Miscellaneous UNIX notes

System errors defined

System errors are defined in /usr/include/sys/errno.h on Solaris systems and /usr/include/asm/errno.h on Red Hat Linux systems. The information in this file is helpful in interpreting output of the truss command (Solaris) or strace (Linux).

Example:
#define EPERM            1      /* Operation not permitted */
#define ENOENT           2      /* No such file or directory */
#define ESRCH            3      /* No such process */
#define EINTR            4      /* Interrupted system call */
#define EIO              5      /* I/O error */
#define ENXIO            6      /* No such device or address */
#define E2BIG            7      /* Arg list too long */
#define ENOEXEC          8      /* Exec format error */
#define EBADF            9      /* Bad file number */
#define ECHILD          10      /* No child processes */

Limiting find to one file system

The find command's -xdev argument can be used to limit searches to one file system.

Example: find all files on the root file system sorted smallest-to-largest. Do not descend other file systems (etc. /usr, /var).

find / -xdev -ls | sort -n -k 7

Disabling ssh1 compatibility with ssh.com server

ssh protocol 1 is vulnerable to man-in-the-middle attacks with tools like dsniff, and should not be used unless absolutely necessary.

To disable ssh protocol 1 with an ssh.com server,

1. Edit the /etc/ssh2/sshd2_config configuration file.

2.
Change:
        Ssh1Compatibility               yes
To:
        Ssh1Compatibility               no

3. Send the sshd process a SIGHUP for the change to take effect.

Zombie processes

A zombie process is a process that has exited, but whose exit code has not reached its parent process. The parent process has to perform a wait system call to read the exit code of a child. Until the parent receives the exit code, the child process will remain in "zombie" state.

Zombie processes are already dead and cannot be "killed." They consume no system resources except an entry in the system process table (seen in the proc-sz column with the sar -v command).

The only way to remove a zombie process is to kill its parent process.

More information:
http://groups.google.com/groups?q=zombie+processes+wait&hl=en&lr=&ie=UTF-8&selm=1993Feb14.021655.13721%40acd4.acd.com&rnum=9

/etc/hosts on Windows

The file on Windows that provides the same functionality as /etc/hosts in UNIX is %SystemRoot%\system32\drivers\etc\hosts

stty: : Invalid argument

This message is often caused when running stty in the C shell initialization script .cshrc with a non-interactive shell (ex. an ssh, scp, rsh, or rsync command). stty should only be run in an interactive shell.

Example change in .cshrc to check for an interactive shell:

Change:
stty erase ^?

To:
if ( $?prompt && { tty -s } ) stty erase ^?

xterm Xt error: Can't open display:

If you receive this message when tunneling X11 traffic over an ssh tunnel, (1) make sure that the remote ssh server allows X11 forwarding with the X11Forwarding yes directive in the server configuration file (OpenSSH example), and (2) make sure that you are enabling X11 forwarding on your ssh client with the -X flag.

[hutch@hutch hutch]$ ssh hutch@server
[hutch@server hutch]$ echo $DISPLAY

[hutch@server hutch]$

[hutch@hutch hutch]$ ssh -X hutch@server
[hutch@server hutch]$ echo $DISPLAY
localhost:10.0
[hutch@server hutch]$

X11 tunneling after su -

In order to run X clients over an ssh tunnel after running su - for a root login shell, you have to manually specify the DISPLAY and XAUTHORITY environment variables. These steps are not needed when running su, su -m, or su -p.

Example:

/bin/su -
DISPLAY=localhost:10.0 XAUTHORITY=~hutch/.Xauthority X_client
-- or --
export DISPLAY=localhost:10.0 XAUTHORITY=~hutch/.Xauthority
X_client

sftp problems

When attempting to login to an OpenSSH sftp server, I received the following error:

Request for subsystem 'sftp' failed on channel 0
Couldn't read packet: Connection reset by peer

When receiving this error, make sure to check the permissions of sftp-server. In this case, the permissions on the directory containing sftp-server were incorrect:

# grep sftp-server /usr/local/etc/sshd_config
Subsystem       sftp    /usr/local/libexec/sftp-server

# ls -ld /usr/local/libexec /usr/local/libexec/sftp-server
drwx------   2 root     other        512 Oct  7  2003 /usr/local/libexec
-rwxr-xr-x   1 root     other      28292 Oct  7  2003 /usr/local/libexec/sftp-server

To correct the problem:
chmod 755 /usr/local/libexec

X11 forwarding problems

When attempting to run an X client, I received the following errors:

debug1: X11 connection uses different authentication protocol.
X11 connection rejected because of wrong authentication.

In this case, the file system housing the user's home directory was full, resulting in a 0-byte ~/.Xauthority file. Freeing up space in the user's home directory fixed the problem.

sudo: must be setuid root

If you receive this error when executing sudo, first check to make sure that sudo is setuid root. A less obvious cause of this error is that sudo is located on a file system mounted nosuid. If this is the case, you will have to remount the file system suid if sudo is needed. Note that mount -o remount,suid file_system may not work; you may have to actually unmount the file system and remount it to fix the problem.

More information in this post.

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Last modified: 07/26/2005