I like this on first read. In fact I thought this already existed.
I'll actually look more closely at the code tomorrow.

.... Ken

On Thu, Jul 26, 2012 at 10:09:28PM -0400, Ted Unangst wrote:
> I have a system with two network interfaces (em0 and em1), running dhcp
> on both. Both dhcp servers provide me with a nameserver, but only one
> of them works (I can't fix this).  There is a config file for dhclient
> I can use, but it only supports the supersede keyword.  I don't want
> to statically configure a nameserver override for em1, because the
> whole point is that the good nameserver on em0 can change.  I just
> want to say "pretend this option did not arrive."
> 
> Diff below adds a little support for an ignore keyword.  Like
> supersede, except don't actually use the supplied value.
> 
> Index: clparse.c
> ===================================================================
> RCS file: /cvs/src/sbin/dhclient/clparse.c,v
> retrieving revision 1.38
> diff -u -p -r1.38 clparse.c
> --- clparse.c 10 Dec 2011 17:15:27 -0000      1.38
> +++ clparse.c 27 Jul 2012 01:59:10 -0000
> @@ -170,6 +170,11 @@ parse_client_statement(FILE *cfile)
>               if (code != -1)
>                       config->default_actions[code] = ACTION_SUPERSEDE;
>               return;
> +     case TOK_IGNORE:
> +             code = parse_option_decl(cfile, &config->defaults[0]);
> +             if (code != -1)
> +                     config->default_actions[code] = ACTION_IGNORE;
> +             return;
>       case TOK_APPEND:
>               code = parse_option_decl(cfile, &config->defaults[0]);
>               if (code != -1)
> Index: conflex.c
> ===================================================================
> RCS file: /cvs/src/sbin/dhclient/conflex.c,v
> retrieving revision 1.14
> diff -u -p -r1.14 conflex.c
> --- conflex.c 10 Dec 2011 17:36:40 -0000      1.14
> +++ conflex.c 27 Jul 2012 01:15:19 -0000
> @@ -337,6 +337,7 @@ static const struct keywords {
>       { "filename",                           TOK_FILENAME },
>       { "fixed-address",                      TOK_FIXED_ADDR },
>       { "hardware",                           TOK_HARDWARE },
> +     { "ignore",                             TOK_IGNORE },
>       { "initial-interval",                   TOK_INITIAL_INTERVAL },
>       { "interface",                          TOK_INTERFACE },
>       { "lease",                              TOK_LEASE },
> Index: dhclient.c
> ===================================================================
> RCS file: /cvs/src/sbin/dhclient/dhclient.c,v
> retrieving revision 1.146
> diff -u -p -r1.146 dhclient.c
> --- dhclient.c        9 Jul 2012 16:21:21 -0000       1.146
> +++ dhclient.c        27 Jul 2012 01:59:35 -0000
> @@ -1535,6 +1535,9 @@ priv_script_write_params(char *prefix, s
>               if (config->defaults[i].len) {
>                       if (lease->options[i].len) {
>                               switch (config->default_actions[i]) {
> +                             case ACTION_IGNORE:
> +                                     /* handled below */
> +                                     break;
>                               case ACTION_DEFAULT:
>                                       dp = lease->options[i].data;
>                                       len = lease->options[i].len;
> @@ -1588,6 +1591,9 @@ supersede:
>                       len = lease->options[i].len;
>                       dp = lease->options[i].data;
>               } else {
> +                     len = 0;
> +             }
> +             if (len && config->default_actions[i] == ACTION_IGNORE) {
>                       len = 0;
>               }
>               if (len) {
> Index: dhclient.conf.5
> ===================================================================
> RCS file: /cvs/src/sbin/dhclient/dhclient.conf.5,v
> retrieving revision 1.21
> diff -u -p -r1.21 dhclient.conf.5
> --- dhclient.conf.5   9 Apr 2011 19:53:00 -0000       1.21
> +++ dhclient.conf.5   27 Jul 2012 02:05:28 -0000
> @@ -244,6 +244,14 @@ in the
>  .Ic supersede
>  statement.
>  .It Xo
> +.Ic ignore No { Op Ar option declaration
> +.Oo , Ar ... option declaration Oc }
> +.Xc
> +If for some set of options the client should always ignore the
> +value supplied by the server, these values can be defined in the
> +.Ic ignore
> +statement.
> +.It Xo
>  .Ic prepend No { Op Ar option declaration
>  .Oo , Ar ... option declaration Oc }
>  .Xc
> Index: dhcpd.h
> ===================================================================
> RCS file: /cvs/src/sbin/dhclient/dhcpd.h,v
> retrieving revision 1.76
> diff -u -p -r1.76 dhcpd.h
> --- dhcpd.h   9 Jul 2012 16:21:21 -0000       1.76
> +++ dhcpd.h   27 Jul 2012 01:18:18 -0000
> @@ -130,6 +130,7 @@ struct client_config {
>       struct option_data      defaults[256];
>       enum {
>               ACTION_DEFAULT,
> +             ACTION_IGNORE,
>               ACTION_SUPERSEDE,
>               ACTION_PREPEND,
>               ACTION_APPEND
> Index: dhctoken.h
> ===================================================================
> RCS file: /cvs/src/sbin/dhclient/dhctoken.h,v
> retrieving revision 1.5
> diff -u -p -r1.5 dhctoken.h
> --- dhctoken.h        15 May 2006 08:10:57 -0000      1.5
> +++ dhctoken.h        27 Jul 2012 01:15:54 -0000
> @@ -79,6 +79,7 @@
>  #define TOK_REJECT           292
>  #define TOK_FDDI             293
>  #define TOK_LINK_TIMEOUT     294
> +#define TOK_IGNORE           295
>  
>  #define is_identifier(x)     ((x) >= TOK_FIRST_TOKEN &&      \
>                                (x) != TOK_STRING &&   \

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