TIL: You can make HTTP requests without curl using Bash /dev/TCP

167

mmrshu about 4 hours ago 90 commentsRead Article on mareksuppa.com

⚡ Community Insights

Discussion Sentiment

85% Positive

Analyzed from 2260 words in the discussion.

Discussion (90 Comments)Read Original on HackerNews

basilikum•about 4 hours ago

> As it turns out, bash can speak HTTP by itself.

No, it can not. Bash lets you open TCP sockets.

What you are doing here is trying to speak HTTP yourself, which is fine for testing and debugging, and hella cool for fun to do by hand, but you will shoot yourself in the foot if you try to use this pseudo http client unattended in reality. This toy code does not parse HTTP properly and will break.

You could of course write a full http/1.1 client in bash, you can even do a full http server in pure bash: https://github.com/bahamas10/bash-web-server

For less insane, non-bash shells there is always nc which is usually probably the wiser choice.

iam-TJ•about 1 hour ago

Need to be clear that "full http server in pure bash" is incorrect. Bash cannot listen on a TCP/UDP socket for incoming connections.

bash-web-server project builds a C language socket listener [0] that is dynamically loaded at run-time as a "built-in" module that makes the functionality available.

[0] https://github.com/bahamas10/bash-web-server/tree/main/loada...

a-dub•about 4 hours ago

it's not that insane. i've been manually typing http requests in since before http/1.1 and the mandatory host header.

it is insane to use it for anything serious (also the opposite, implementing webservers in bash), but for quick testing it's pretty great!

bitmasher9•about 3 hours ago

Why wouldn’t you use curl for the quick test?

hnav•about 3 hours ago

Sometimes you want to do something that curl cannot express, e.g. timing, protocol oddities, etc. For example you may want to issue a CONNECT to an echo server through a proxy and observe the bytes flowing back and forth. You may want to see what happens when conflicting hop-by-hop headers are specified without worrying about the client's (curl's) interpretation of them. A simple nc -c (or openssl s_client -crlf) lets you do all of that.

asmnzxklopqw•about 1 hour ago

Because curl is not installed in minimal docker images.

a-dub•about 2 hours ago

because in those days there was no curl, or wget. and then when there was, there was no guarantee they'd be installed.

telnet was always there though. it also worked for speaking all the other plaintext internet protocols. (imap, pop, smtp, etc)

andelink•about 2 hours ago

Nice parameter expansion examples in that bash-web-server. It uses the $_ parameter in ways I hadn’t thought to before, often preceded by a single : ${x} line for pre-processing of the variable.

mrshu•about 4 hours ago

> No, it can not. Bash lets you open TCP sockets.

Very fair pushback -- I did get carried away and will update the article to be more precise. Thanks for raising it!

> For less insane, non-bash shells there is always nc which is usually probably the wiser choice.

For completeness, `nc` or any netcat equvialent I could think of was not available in the image I was trying this with. It would certainly be a better option though.

bearjaws•about 3 hours ago

This is the most Claude pilled comment I've seen here.

thih9•about 3 hours ago

This worries me. Some AI writing styles became mainstream; at first it was the em-dashes, now it’s “A, not B” patterns and excessive acknowledging. There will be more.

Was grandparent comment written by an LLM?

Or is this a human who copies a style they saw in a blog post, unaware that they’re copying an AI?

Or is this a human who spent too much time talking to an AI and now they just talk like this?

Or is this an organic human response and we’re all paranoid by now?

I don’t know which would be worse.

mrshu•about 2 hours ago

It's pretty rough to learn I sound like Claude. Will need to do something about it then.

(For what it's worth I did write the message above manually but I understand why no one would believe that now. At least I did not call netcat "load-bearing" [https://mareksuppa.com/til/load-bearing/] or something...)

nialv7•about 3 hours ago

what would be a non-pilled way of saying the same thing?

scubbo•about 1 hour ago

FWIW, I didn't read this as AI-like. Even on a re-read, it's only the quasi-em-dash, and _maybe_ the polite acknowledgement of "Very fair pushback" (just good etiquette, IMO!) that would ring any alarm bells. You're fine.

throwrioawfo•about 3 hours ago

Bro really replaced the em-dash with "--"

mrshu•about 2 hours ago

An old habit that unfortunately makes one indistinguishable from LLMs these days...

TZubiri•about 3 hours ago

>No, you can't write 10 lines of code, you have to import a 100k LOC dependency

Common misconception, if you want to replace a dependency on a swiss knife you don't need to implement a swiss knife, sometimes you can just implement the last helix of the corkscrew.

cyanydeez•about 2 hours ago

it's curious what you'd be building where you think you can hit the reliability of curl with a bash script.

pillmillipedes•about 2 hours ago

a script ten lines long perhaps?

TZubiri•about 1 hour ago

health check, check that website/webapp returns 4xx and some known keyword

api, GET url, content-type aplication/json, parse json

you can even invert it and make a server

morpheuskafka•about 3 hours ago

> No, it can not. Bash lets you open TCP sockets.

I thought you had to use a program called netcat for that--if not then what is the point of that binary? And for that matter, can't you also use telnet to manually send HTTP?

some_random•about 3 hours ago

nc is basically just a nicer interface for the same thing, in the same way that curl is.

https://linux.die.net/man/1/nc

xenadu02•about 2 hours ago

As a kid in the late 90s my mind was blown when I realized I could telnet to port 80, 25, or 110 and interact with the servers manually.

Simple get: GET / HTTP/1.1 Content-Type: text/html User-Agent: l33t hax0rs lol X-Funny-Monkey: farts

For sending a mail message on port 25: HELO mail-from: whoever@whatever.com mail-to: sysadmin@yaya.com <other headers> <blank line> Body of the message yay. <two blank lines to end>

POP3 was so long ago I forgot but you could list the mailboxes then get individual messages and so on.

This revelation was the beginning of "there is no magic" for me. The realization that every part of the computer was built by human beings and was at some level understandable if one undertook the effort.

Perhaps most people in the future won't bother. They'll just let agents do it all. I'm sure that will leave some interesting holes in various systems for people willing to actually learn how they work without the filter of a model (or its safety rails).

kps•about 1 hour ago

Last century I would read and send personal email from work using telnet to pop3 and smtp respectively.

jazz9k•about 1 hour ago

When I was 12, I learned about open SMTP relays and how to spoof email this way. I once spoofed an email between two rivals on a community I was a part of and started a flame war.

Good times.

sejje•21 minutes ago

I once made an enemy on AOL and he was a spammer--he put my email in the from: field and I got a lot of hostile emails.

But the joke's on him--it led directly to me meeting a lifelong friend & mentor.

mrshu•about 4 hours ago

I ran into this while checking connectivity between containers on an internal Docker network where the image had neither curl nor wget.

The main surprise was that Bash has /dev/tcp which lets you do the equivalent of an HTTP request with a bit of shell magic, for instance:

  exec 3<>/dev/tcp/service/8642
  printf 'GET /health HTTP/1.1\r\nHost: service\r\nConnection: close\r\n\r\n' >&3
  cat <&3

Where `service` is just the hostname of whatever you’re talking to and 8642 is the port you are trying to talk HTTP to.

Pretty cool!

sevenzero•about 4 hours ago

It seems pretty cool, but I am wondering if there's any drawback on just using images that support curl? I can't think of any and to me it's kinda a must have, even on production images

OptionOfT•about 3 hours ago

I always recommend to not have any dependencies outside of the code.

So we start at compiling the codebase (Rust) against MUSL. That way we can run it with FROM scratch images.

If we need more tooling available at runtime, then we look at alpine, but still using MUSL.

If MUSL itself is proving problematic, or if some of the libraries we use need glibc then we can look at using some locked down image.

The cool part about FROM scratch images is that you'll never have to update your base image to address CVEs. Only your software and its (compiled) dependencies.

xmodem•about 3 hours ago

> The cool part about FROM scratch images is that you'll never have to update your base image to address CVEs. Only your software and its (compiled) dependencies.

What's the benefit really, though? If you still need to be able to rapidly deploy a new image in response to a dependency CVE, what have you gained?

monkpit•37 minutes ago

You might not have any say on what image is in use, for example, in a cicd library project.

xmodem•about 3 hours ago

More than one ~500 employee company I've worked at has had security policies either encouraging or requiring the use of "distro-less" images - images with no OS components other than the absolute minimum required to run the application. For go binaries this meant literally nothing in the container apart from the executable.

In theory it has a couple of benefits. You don't have to re-deploy your image to patch CVE's in OS components if you don't have any OS components. And it provides some measure of defence-in-depth - one could certainly theory-craft a scenario where an attacker gains some limited control over your application and then uses some OS component to escalate.

These days if a security engineer is proposing my team adopt distro-less containers to receive these benefits, I would point out that we need to weigh them against the very real drawbacks of not having standard debugging tools available where and when we need them. And also to consider the relative impact of other defence-in-depth measures they could be pursuing instead - such as any sort of network policy to limit network traffic.

NewJazz•about 1 hour ago

Debug containers are a thing.

Add an ephemeral container to an already running pod, for example to add debugging utilities without restarting the pod.

https://kubernetes.io/docs/reference/kubectl/generated/kubec...

mrshu•about 4 hours ago

That is indeed a solid pushback! :)

For what its worth, this container used `python:3.12.2-slim-bookworm` and I really would not expect that sort of an image to bundle `curl` -- even if it is intended for production.

TZubiri•about 2 hours ago

You can also use the sockets lib in that case, you depend on POSIX instead of Linux

sevenzero•about 4 hours ago

Ah I see so it was basically a minimal image that bundles just python? I can see why it wouldn't bundle curl! Thought it was a custom Image for some reason, hence my original comment

figmert•about 4 hours ago

This of course only supports http, not https. It's great for health checks e.g. in a docker environment. To do https, you'd have to use something like socat, but of course that doesn't use bash only.

TZubiri•about 2 hours ago

Https is almost always terminated separately from the application code.

giobox•about 4 hours ago

It's also a two line Dockerfile to add wget or curl to almost any pre-existing container image. This is a fun idea though.

simonw•about 3 hours ago

Neat, works against example.com

  exec 3<>/dev/tcp/example.com/80
  printf 'GET / HTTP/1.1\r\nHost: example.com\r\nConnection: close\r\n\r\n' >&3
  cat <&3

Outputs:

  HTTP/1.1 200 OK
  Date: Tue, 16 Jun 2026 17:37:45 GMT
  Content-Type: text/html
  ...

I always end up on example.com for this kind of thing because there are so few domains these days that don't enforce https!

QuantumNomad_•about 3 hours ago

example.com is also great for that reason when something fails about a captive portal on a public WiFi.

I open my web browser and go to http://example.com and get redirected to the captive portal page again and retry completing what they need from me to get internet access.

some_random•about 3 hours ago

Fun fact, this is almost exactly how active portal detection is done in the OS/browser!

https://gist.github.com/skull-squadron/edb8c0122f902013304c0...

QuantumNomad_•about 2 hours ago

Yep :) I just find example.com easier to remember and quicker to type than any of the OS or browser makers own URLs like

- http://captive.apple.com/

- http://connectivitycheck.gstatic.com/generate_204

- http://detectportal.brave-http-only.com/

Plus, it feels nice to depend on the reserved domain name example.com instead of relying on a domain that any one specific corporation has to maintain :D

LeoPanthera•about 2 hours ago

I use neverssl.com for this purpose because it is designed to resist caching.

gabrielsroka•about 3 hours ago

This works too

  exec 3<>/dev/tcp/example.com/80
  printf 'GET / HTTP/1.1\r
  Host: example.com\r
  Connection: close\r
  \r
  ' >&3
  cat <&3

You can even take out the \r though they should be there

chaps•21 minutes ago

Once had a coworker tell me to never to use this because "you never know when the customer doesn't have bash installed; use python instead" even though our contract required that the customer had bash. I'm still laughing at that.

pgtan•about 1 hour ago

It is a KornShell feature since ca. 1997

https://github.com/ksh93/ast-open-archive/blame/master/src/c...

timwis•43 minutes ago

You could also use nsenter if curl is installed on the host, eg

docker inspect -f '{{.State.Pid}}' container-name

# let's imagine that outputs 814538

nsenter -t 814538 -n curl example.com

sam_lowry_•about 3 hours ago

A few years ago I had to do this for a SpringBoot health check from a Docker container:

FROM openjdk:11-jre-slim HEALTHCHECK --start-period=10s --timeout=3s --retries=5 \ CMD perl -e "use IO::Socket; $sock = IO::Socket::INET->new(Proto => 'tcp', PeerAddr => 'localhost', PeerPort => '8888') or die $@; $sock->autoflush(1); print $sock 'GET /actuator/health HTTP/1.1' . chr(0x0a) . chr(0x0d) . 'Host: localhost:8888' . chr(0x0a) . chr(0x0d) . 'Connection: close' . chr(0x0a) . chr(0x0d) . chr(0x0a) . chr(0x0d); while (my $line = $sock->getline ) { if ($line =~ /UP/) {exit;} }; close $sock; exit 1;"

hn92726819•about 3 hours ago

Note that this is not what the article is about. Bash has a fake /dev/tcp path that opens sockets. What you have there is just perl opening a socket normally. Great solution, but the interesting bit is that fake path.

saidinesh5•about 2 hours ago

Fun story: A few years ago, I worked for a small company that customized off the shelf routers to enable businesses provide Wifi Hotspots.

The routers were very basic model with very limited flash memory (~4MB?). I was brought in to build firmware for those routers. I ended up customising openwrt - removed all kinds of packages to make their packages fit on those routers. At the end, I had less than 4KB space, And I needed to implement a "heart beat" service. A lot of routers were behind firewalls that only allowed http, https and a couple of other protocols. Libcurl was too heavy. So I ended up writing a shell script that used this feature of bash to send out heart beats.

Fun times...

dchest•about 3 hours ago

It's interesting that most of the comments here are about using this feature to bypass security restrictions (whether valid or not). It says a lot about the attack surface of GNU utilities caused by featuritis.

AndrewStephens•about 4 hours ago

This is pretty neat if all you need is to ping a local server but please use curl (or something equivalent) for contacting remote services. HTTP1.1 seems like such a simple protocol but in the real world you need to deal with proxies, different encodings, and redirects. Curl takes care of that (and a host of other annoying stuff) for you.

mrshu•about 4 hours ago

Totally!

I was really just trying to see if intra-container connectivity works, and this ended up being a very quick way of doing so. (The alternative being building and deploying a new image, which would likely take significantly longer.)

KomoD•about 3 hours ago

> The alternative being building and deploying a new image, which would likely take significantly longer

You said the image was Python, though? Using that is way easier and faster. https://news.ycombinator.com/item?id=48558763

If all you need to know is that it can connect:

python3 -c 'import socket as s;s.create_connection(("8.8.8.8",53))'

or http:

python3 -c 'from urllib.request import*;print(urlopen("http://example.com").status)'

mrshu•about 2 hours ago

You are right, I am not sure why I did not realize Python is the whole point of the image. This is indeed much faster and easier.

nesarkvechnep•about 1 hour ago

I find /dev/udp much more useful. I can create aliases for fire and forget commands to my daemons without actually writing *ctl program.

washbasin•about 2 hours ago

This is an old post-compromise trick used when an attacker needs to download a payload or make a network connection and curl, wget and nc are all not available.

Retr0id•about 2 hours ago

It's a fun trick, but I really don't like that bash does this. It's such an un-clean interface, and I'm not aware of any use cases beyond trying to exfiltrate data from a badly locked-down shell.

orthogonal_cube•about 3 hours ago

It was fun exploring this to make a native-shell-only peer-to-peer file transfer utility at work for some automation scripts. At least, it was until trying to replicate it in Powershell was somehow triggering Crowdstrike and the corporate Cybersecurity team thought I was writing malware.

geoctl•about 3 hours ago

I discovered this bash trick by chance when I was once trying to healthCheck the Envoy's official OCI image container which didn't include curl or wget while forcing the envoy admin interface to listen on localhost which breaks the traditional k8s httpGet checks.

devsda•about 3 hours ago

Yes, it used to be my goto few times when some devices tried to lockdown everything with bare minimum core utils and no network capable tools like curl etc.

m3047•about 2 hours ago

At least on my systems there's also /dev/udp...

sc68cal•about 4 hours ago

That's pretty neat, thanks for sharing

alienbaby•about 3 hours ago

Reminds me of telnetting to port 80 to make a get request years and years ago

Steeeve•about 3 hours ago

brb. recompiling bash in all my base images.

alienbaby•about 3 hours ago

Reminds me of using telnet to port 80 to make get requests aeons ago