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Running a UDP Remote Console with Linux 2.6

Posted by simoniker on from the so-there dept.

Bruce Perens writes "Many system admins have learned how to use syslog to log events of remote systems. But when the kernel crashes, its final messages never get to syslog. If you don't have a remote console manager, you won't see them, unless you run netconsole."

17 of 39 comments (clear)

  1. Re: First Post? by pizza_milkshake · · Score: 5, Funny

    did you send them via UDP? perhaps your first attempt hasn't made it yet.

  2. Re:Tempting... by Bruce+Perens · · Score: 3, Funny
    Writing another client daemon would not be a big job. This is at most a 200-line program. No doubt a Perl hacker could do it in one line - but I might lose my lunch from looking at that :-)

    Bruce

    --
    Bruce Perens.
  3. Re:Huh? by Bruce+Perens · · Score: 3, Funny
    Well, I've got to say that I switched from the stable 2.4 kernel to the unstable 2.6 one to use netconsole, and so far 2.6 hasn't crashed and I haven't been able to see how far into the panic I'll get the messages. The system is being slashdotted now, maybe that will help.

    Bruce

    --
    Bruce Perens.
  4. some conditions.. by molo · · Score: 4, Informative

    This only works/is helpful in certain situations:

    1. network driver is initialized
    2. kernel messages are what you want to capture
    3. the kernel doesn't get itself foobarred so badly that it can't send packets
    4. interrupts are enabled (right? can't talk to the network card otherwise)

    Also, if you change mac addresses or network cards of the first hop between the sender and receiver, this all needs to be reconfigured.

    This can certainly be useful, but recognize its limitations.

    -molo

    --
    Using your sig line to advertise for friends is lame.
    1. Re:some conditions.. by Bruce+Perens · · Score: 2, Interesting
      Yes. I really want a console server. I can't get one without paying a lot more or getting less for what I'm paying.

      Bruce

      --
      Bruce Perens.
    2. Re:some conditions.. by molo · · Score: 4, Informative

      Yes, serial console servers are quite useful for remote administration.

      Another option might be to build your own - take an older spare machine and stick a PCI USB controller in it and add a bunch of USB-to-serial dongle devices (possibly with a USB hub). These show up as /dev/ttyUSB#.

      You can then ssh in and use minicom or whatever. You get added security since many terminal servers don't support ssh, plus you have a fully configurable system to manage stuff. You could write a program to page/email you on certain kernel events for example. It could be pretty flexible if that is what you need.

      Just my two cents.

      BTW, Bruce, thanks for your contributions to the community and to Debian. We all appreciate it.

      -molo

      --
      Using your sig line to advertise for friends is lame.
    3. Re:some conditions.. by Paul+Jakma · · Score: 4, Informative

      1. Correct
      2. Correct, the purpose of netconsole being to capture kernel messages that otherwise would not be sent due to a hung kernel - all other messages, syslog can take of.
      3. Unlikely. If the kernel can panic, it can use netconsole. Used with nmi_watchdog, you can even get a stack trace from a stuck interrupt handler
      4. No, netconsole uses polling-mode and drives the nic driver directly.

      Note that netconsole can also dump the entire contents of RAM.

      --
      I use Friend/Foe + mod-point modifiers as a karma/reputation system.
    4. Re:some conditions.. by emag · · Score: 2, Insightful

      3. Ok, seemingly hard lockups are out - but these are more and more uncommon with 2.6 - but nearly everything else is dumpable, unless it screws with the NIC driver itself?

      Well, a friend of mine using 2.6 is experiencing random lockups (we suspect either hardware problems, or ghosts), with nothing ever logged. However, he can still ping the machine even after it's locked up, but the console also doesn't respond. I've pointed him at this article, and he's going to try enabling netconsole to see if anything from the kernel is getting lost. Hopefully it'll help figure out what the problem is.

      --
      "The urge to save humanity is almost always a false front for the urge to rule." --H.L. Mencken
  5. Same thing as sending syslog to remote loghost? by menscher · · Score: 2, Insightful
    I really don't see much benefit here. Seems like someone just took syslogd and made it a kernel module. Just one more piece of the kernel to crash. Personally I'd rather keep the kernel as small as possible. Less to go wrong that way.

    While on the topic, though, it would be nice if Linux did something similar to IRIX, where a crash would save the kernel coredump to the swap partition. Yes, I know the Linux Kernel Crash Dump project does this, but they're not exactly stable yet (hell, their website won't even load right now).

    1. Re:Same thing as sending syslog to remote loghost? by drdink · · Score: 4, Insightful

      This is a useful tool for those of us who have a slew of machines on a single network segment and don't want to have an expensive KVM or 47 monitors. The ability to put your consoles over the same wire you put your normal network traffic over is great. It is like those cable companies that also provide phone service over your cable line, sort of...
      IRIX isn't the only operating system that does crashdumps. I've found them to be an invaluable tool for debugging FreeBSD. It has supported them for quite a long time. The ability to gdb your kernel and the vm after you reboot and see exactly where things went wrong is great. I still don't understand why Linux lacks this.

      --
      Beware, Nugget is watching... See?
  6. Re:Tempting... by hattmoward · · Score: 4, Informative

    It looks like the packet format is syslog compatible in the recent 2.6 kernels. I think the work is already done. =)

  7. Re:Tempting... by hattmoward · · Score: 3, Informative

    Neither does the syslogd provided with slackware. The snippet below will allow you to inclusively list log hosts.

    iptables -A INPUT -s $LOGHOST -p udp --destination-port 514 -j ACCEPT
    iptables -A INPUT -p udp --destination-port 514 -j DROP

    You can repeat the first line as many times as needed, replacing $LOGHOST with an IP or a resolvable domain name. Just make sure the DROP rule goes last.

  8. For FreeBSDers, try ethercons by drdink · · Score: 4, Interesting

    If you use FreeBSD and find this interesting, you should take a look at rwatson's ethercons patches. Basically, it is this but supports bidirectional communication. That means you can run a getty on it and login over ethernet console. Rather slick.

    --
    Beware, Nugget is watching... See?
  9. Re:Tempting... by hattmoward · · Score: 4, Interesting

    It may not be as surprising when you note that sysklogd was written in a time where the network was considered more trustworthy, though there is another reason, which is mentioned below. You should also find the syslog-ng package available on your Debian system, which can filter by sending host, although it will spend more CPU time to do the blocking at that level. Both solutions work, though.

    The reason you don't see this feature made more prominent is that UDP packets are easily spoofed, since there is no handshake required to get the payload through. With TCP, someone has to be at the "sending" host to reply to the handshake, while UDP simply accepts what it receives in the single packet (barring the sender being a local-net host, with strict ARP checking on the receiver). So anyway, filtering hosts doesn't completely cover you from log spoofing or DOSing, but it's still a good idea.

  10. Re:Tempting... by Bruce+Perens · · Score: 2, Interesting
    That's a good answer, thanks! I had not thought about the ease of spoofing a UDP packet. I'm actually surprised that so far I am not losing any of them between Texas and California.

    If I had another machine in the same data center, I'd not be sending UDP over the internet.

    Thanks

    Bruce

    --
    Bruce Perens.
  11. Re:Oh I'll see them... by Bruce+Perens · · Score: 4, Funny
    My console is a 300 baud teletype you insensitive clod!

    THERE'S STILL LOTS OF LIFE LEFT IN MY 5-LEVEL BAUDOT MACHINE.

    BRUCE

    --
    Bruce Perens.
  12. That's A Phrase You Never Hear by RogL · · Score: 2, Funny

    Never heard this before, probably never hear it again:

    "The system is being slashdotted now, maybe that will help."