script "</dev/tcp/127.0.0.1/80" relies on /bin/sh being bash

[Paride Legovini]

The /usr/share/doc/keepalived/samples/keepalived.conf.vrrp.localcheck sample configuration file shows how to check if a TCP port is open using a `script`, e.g.:

vrrp_script chk_http_port {
       script "</dev/tcp/127.0.0.1/80" # connects and exits
       interval 1 # check every second
       weight -2 # default prio: -2 if connect fails
}

however this works only on systems where `/bin/sh` is Bash, and therefore doesn't work on Ubuntu. This happens because `script` basically passes the script name to stdlib's system(), which in turn does (see system(3)):

    execl("/bin/sh", "sh", "-c", command, (char *) NULL);

When `/bin/sh` is Bash then `script "</dev/tcp/127.0.0.1/80"` works because `/dev/tcp/host/port` is a special syntax telling Bash to tcp-connect to `host:port`, however other shells will just try to open `/dev/tcp/127.0.0.1/80` as a file and fail.

As the sample file comes from upstream I filed an upstream issue [1].

Possible workarounds are (warning: untested!):

    script "bash -c '</dev/tcp/127.0.0.1/80'"
    script "nc -z 127.0.0.1 80"

[1] https://github.com/acassen/keepalived/issues/1926

[Paride Legovini (paride)]

Note for context: this is a spin-off from LP: #1795420.

[Christian Ehrhardt  (paelzer)]

https://github.com/acassen/keepalived/commit/8f658000ff9fa8b67a81d29b4a73c6a3c1fc2c2b
was accepted upstream and is in
 keepalived | 1:2.2.4-0.2 | jammy | source, amd64, arm64, armhf, ppc64el, riscv64, s390x

I do not think this needs backports, setting fix released.