What causes "sysctlbyname(kern.ostype) failed" in nginx/nginx?

Verify natively with `sysctl kern.ostype` on the same host/jail nginx runs in

NGX_LOG_ALERT

NGX_LOG_ALERT

Error message

sysctlbyname(kern.ostype) failed

What it means

During ngx_os_specific_init() at nginx startup on FreeBSD, the kernel description string kern.ostype is read with sysctlbyname() into a fixed 32-byte buffer. Only a too-small-buffer result (ENOMEM) is tolerated by NUL-terminating the truncated value; any other errno aborts startup with NGX_ERROR.

Source

Thrown at src/os/unix/ngx_freebsd_init.c:112

    if (mo && ngx_strchr(mo, 'J')) {
        ngx_debug_malloc = 1;
    }
#endif
}


ngx_int_t
ngx_os_specific_init(ngx_log_t *log)
{
    int         version;
    size_t      size;
    ngx_err_t   err;
    ngx_uint_t  i;

    size = sizeof(ngx_freebsd_kern_ostype);
    if (sysctlbyname("kern.ostype",
                     ngx_freebsd_kern_ostype, &size, NULL, 0) == -1) {
        ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
                      "sysctlbyname(kern.ostype) failed");

        if (ngx_errno != NGX_ENOMEM) {
            return NGX_ERROR;
        }

        ngx_freebsd_kern_ostype[size - 1] = '\0';
    }

    size = sizeof(ngx_freebsd_kern_osrelease);
    if (sysctlbyname("kern.osrelease",
                     ngx_freebsd_kern_osrelease, &size, NULL, 0) == -1) {
        ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
                      "sysctlbyname(kern.osrelease) failed");

        if (ngx_errno != NGX_ENOMEM) {
            return NGX_ERROR;
        }

View on GitHub (pinned to 3f6f7824d4)

Solutions

Verify natively with `sysctl kern.ostype` on the same host/jail nginx runs in
Run the binary on a native FreeBSD kernel of the matching ABI instead of an emulation layer
Adjust jail/hardening policy so sysctlbyname reads of kern.* are permitted
Rebuild/reinstall the matching libc and world if sysctl is broken system-wide

Defensive patterns

Strategy: validation

Validate before calling

char buf[32]; size_t len = sizeof(buf);
if (sysctlbyname("kern.ostype", buf, &len, NULL, 0) == -1
    && errno != ENOMEM) {
    /* nginx startup will abort here: fix the environment first */
}

Try / catch

Startup abort has no runtime catch: preflight the same sysctl in the deploy environment (shell or C probe above). If the probe fails under a jail/emulator but succeeds on the host, run nginx natively or open the sysctl in policy, then start nginx.

Prevention

Never run FreeBSD nginx binaries under OS emulation layers
Add a sysctl kern.ostype preflight to install/start scripts on FreeBSD
Keep jail security policies permissive for read-only kern.* sysctls

When it happens

Trigger: Running a FreeBSD nginx binary under an emulation layer (e.g. Linuxulator) that lacks kern.ostype; jails or hardened kernels restricting sysctlbyname(); EINVAL/EFAULT from a broken libc; a stripped custom kernel without the kern.ostype MIB.

Common situations: FreeBSD binary executed on a non-native kernel or in a restricted sandbox; exotic custom kernel builds; mismatched libc after a partial upgrade.

Related errors

AI-assisted analysis of nginx/nginx@3f6f7824d4 (2026-08-22). Data as JSON: /api/errors/725f3e116ed09c64. Report an issue: GitHub.