From 1d41b0c1ec66d38355a1e76c29dd2200433335f6 Mon Sep 17 00:00:00 2001 From: Anthony Liguori Date: Thu, 22 Oct 2009 17:43:48 +0100 Subject: [PATCH] Work around dhclient brokenness With the latest GSO/csum offload patches, any guest using an unpatched version of dhclient (any Ubuntu guest, for instance), will no longer be able to get a DHCP address. dhclient is actually at fault here. It uses AF_PACKET to receive DHCP responses but does not check auxdata to see if the packet has a valid csum. This causes it to throw out the DHCP responses it gets from the virtio interface as there is not a valid checksum. Fedora has carried a patch to fix their dhclient (it's needed for Xen too) but this patch has not made it into a release of dhclient. AFAIK, the patch is in the dhclient CVS but I cannot confirm since their CVS is not public. This patch, suggested by Rusty, looks for UDP packets (of a normal MTU) and explicitly adds a checksum to them if they are missing one. This allows unpatched dhclients to continue to work without needing to update the guest kernels. Signed-off-by: Anthony Liguori Signed-off-by: Mark McLoughlin Signed-off-by: Anthony Liguori --- hw/virtio-net.c | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+) diff --git a/hw/virtio-net.c b/hw/virtio-net.c index 5e9520de7f..064ec2e609 100644 --- a/hw/virtio-net.c +++ b/hw/virtio-net.c @@ -372,6 +372,34 @@ static int virtio_net_can_receive(VLANClientState *vc) return do_virtio_net_can_receive(n, VIRTIO_NET_MAX_BUFSIZE); } +/* dhclient uses AF_PACKET but doesn't pass auxdata to the kernel so + * it never finds out that the packets don't have valid checksums. This + * causes dhclient to get upset. Fedora's carried a patch for ages to + * fix this with Xen but it hasn't appeared in an upstream release of + * dhclient yet. + * + * To avoid breaking existing guests, we catch udp packets and add + * checksums. This is terrible but it's better than hacking the guest + * kernels. + * + * N.B. if we introduce a zero-copy API, this operation is no longer free so + * we should provide a mechanism to disable it to avoid polluting the host + * cache. + */ +static void work_around_broken_dhclient(struct virtio_net_hdr *hdr, + const uint8_t *buf, size_t size) +{ + if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */ + (size > 27 && size < 1500) && /* normal sized MTU */ + (buf[12] == 0x08 && buf[13] == 0x00) && /* ethertype == IPv4 */ + (buf[23] == 17) && /* ip.protocol == UDP */ + (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */ + /* FIXME this cast is evil */ + net_checksum_calculate((uint8_t *)buf, size); + hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM; + } +} + static int iov_fill(struct iovec *iov, int iovcnt, const void *buf, int count) { int offset, i; @@ -399,6 +427,7 @@ static int receive_header(VirtIONet *n, struct iovec *iov, int iovcnt, if (n->has_vnet_hdr) { memcpy(hdr, buf, sizeof(*hdr)); offset = sizeof(*hdr); + work_around_broken_dhclient(hdr, buf + offset, size - offset); } /* We only ever receive a struct virtio_net_hdr from the tapfd,