Is there any easy & fast way to add TCP options to the SKB packet like Window scaling factor or Time stamp with C on netfilter.
Or if any body has example it would be perfect to see.
Thank you
One of the fastest way is by using sysctl and following are various options, but as you are interested in window size and timestamp are below:
net.ipv4.tcp_slow_start_after_idle
net.ipv4.tcp_timestamps
The command to control them is:
sudo sysctl -w <option=123>
Otherwise you can control it programatically like the way timestamp is controlled either in software or hardware in this guide.
Related
I keep getting the SysRq HELP printout, basically seems i can send over serial the sysrq but it won't accept the next key within 5 seconds ie the command key (b) to reboot.
I need to send the command programmatically over serial console connection to reboot system.
I can reboot the system via echo b > /proc/sysrq-trigger and cat /proc/sys/kernel/sysrq is 1 (ie full sysrq is enabled)
But I notice that kernel (2.6.32) image I'm booting with only has CONFIG_MAGIC_SYSRQ=y and there's no mention of CONFIG_MAGIC_SYSRQ_SERIAL. I'd like to know if that setting is required for 2.6.32 or if it was "assumed enabled" and its only required in new kernels.
According to this, I don't need that in my kernel since it was only added to apparently optionally disable sysrq over serial to prevent unwanted triggers.
Anyway I don't really care if PERL is used or PYTHON or C code with tcsendbreak or any programmatic method to send alt-sysrq-b over /dev/ttyUSB0 to reboot linux over serial. So far all i can do is send break sequence and see output of:
SysRq : HELP : loglevel(0-9) reBoot Crash terminate-all-tasks(E) memory-full-oom
l-active-cpus(L) show-memory-usage(M) nice-all-RT-tasks(N) powerOff show-registe
-blocked-tasks(W)
But the command key sent afterward never does anything. So I'm not sure what's wrong. FYI, the system I'm trying to send sysrq over serial to is an embedded linux system that boots via uboot with uimage and dtb file.
Instead of using a break signal I would prefer a technique where the code actually sends the Alt-SysRq-b keyboard keys over the serial console connection.
Using wpa_supplicant 2.4 on ARM Debian.
Is there a way to get signal level, in decibels or percents, of the wireless network I’m currently connected to?
STATUS command only returns the following set of values: bssid, freq, ssid, id, mode, pairwise_cipher, group_cipher, key_mgmt, wpa_state, ip_address, p2p_device_address, address, uuid
I can run SCAN afterwards, wait for results and search by SSID. But that’s slow and error-prone, I'd like to do better.
The driver should already know that information (because connected, and adjusting transmit levels for energy saving), is there a way to just query for that?
This question is not about general computing hardware and software. I'm using wpa_supplicant through a C API defined in wpa_ctrl.h header, interacting with the service through a pair of unix domain sockets (one for commands, another one for unsolicited events).
One reason I don’t like my current SCAN + SCAN_RESULT solution, it doesn’t work for hidden SSID networks. Scan doesn’t find the network, therefore I’m not getting signal level this way. Another issue is minor visual glitch at application startup. My app is launched by systemd, After=multi-user.target. Unless it’s the very first launch, Linux is already connected to Wi-Fi by then. In my app’s GUI (the product will feature a touch screen), I render a phone-like status bar, that includes WiFi signal strength icon. Currently, it initially shows minimal level (I know it's connected because STATUS command shows SSID), only after ~1 second I’m getting CTRL-EVENT-SCAN-RESULTS event from wpa_supplicant, run SCAN_RESULT command and update signal strength to the correct value.
On the API level my code is straightforward. I have two threads for that, both call wpa_ctrl_open, the command thread calls wpa_ctrl_request, the event thread has an endless loop that calls poll passing wpa_ctrl_get_fd() descriptor and POLLIN event mask, followed by wpa_ctrl_pending and wpa_ctrl_recv.
And here's the list of files in /sys/class/net/wlan0:
./mtu
./type
./phys_port_name
./netdev_group
./flags
./power/control
./power/async
./power/runtime_enabled
./power/runtime_active_kids
./power/runtime_active_time
./power/autosuspend_delay_ms
./power/runtime_status
./power/runtime_usage
./power/runtime_suspended_time
./speed
./dormant
./name_assign_type
./proto_down
./addr_assign_type
./phys_switch_id
./dev_id
./duplex
./gro_flush_timeout
./iflink
./phys_port_id
./addr_len
./address
./operstate
./carrier_changes
./broadcast
./queues/rx-0/rps_flow_cnt
./queues/rx-0/rps_cpus
./queues/rx-1/rps_flow_cnt
./queues/rx-1/rps_cpus
./queues/rx-2/rps_flow_cnt
./queues/rx-2/rps_cpus
./queues/rx-3/rps_flow_cnt
./queues/rx-3/rps_cpus
./queues/tx-0/xps_cpus
./queues/tx-0/tx_maxrate
./queues/tx-0/tx_timeout
./queues/tx-0/byte_queue_limits/limit
./queues/tx-0/byte_queue_limits/limit_max
./queues/tx-0/byte_queue_limits/limit_min
./queues/tx-0/byte_queue_limits/hold_time
./queues/tx-0/byte_queue_limits/inflight
./queues/tx-1/xps_cpus
./queues/tx-1/tx_maxrate
./queues/tx-1/tx_timeout
./queues/tx-1/byte_queue_limits/limit
./queues/tx-1/byte_queue_limits/limit_max
./queues/tx-1/byte_queue_limits/limit_min
./queues/tx-1/byte_queue_limits/hold_time
./queues/tx-1/byte_queue_limits/inflight
./queues/tx-2/xps_cpus
./queues/tx-2/tx_maxrate
./queues/tx-2/tx_timeout
./queues/tx-2/byte_queue_limits/limit
./queues/tx-2/byte_queue_limits/limit_max
./queues/tx-2/byte_queue_limits/limit_min
./queues/tx-2/byte_queue_limits/hold_time
./queues/tx-2/byte_queue_limits/inflight
./queues/tx-3/xps_cpus
./queues/tx-3/tx_maxrate
./queues/tx-3/tx_timeout
./queues/tx-3/byte_queue_limits/limit
./queues/tx-3/byte_queue_limits/limit_max
./queues/tx-3/byte_queue_limits/limit_min
./queues/tx-3/byte_queue_limits/hold_time
./queues/tx-3/byte_queue_limits/inflight
./tx_queue_len
./uevent
./statistics/rx_fifo_errors
./statistics/collisions
./statistics/rx_errors
./statistics/rx_compressed
./statistics/rx_dropped
./statistics/tx_packets
./statistics/tx_errors
./statistics/rx_missed_errors
./statistics/rx_over_errors
./statistics/tx_carrier_errors
./statistics/tx_heartbeat_errors
./statistics/rx_crc_errors
./statistics/multicast
./statistics/tx_fifo_errors
./statistics/tx_aborted_errors
./statistics/rx_bytes
./statistics/tx_compressed
./statistics/tx_dropped
./statistics/rx_packets
./statistics/tx_bytes
./statistics/tx_window_errors
./statistics/rx_frame_errors
./statistics/rx_length_errors
./dev_port
./ifalias
./ifindex
./link_mode
./carrier
You can get the signal level of the connected wifi by wpa_supplicant cmd SIGNAL_POLL
The wpa_supplicant would return:
RSSI=-60
LINKSPEED=867
NOISE=9999
FREQUENCY=5745
The value of the RSSI is the signal level.
You can get the signal level of the connected wifi by wpa_supplicant cmd BSS <bssid>.
About the bssid of the connected wifi, you can get from wpa_supplicant cmd STATUS.
https://android.googlesource.com/platform/external/wpa_supplicant_8/+/622b66d6efd0cccfeb8623184fadf2f76e7e8206/wpa_supplicant/ctrl_iface.c#1986
For iw compatible devices:
Following command gives the current station(aka AP) signal strength:
iw dev wlp2s0 station dump -v
If you need C API, just dig the source code of iw.
After a quick glance, the function you need is here
For broadcom devices, try search broadcom wl. It is close source, don't know if C API is provided.
I am using v4l2-ctl from command line to change exposure values of usb camera but I cannot change the device from built in webcam
When I am using v4l2-ctl d /dev/video1 - it gives no error but it does nothing at all
You might be using the wrong cmd.
First of all, you need to specify -d to select a different device (mind the --prefix; it is missing in the Q).
but simply running v4l2-ctl -d /dev/video1 will not do anything with the device (you don't specify what to do)
So you also need to tell v4l2-ctl to change the exposure-time (or whatever you want to do) with the -c <ctrl>=<val> switch
So your command should look like:
v4l2-ctl -d /dev/video1 -c exposure_absolute=3000
but then, your device simply may not support setting the exposure time and simply ignore any requests (it should not announce support for setting the exposure if it cannot change it, but often device drivers are a bit easygoing)
my program get events from remote systems, every event contains an timestamp.
I want to log this events to syslog using the event timestamp instead of systemtime.
Is there any way to send a custom header to syslog deamon ?
I'm using rsyslog on debian
EDIT:
The "events" are generated by some "bare-metal" devices.
My application is a gateway between a realtime-ethernet (EthernetPOWERLINK) and a normal network.
I want to save them in micro-second precision, because its important to know in wich sequence they are occoured.
So i need the exact timestamp created by the bare-metal devices.
I'like to put this events into syslog.
I did not found any lib (except syslog.h) to write into syslog).
I really need to build the packages myself and send them to rsyslog deamon ?
No, don't open that can of worms.
If you allow the sender to specify the timestamp, you allow an attacker to spoof the timestamps of events they wish to hide. That kind of defeats the entire purpose (security-wise) of using a separate machine for logging.
What you can do, however, is compare the current time and the timestamp, and include that at the start of every logged message, using something like
struct timespec now;
struct timespec timestamp;
double delta;
int priority = facility | level;
const char *const message;
delta = difftime(timestamp.tv_sec, now.tv_sec)
+ ((double)timestamp.tv_nsec - now.tv_nsec) / 1000000000.0;
syslog(priority, "[%+.0fs] %s\n", delta, message);
On a typically configured Linux machine, that should produce something similar to
Jan 18 08:01:02 hostname service: [-1s] Original message
assuming the message took at least half a second to arrive. If hostname has its clock running fast, the delta would be positive. Normally, the delta is zero. In the case of a very slow network, the delta is negative, since the original event happened in the past relative to the timestamp shown.
If you already have infrastructure in place to monitor the logged messages, you can have a daemon or a cron script read the log files, and generate new log files (not via syslog(), but simply with string and file operations) with the timestamps adjusted by the specified delta. However, that must be done with extreme care, recognizing unacceptable or unexpectedly changing deltas, or maybe flagging them somehow.
If you write your log file monitoring/display widgets, then you can very easily let the user switch between "actual" (syslog) or "derived" (syslog + delta) timestamps, as the delta is trivial to extract from the logged lines if always present; even then, you must be careful to let the user know if a delta is out of bounds or changes unexpectedly, as such a change is most always informative to the user. (If it is not nefarious, it does mean there is something iffy with the machine timekeeping; time should not just jump around. Even NTP adjustments should be quite smooth.)
If you insist on opening that can of worms, just produce your own log files. Many applications do. It's not like syslog() was a magic bullet or a strict requirement for reliable logging, after all.
If your log-receiving application runs as a specific user and group, you can create /var/log/yourlogs/ owned by root user and that group, and save your log files there. Set the directory mode to 02770 (drwxrws--- or u=rwx,g=rwxs,o=), and all files created in that directory will automatically be owned by the same group (that's what the setgid bit, s, does for directories). You just need to make sure your service sets umask to 002 (and uses 0666 or 0660 mode flags when creating log files), so that they stay group-readable and group-writable.
Log rotation (archiving and/or deleting old log files, mailing logs) is usually a separate service, provided by the logrotate package, and configured by dropping a service-specific configuration file in /etc/logrotate.d/ at installation time. In other words, even if you write your own log files, do not rotate them; use the existing service for this. It makes life much easier for your users, us system administrators. (Note: Setting umask 002 at the start of the log rotate scripts is very useful in the above directory case; created files will then be group-writable. umask 022 will make them group-read-only.)
Ok've solved this, by enabling networking support (TCP) and micro seconds timer in rsyslog configuration.
Accroding to RFC 5424 my application build raw syslog messages and sends them via TCP (port 514) to the deamon.
Thanks to Nominal Animal, but i've no choice...
You can write a raw log message to the /dev/log file. This is a Unix domain socket from where the syslog server reads the messages, as they are written with the syslog() function.
I'm not sure about portability since the message format written by syslog() does not seem to follow the RFC 5424. I can only share my findings with busybox and its syslogd and nc utilities.
syslog() function writes messages as datagrams in the form <PRI>Mon DD HH:MM:SS message, where PRI is a priority, i.e. a decimal number computed as facility | severity, followed by a timestamp and a message.
With nc -u local:/dev/log, you can write UDP datagrams to the domain socket directly. For example, writing <84>Apr 3 07:27:20 hello world results in a Apr 3 07:27:20 hostname authpriv.warn hello world line in /var/log/messages.
Then you are free to extend the timestamp with the microseconds precision. Anyway, you need to make sure your syslog server implementation accepts such form. In case of busybox, I had to modify the source code.
Note: Busybox needs to be configured with enabled CONFIG_NC_EXTRA, CONFIG_NC_110_COMPAT and CONFIG_FEATURE_UNIX_LOCAL options to allow for opening /dev/log with nc.
I am trying a program in c that controls keyboard handler to blink NUMLOCK & CAPSLOCK LED's as a reaction of ctrl+alt+del push... please help me..
I kinda agree with KP. This is funny...
But if yer serious...
First:
There's a setleds program that might help you get started. It's been around for ages... Try man setleds.
Also, xset can be used (under X-windows) to change leds... (You may have to see which leds are enabled for changing in the X-config file.)
Second:
Detecting ctrl+alt+del is more of an issue as it's flagged specially by init. Look in /etc/inittab or /etc/init/control-alt-delete.conf or someplace like that, and you'll see lines like:
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -h now
Or:
# control-alt-delete - emergency keypress handling
#
# This task is run whenever the Control-Alt-Delete key combination is
# pressed. Usually used to shut down the machine.
start on control-alt-delete
exec /sbin/shutdown -r now "Control-Alt-Delete pressed"
So you'd have to disable that... Or simply have it run your keyboard-blink program rather than /sbin/shutdown.
Also, watch out for "Control-Alt-Backspace" -- Many X11 config setups enable this combination to shutdown the X server. (Option "DontZap".)
Third:
Now you need to find a way to pickup the control-alt-delete keypress. It's not impossible, but it may not be as simple as getc(). (The again, I could be wrong...)
Of course, if you don't want your program to have the keyboard focus. If you want this to happen while other programs are running in the foreground with keyboard focus... Well then yer looking at tweaking the kernel or some kernel driver. (Or having inittab run yer program instead of /sbin/shutdown.)
Any way you slice it, this is not a good Hello World type exercise.
Option:
Find the right place to trap Alt+Ctrl+Del and register a handler.
Use the KDGETLED/KDSETLED ioctl on /dev/console to changes the keyboard LEDs.
Good Luck!