Currently showing posts tagged: networking

Tool-building hacks #1: audible pings

By , February 11, 2012 8:07 pm

I think I’m genetically a tool-builder. My dad and uncle both take great pleasure in carefully selecting and buying or building tools for their workshops, my mum’s an expert woodcarver with a fair array of sharp pointy things, and these are not the only examples in the family. For me, the habit has manifested in electronic form, and I really enjoy programming new scripts etc. to help me work more efficiently. In fact I’ve collected such a vast array of them over the years that I had to start tracking them under version control back in 1999. (CVS did me proud for many years, but it did not age well, and I finally migrated them to git a few months ago.)

I can’t claim to be remotely unusual in this respect though – there’s a whole subculture of programmers (“hackers”) who can relate to this mindset. Sometimes when I build a new tool, I get the impulse to share it with the world in case it turns out to be of use to anyone else. In the past, these hacks have ended up on my software web page, but I think this blog might be a better medium.

So, without further ado, here’s a cute hack I just built: bping, a wrapper around ping(8) which makes it do a lot of beeping ;-) – one beep per packet received, with pitch going up an octave for every doubling of the response latency. (See below for installation instructions.) Why did I want this?

I use ethernet over power to connect the machines in my bedroom to the equipment in the lounge. In a high-rise apartment block with about 100 wireless networks fighting over the same spectrum band, this is (normally) a much more reliable option. However yesterday the connections in my home network started behaving very weirdly. I tried pinging various machines on the network from each other to narrow down the problem, but it was annoying to have to keep going between rooms to visually monitor the output from the various pings when it would have been quicker to be able to hear the quality of the connections being tested. So I wrote bping, which also turns a laptop into a sort of Geiger counter for wifi.

Currently bping uses bip (here’s a suggested approach to installation) which in turns uses sox to make the beeps. This works fine on Fedora 15 but unfortunately for some reason takes longer than one second to run on Ubuntu 11.10 and openSUSE 12.1, regardless of how short the beep is. I think it’s something to do with pulseaudio, but I haven’t bothered to figure it out yet. Answers on a e-postcard please …

By the way, many of my hacks are already publically available on my github account, but most aren’t documented yet. The current exceptions are:

I’ll try to document some of the others soon. Watch this space!


port redirection from kvm host to guest

By , January 23, 2012 2:58 am

I’ve just started using kvm in earnest, and immediately ran into the challenge of how to access my guest via ssh. My first instinct was to configure the guest in bridged mode, but this doesn’t work well (or at all) with wireless interfaces.

So plan B was to set up port redirection from the host to the guest, e.g. so that ssh’ing to localhost port 2222 would redirect to the guest’s port 22.

After a quick google, some fiddling with iptables, and a glance at the libvirt Networking wiki page, I was still having no luck. Then it hit me – my guest was using user-mode networking, and rather than getting its DHCP-allocated IP from the libvirtd-launched dnsmasq instance on the host, was receiving a hardcoded allocation of from the host which is on This can be extremely puzzling at first, because no network commands run on the host (such as ifconfig, iptables, brctl, route) will reveal this magic address, yet the host is still accessible from the guest via it.

After a lot more googling, I stumbled across a technique for configuring host to guest port redirection on a running VM. This sounded very promising, but virt-manager refused to accept the magic Control-Alt-2 key combination to switch to QEMU monitor mode. It turns out that this is no accident. However, since libvirt 0.8.8, the QEMU monitor can be accessed via virsh.
Note that the --hmp option is required, otherwise the monitor expects the command in JSON format, so omitting it leads to errors like error: internal error cannot parse json ... lexical error: invalid char in json text.

The final hurdle was figuring out the correct monitor command. The host_net_redir command as mentioned in the above article is no longer recognized. Luckily the QEMU monitor interface helped me out here – I spotted an encouraging sounding command hostfwd_add:

# virsh qemu-monitor-command --hmp sles11 'help hostfwd_add'
hostfwd_add [vlan_id name] [tcp|udp]:[hostaddr]:hostport-[guestaddr]:guestport -- redirect TCP or UDP connections from host to guest (requires -net user)

and google confirmed that the latter had superceded the former.

So finally we have the complete solution:

# virsh qemu-monitor-command --hmp sles11 'hostfwd_add ::2222-:22'
# ssh -p 2222 localhost
Last login: Mon Jan 23 00:37:44 2012
linux-mnsh:~ #


UPDATE: just found another very simple solution – add a new NIC to the VM which doesn’t use user-mode networking. Then it will get a IP (on by default) which is still NAT’d but also routable via virbr0 on the host, meaning no redirection is necessary; just ssh directly to the guest’s IP from the host. A minor disadvantage of this is that the guest won’t be directly reachable from outside the host, but that’s unlikely to be an issue in most scenarios.


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