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Odi's astoundingly incomplete notes

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Relaying UDP broadcasts

iptables -t mangle -A INPUT -i eth0 -d -j TEE --gateway
The above iptables rule copies broadcast traffic received on the eth0 network interface to another network interface (the one whose broadcast address is Note that this is one-way only. We can't add a second rule for the other direction without creating an infinite packet loop. We need to play tricks with the TTL for that!

Incoming broadcast packets typically have a TTL of 64 or 128. TEE uses the kernel function nf_dup_ipv4() to copy the packet, which already decrements the TTL if the rule is in INPUT or PREROUTING. Note that a packet with TTL=0 will still be accepted by the destination, but will no longer be routed. But TEE itself does not check for TTL=0 and happily copies such packets. So we need to prevent that too, since what we do is effectively routing.

The improved rule adds TTL sanity check:
iptables -t mangle -A INPUT -i eth0 -d -m ttl --ttl-gt 0 -j TEE --gateway
If we want to add a rule for the other direction as well...
iptables -t mangle -A INPUT -i eth1 -d -m ttl --ttl-gt 0 -j TEE --gateway
then we easily create a packet loop, since the copy of a packet on eth0 will now also match the rule on eth1. To prevent that we need to ensure that the copied packet has TTL=0. We can do that by simply setting the TTL=1 of all incoming broadcasts before passing them to TEE. Then no more loops should occur. The complete rule set for merging a broadcast domain across networks is then:
iptables -t mangle -A INPUT -i eth0 -d -m ttl --ttl-gt 0 -j TTL --ttl-set 1
iptables -t mangle -A INPUT -i eth1 -d -m ttl --ttl-gt 0 -j TTL --ttl-set 1
iptables -t mangle -A INPUT -i eth0 -d -m ttl --ttl-gt 0 -j TEE --gateway
iptables -t mangle -A INPUT -i eth1 -d -m ttl --ttl-gt 0 -j TEE --gateway
Make sure to monitor your broadcast traffic to detect any misconfiguration after that change:
tcpdump -vnpi eth0 ip broadcast

posted on 2018-12-18 18:36 CET in Code | 0 comments | permalink

preventing CUPS password prompt

Add Unix groups to CUPS:
SystemGroup lpadmin root
Set permissions to access the local secret:
chgrp -R lpadmin /run/cups/certs
This directory contains a secret that is read by CUPS utilities like cupsenable. They pass that secret in HTTP Authorization headers to the local HTTP socket when sending command.

Add your user to that group:
usermod -a -G lpadmin myuser

posted on 2018-10-29 15:48 CET in Code | 0 comments | permalink

Be bloody careful with CNAME records

Be careful when doing something stupid!

CNAME records are useful. Especially together with external hosting services.
You run your domain and control DNS yourself but you host a website on an external service. They manage the IP of that site and give you a name that may look like examplesite383.hostingprovider.biz and already points to the correct IP. You want to map it into your DNS zone with a nice name like www.fancyproduct.com so creating a CNAME that points tha name www to your hosting provider's name is a practical way to go. You also want to do the same without the www prefix, but mind you! Creating a CNAME for fancyproduct.com would redirect the complete zone to a different one!

Never create a CNAME record for the zone name!

Also when caches start picking up the SOA information of the wrong zone, they apply the TTL values of that zone. So getting control of your zone back may not be easy and is not under your control anymore.

Also any secondary DNS server will pickup the zone redirect and may completely stop updating the zone from your authorative server. You need to manually fix that on the secondary DNS!

posted on 2018-10-02 10:01 CEST in Code | 0 comments | permalink

When Oracle does not register with the listener

If your Oracle database does not register with the listener, you get ORA-12505. Normally the DB registeres automatically after a while. But you can force it to do that.

However, if it did not register automatically, chances are that this does not help either.

Check the listener status:
lsnrctl status
Check the listener's log file (filename is in the status output).

Check which listener the DB wants to register with:
select name, value from v$parameter where name in ('local_listener', 'remote_listener');
This gives you a TNS name. Check that this TNS name is defined in $ORACLE_HOME/network/admin/tnsnames.ora Such an entry looks like:
LISTENER_SID = (ADDRESS = (PROTOCOL = TCP)(HOST = oracle.example.com)(PORT = 1521))

posted on 2018-09-12 15:09 CEST in Code | 0 comments | permalink

ORDImage leaks files in /tmp

In Oracle the following call leaks an imageio file in /tmp. Not nice.

ORDImage.processCopy(pImageBlob, 'maxscale='100 100', destBlob);

posted on 2018-08-29 13:38 CEST in Code | 0 comments | permalink

GUI code is prone to memory leaks

Have you ever seen GUI code that has no leaks? Me neither.

posted on 2018-08-20 11:14 CEST in Code | 0 comments | permalink

Synology load

Are they sure they want to run all sorts of crap as root without any limits?
top - 15:12:11 up 12 min,  1 user,  load average: 98.10, 40.27, 17.04
Tasks: 271 total, 112 running, 159 sleeping,   0 stopped,   0 zombie
%Cpu(s): 17.0 us, 81.7 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.7 hi,  0.6 si,  0.0 st
GiB Mem :    0.113 total,    0.008 free,    0.066 used,    0.039 buff/cache
GiB Swap:    2.000 total,    1.801 free,    0.199 used.    0.006 avail Mem

  PID USER      PR  NI    VIRT    RES %CPU %MEM     TIME+ S COMMAND                                                             
 8000 root      20   0   61.2m   4.8m  1.1  4.2   0:00.79 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get                       
 8014 root      20   0   61.2m   4.8m  1.1  4.2   0:00.79 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get                       
 8020 root      20   0   61.2m   4.8m  1.1  4.2   0:00.80 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get                       
 8022 root      20   0   61.2m   4.8m  1.1  4.2   0:00.78 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get                       
 8024 root      20   0   61.2m   4.8m  1.1  4.2   0:00.78 R synoscgi_SYNO.Core.Package.Server_1_get_timestamp                   
 8025 root      20   0   61.2m   4.8m  1.1  4.2   0:00.85 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get                       
 7994 root      20   0   61.2m   4.8m  1.1  4.2   0:00.74 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get                       
 7995 root      20   0   61.2m   4.8m  1.1  4.2   0:00.85 R synoscgi_SYNO.Core.Package.Thumb.Server_1_get    

posted on 2018-08-16 15:16 CEST in Code | 1 comments | permalink
.. well, as far as they do (sometimes?) not really know what they are doing, it seems so.
This is why synology nas no longer are my favourites and I am not willing to recommend them.

Gentoo intel-microcode reorg

Gentoo has given more love to its sys-firmware/intel-microcode package. It's now easier than ever to update microcode automatically early on boot. This is a valuable alternative to shipping microcode within the BIOS. I mean honestly, who regularly checks for BIOS updates? Much easier by doing that through an ebuild.

The new USE flag initramfs now builds a cpio archive at /boot/intel-uc.img ready to be used by grub. In /boot/grub/grub.cfg:
menuentry 'Gentoo Linux 4.14' {
  linux /boot/linux-4.14.12 root=LABEL=ROOT ro rootfstype=ext4 net.ifnames=0
  initrd /boot/intel-uc.img /boot/initrd.img
Note how the microcode intitramfs is simply prepended to the boot initramfs (initrd). A kernel that has microcode laoding support enabled will find it there, upload the microcode into the cpu and then discard the initramfs blob, and continue booting with the initrd.img. The first line in your dmesg output will show:
microcode: microcode updated early to revision 0x80, date = 2018-01-04

posted on 2018-01-15 10:10 CET in Code | 0 comments | permalink

Gentoo enables PIE

Gentoo has new profiles that require you to "recompile everything". That is technically not really necessary. Only static libraries really need recompiling.

Here is why:
A static library is just an archive of .o files (similar to tar), nothing more, and linking against a static library is roughly the same as just adding more .o files to the linker line. You can also link a static library into a shared library - the code in the static library is then just copied into the shared library (but the code then must be compiled with -fPIC, as with all other code that is used in shared libraries).

You can find static libs like so:
equery b $(find /usr/lib/ /lib/ -name *.a) | awk '{ print $1; }' | sort | uniq
Typically this yields packages like elfutils, libbsd, nss, iproute2, keyutils, texinfo, flex, db, numactl.

posted on 2017-12-07 11:59 CET in Code | 0 comments | permalink

pulseaudio: xmalloc.c: Assertion 'size < (1024*1024*96)' failed

I noticed that Firefox was not playing any sound any more. Apparently pulseaudio stopped working. Manually running pa showed the following error:

xmalloc.c: Assertion 'size < (1024*1024*96)' failed at /var/tmp/portage/media-sound/pulseaudio-10.0/work/pulseaudio-10.0/src/pulse/xmalloc.c:72, function pa_xmalloc0(). Aborting.

This means it is trying to allocate a ridiculous amount of memory.

Looking at the backtrace in gdb reveals:
(gdb) bt
#0  0x00007ffff6a5ef50 in raise () from /lib64/libc.so.6
#1  0x00007ffff6a60bfa in abort () from /lib64/libc.so.6
#2  0x00007ffff7914b32 in pa_xmalloc0 () from /usr/lib64/libpulse.so.0
#3  0x00007ffff7ba1db1 in pa_database_open () from /usr/lib64/pulseaudio/libpulsecore-10.0.so
#4  0x00007fffeed60468 in module_card_restore_LTX_pa__init () from /usr/lib64/pulse-10.0/modules/module-card-restore.so
#5  0x00007ffff7b5ac98 in pa_module_load () from /usr/lib64/pulseaudio/libpulsecore-10.0.so
#6  0x00007ffff7b49751 in ?? () from /usr/lib64/pulseaudio/libpulsecore-10.0.so
#7  0x00007ffff7b4fd2c in pa_cli_command_execute_line_stateful () from /usr/lib64/pulseaudio/libpulsecore-10.0.so
#8  0x00007ffff7b50551 in pa_cli_command_execute_file_stream () from /usr/lib64/pulseaudio/libpulsecore-10.0.so
#9  0x0000000000406e55 in main ()
So it is trying to read some database. What database?

strace tells us quickly:
open("/home/xoxo/.pulse/91b2f1e2678a89a9c38b83075061a39a-card-database.x86_64-pc-linux-gnu.simple", O_RDONLY|O_CLOEXEC) = 9
So likely this thingy is corrupt. Solved this with:

rm -rf .pulse

posted on 2017-12-01 15:05 CET in Code | 0 comments | permalink
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