 
>>> nc-1.log])
>>> +AT_CHECK([ovs-appctl dpctl/dump-conntrack | grep 
>>> "orig=.src=10\.1\.1\.1,"], [0], [dnl
>>> +tcp,orig=(src=10.1.1.1,dst=10.1.1.2,sport=30000,dport=80),reply=(src=10.1.1.2,dst=10.1.1.1,sport=80,dport=30000),protoinfo=(state=TIME_WAIT)
>>> +])
>>> +
>>> +dnl - Test to make sure src nat is done when needed
>>> +NS_CHECK_EXEC([at_ns0], [echo "TEST2" | nc -p 30001 172.1.1.2 80 > 
>>> nc-2.log])
>>> +NS_CHECK_EXEC([at_ns0], [echo "TEST3" | nc -p 30001 10.1.1.2 80 > 
>>> nc-3.log])
>>> +AT_CHECK([ovs-appctl dpctl/dump-conntrack | grep 30001 | grep 
>>> "orig=.src=10\.1\.1\.1," | sed -e 's/port=30001/port=<clnt_s_port>/g' 
>>> -e 's/sport=80,dport=[[0-9]]\+/sport=80,dport=<rnd_port>/g' | sort], 
>>> [0], [dnl
>>> +tcp,orig=(src=10.1.1.1,dst=10.1.1.2,sport=<clnt_s_port>,dport=80),reply=(src=10.1.1.2,dst=10.1.1.1,sport=80,dport=<rnd_port>),protoinfo=(state=TIME_WAIT)
>>> +tcp,orig=(src=10.1.1.1,dst=172.1.1.2,sport=<clnt_s_port>,dport=80),reply=(src=10.1.1.2,dst=10.1.1.1,sport=80,dport=<clnt_s_port>),protoinfo=(state=TIME_WAIT)
>>> +])
>>> +
>>> +OVS_TRAFFIC_VSWITCHD_STOP
>>> +AT_CLEANUP
>>> +
>>> +
>>>  AT_SETUP([conntrack - simple DNAT])
>>>  CHECK_CONNTRACK()
>>>  CHECK_CONNTRACK_NAT()
>>> diff --git a/tests/system-userspace-macros.at 
>>> b/tests/system-userspace-macros.at
>>> index 34f82cee3..71acc8618 100644
>>> --- a/tests/system-userspace-macros.at
>>> +++ b/tests/system-userspace-macros.at
>>> @@ -96,6 +96,16 @@ m4_define([CHECK_CONNTRACK_FRAG_OVERLAP])
>>>  #
>>>  m4_define([CHECK_CONNTRACK_NAT])
>>>
>>> +# CHECK_CONNTRACK_NULL_SNAT()
>>> +#
>>> +# Perform requirements checks for running conntrack SNAT NULL tests.
>>> +# The userspace datapath does not support NULL SNAT.
>>> +#
>>> +m4_define([CHECK_CONNTRACK_NULL_SNAT],
>>> +[
>>> +    AT_SKIP_IF([:])
>>> +])
>>> +
>>>  # CHECK_CONNTRACK_TIMEOUT()
>>>  #
>>>  # Perform requirements checks for running conntrack customized 
>>> timeout tests.



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