misc/187094: FreeBSD DHCP ignores dhcp option interface-mtu

[Robert Blayzor]

The following reply was made to PR misc/187094; it has been noted by GNATS.

From: Robert Blayzor <rblayzor at inoc.net>
To: "bug-followup at freebsd.org" <bug-followup at FreeBSD.org>
Cc:  
Subject: Re: misc/187094: FreeBSD DHCP ignores dhcp option interface-mtu
Date: Thu, 27 Feb 2014 13:24:11 -0500

 I would like to submit the following patch that corrects two major issues:
 
 -  Look for and set MTU according to  DHCP option tag 26 for Interface MTU. 
    This allows booting interface to be used on a jumbo frame enabled network.
 
    Currently this is broken and cannot be overridden or set later.
 
 -  Remove ancient proxy ARP setting.  Currently it is more of a problem that 
    a host is multi-homed and booting interface network may not have a router.
    (ie: default route is on another interface/network)
 
    The default today is to use DHCP router tag and if no router tag is
    supplied by DHCP, default route will be set to hosts self.  Not supplying
    a route is a completely valid option, especially since many hosts
    multi-homed.
 
 
 Patch is against 10.0-RELEASE
 
 Index: bootp_subr.c
 ===================================================================
 --- bootp_subr.c	(revision 261846)
 +++ bootp_subr.c	(working copy)
 @@ -196,6 +196,8 @@
  #define TAG_HOSTNAME	 12  /* Client host name */
  #define TAG_ROOT	 17  /* Root path */
 
 +#define TAG_INTF_MTU     26  /* Interface MTU Size (RFC2132) */
 +
  /* DHCP specific tags */
  #define TAG_OVERLOAD	 52  /* Option Overload */
  #define TAG_MAXMSGSIZE   57  /* Maximum DHCP Message Size */
 @@ -229,6 +231,8 @@
  #endif
 
  static char bootp_cookie[128];
 +static unsigned int bootp_ifmtu = 0;
 +
  static struct socket *bootp_so;
  SYSCTL_STRING(_kern, OID_AUTO, bootp_cookie, CTLFLAG_RD,
  	bootp_cookie, 0, "Cookie (T134) supplied by bootp server");
 @@ -1030,7 +1034,22 @@
  		return (0);
  	}
 
 -	printf("Adjusted interface %s\n", ifctx->ireq.ifr_name);
 +	printf("Adjusted interface %s", ifctx->ireq.ifr_name);
 +
 +	/* Do BOOTP interface options */
 +	if (bootp_ifmtu != 0) {
 +	        printf(" (MTU=%d", bootp_ifmtu);
 +	        if (bootp_ifmtu > 1514)
 +	                printf("/JUMBO");
 +	        printf(")");
 +
 +		ifr->ifr_mtu = bootp_ifmtu;
 +		error = ifioctl(bootp_so, SIOCSIFMTU, (caddr_t) ifr, td);
 +		if (error != 0)
 +		        panic("%s: SIOCSIFMTU, error=%d", __func__, error);
 +	}
 +        printf("\n");
 +
  	/*
  	 * Do enough of ifconfig(8) so that the chosen interface
  	 * can talk to the servers.  (just set the address)
 @@ -1053,7 +1072,12 @@
 
  	/* Add new default route */
 
 -	if (ifctx->gotgw != 0 || gctx->gotgw == 0) {
 +        /*  Only set default route if we received one in the request.
 +            Proxy ARP considered obsolete.  More valid to NOT set
 +            a router in request as the host may be multi-homed and
 +            gateway may not be on this interface.
 +	*/
 +	if (ifctx->gotgw != 0 || gctx->gotgw != 0) {
  		clear_sinaddr(&defdst);
  		clear_sinaddr(&defmask);
  		/* XXX MRT just table 0 */
 @@ -1518,6 +1542,11 @@
  		p[i] = '\0';
  	}
 
 +        p = bootpc_tag(&gctx->tag, &ifctx->reply, ifctx->replylen,
 +		       TAG_INTF_MTU);
 +        if (p != NULL) {
 +	        bootp_ifmtu = (((unsigned char)p[0] << 8) + (unsigned char)p[1]);
 +	}
 
  	printf("\n");