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

Paolo Valerio pvalerio at redhat.com
Mon Mar 22 18:50:20 UTC 2021


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])

> +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)
> +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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