When all debugging routes have failed: network scans and/or code tracing

In the world of car, bike and motorbike mechanics there is a versatile tool which is something of a last resort: the vice-grips (sometimes referred to as the bodger’s tool, because of people’s tendency to shear bolts with them).  In the world of operating systems there are two tools I have found to be like vice-grips, but not potentially harmful: Network scanning and code tracing.

Network scanning

Most operating systems have a way of scanning the network:

  • Linux: tcpdump, Wireshark
  • Darwin (MacOS): tcpdump, Wireshark
  • Solaris: snoop, tcpdump, Wireshark
  • Windows: Wireshark (there is also a version of tcpdump for Windows)

So, why is network scanning useful?  Well consider the situation where you have installed the monitoring software, Xymon.  The server is already working and most of the clients are responding, but the server isn’t receiving data from one of the clients.  Xymon uses port 1984 so you can check to watch the traffic going to and from the server:

[root@host1 etc]# tcpdump port 1984
tcpdump: verbose output suppressed, use -v or for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
15:08:00.857457 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: S 1387852790:1387852790(0) win 5840 <mss 1460,sackOK,timestamp 119364978 0,nop,wscale 2>
15:08:00.864380 IP xymonserver.linuxtech.ie.1984 > host2.linuxtech.ie.32821: S 3491816971:3491816971(0) ack 1387852791 win 5792 <mss 1460,sackOK,timestamp 8108268 119364978,nop,wscale 0>
15:08:00.864553 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: . ack 1 win 1460 <nop,nop,timestamp 119364993 8108268>15:08:00.865187 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: . 1:1449(1448) ack 1 win 1460 <nop,nop,timestamp 119364993 8108268>
15:08:00.865419 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: . 1449:2897(1448) ack 1 win 1460 <nop,nop,timestamp 119364994 8108268>
15:08:00.867342 IP xymonserver.linuxtech.ie.1984 > host2.linuxtech.ie.32821: . ack 1449 win 8688 <nop,nop,timestamp 8108268 119364993>
15:08:00.867486 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: P 2897:4345(1448) ack 1 win 1460 <nop,nop,timestamp 119364996 8108268>
15:08:00.867684 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: . 4345:5793(1448) ack 1 win 1460 <nop,nop,timestamp 119364996 8108268>
15:08:00.868361 IP xymonserver.linuxtech.ie.1984 > host2.linuxtech.ie.32821: . ack 2897 win 11584 <nop,nop,timestamp 8108268 119364994>
15:08:00.869032 IP host2.linuxtech.ie.32821 > xymonserver.linuxtech.ie.1984: . 5793:7241(1448) ack 1 win 1460 <nop,nop,timestamp 119364997 8108268>

So in this example the traffic is going from host1 to the Xymon server’s port, so it looks like Xymon is receiving the data.  What’s wrong is that DNS knows this host as host2.linuxtech.ie not host1 so Xymon doesn’t realise it’s receiving data for host1.  So there are a few solutions, for example you can configure host1 to explicitly tell Xymon that it is host1.

Another example was when I was trying to get some commercial software working in a firewall, where the DNS servers were locked down to resolve only addresses we allowed them too.  The documentation said that it would need to be able to resolve, say, swcheck.sweet.ie, but it still wasn’t working.  So gave it just one DNS server and watched what addresses it asked for and sure enough it was asking for swcheck.sweet.ie, but also for, say, dwnld.sweet.ie.  So I needed to make sure that was added to the list of addresses it could resolve.

Another nice thing about tcpdump in particular is its data can be saved to a file which can be imported into Wireshark on another server.  This is very handy if you have a sensitive host where you can’t run the GUI of Wireshark.

There’s a lot to this subject but I hope this helps.

Code tracing

When I say code tracing, I mean tracing system and library calls.  Most operating systems have a way to do this:

  • Linux: strace, ltrace
  • Darwin: dtruss, dtrace (both require root/sudo)
  • Solaris: truss, dtrace
  • Windows: (none that I can find)

In my opinion Linux has the best implementation of code tracing.  (Darwin/FreeBSD/Solaris’s DTrace  and Linux’s SystemTap are exceedingly powerful, but beyond the scope of this post.)  Suppose you want to see what environment variables a program is using:

[admin2@centos6 ~]$ ltrace -e getenv -o /tmp/tmp.adm2.ltrace vi
[admin2@centos6 ~]$ ls -l /tmp/tmp.adm2.ltrace
-rw-rw-r--. 1 admin2 admin2 1777 Oct 2 05:08 /tmp/tmp.adm2.ltrace
[admin2@centos6 ~]$ vim /tmp/tmp.adm2.ltrace
[admin2@centos6 ~]$ cat /tmp/tmp.adm2.ltrace
(0, 0, 0, 0x7fcf69b6d918, 88) = 0x3b6ec21160
getenv("HOME") = "/home/admin2"
getenv("VIM_POSIX") = NULL
getenv("SHELL") = "/bin/bash"
getenv("TMPDIR") = NULL
getenv("TEMP") = NULL
getenv("TMP") = NULL
getenv("VIMRUNTIME") = NULL
getenv("VIM") = NULL
getenv("VIM") = NULL
getenv("VIMRUNTIME") = "/usr/share/vim/vim72"
getenv("VIM") = "/usr/share/vim"
getenv("TERM") = "xterm"
getenv("COLORFGBG") = NULL
getenv("VIMINIT") = NULL
getenv("HOME") = "/home/admin2"
getenv("EXINIT") = NULL
getenv("HOME") = "/home/admin2"
(0x3b6ec21160, 0, 0, 0x3b6ec21160, 0) = 140608
(0, 0, 0, 3, 0x963cf85) = 0x3b6ec21160
+++ exited (status 0) +++
[admin2@centos6 ~]$

So consider you have a program which is reading a configuration file from somewhere but you can’t figure out where.  The best thing is to check its open() (which will cover fopen() too), stat() and lstat().  stat and lstat check existence, permissions etc. of a closed file.  So this example uses vi (even though the esteemed Meneer Bram Moolenaar has so extensively documented vim this is a redundant example):

[admin2@centos6 ~]$ strace -e stat,lstat,open -o /tmp/tmp.adm2.strace vi
[admin2@centos6 ~]$ cat /tmp/tmp.adm2.strace
open("/etc/ld.so.cache", O_RDONLY) = 3
open("/lib64/libm.so.6", O_RDONLY) = 3
open("/lib64/libselinux.so.1", O_RDONLY) = 3
open("/lib64/libncurses.so.5", O_RDONLY) = 3
open("/lib64/libacl.so.1", O_RDONLY) = 3
open("/lib64/libc.so.6", O_RDONLY) = 3
open("/lib64/libtinfo.so.5", O_RDONLY) = 3
open("/lib64/libdl.so.2", O_RDONLY) = 3
open("/lib64/libattr.so.1", O_RDONLY) = 3
open("/usr/lib/locale/locale-archive", O_RDONLY) = 3
stat("/usr/share/vim/vim72", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/usr/share/vim", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/home/admin2/.terminfo", 0x7fff4b7dbb00) = -1 ENOENT (No such file or directory)
stat("/etc/terminfo", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
stat("/usr/share/terminfo", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/usr/share/terminfo/x/xterm", O_RDONLY) = 3
open(".", O_RDONLY) = 3
stat("/etc/virc", {st_mode=S_IFREG|0644, st_size=1962, ...}) = 0
open("/etc/virc", O_RDONLY) = 3
open(".", O_RDONLY) = 3
stat("/home/admin2/.vimrc", 0x7fff4b7dd460) = -1 ENOENT (No such file or directory)
open("/home/admin2/.vimrc", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/admin2/_vimrc", O_RDONLY) = -1 ENOENT (No such file or directory)
open(".", O_RDONLY) = 3
stat("/home/admin2/.exrc", 0x7fff4b7dd460) = -1 ENOENT (No such file or directory)
open("/home/admin2/.exrc", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/etc/nsswitch.conf", O_RDONLY) = 3
open("/etc/ld.so.cache", O_RDONLY) = 3
open("/lib64/libnss_files.so.2", O_RDONLY) = 3
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
[admin2@centos6 ~]$

It is my belief that the true mastery of a skill is to take from the specific to the general and back to the specific again.  So these are specific examples of using these tools, which I hope gives you an insight into the general principles so you can apply them to your specific problems.

 

2 thoughts on “When all debugging routes have failed: network scans and/or code tracing

  1. lisleman

    Hi Philip – Your tool tips could be very handy. As the software/system onion adds layers it does help to know basic things to check in trouble shooting.
    I always forget which animal my MacOS is. I wish the “about this Mac” would use the animal name along with the version number. I’m at 10.6.8 right now which I think is a Leopard type.

    Reply
    1. Philip Kearns Post author

      Hi Bill,

      Thanks for your comment. You’re right about the MacOS animal name; 10.6.8 is Snow Leopard which, incidentally, is on reduced support so you should upgrade to Lion or Mountain Lion. Reduced support means you will get updates only for a subset of problems, e.g. major security issues with Java. Apple really should explicitly state when an OS is out of support, or under reduced support.

      Philip

      Reply

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