vhost-net: add dhclient work-around from userspace

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin <[email protected]>

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin <[email protected]>

---

Dave, I'm going to put this patch on the vhost tree, no need for you to bother merging it - you'll get it with a pull request.

 drivers/vhost/net.c |   44 +++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 43 insertions(+), 1 deletions(-)

drivers/vhost/net.c +4 -0

index cc19595..03bba6a 100644
@@ -24,6 +24,10 @@
 #include <linux/if_tun.h>
 #include <linux/if_macvlan.h>
 
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <linux/netdevice.h>
+
 #include <net/sock.h>
 
 #include "vhost.h"

drivers/vhost/net.c +19 -0

+{
+	struct sk_buff *skb;
+
+	lock_sock(sk);
+	skb = skb_peek(&sk->sk_receive_queue);
+	if (unlikely(!skb)) {
+		release_sock(sk);
+		return 0;
+	}
+	/* Userspace virtio server has the following hack so
+	 * guests rely on it, and we have to replicate it, too: */
+	/* Use port number to detect incoming IPv4 DHCP response packets,
+	 * and fill in the checksum. */
+
+	/* The issue we are solving is that on linux guests, some apps
+	 * that use recvmsg with AF_PACKET sockets, don't know how to
+	 * handle CHECKSUM_PARTIAL;
+	 * The interface to return the relevant information was added in
+	 * 8dc4194474159660d7f37c495e3fc3f10d0db8cc,

drivers/vhost/net.c +9 -1

+		skb_checksum_help(skb);
+		/* Restore ip_summed value: tun passes it to user. */
+		skb->ip_summed = CHECKSUM_PARTIAL;
+	}
+	release_sock(sk);
+	return 1;
+}
+
 /* Expects to be always run from workqueue - which acts as
  * read-size critical section for our kind of RCU. */
 static void handle_rx(struct vhost_net *net)

@@ -222,7 +264,7 @@ static void handle_rx(struct vhost_net *net)
 	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
 		vq->log : NULL;
 
-	for (;;) {
+	while (peek_head(sock->sk)) {
 		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
 					 ARRAY_SIZE(vq->iov),
 					 &out, &in,

Quoted 46 lines · Michael S. Tsirkin

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin <[email protected]>

---

Dave, I'm going to put this patch on the vhost tree, no need for you to bother merging it - you'll get it with a pull request.

 drivers/vhost/net.c |   44 +++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 43 insertions(+), 1 deletions(-)

drivers/vhost/net.c +4 -0

index cc19595..03bba6a 100644
@@ -24,6 +24,10 @@
 #include <linux/if_tun.h>
 #include <linux/if_macvlan.h>
 
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <linux/netdevice.h>
+
 #include <net/sock.h>
 
 #include "vhost.h"

drivers/vhost/net.c static void handle_tx(struct vhost_net *net) · line 193

Quoted 4 lines

drivers/vhost/net.c +1 -0

@@ -186,6 +190,44 @@ static void handle_tx(struct vhost_net *net)
 	unuse_mm(net->dev.mm);
 }
 
+static int peek_head(struct sock *sk)

This routine is doing more than just peeking the head of sk's receive queue. May be this should be named similar to what qemu calls 'work_around_broken_dhclient()'

Quoted 24 lines

drivers/vhost/net.c +24 -0

+{
+	struct sk_buff *skb;
+
+	lock_sock(sk);
+	skb = skb_peek(&sk->sk_receive_queue);
+	if (unlikely(!skb)) {
+		release_sock(sk);
+		return 0;
+	}
+	/* Userspace virtio server has the following hack so
+	 * guests rely on it, and we have to replicate it, too: */
+	/* Use port number to detect incoming IPv4 DHCP response packets,
+	 * and fill in the checksum. */
+
+	/* The issue we are solving is that on linux guests, some apps
+	 * that use recvmsg with AF_PACKET sockets, don't know how to
+	 * handle CHECKSUM_PARTIAL;
+	 * The interface to return the relevant information was added in
+	 * 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
+	 * and older userspace does not use it.
+	 * One important user of recvmsg with AF_PACKET is dhclient,
+	 * so we add a work-around just for DHCP. */
+	if (skb->ip_summed == CHECKSUM_PARTIAL &&
+	    skb_headlen(skb) >= skb_transport_offset(skb) +

drivers/vhost/net.c static void handle_tx(struct vhost_net *net) · line 222

Quoted 5 lines

drivers/vhost/net.c +5 -0

+				sizeof(struct udphdr) &&
+	    udp_hdr(skb)->dest == htons(68) &&
+	    skb_network_header_len(skb) >= sizeof(struct iphdr) &&
+	    ip_hdr(skb)->protocol == IPPROTO_UDP &&
+	    skb->protocol == htons(ETH_P_IP)) {

Isn't it more logical to check for skb->protocol, followed by ip_hdr and then udp_hdr?

Quoted 20 lines

drivers/vhost/net.c +9 -1

+		skb_checksum_help(skb);
+		/* Restore ip_summed value: tun passes it to user. */
+		skb->ip_summed = CHECKSUM_PARTIAL;
+	}
+	release_sock(sk);
+	return 1;
+}
+
 /* Expects to be always run from workqueue - which acts as
  * read-size critical section for our kind of RCU. */
 static void handle_rx(struct vhost_net *net)

@@ -222,7 +264,7 @@ static void handle_rx(struct vhost_net *net)
 	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
 		vq->log : NULL;
 
-	for (;;) {
+	while (peek_head(sock->sk)) {
 		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
 					 ARRAY_SIZE(vq->iov),
 					 &out, &in,

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin <[email protected]>

From: "Michael S. Tsirkin" <[email protected]>
Date: Mon, 28 Jun 2010 13:08:07 +0300

Quoted 15 lines

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Quoted 5 lines

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin <[email protected]>

Yikes, this is awful too.

Nothing in the kernel should be mucking around with procotol packets like this by default. In particular, what the heck does port 67 mean? Locally I can use it for whatever I want for my own purposes, I don't have to follow the conventions for service ports as specified by the IETF.

But I can't have the packet checksum state be left alone for port 67 traffic on a box using virtio because you have this hack there.

And yes it's broken on machines using the qemu thing, but at least the hack there is restricted to userspace.

I really don't want anything in the kernel that looks like this.

These applications are broken, and we've provided a way for them to work properly. What's the point of having fixed applications if all of these hacks grow like fungus over every virtualization transport?

It just means that people won't fix the apps, since they don't have to. There is no incentive, and the mechanism we created to properly handle this loses it's value.

At best, you can write a netfilter module that mucks up the packet checksum state in these situations. At least in that case, you can make it generic (it mangles iff a packet matches a certain rule, so for your virtio guests you'd make it match for DHCP frames) instead of being some hard-coded DHCP thing by design.

And since this is so cleanly seperated and portable you don't even need to push it upstream. It's a temporary workaround for a temporary problem. You can just delete it as soon as the majority of guests have the fixed dhcp. The qemu crap should disappear similarly.

Since using the module involves updating the management tools as well, if we go down this route it will be much less painful for everyone to do push it upstream.

Quoted 36 lines · David Miller

From: "Michael S. Tsirkin"<[email protected]>
Date: Mon, 28 Jun 2010 13:08:07 +0300

Quoted 15 lines

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Quoted 6 lines

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin<[email protected]>

Yikes, this is awful too.

Nothing in the kernel should be mucking around with procotol packets like this by default. In particular, what the heck does port 67 mean? Locally I can use it for whatever I want for my own purposes, I don't have to follow the conventions for service ports as specified by the IETF.

But I can't have the packet checksum state be left alone for port 67 traffic on a box using virtio because you have this hack there.

Quoted 3 lines

And yes it's broken on machines using the qemu thing, but at least the hack there is restricted to userspace.

Yes, and I think it was a mistake to add the hack there. This is what prevented applications from using the new interface in the 3 years since it was first introduced.

Quoted 32 lines · Michael S. Tsirkin

Quoted 31 lines · David Miller

From: "Michael S. Tsirkin"<[email protected]>
Date: Mon, 28 Jun 2010 13:08:07 +0300

Quoted 15 lines

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Quoted 5 lines

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin<[email protected]>

Yikes, this is awful too.

Nothing in the kernel should be mucking around with procotol packets like this by default. In particular, what the heck does port 67 mean? Locally I can use it for whatever I want for my own purposes, I don't have to follow the conventions for service ports as specified by the IETF.

Quoted 8 lines

Quoted 5 lines

But I can't have the packet checksum state be left alone for port 67 traffic on a box using virtio because you have this hack there.

And yes it's broken on machines using the qemu thing, but at least the hack there is restricted to userspace.

Yes, and I think it was a mistake to add the hack there. This is what prevented applications from using the new interface in the 3 years since it was first introduced.

It's far more complicated than that. dhclient is part of ISC's DHCP package. They do not have a public SCM and instead require you to join their Software Guild to get access to it.

This problem was identified in one distribution and the patch was pushed upstream but because they did not have a public SCM, most other distributions did not see the fix until it appeared in a release. ISC has a pretty long release cycle historically.

ISC's had the fix for a long time but there was a 3-year gap in their releases and since their SCM isn't public, users are stuck with the last release.

This hack makes sense in QEMU as we have a few hacks like this to fix broken guests.
A primary use of virtualization is to run old
applications so it makes sense for us to do that.

I don't think it makes sense for vhost to do this. These guests are so old that they don't have the requisite features to achieve really high performance anyway.

I've always thought making vhost totally transparent was a bad idea and this is one of the reasons.

We can do a lot of ugly things in
userspace that we shouldn't be doing in the kernel.

Regards,

Anthony Liguori

Quoted 56 lines · Anthony Liguori

Quoted 38 lines · Michael S. Tsirkin

Quoted 36 lines · David Miller

From: "Michael S. Tsirkin"<[email protected]>
Date: Mon, 28 Jun 2010 13:08:07 +0300

Quoted 15 lines

Userspace virtio server has the following hack so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets, and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps that use recvmsg with AF_PACKET sockets, don't know how to handle CHECKSUM_PARTIAL;
The interface to return the relevant information was added in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient, so we add a work-around just for DHCP.

Quoted 6 lines

Don't bother applying the hack to IPv6 as userspace virtio does not have a work-around for that - let's hope guests will do the right thing wrt IPv6.

Signed-off-by Michael S. Tsirkin<[email protected]>

Yikes, this is awful too.

Nothing in the kernel should be mucking around with procotol packets like this by default. In particular, what the heck does port 67 mean? Locally I can use it for whatever I want for my own purposes, I don't have to follow the conventions for service ports as specified by the IETF.

But I can't have the packet checksum state be left alone for port 67 traffic on a box using virtio because you have this hack there.

Quoted 7 lines

Quoted 3 lines

And yes it's broken on machines using the qemu thing, but at least the hack there is restricted to userspace.

Yes, and I think it was a mistake to add the hack there. This is what prevented applications from using the new interface in the 3 years since it was first introduced.

It's far more complicated than that. dhclient is part of ISC's DHCP package. They do not have a public SCM and instead require you to join their Software Guild to get access to it.

This problem was identified in one distribution and the patch was pushed upstream but because they did not have a public SCM, most other distributions did not see the fix until it appeared in a release. ISC has a pretty long release cycle historically.

Quoted 9 lines

ISC's had the fix for a long time but there was a 3-year gap in their releases and since their SCM isn't public, users are stuck with the last release.

This hack makes sense in QEMU as we have a few hacks like this to fix broken guests.
A primary use of virtualization is to run old
applications so it makes sense for us to do that.

IMO it was wrong to put it in qemu: originally, if a distro shipped a broken virtio/dhclient combo, it was it's own bug to fix. But now that qemu has shipped the work-around for so long, broken guests seemed work.

So we *still* see the bug re-surface in new guests.

And since they are fairly new, it is interesting to get decent performance from them now.

Quoted 4 lines

I don't think it makes sense for vhost to do this. These guests are so old that they don't have the requisite features to achieve really high performance anyway.

Quoted 3 lines

I've always thought making vhost totally transparent was a bad idea and this is one of the reasons.

It does not have to be fully transparent. You can insert your own ring in the middle, and copy descriptors around. And we stop on errors and let userspace handle. This will come handy if we get e.g. virtio bug that we need to work around.

Quoted 7 lines

We can do a lot of ugly things in
userspace that we shouldn't be doing in the kernel.

Regards,

Anthony Liguori

QEMU is only userspace for the host. It is the hardware for the guest. So IMO we should not be doing the ugly things there either.

Hi,

I think I am hitting this, in a weird way :D. Running 0.12.90, vhost_net and kernel 2.6.35.4, I have troubles to get dhcp clients to work except an old dhcp client "pump" seems to work out of the box. Below are the output of dhcpcd, isc's dhcp client and busybox' udhcpc.

My dhcp server is in a guest with vhost_net running dnsmasq. Clients which do not use vhost_net (plain virtio, e1000 and physical machines) do not have any problems. The dhcp server itself is a dhcp client and uses dhcpcd to get lease from ADSL modem w/o issues.
When the dhcp server is not using vhost_net, then all clients can get a lease.

So my conclusion so far is that when the server is using vhost_net and clients are using vhost_net (and above mentioned software), then it's getting invalid/checksum error UDP. the only working DHCP client is pump.

# dhcpcd
dhcpcd[9089]: version 5.2.7 starting
dhcpcd[9089]: eth0: broadcasting for a lease
dhcpcd[9089]: eth0: invalid UDP packet from 192.168.1.1
dhcpcd[9089]: eth0: invalid UDP packet from 192.168.1.1
dhcpcd[9089]: eth0: invalid UDP packet from 192.168.1.1
dhcpcd[9089]: eth0: invalid UDP packet from 192.168.1.1
dhcpcd[9089]: timed out
dhcpcd[9089]: allowing 8 seconds for IPv4LL timeout
dhcpcd[9089]: eth0: probing for an IPv4LL address
dhcpcd[9089]: eth0: checking for 169.254.195.227
dhcpcd[9089]: eth0: using IPv4LL address 169.254.195.227
dhcpcd[9089]: forked to background, child pid 9109

# dhclient
Internet Systems Consortium DHCP Client V3.1.2p1-Gentoo
Copyright 2004-2009 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

Listening on LPF/eth0/00:16:3e:00:07:01
Sending on LPF/eth0/00:16:3e:00:07:01
Sending on Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 7 DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 7 DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 7 DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 13 DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 13 5 bad udp checksums in 5 packets
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 14 No DHCPOFFERS received.
No working leases in persistent database - sleeping.

# busybox udhcpc
### adapter index 2
### adapter hardware address 00:16:3e:00:07:01
udhcpc (v1.13.2) started
### vfork'ing and execle'ing /usr/share/udhcpc/default.script
udhcpc: exec /usr/share/udhcpc/default.script: No such file or directory
### entering raw listen mode
### opening raw socket on ifindex 2
### got raw socket fd 5
### attached filter to raw socket fd 5
### bound to raw socket fd 5
### adding option 0x35
### adding option 0x3d
### adding option 0x3c
### adding option 0x39
Sending discover...
### Waiting on select...
udhcpc: packet with bad UDP checksum received, ignoring
### Waiting on select...
### adding option 0x35
### adding option 0x3d
### adding option 0x3c
### adding option 0x39
Sending discover...
### Waiting on select...
udhcpc: packet with bad UDP checksum received, ignoring
### Waiting on select...
### adding option 0x35
### adding option 0x3d
### adding option 0x3c
### adding option 0x39
Sending discover...
### Waiting on select...
udhcpc: packet with bad UDP checksum received, ignoring
### Waiting on select...
### vfork'ing and execle'ing /usr/share/udhcpc/default.script
udhcpc: exec /usr/share/udhcpc/default.script: No such file or directory
### Waiting on select...
### adding option 0x35
### adding option 0x3d
### adding option 0x3c
### adding option 0x39
Sending discover...
### Waiting on select...
udhcpc: packet with bad UDP checksum received, ignoring
### Waiting on select...
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Newer versions of dhclient should also be OK: they detect that checksum is missing in the packet. Try it e.g. with a recent fedora guest as a client.

To solve the problem for old clients, recent kernels and iptables have support for CHECKSUM target.

You can use this target to compute and fill in the checksum in a packet that lacks a checksum.

Typical expected use:
iptables -A POSTROUTING -t mangle -p udp --dport bootpc \
-j CHECKSUM --checksum-fill

libvirt will program these for you if it sets up the server, maybe there needs a flag to tell it that server is local.

Quoted 3 lines

Newer versions of dhclient should also be OK: they detect that checksum is missing in the packet. Try it e.g. with a recent fedora guest as a client.

I don't have fedora, but with the latest release (4.1.1-P1) on isc.org it still behaves the same (see output at the bottom).

Quoted 3 lines

To solve the problem for old clients, recent kernels and iptables have support for CHECKSUM target.

Quoted 6 lines

You can use this target to compute and fill in the checksum in a packet that lacks a checksum.

Typical expected use:
iptables -A POSTROUTING -t mangle -p udp --dport bootpc \
-j CHECKSUM --checksum-fill

Nice trick :D

Quoted 2 lines

libvirt will program these for you if it sets up the server, maybe there needs a flag to tell it that server is local.

I don't use libvirt.

My point is, there doesn't seem to be much working client and the only working client is a ver very old one (pump), newer client do not work, as opposite to what you have explained.

To repeat myself, here is the situation:

- DHCP server with vhost_net, all client w/o vhost_net work, clients with vhost_net do not work except pump
- DHCP server w/o vhost_net, all clients work

- physical DHCP server, client with vhost *do* work.