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Binary package hint: ifupdown
release: Ubuntu 10.04 LTS (but probably all releases back to Hardy and forward
to Maverick)
package: ifupdown: Installed: 0.6.8ubuntu29
Since upgrading my systems to lucid and cleaning up the configurations,
I've noticed strange behaviour where client tcp or udp sessions to local
tcp/ip servers have timed out even though the servers were running and
netstat showed they were listening on all interfaces. Wireshark traces
showed that successful connections went to 127.0.0.1, while any
connection attempt to 127.0.1.1 would fail.
The systems are fully controlled by NetworkManager <offtopic>a long and painful
story!</offtopic> and so /etc/network/interfaces contains only two lines:
auto lo
iface lo inet loopback
This "minimal configuration" is not only highly recommended to avoid
wierd behaviour with NetworkManager, but also to avoid problems with
upstart (e.g.
https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/497299). All the
other network interfaces are managed by NM.
"In the old days", a typical /etc/hosts file would define all the local
hostnames as follows:
127.0.0.1 myhostname.localdomain myhostname localhost.localdomain
localhost
For several releases (back as far as Hardy), ubuntu installations have been
creating a more sophisticated version of the hosts file (presumably because of
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=316099), e.g.
127.0.0.1 localhost
127.0.1.1 myhostname.mydomain myhostname
This modern variant allows the system to always resolve its own hostname
and canonical name, even when there is no external network.
It appears to work really well... e.g. "ping hostname" and "ping
localhost" both succeed. "hostname" and "hostname -f" return the
expected values. (note: I don't fully understand why ping hostname
actually works - perhaps because the class A 127.0.0.0 subnet has been
defined for ever as a loopback subnet?).
The problem comes when a local client tries to connect to a local server... a
well-behaved server will listen on "all available network interfaces", e.g.
brian@myhostname:~$ netstat -ln | grep 53
tcp 0 0 10.1.252.115:53 0.0.0.0:* LISTEN
tcp 0 0 192.168.252.115:53 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:53 0.0.0.0:* LISTEN
udp 0 0 10.1.252.115:53 0.0.0.0:*
udp 0 0 192.168.252.115:53 0.0.0.0:*
udp 0 0 127.0.0.1:53 0.0.0.0:*
... so, when a client tries to connect to the local server (in my not
very realistic example you would have to edit /etc/resolv.conf to point
to myhostname instead of the default localhost), the nslookup connection
would never be received by the local bind server and so will eventually
time out.
If we are to keep the "new style" hosts file structure (and I think we
should), then we MUST ensure that local servers are able to access all
available interfaces, including BOTH of the loopback addresses mentioned
in the hosts file, so we need to define an additional interface for the
myhostname loopback address.
This can be easily demonstrated by defining a second (virtual) address for the
lo interface:
sudo ifconfig lo:0 127.0.1.1 netmask 255.0.0.0
Stop the server and it will discover the extra interface when it is restarted:
brian@myhostname:~$ netstat -ln | grep 53
tcp 0 0 10.1.252.115:53 0.0.0.0:* LISTEN
tcp 0 0 192.168.252.115:53 0.0.0.0:* LISTEN
tcp 0 0 127.0.1.1:53 0.0.0.0:* LISTEN
tcp 0 0 127.0.0.1:53 0.0.0.0:* LISTEN
udp 0 0 10.1.252.115:53 0.0.0.0:*
udp 0 0 192.168.252.115:53 0.0.0.0:*
udp 0 0 127.0.1.1:53 0.0.0.0:*
udp 0 0 127.0.0.1:53 0.0.0.0:*
Of course, this circumvention will only last until the next reboot. The
solution can be made permanent by updating the
/etc/networking/interfaces file to automatically bring up both loopback
interfaces early in the boot process:
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# automatically bring up both these interfaces at boot (ifup -a == all)
auto lo lo:0
# The standard loopback network interface
iface lo inet loopback
# another loopback interface for that pesky dual loopback hosts file
iface lo:0 inet static
address 127.0.1.1
netmask 255.0.0.0
I do not know if there is a more elegant solution to the same problem
(but remember that some of the server processes are started very early,
hence the upstart bug reference above).
If acceptable, this bug should be pushed upstream to encompass the other
distros that use the same multiple loopback interface /etc/hosts file
organisation.
** Affects: bind9 (Ubuntu)
Importance: Undecided
Status: Invalid
** Affects: ifupdown (Debian)
Importance: Unknown
Status: New
** Tags: patch udd-find
--
network servers do not listen on 127.0.1.1
https://bugs.launchpad.net/bugs/604283
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