SchOS/cmd/resolvconf/main.c

// SPDX-License-Identifier: Apache-2.0
//
// resolvconf -- put the nameserver where the resolver will look.
//
// SchOS has no DNS client, and the interesting part is that it does not need
// one. musl contains a complete resolver: `getaddrinfo` reads
// /etc/resolv.conf, sends a query over an ordinary UDP socket and parses the
// reply, and every one of those pieces already worked here. The single missing
// fact was *which* nameserver -- which arrives in a DHCP reply the kernel
// handled and never told anyone about.
//
// That is what made `wget` look broken. It was not wget, and it was not the
// core utilities: nothing on this system could turn a name into an address, so
// every program that took a hostname failed the same way and each one looked
// like its own separate bug.
//
//     resolvconf          wait for a nameserver, write it, exit
//     resolvconf -n       write whatever there is now, do not wait
//     resolvconf -p       print what the kernel has, change nothing
//
// The kernel publishes /proc/resolv.conf and stops there, deliberately: it
// renders resolv.conf's exact syntax so this is a copy rather than a
// translation, and *whether* /etc/resolv.conf should be overwritten is a
// policy question. A machine with a hand-written one should keep it, which is
// why -f exists and why it is not the default.

#include "types.h"
#include "user.h"
#include "fcntl.h"

#define SRC "/proc/resolv.conf"
#define DST "/etc/resolv.conf"

// DHCP takes a few seconds on a quiet network and longer on a busy one. Two
// minutes is past any of that and short enough that a machine with no DHCP
// server does not hold a boot job open for ever.
#define WAIT_SECONDS 120

static void
puts_(const char *s)
{
    int n = 0;
    while (s[n]) n++;
    write(1, (void *)s, n);
}

static int
same(const char *a, const char *b, int n)
{
    for (int i = 0; i < n; i++)
        if (a[i] != b[i])
            return 0;
    return 1;
}

static int
slurp(const char *path, char *buf, int cap)
{
    int fd = open((char *)path, O_RDONLY);
    int n, len = 0;

    if (fd < 0)
        return -1;
    while (len < cap - 1 && (n = read(fd, buf + len, cap - 1 - len)) > 0)
        len += n;
    close(fd);
    buf[len] = 0;
    return len;
}

int
main(int argc, char *argv[])
{
    char buf[512];
    int i, n, fd, wait = 1, force = 0, print = 0;

    for (i = 1; i < argc; i++) {
        if (argv[i][0] != '-' || argv[i][2] != 0)
            goto usage;
        switch (argv[i][1]) {
        case 'n': wait = 0;  break;
        case 'f': force = 1; break;
        case 'p': print = 1; wait = 0; break;
        default:
        usage:
            puts_("usage: resolvconf [-n] [-f] [-p]\n"
                  "  -n  do not wait for DHCP\n"
                  "  -f  overwrite an existing " DST "\n"
                  "  -p  print what the kernel has and exit\n");
            return 1;
        }
    }

    // Wait by polling rather than by blocking on the file: /proc has no
    // readiness to report and inventing one for this would be a worse
    // interface than a loop that ends.
    for (i = 0; ; i++) {
        n = slurp(SRC, buf, sizeof(buf));
        if (n < 0) {
            puts_("resolvconf: no " SRC " -- this kernel has no network\n");
            return 1;
        }
        if (n > 0 || !wait || i >= WAIT_SECONDS)
            break;
        sleep(100);                      // one second, at 100 Hz
    }

    if (print) {
        if (n > 0) write(1, buf, n);
        else       puts_("resolvconf: the kernel has no nameserver yet\n");
        return n > 0 ? 0 : 1;
    }

    if (n == 0) {
        puts_("resolvconf: no nameserver after ");
        puts_(wait ? "two minutes" : "no wait");
        puts_(" -- is there a DHCP server?\n");
        return 1;
    }

    // An existing file is somebody's decision until told otherwise.
    //
    // Saying so every boot is the wrong volume for it. On a machine that has
    // been up before, /etc/resolv.conf always exists and always says the same
    // thing, so this printed a line about having done nothing on every single
    // start -- which is exactly the kind of message that teaches people to
    // stop reading their own boot.
    //
    // Quiet when the file already says what this would have written. It is
    // still worth a word when the two *disagree*, because then the nameserver
    // has changed and the machine is using the old one.
    if (!force) {
        char old[512];
        int on = slurp(DST, old, sizeof(old));
        if (on > 0) {
            if (on == n && same(old, buf, n))
                return 0;                        // already right; nothing to say
            puts_("resolvconf: " DST " differs from what DHCP gave;"
                  " -f to replace it\n");
            return 0;
        }
    }

    fd = open(DST, O_WRONLY | O_CREAT | O_TRUNC);
    if (fd < 0) {
        puts_("resolvconf: cannot write " DST "\n");
        return 1;
    }
    if (write(fd, buf, n) != n) {
        puts_("resolvconf: short write to " DST "\n");
        close(fd);
        return 1;
    }
    close(fd);

    // Worth one line the first time, because it is the moment names start
    // working and somebody may be waiting to know it.
    puts_("resolvconf: ");
    write(1, buf, n);
    return 0;
}