Bug 16118 - alarm syscall fails for tcpd's children under certain conditions when rfc931 is in use
alarm syscall fails for tcpd's children under certain conditions when rfc931 ...
Product: Red Hat Linux
Classification: Retired
Component: tcp_wrappers (Show other bugs)
i386 Linux
medium Severity medium
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Assigned To: Thomas Woerner
Depends On:
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Reported: 2000-08-13 17:16 EDT by waoki
Modified: 2007-04-18 12:28 EDT (History)
0 users

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Doc Type: Bug Fix
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Last Closed: 2004-10-07 09:57:41 EDT
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Description waoki 2000-08-13 17:16:39 EDT
On test systems, /etc/hosts.allow contains this line:

all:all: rfc931 5

which causes an ident lookup to be performed for all inbound TCP
connections that go through tcpd.

This is a program called alarm.c, used with time to test if alarm works:

int main () {
  while (1) {
    /* noop */

This is what normally happens:

$ /usr/bin/time ./alarm
Command terminated by signal 14
3.01user 0.00system 0:03.00elapsed 100%CPU (0avgtext+0avgdata
0inputs+0outputs (81major+11minor)pagefaults 0swaps

This is what happens when alarm() fails (program terminated with ^C after
~8 seconds):

$ /usr/bin/time ./alarm
Command terminated by signal 2
8.76user 0.00system 0:08.76elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (81major+11minor)pagefaults 0swaps

This behavior happens iff the program which uses alarm is a child process
of a process invoked via tcp_wrappers when rfc931 is in use, such as
in.telnetd through inetd, or that uses libwrap, such as sshd, where the
remote host (the client) ignores connections on port 113 (in my test case,
the client is behind a Cisco 675 ADSL router doing NAT).

This is a tcpdump of traffic generated when a user behind a 675
( tries to telnet in to a host (raven) using tcp_wrappers:

14:13:59.934475 > raven.2726 > S 489162589:489162589(0)
win 32120 <mss 1460,sackOK,timestamp 111563722 0,nop,wscale 0> (DF)
14:14:02.929819 > raven.2726 > S 489162589:489162589(0)
win 32120 <mss 1460,sackOK,timestamp 111564022 0,nop,wscale 0> (DF)

Note that nothing is sent in response to the inbound packets; they are
silently ignored. Upon recipt of packets trying to set up a connection,
rather than refusing it, the inbound packets are dropped on the floor.

According to an ltrace, this is the last time an alarm is set successfully:

[pid 13442] alarm(5)                              = 0
[pid 13442] htons(0, 5, 14, 0x0804a640, 0xbffffb1c) = 0
[pid 13442] htons(113, 0, 5, 14, 0x0804a640)      = 28928
[pid 13442] fileno(0x0804ebd0)                    = 3
[pid 13442] bind(3, 0xbfffb24c, 16, 113, 0)       = 0
[pid 13442] fileno(0x0804ebd0)                    = 3
[pid 13442] connect(3, 0xbfffb25c, 16, 0xbffffb1c, 0xbffffb20 <unfinished
[pid 529] --- SIGCHLD (Child exited) ---
[pid 529] wait3(0xbffffbc8, 1, 0, 19, 0)          = 13433
[pid 529] snprintf("/usr/sbin/tcpd (pid 13433)", 64, "%s (pid %d)",
"/usr/sbin/tcpd", 13433) = 26
[pid 529] wait3(0xbffffbc8, 1, 0, 19, 0)          = -1
[pid 529] breakpointed at 0x400c817d (?)
[pid 529] sigprocmask(0, 0x0804e2c0, 0, 0xbffffda8, 0x08049cb2) = 0
[pid 529] __errno_location()                      = 0x4010bd60
[pid 529] sigprocmask(2, 0x0804e340, 0, 0xbffffda8, 0x08049c93) = 0
[pid 529] select(19, 0xbffffd28, 0, 0, 0 <unfinished ...>
[pid 13442] --- SIGALRM (Alarm clock) ---

this activity is occuring when tcpd uses alarm to cause its ident attempt
to time out. (inetd is pid 529)

alarm works fine for users coming in via ssh or telnet from hosts that have
identd running or from hosts that reject connections to the ident port. It
only fails when hosts ignore the inbound connection attempt, causing the
connect to time out.

This behavior was first noticed on a RedHat 6.2 system with tcp_wrappers
7.6-10 when a daemon failed after being restarted remotely; it has also
been observed on two RedHat 5.1-derived systems (upgraded with individual
packages from later distributions and with custom packages) with
tcp_wrappers 7.6-4.
Comment 1 waoki 2000-08-14 20:26:59 EDT
To help replicate the bug when such beasts as a 675 are unavailable, the 

ipchains -A input -j DENY -p tcp --destination-port 113
ipchains -A output -j DENY -p tcp --source-port 113

will cause a system to ignore inbound connections to and outbound connections 
from port 113 and can be used on a client prior to telnetting to a server to 
trigger the bug.
Comment 2 Alan Cox 2002-12-14 20:06:24 EST
Looks like something is masking SIGALRM in the caller. Is this still the case in
newer kernels ?
Comment 3 waoki 2002-12-18 02:02:39 EST
It's still occuring as of 2.2.19. I don't have the opportunity to
upgrade any of my remaining Redhat machines to 2.4, so I can't test with
anything newer.

I don't want to touch the kernels on two of them, but I could probably
schedule downtime on the other to upgrade elsewhere in the 2.2 series.

It dosen't happen on my 2.4.x Debian boxes, but it also dosen't happen
on Debian boxes running 2.2.x, so I can't chalk it up to the kernel
version alone.

I've not thought about this bug for nearly two years, but looking back
at it, I'd start by comparing how tcp-wrappers and the kernel are built
the two machines, and I would tend to suspect tcp_wrappers more than the

The Debian tcp-wrappers package applies the patch
and then says 'make linux' to build.

I assume that you have details about how the Redhat tcp_wrappers is
built. The machine is currently running tcp_wrappers-7.6-4.

On one of the Redhat machines, the kernel was configured as per
http://www.umnh.utah.edu/temp-files/raven-config-2.2.19 .

On one of the Debian machines, the kernel was configured as per
http://www.g6net.com/tmp/oxygen-config-2.2.20 . On the other, the kernel
was configured as per
http://www.umnh.utah.edu/temp-files/phoenix-config-2.2.19pre17 .

I can dig up more configs, but it'll take time.

If I get a chance, I'll try to hunt down the options that RedHat builds
with and then try to tickle the bug on the Debian boxes by building with
those config options.

Comment 4 Thomas Woerner 2004-10-07 09:57:41 EDT
Please verify this with a newer version of Red Hat Enterprise Linux or
Fedora Core and reopen it against the new version if it still occurs.

Closing as "not a bug" for now.

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