vault backup: 2025-01-08 10:48:09

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diff --git a/3 resources/linux/Watch.md b/3 resources/linux/Watch.md
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+`watch (1)` periodically executes the executable on argv. It can be used to reload the output of a command, like `sudo watch ovs-dpctl dump-flows` .
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diff --git a/3 resources/linux/apt.md b/3 resources/linux/apt.md
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+reinstall packages:
+`apt reinstall <pkg>`
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diff --git a/3 resources/linux/bootloader.md b/3 resources/linux/bootloader.md
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+[[TODO]]
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diff --git a/3 resources/linux/device-drivers.md b/3 resources/linux/device-drivers.md
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+major, minor numbers
+major = driver (used by kernel when opening device to dispatch to correct driver)
+minor = used by driver to differentiate device(s)
+
+after disk the major, minor numbers are shown:
+```
+[jasras@n04.compute.vps2-lej1 ~]$ ls -l /dev/dm-*
+brw-rw---- 1 root disk 253,  0 Oct 25 10:10 /dev/dm-0
+brw-rw---- 1 root disk 253,  1 Oct 25 10:11 /dev/dm-1
+brw-rw---- 1 root root 253, 11 Oct 28 19:58 /dev/dm-11
+brw-rw---- 1 root root 253, 12 Oct 28 19:58 /dev/dm-12
+```
+
+available devices: `cat /proc/devices`
+
+character devices vs block devices; what is the difference?
+
+---
+[oreilly-device-drivers](https://www.oreilly.com/library/view/linux-device-drivers/0596000081/ch03s02.html)
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diff --git a/3 resources/linux/device-mapper.md b/3 resources/linux/device-mapper.md
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+Device mapper is a kernel driver to map physical block devices to virtual ones; this means that for example we can have many physical devices be represented by a single virtual one (linear mapping).
+
+How does it work?
+[[TODO]]
+What is its function in relation to multipath?
+
+Logical devices using device-mapper can be managed using `man 8 dmsetup`
+```
+[jasras@n04.compute.vps2-lej1 ~]$ sudo dmsetup info 3600a098038314d736724566a67346538
+Name:              3600a098038314d736724566a67346538
+State:             ACTIVE
+Read Ahead:        256
+Tables present:    LIVE
+Open count:        1
+Event number:      3
+Major, minor:      253, 12
+Number of targets: 1
+UUID: mpath-3600a098038314d736724566a67346538
+```
+
+If dmsetup cannot remove a device because a process still has it open, but lsof does not show any processes that open it, use `-f` which replaces the device with a fake that rejects all I/O.
+
+---
+[device-mapper](https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/7/html/logical_volume_manager_administration/device_mapper#device_mapper)
+[dmsetup](https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/7/html/logical_volume_manager_administration/dmsetup#dmsetup)
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diff --git a/3 resources/linux/filesystems/tmpfs.md b/3 resources/linux/filesystems/tmpfs.md
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+Is a [[virtual filesystem]] that is stored in memory; it uses both RAM and swap space.
+> Kernel option: CONFIG_TMPFS
+
+`mount -t tmpfs source target`
+
+A tmpfs can only grow to 50% of RAM avail. unless overridden with size=nbytes mount option. 
+Only consumes RAM for actual usage.
+Data is ephemeral/volatile.
+
+Mount stacking: mount over existing /tmp, to improve speed of apps using alot of tmp files, then unmount to delete data.
+
+Kernel uses tmpfs for [[system v shared memory]] and [[shared anonymous memory mappings]].
+`/dev/shm` or `/run/shm`: [[glibc]] of [[POSIX]] shared memory and [[POSIX]] semaphores.
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diff --git a/3 resources/linux/filesystems/virtual filesystem.md b/3 resources/linux/filesystems/virtual filesystem.md
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+++ b/3 resources/linux/filesystems/virtual filesystem.md
diff --git a/3 resources/linux/glibc.md b/3 resources/linux/glibc.md
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+++ b/3 resources/linux/glibc.md
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+The GNU C standard library.
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diff --git a/3 resources/linux/lsof.md b/3 resources/linux/lsof.md
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+```
+lsof -p 1111 # show open files for this PID
+lsof /path/to/file # show which process has this open
+```
+
+A usage in the wild; using lsof to find the qemu process for a specific base image and using that to get instance name and disk path:
+```
+lsof -F p {{ item['path'] }} | cut -b 2- | head -1 | xargs ps | grep -oP '(instance\\-[a-z0-9]+)|(\\/var\\/lib\\/nova\\/instances\\/[a-z0-9\\-]+\\/disk)' | uniq
+```
+
+(man lsof: search "OUTPUT FOR OTHER PROGRAMS")
+output for other programs
+```
+specify -F
+These are the fields that lsof will produce.  The single character listed first is the field identifier.
+            a    file access mode
+            c    process command name (all characters from proc or
+                 user structure)
+            C    file structure share count
+            d    file's device character code
+            D    file's major/minor device number (0x<hexadecimal>)
+            f    file descriptor (always selected)
+            F    file structure address (0x<hexadecimal>)
+            G    file flaGs (0x<hexadecimal>; names if +fg follows)
+            g    process group ID
+            i    file's inode number
+            K    tasK ID
+            k    link count
+            l    file's lock status
+            L    process login name
+            m    marker between repeated output
+            M    the task comMand name
+            n    file name, comment, Internet address
+            N    node identifier (ox<hexadecimal>
+            o    file's offset (decimal)
+            p    process ID (always selected)
+            P    protocol name
+            r    raw device number (0x<hexadecimal>)
+            R    parent process ID
+            s    file's size (decimal)
+            S    file's stream identification
+            t    file's type
+            T    TCP/TPI information, identified by prefixes (the
+                 `=' is part of the prefix):
+                     QR=<read queue size>
+                     QS=<send queue size>
+                     SO=<socket options and values> (not all dialects)
+                     SS=<socket states> (not all dialects)
+                     ST=<connection state>
+                     TF=<TCP flags and values> (not all dialects)
+                     WR=<window read size>  (not all dialects)
+                     WW=<window write size>  (not all dialects)
+                 (TCP/TPI information isn't reported for all supported
+                   UNIX dialects. The -h or -? help output for the
+                   -T option will show what TCP/TPI reporting can be
+                   requested.)
+            u    process user ID
+            z    Solaris 10 and higher zone name
+            Z    SELinux security context (inhibited when SELinux is disabled)
+            0    use NUL field terminator character in place of NL
+            1-9  dialect-specific field identifiers (The output
+                 of -F? identifies the information to be found
+                 in dialect-specific fields.)
+
+```
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diff --git a/3 resources/linux/man pages.md b/3 resources/linux/man pages.md
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+I need to more consistently read man pages. Example is `man task` to see how taskwarrior works, after reading it today I found that it is actually quite easy.
+They often contain good information on how to use a certain tool.
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diff --git a/3 resources/linux/networking/dhcp.md b/3 resources/linux/networking/dhcp.md
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+Dynamic Host Configuration Protocol
+Allows hosts dynamically receive an IP address.
+
+[[TODO]] learn exactly how it works
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diff --git a/3 resources/linux/networking/ip-masquerade-nat.md b/3 resources/linux/networking/ip-masquerade-nat.md
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+A function in the kernel that allows guests with internal/private IPs to access the internet by using the hosts' public IP similar to 1 to many NAT.
+
+Differences with Proxy, IP Masq, NAT:
+
+**Proxy**
+Requires special configuration on internal client side. Can be used for caching.
+
+**IP Masq**
+A form of NAT used primarily in Linux devices. Translates IP address and uses the connection tracking table and ports to map responses back to the internal connection.
+
+**NAT**
+Has different types with different use-cases.
+Static NAT: 1-1 mapping of pub/private IP. Dynamic NAT: pool of pub ips to dynamically map private IPs to based on availability. 
+PAT (Port): similar to IP Masq; 1-many using ports.
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diff --git a/3 resources/linux/networking/nbp.md b/3 resources/linux/networking/nbp.md
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+Network Bootstrap Program
+
+Is a bootloader that is obtained via the network typically via [[TFTP]]. It handles setting up an environment in which an OS can run or install. For example, it displays a boot menu and downloads the kernel images. It sets up hardware, for example the [[NIC]]s.
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diff --git a/3 resources/linux/networking/pxe-boot.md b/3 resources/linux/networking/pxe-boot.md
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+PXE = Preboot Execution  Environment. 
+
+PXE boot process:
+1. Modified [[DHCP]] request: request to include boot information (PXE boot request).
+2. Process DHCP response which includes an IP address of [[TFTP]] server and filename of [[NBP]].
+3. Download NBP over TFTP.
+4. NBP executes
+	1. Loads additional stuff over network, like kernel images.
+	2. Handles the boot menu
+5. Kernel takes over boot process and potentially downloads additional files required to complete booting.
+
+Popular NBP's:
+- PXELINUX
+- iPXE
+- GRUB2
diff --git a/3 resources/linux/networking/udp.md b/3 resources/linux/networking/udp.md
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+++ b/3 resources/linux/networking/udp.md
diff --git a/3 resources/linux/nic.md b/3 resources/linux/nic.md
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+++ b/3 resources/linux/nic.md
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+Network Interface Card
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diff --git a/3 resources/linux/shared anonymous memory mappings.md b/3 resources/linux/shared anonymous memory mappings.md
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+[[TODO]]
+Linux API book: chapter 49
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diff --git a/3 resources/linux/system v shared memory.md b/3 resources/linux/system v shared memory.md
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+++ b/3 resources/linux/system v shared memory.md
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+[[TODO]]
+Linux API book chapter 48
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diff --git a/3 resources/linux/tftp.md b/3 resources/linux/tftp.md
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+++ b/3 resources/linux/tftp.md
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+Trivial File Transfer Protocol
+[[TODO]]
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