Networking with a gateway not on the local subnet on NetBSD at OVH

Peter

NetBSD has a FAQ for networking that outlines how to do Networking with a gateway not on the local subnet, unfortunately the recipe that they provide doesn't actually work "in the real world." The route command they provide does not make the network stack send an ARP who-has for the IP address and requires that you statically set the MAC address of the gateway.

I figured out a work-around for this, based on some insight from people on the NetBSD tech-talk mailing list. This allows you to use NetBSD as a guest operating system on providers such as OVH and Hetzner:

# ifconfig fxp0 inet 10.0.0.1 
# route add -net 192.168.0.1/32 -cloning -link fxp0 -iface 
# route add default -ifa 10.0.0.1 192.168.0.1

The trick was to specify use route cloning, and use a net definition instead of a host definition. Now NetBSD will send an ARP who-has request for the gateway IP address.

To supplement the OVH bridge client guide that is available on their Wiki, it would fit into the following template:

# ifconfig fxp0 inet Fail.over.IP netmask 255.255.255.255 broadcast Fail.over.IP 
# route add -net Your.Server.IP.254/32 -cloning -link fxp0 -iface 
# route add default -ifa Fail.over.IP Your.Server.IP.254

This should allow you to use NetBSD as a guest and not get blocked by OVH robots that check for too many ARP requests.

What Linux/*BSD distributions have Syncookies enabled by default?

Peter

In light of the recently published article on Quick Blind TCP Connection Spoofing with SYN Cookies, I wanted to see what operating systems and distributions have Syncookies enabled by default.

Distribution Sysctl Default
Ubuntu Linux 12.04 net.ipv4.tcp_syncookies On
Debian Linux 6 Off
Debian Linux 7 On
CentOS 5 On
CentOS 6 On
FreeBSD 8 net.ipv4.tcp_syncookies On
Solaris 10 Not Implemented Off
OpenBSD 5.3 Not Implemented Off

I'm not sure that turning off Syncookies is the best idea, due to the potential DoS effects from disabling them – applications should use something besides IP addresses for authentication.

KVM PCI Passthrough of an AHCI SATA controller to a guest causing data corruption

Peter

I recently migrated from VMware ESXi to Linux KVM, where I was using PCI Passthrough under VMware ESXi to pass through an Intel AHCI SATA controller to a guest. I implemented the same setup by enabling IOMMU on the KVM host, and passed through the AHCI SATA controller to the guest.

After a week or two, I started seeing the following messages in /var/log/syslog on the guest:

Aug  6 13:25:28 yama kernel: [78351.258573] XFS (md0): Corruption detected. Unmount and run xfs_repair
Aug  6 13:25:28 yama kernel: [78351.259102] XFS (md0): Corruption detected. Unmount and run xfs_repair
Aug  6 13:25:28 yama kernel: [78351.259616] XFS (md0): metadata I/O error: block 0x31214bd0 ("xfs_trans_read_buf_map") error 117 numblks 16
Aug  6 13:25:28 yama kernel: [78351.260203] XFS (md0): xfs_imap_to_bp: xfs_trans_read_buf() returned error 117.
Aug  6 13:29:10 yama kernel: [78573.533933] XFS (md0): Invalid inode number 0xfeffffffffffffff
Aug  6 13:29:10 yama kernel: [78573.533940] XFS (md0): Internal error xfs_dir_ino_validate at line 160 of file /build/buildd/linux-lts-raring-3.8.0/fs/xfs/xfs_dir2.c.  Caller 0xffffffffa045cd96
Aug  6 13:29:10 yama kernel: [78573.533940]
Aug  6 13:29:10 yama kernel: [78573.538440] Pid: 1723, comm: kworker/0:1H Tainted: GF            3.8.0-27-generic #40~precise3-Ubuntu
Aug  6 13:29:10 yama kernel: [78573.538443] Call Trace:
Aug  6 13:29:10 yama kernel: [78573.538496]  [<ffffffffa042316f>] xfs_error_report+0x3f/0x50 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538537]  [<ffffffffa045cd96>] ? __xfs_dir2_data_check+0x1e6/0x4a0 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538560]  [<ffffffffa045a150>] xfs_dir_ino_validate+0x90/0xe0 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538579]  [<ffffffffa045cd96>] __xfs_dir2_data_check+0x1e6/0x4a0 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538598]  [<ffffffffa045d0ca>] xfs_dir2_data_verify+0x7a/0x90 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538637]  [<ffffffff810135aa>] ? __switch_to+0x12a/0x4a0
Aug  6 13:29:10 yama kernel: [78573.538664]  [<ffffffffa045d195>] xfs_dir2_data_reada_verify+0x95/0xa0 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538675]  [<ffffffff8108e2aa>] ? finish_task_switch+0x4a/0xf0
Aug  6 13:29:10 yama kernel: [78573.538697]  [<ffffffffa042133f>] xfs_buf_iodone_work+0x3f/0xa0 [xfs]
Aug  6 13:29:10 yama kernel: [78573.538706]  [<ffffffff81078c21>] process_one_work+0x141/0x490
Aug  6 13:29:10 yama kernel: [78573.538710]  [<ffffffff81079be8>] worker_thread+0x168/0x400
Aug  6 13:29:10 yama kernel: [78573.538714]  [<ffffffff81079a80>] ? manage_workers+0x120/0x120
Aug  6 13:29:10 yama kernel: [78573.538721]  [<ffffffff8107f0f0>] kthread+0xc0/0xd0
Aug  6 13:29:10 yama kernel: [78573.538726]  [<ffffffff8107f030>] ? flush_kthread_worker+0xb0/0xb0
Aug  6 13:29:10 yama kernel: [78573.538730]  [<ffffffff816fc6ac>] ret_from_fork+0x7c/0xb0
Aug  6 13:29:10 yama kernel: [78573.538735]  [<ffffffff8107f030>] ? flush_kthread_worker+0xb0/0xb0

I initially used xfs_repair on the file system, thinking that the issue was caused by a number of power failures that happened when the machine was running ESXi. However, this did not resolve the issue and made the problem worse. Eventually I decided that I wanted to scrap the file system, and pulled a drive from the array to backup the data and re-create the file system.

The drive that I pulled from the array for backups started showing the same issues with XFS corruption.

After further investigation via trial-and-error, I determined that KVM PCI Passthrough was causing the issue and decided to just pass through an array to the guest using vrtio-block – This solved the corruption problem and I haven't had any issues (knock on wood) since!

Accessing USB devices as non-root: writing udev rules the easy way

Peter

I recently purchased a TEMPered USB thermometer, which I wanted to use as non-root using an open source utility called TEMPered. All the recipes I found, required that I use root to access the /dev/hidraw0 device that the particular TEMPered USB device exposed – of course this was not acceptable.

systemd (and udev, in general – I believe) has a handy utility called udevadm. You can use this tool to query a device on your system, for example:

udevadm info --query=all --name=/dev/hidraw0 --attribute-walk

Which allows you to retrieve all the required attributes to craft a file to put in /etc/dev/rules.d. I have created the following to expose PCsensor TEMPerV1.4 to a user that is part of the group temper:

# TEMPer1.4 USB thermometer
SUBSYSTEM=="hidraw", ATTRS{idVendor}=="0c45", ATTRS{idProduct}=="7401", GROUP="temper", MODE="0660"

I placed this in a file called /etc/udev/rules.d/60-temper.rules. You can now use TEMPered as a non-root user, which is a member of the group in question!

Experience migrating from VMware ESXi to KVM in a production environment

Peter

My notes from setting up a production KVM environment, after migrating from VMware ESXi 5.1 to Ubuntu 12.04.2 64-bit with Linux Kernel 3.2 and QEMU 1.4.2, and open vSwitch.

General

Disk I/O throughput and performance characteristics

  • Always use LVM backed storage (which is aligned), with cache='none' and io='native' (aio) for guests. Disabling cache allows the host system to properly schedule disk reads and writes
  • Use deadline I/O scheduler for host systems, and vm.swappiness = 0 in on host or equivalent to reduce pressure on I/O resources and make use of host memory
  • Use virtio for bus type to allow direct access to storage instead of going through QEMU, if supported by guest operating system drivers

Processor

  • Pass through CPU flags to guest to take advantage of newer instruction sets, assuming host hardware is the same or migration is not going to be used (-cpu host)

Network

  • Use virtio Network adapters (except with Windows) to realise full throughput and lower latency on guest operating systems, where support is available (Linux 2.6+, FreeBSD)
  • Load vhost_net kernel module on host, which permits direct access to network devices skipping QEMU (libvirt will detect if vhost_net is enabled, and add vhost=on to qemu command line by default)

Software

  • Linux Kernel 3.5, distributed with Ubuntu 12.04.2 does not support building open-vswitch – you must install Kernel 3.5 for the DKMS to properly build
  • Build QEMU from source to include new functionality and Hyper-V enhancements for Windows guests, using the 1.4 stable branch – 1.5 does not work with libvirt due to the way QEMU help parameters are parsed by the library

Guest

Linux

  • vm.swappiness = 0

FreeBSD

  • vm.defer_swapspace_pageouts = 1
  • kern.timecounter.hardware=ACPI-fast
  • kern.timecounter.smp_tsc="1"
  • kern.timecounter.invariant_tsc="1"
  • Use prebuilt virtio drivers, or compile from ports under emulators/virtio-kmod after each system upgrade

Windows

  • Disable HPET via qemu (-no-hpet) or libvirt configuration, force use of TSC to reduce time drift in guest
  • If you are using qemu 1.4 or greater, enable CPU flags (Hyper-V shims) hv_vapic,hv_relaxed,hv_spinlocks=0xffff on top of disabling HPET
  • Install Memory Ballooning service and drivers, SCSI (virtio) drivers using the stable branch from Redhat
  • Use e1000 Ethernet for Windows 2008 R2 to avoid high latency/freezing of guest operating system

libvirt

  • Add xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0' to domain type, if you are going to add custom qemu command args
  • If you are using Ubuntu, and want to change the version of qemu you are going to use – you will either need to disable AppArmor, or update the profile to include the directory you've installed the alternative qemu version to

Migration

  • Make sure to uninstall VMware Tools in the guest environment after you have migrated, and install acpid on the guest to allow graceful shutdowns (if Linux)
  • Take a snapshot of the virtual machine – transfer the VMDK, and use a utility such as vmdksync to merge the deltas after shutting down the VM for the file migration to reduce downtime
  • For Windows VMs, make sure to add a dummy virtio SCSI and Ethernet device so you can install drivers and then switch the root drive to virtio