DEV Community: Li

Use proxychains in WSL

Li — Sat, 23 Dec 2023 15:49:32 +0000

A WSL distro changes its IP and gateway address every time it restarts, so using proxychains would be a painful experience because the proxychains config file can not use domain names. The proxy runs on the host, forwarding traffic to a remote server.

So a script to dynamically generate proxychains config file would be helpful.

Finally, I figure out how to do shift + ins on HP laptop

Li — Fri, 10 Mar 2023 15:34:19 +0000

HP ELITEBOOK laptop combines F10 and insert keys together. It will kill you if you just want to do a shift + ins for pasting something in terminals. Fn + F10, Win + F10, shift + Win + F10, … , none of those work.

Finally, I figure out how to do a shift + ins. Just

right shift + Fn + E

Note that left shift is combined with fn lock, so you must use right shift.

As for ctrl + ins, just use ctrl + Fn + E, both ctrl keys work.

Get the host address for WSL2

Li — Fri, 03 Mar 2023 08:54:27 +0000

For WSL2, the ip address of a running distro is not static, neither the subset WSL2 uses is fixed. If you need to do something like accessing the services on the WSL2 host, you need to get the address by yourself.

Get the ip address the current running distro:

ip -4 -br route get 8.8.8.8 | head -n 1|awk '{print $7}'

Get the ip address the host(the gateway):

ip -4 -br route get 8.8.8.8 | head -n 1|awk '{print $3}'

And get the netmask:

ifconfig eth0 |grep netmask|awk '{print $4}'

Convert String to CStr and back in rust

Li — Wed, 10 Aug 2022 09:31:09 +0000

Just some code snippets to convert rust String to std::ffi::CStr/std::ffi::CString and back.

String to CString/CStr

use std::ffi::CStr;
use std::ffi::CString;

fn main() {
   let s = "Hello World!".to_string();
   let c_string: CString = CString::new(s.as_str()).unwrap();
   let c_str: &CStr = c_string.as_c_str();
   println!("{:?}", c_str);
}

CStr to String

use std::ffi::CStr;

fn main() {
   let c_str: &CStr = CStr::from_bytes_with_nul(b"Hello World!\0").unwrap();
   let s: String = c_str.to_string_lossy().into_owned();
   println!("{:?}", s);
}

Sending real-time server logs to remote server with nc

Li — Thu, 07 Jan 2021 06:15:32 +0000

Remote debugging is hard, especially when you can't ssh into remote server, monitor the log generating. Maybe it is the server inside an office, or the security policy of your client doesn't allow remote login. But still, we can use nc to deliver the logs, real-timely. No complex setup required, just a little more than the beloved tail -f command.

nc(netcat) should be available in most the servers, so additional software are not required(and maybe your client doesn't have the permissions to install new software).

Only two steps, first, on your own server, you may setup a server to listen for connections:

nc -lvk <PORT_NUMBER> | tee remote.log

Second, just let your client use the nc to sync the logs to your server, and now you get logs.

tail -f server.log | nc <SERVER_IP> <PORT_NUMBER>

Actually, even there is no nc at your client's server. You may actually send the by the following command

tail -f server.log > /dev/tcp/<SERVER_IP>/<PORT_NUMBER>

And remember to close the nc service when are done.

About DoesNotExist Exception with Django ForeignKey

Li — Wed, 16 Sep 2020 04:50:05 +0000

For a model defined in Django，consider, a foreign key field pointed to User:

class Article(Model):
    title = CharField()
    content = TextField()
    user = ForeignKey(to=get_user_model())

If article is an instance of Article, article.user would get a User instance(base on get_user_model()) based on the foreign key defined.

If you happen to a project where the data consistency is not well maintained, e.g., the admin had deleted several old users despite the data dependency. When calling article.user, you may get a DoesNotExist error, mostly look like

DoesNotExist: User matching query does not exist.

From the source of Django, I found a comment in db/models/fields/related_descriptors.py:

# Assuming the database enforces foreign keys, this won't fail.

This means when dealing with the consistency of ForeignKey, Django tends to simply raise an error and leave it to the database admin. But in some cases, if we can resolve article.user as a None object, it would be easier for the code to handle, or more compatible with other serializers/validators.

I monkey-patched the module with the following code:

from django.db.models.fields.related_descriptors import ForwardManyToOneDescriptor

def get_object(self, instance):
    qs = self.get_queryset(instance=instance)
    # Assuming the database enforces foreign keys, this won't fail.
    return qs.filter(self.field.get_reverse_related_filter(instance)).first()

ForwardManyToOneDescriptor.get_object = get_object

The code replaces query_set.get with query_set.first, no exceptions raised but return a None when object does not exist. By the way, when if the uniqueness of the foreign key object can be assured, e.g. the foreign key is the primary key in the table related, query_set.first is faster than query_set.get, since it stops scanning the table on the first row matched.