new post: ssh-keyscan: fdlim_get: bad value
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_posts/2023-06-20-ssh-keyscan-fdlim-get-bad-value.md
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_posts/2023-06-20-ssh-keyscan-fdlim-get-bad-value.md
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permalink: "/{{ year }}/{{ month }}/{{ day }}/ssh-keyscan-fdlim-get-bad-value"
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title: "ssh-keyscan: fdlim_get: bad value"
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published_date: "2023-06-20 15:00:00 +0200"
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layout: post.liquid
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data:
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route: blog
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excerpt: |
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`ssh-keyscan: fdlim_get: bad value` - That's the error message I got the other day when I was trying out some project.
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The web was incredibly useless in telling me what the hell was going wrong here.
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So I set out to find why this was happening, how to fix it and hopefully make this error message findable on the web.
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And this is the story how I found a type confusion bug in some 20-year old OpenSSH code.
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---
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```
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ssh-keyscan: fdlim_get: bad value
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```
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That's the error message I got the other day when I was trying out some project.
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The web was incredibly useless in telling me what the hell was going wrong here.
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So I set out to find why this was happening, how to fix it and hopefully make this error message findable on the web.
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And this is the story how I found a type confusion bug in some 20-year old OpenSSH code.
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## What is `ssh-keyscan`?
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`ssh-keyscan` is a small utility to "gather SSH public keys from servers" and part of the OpenSSH package (see [the man page][manpage]).
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The one that (most likely) provides you the SSH client and server.
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You run it like this:
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```text
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$ ssh-keyscan github.com
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# github.com:22 SSH-2.0-babeld-dca4d356
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github.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEmKSENjQEezOmxkZMy7opKgwFB9nkt5YRrYMjNuG5N87uRgg6CLrbo5wAdT/y6v0mKV0U2w0WZ2YB/++Tpockg=
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<snip>
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```
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and get all public keys from that host.
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Or if you only need a specific type you pass that:
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```text
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$ ssh-keyscan -t ed25519 github.com
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# github.com:22 SSH-2.0-babeld-dca4d356
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github.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOMqqnkVzrm0SdG6UOoqKLsabgH5C9okWi0dh2l9GKJl
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```
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[manpage]: https://manpages.debian.org/bookworm/openssh-client/ssh-keyscan.1.en.html
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## What went wrong?
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Running it on my freshly booted M1 MacBook errors out:
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```
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$ ssh-keyscan github.com
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ssh-keyscan: fdlim_get: bad value
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$ echo $?
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255
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```
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Yeah, not particularly helpful.
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What's `fdlim_get`? What bad value did it encounter?
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Is this a Mac problem? Or a problem in ssh-keyscan?
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So I tried from two of my Linux machines. No issues.
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## What's the code?
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`fdlim_get` is a function in the `ssh-keyscan` code base in OpenSSH.
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You can find it [in `ssh-keyscan.c` on GitHub](https://github.com/openssh/openssh-portable/blob/b4ac435b4e67f8eb5932d8f59eb5b3cf7dc38df0/ssh-keyscan.c#L129-L144).
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It's supposed to get the maximum and current limit for file descriptors the program can use.
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The error we're seeing is [from a `fdlim_get(1)` call further down in that file][fdlim_get_call].
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[fdlim_get_call]: https://github.com/openssh/openssh-portable/blob/b4ac435b4e67f8eb5932d8f59eb5b3cf7dc38df0/ssh-keyscan.c#L830-L832
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```c
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maxfd = fdlim_get(1);
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if (maxfd < 0)
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fatal("%s: fdlim_get: bad value", __progname);
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```
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Time to compile my own `ssh-keyscan`, so I can modify and debug it:
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```
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git clone https://github.com/openssh/openssh-portable
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cd openssh-portable
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autoreconf
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./configure --with-ssl-dir=/opt/homebrew/Cellar/openssl@1.1/1.1.1u
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make ssh-keyscan
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```
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And now I can run it locally:
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```
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$ ./ssh-keyscan
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usage: ssh-keyscan [-46cDHv] [-f file] [-O option] [-p port] [-T timeout]
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[-t type] [host | addrlist namelist]
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```
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That was surprisingly easy.
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Let's dive into the code and try to understand it:
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```c
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static int
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fdlim_get(int hard)
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{
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#if defined(HAVE_GETRLIMIT) && defined(RLIMIT_NOFILE)
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struct rlimit rlfd;
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if (getrlimit(RLIMIT_NOFILE, &rlfd) == -1)
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return (-1);
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if ((hard ? rlfd.rlim_max : rlfd.rlim_cur) == RLIM_INFINITY)
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return SSH_SYSFDMAX;
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else
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return hard ? rlfd.rlim_max : rlfd.rlim_cur;
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#else
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return SSH_SYSFDMAX;
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#endif
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}
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```
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Using some printf-debugging is a quick way to see some of those values.
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Adding the following lines right after the `getrlimit` call should tell me more:
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```c
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printf("int size=%lu\n", sizeof(int));
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printf("type size=%lu\n", sizeof(typeof(rlfd.rlim_max)));
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printf("rlfd.rlim_max=%llu\n", rlfd.rlim_max);
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printf("rlfd.rlim_cur=%llu\n", rlfd.rlim_cur);
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printf("RLIM_INFINITY=%llu\n", RLIM_INFINITY);
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printf("SSH_SYSFDMAX=%ld\n", SSH_SYSFDMAX);
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```
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After a `make ssh-keyscan` and `./ssh-keyscan github.com` cycle I get:
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```
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$ ssh-keyscan github.com
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int size=4
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type size=8
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rlfd.rlim_max=9223372036854775807
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rlfd.rlim_cur=9223372036854775807
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RLIM_INFINITY=9223372036854775807
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SSH_SYSFDMAX=9223372036854775807
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ssh-keyscan: fdlim_get: bad value
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```
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Remember the `fdlim_get(1)` call and check [later][fdlim_get_call] looked like this:
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```c
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maxfd = fdlim_get(1);
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if (maxfd < 0)
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fatal("%s: fdlim_get: bad value", __progname);
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```
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And `fdlim_get` is defined to return an `int`, which is only 4 byte wide (that's 32 bit).
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What's the biggest number one can fit into an int?
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```c
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printf("INT_MAX=%d\n", INT_MAX);
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```
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```
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INT_MAX=2147483647
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```
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That's smaller than `9223372036854775807`.
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What's `9223372036854775807` as a 32-bit integer?
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```c
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printf("int(SSH_SYSFDMAX)=%d\n", (int)SSH_SYSFDMAX);
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```
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```
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int(SSH_SYSFDMAX)=-1
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```
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So from `getrlimit` I get pretty large values, but because `ssh-keyscan` stuffs them into a smaller type, it wraps around and returns `-1`.
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And that's smaller than `0` and thus a `bad value`.
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## What now?
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Why am I getting such large values to begin with?
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```
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$ ulimit -n
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unlimited
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```
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_(`ulimit -n` shows the file descriptor limit for the current shell)_
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That's probably a large value.
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How does one change that in macOS?
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Multiple ways!
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First let's ask the OS what is configured:
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```
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$ launchctl limit maxfiles
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maxfiles 256 unlimited
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```
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The first number, `256`, is a soft limit and the other, `unlimited`, the hard limit per process.
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Soft limit? Hard limit?
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The soft limit is configurable by the user up to the hard limit, which can only be changed by `root`.
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But there's also a kernel configuration for it:
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```
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$ sysctl -a | grep maxfiles
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kern.maxfiles: 122880
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kern.maxfilesperproc: 61440
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```
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That's the hard limit for a single process (`maxfilesperproc=61440`) and for all processes (`maxfiles=122880`).
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This doesn't even match the `launchctl` output.
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Let's change this using `launchctl`[^1]:
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```
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$ sudo launchctl limit maxfiles 245760 491520
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$ launchctl limit maxfiles
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maxfiles 245760 491520
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$ sysctl -a | grep maxfiles
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kern.maxfiles: 491520
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kern.maxfilesperproc: 245760
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```
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Now both outputs match.
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Did that help with our `ssh-keyscan` problem?
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```
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$ ulimit -n
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unlimited
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```
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Still unlimited, I don't have high hopes now.
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```
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$ ./ssh-keyscan github.com
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int size=4
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type size=8
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rlfd.rlim_max=9223372036854775807
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rlfd.rlim_cur=9223372036854775807
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RLIM_INFINITY=9223372036854775807
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SSH_SYSFDMAX=9223372036854775807
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ssh-keyscan: fdlim_get: bad value
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```
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And indeed it still fails and I get large values.
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What if I change the limit just for this shell session?
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```
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$ ulimit -n 245760
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$ ulimit -n
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245760
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$ ./ssh-keyscan github.com
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int size=4
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type size=8
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rlfd.rlim_max=9223372036854775807
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rlfd.rlim_cur=245760
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RLIM_INFINITY=9223372036854775807
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SSH_SYSFDMAX=245760
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int size=4
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type size=8
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rlfd.rlim_max=9223372036854775807
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rlfd.rlim_cur=245760
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RLIM_INFINITY=9223372036854775807
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SSH_SYSFDMAX=245760
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# github.com:22 SSH-2.0-babeld-dca4d356
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github.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEmKSENjQEezOmxkZMy7opKgwFB9nkt5YRrYMjNuG5N87uRgg6CLrbo5wAdT/y6v0mKV0U2w0WZ2YB/++Tpockg=
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<snip>
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```
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It works!
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_(We get the whole debug output twice, because `fdlim_get` is called twice)_
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Wait, why did `SSH_SYSFDMAX` change? Isn't that a constant?
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Yes and no:
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```
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$ grep -R "define SSH_SYSFDMAX" .
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./defines.h:# define SSH_SYSFDMAX sysconf(_SC_OPEN_MAX)
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./defines.h:# define SSH_SYSFDMAX 10000
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```
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In [`defines.h`](https://github.com/openssh/openssh-portable/blob/b4ac435b4e67f8eb5932d8f59eb5b3cf7dc38df0/defines.h#L728-L733):
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```c
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/* Maximum number of file descriptors available */
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#ifdef HAVE_SYSCONF
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# define SSH_SYSFDMAX sysconf(_SC_OPEN_MAX)
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#else
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# define SSH_SYSFDMAX 10000
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#endif
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```
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It's `sysconf(_SC_OPEN_MAX)`, a function call!
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And `_SC_OPEN_MAX` is [defined as](https://manpages.debian.org/bullseye/manpages-dev/sysconf.3.en.html#OPEN_MAX):
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> The maximum number of files that a process can have open at any time. Must not be less than `_POSIX_OPEN_MAX (20)`.
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So it's the limit I configured using `ulimit -n 245760` above.
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I am still confused why `launchctl limit maxfiles` and `sysctl -a` are different on a freshly booted machine,
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but configuring values with `launchctl` then touches those `sysctl` values too.
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According to ~~some people~~ everyone I asked `ulimit -n` gives them `256`, a small but much more sensible value.
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I still have no clue why it's `unlimited` on my machine.
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Turns out I ran into that problem 2 years ago in another project (and got it fixed):
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[entr: Segmentation fault on MacBook M1 due to unlimited file descriptors](https://github.com/eradman/entr/issues/63).
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This MacBook is cursed.
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At least now there will be search results for `fdlim_get: bad value` on the internet.
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---
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_Footnotes:_
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[^1]: I cannot recommend to run `sudo launchctl limit maxfiles 1024 1024`. You won't be able to shut down your system anymore.
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