Firefox bookmark keywords

No real equivalent exists in Chrome/ium.

SSH keys

A more convenient and secure alternative to passwords.

Many servers nowadays are configured to reject allow password authentication by SSH.

If you use Git on the command line with GitHub/GitLab/etc., you already have an SSH key!

SSH key generation

1. ssh-keygen -t ed25519

   (or -t rsa -b 4096 if you need RSA)
2. Default location is OK
3. Enter a passphrase

Your private key is in .ssh/id_ed25519 (KEEP THIS SAFE), and public key .ssh/id_ed25519.pub

SSH key generation

• On Windows >=10 1803, you can just use ssh-keygen.
• Alternatively, use PuTTY/PuTTYgen.

SSH key agents

A daemon that runs in the background and keeps your keys unlocked for use, so you don't have to keep keying in passwords.

Try:

eval $(ssh-agent)
ssh-add
ssh ...

Typically we configure our profile/bashrc/etc. to automatically start an agent if needed and include the environment variables in each shell.

SSH key agents

On Windows with PuTTY, use Pageant

SSH SOCKS proxy

SSH can act as a SOCKS proxy and let any application forward its connections.

• ssh -D 12345: open a SOCKS proxy locally, forward to remote
• ssh -R 12345: open a SOCKS proxy on the remote, forward to local

You could even use this with your browser, if you needed to e.g. quickly bypass a firewall.

Windows with PuTTY

SSH configuration

Set settings for certain hosts so you can avoid specifying username/other settings every time

Insert to .ssh/config: (for SoC students, this example is for Sunfire)

Host xcn?? xcn??? sunfire sunfire0
  HostName %h.comp.nus.edu.sg
  User soc_username_here

Then you can just ssh sunfire in future!

Other common host options: LocalForward RemoteForward ForwardX11 ProxyJump

SSH configuration

ProxyJump causes "ssh(1) to connect to the target host by first making a ssh(1) connection to the specified ProxyJump host and then establishing a TCP forwarding to the ultimate target from there."

In .ssh/config: (for SoC students: try this with the compute cluster)

Host xcn?? xcn???
ProxyJump soc_username_here@sunfire.comp.nus.edu.sg
User soc_username_here

Then ssh xcnc7, etc.

Windows RDP

Remotely access a Windows computer.

RDP client included with all Windows versions.

Officially only Pro and above editions can be connected to.

But see rdpwrap.

GDB

Command line debugger: gdb myprogram

Common commands:

• r: run program
• start: run and break at entrypoint
• c: continue program
• bt: show backtrace/stacktrace
• b line: set breakpoint at line
• d n: delete breakpoint n
• p expr: print the value of expr
• l: dump source around current line
• disas: dump machine code around current instruction
• help cmd: show help for cmd

GDB

Stepping commands:

• s: step one line
• si: step one instruction
• n: step one line, but don't follow calls
• ni: step one instruction, but don't follow calls
• finish: run until this function returns
• c: continue a paused program
• reverse-cmd: the above stepping commands, but in reverse; only after record
• record: start recording execution

Example program

#include <stdio.h>

int access(int *array, int index) {
    return array[index];
}

int main() {
    int arr[] = { 0, 1, 2 };
    printf("%d\n", access(0, 1));
}

Compile: gcc -g -o test test.c

Run: ./test

Segmentation fault ☹️

Debug it

$ gdb test
GNU gdb (GDB) 8.2.1
(gdb) r
Starting program: test

Program received signal SIGSEGV, Segmentation fault.
0x0000000100001168 in access (array=0x0, index=1) at test.c:4
4	    return array[index];
(gdb) bt
#0  0x0000000100001168 in access (array=0x0, index=1) at test.c:4
#1  0x00000001000011a7 in main () at test.c:9
(gdb) b 4
Breakpoint 1 at 0x100001154: file test.c, line 4.
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/angelsl/test

Breakpoint 1, access (array=0x0, index=1) at test.c:4
4	    return array[index];
(gdb) p array
$1 = (int *) 0x0
(gdb) p index
$2 = 1

Debug it in reverse!

$ gdb test
GNU gdb (GDB) 8.2.1
(gdb) start
...
(gdb) record
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000100001168 in access (array=0x0, index=1) at test.c:4
4	    return array[index];
(gdb) rs
4	    return array[index];
(gdb) rs
main () at test.c:9
9	    printf("%d\n", access(0, 1));
(gdb) rs
8	    int arr[] = { 0, 1, 2 };
(gdb) rs

No more reverse-execution history.
main () at test.c:7
7	int main() {
(gdb)

strace

Look at the syscalls a program is making

Example program:

#include <stdio.h>

int main() {
    printf("Hello world!\n");
}

Compile: gcc -g -o hello hello.c

strace

$ strace ./hello
execve("./hello", ["./hello"], 0x76f6e85d7ab0 /* 47 vars */) = 0
...
write(1, "Hello world!\n", 13)          = 13
exit_group(0)                           = ?
+++ exited with 0 +++

strace -f to follow child processes.

Syscalls are how programs interact with the operating system. Out of scope for today.

You can man 2 syscall to read the manpage for the syscall.

procfs

A virtual filesystem providing system and process information. Many tools we will mention take their information from procfs.

Some files in /proc:

• cpuinfo: lists CPU information
• cmdline: kernel command line
• config.gz: gzipped kernel config

A lot of kernel information is in /sys instead.

procfs

Some files in /proc/pid:

(pid can be self, for current process)

• cmdline: the command line provided to the process
• cwd: the process's current working directory
• exe: the executable being run
• environ: environment variables seen by the process
• status: process status
• fd/fd: open file descriptors

Typically we use tools like top, ps when we want to view process information.

dmesg

View the kernel log: dmesg

journalctl

View the system log (for systemd systems): journalctl

Common flags:

• -b: current boot only
• -k: kernel messages only (same as dmesg)
• -f: follow/tail the log
• -e: jump to end

/var/log

Log files of some applications: Xorg, CUPS, LightDM, ...

Also where systemd journal is stored (/var/log/journal)

tail

To tail a log file (or any file): tail -f file

Windows Event Viewer

lsof/fuser

List open files: lsof

• lsof file: list processes using file
• lsof -p pid: list files used by process pid

List processes using file: fuser file

• fuser -k file: Kill processes using file

Both tools are similar; lsof is more powerful, but fuser is slightly more common.

Windows: Process Hacker

Screenshots are from the website.

Windows: Disk Management

Also diskpart; out of scope

du

View disk usage by folders

Dropbox/ $ du -hd1
250M	./Screenshots
66M	./Backup
2.5G	./Notes
8.0M	./Work
20K	./.dropbox.cache
2.8G	.

• -h: human-readable sizes
• -d n: summarise to n levels
• -s: show only a total for each argument

Windows: WinDirStat

dstat

View system resource statistics

$ dstat
You did not select any stats, using -cdngy by default.
--total-cpu-usage-- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai stl| read  writ| recv  send|  in   out | int   csw
  9   5  85   1   0|  28k   43k|   0     0 |   0     0 | 738  3086
  4   3  90   2   0|   0     0 | 336k 3190B|   0     0 |1979  9753
  5   3  91   0   0|   0     0 | 238B  178B|   0     0 |1919  9188
  6   5  87   2   0|   0     0 | 571B  356B|   0     0 |1916  9077
  5   3  92   0   0|   0   108k|1465B    0 |   0     0 |1863  8952
  4   3  91   2   0|   0     0 | 238B  178B|   0     0 |1716  9049

Windows: Use Process Hacker

ss

View network connections: ss

• -t/-u: TCP/UDP only
• -4/-6: IPv4/IPv6 only
• -l: listening ports only
• -a: all states (listening and non-listening)
• -p: view process information

Windows: Process Hacker

Screenshots are from the website.

nslookup

Use a different nameserver

$ nslookup google.com 8.8.8.8
Server:		8.8.8.8
Address:	8.8.8.8#53

Non-authoritative answer:
Name:	google.com
Address: 74.125.24.138
Name:	google.com
Address: 2404:6800:4003:c01::65

dig

$ dig @8.8.8.8 google.com MX

;; ANSWER SECTION:
google.com.		323	IN	MX	10 aspmx.l.google.com.

;; Query time: 5 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sat Mar 30 02:28:35 +08 2019
;; MSG SIZE  rcvd: 147

What's the difference? nslookup uses the OS's resolver libraries; nslookup uses its own. They are otherwise used for the same purpose.

whois

Look up information on an IP or domain

$ whois nus.edu.sg

----------------------------------------------------------------------
 SGNIC WHOIS Server
----------------------------------------------------------------------

The following data is provided for information purposes only.

Registrar:   SINGNET PTE LTD

    Domain Name:		NUS.EDU.SG
    Creation Date:		03-Sep-1996 00:00:00
    Modified Date:		28-Aug-2018 09:31:22
    Expiration Date:		03-Sep-2019 00:00:00
    Domain Status:		OK
    Domain Status:		VerifiedID@SG-Not Required


    Registrant:

        Name:           NATIONAL UNIVERSITY OF SINGAPORE (SGNIC-ORGNA66731)

whois

$ whois 4.2.2.1

NetRange:       4.0.0.0 - 4.255.255.255
CIDR:           4.0.0.0/8
NetName:        LVLT-ORG-4-8
NetHandle:      NET-4-0-0-0-1
Parent:          ()
NetType:        Direct Allocation
OriginAS:
Organization:   Level 3 Parent, LLC (LPL-141)
RegDate:        1992-12-01
Updated:        2018-02-20
Ref:            https://rdap.arin.net/registry/ip/4.0.0.0