iptables, setting DSCP for DHCP?

Question

I'm trying to set the DSCP bits on DHCP packets. Although the below works fine for udp, the packet is always sent with DSCP of 16. Any ideas?

 iptables -t mangle -A FORWARD -p udp -j DSCP --set-dscp 1
 iptables -t mangle -A OUTPUT -p udp -j DSCP --set-dscp 1
 iptables -t mangle -A PREROUTING -p udp -j DSCP --set-dscp 1
 iptables -t mangle -A POSTROUTING -p udp -j DSCP --set-dscp 1

I'm using dhclient - I don't see any option there to set it either.

Looks like dhclient hard codes it here, fyi:

void assemble_udp_ip_header (interface, buf, bufix,
                             from, to, port, data, len)
        struct interface_info *interface;
        unsigned char *buf;
        unsigned *bufix;
        u_int32_t from;
        u_int32_t to;
        u_int32_t port;
        unsigned char *data;
        unsigned len;
{
        struct ip ip;
        struct udphdr udp;

        memset (&ip, 0, sizeof ip);

        /* Fill out the IP header */
        IP_V_SET (&ip, 4);
        IP_HL_SET (&ip, 20);
        ip.ip_tos = IPTOS_LOWDELAY;

Answer 1

I think DHCP cannot be caught with iptables as it uses RAW sockets that bypass the entire netfilter mechanism. There are some sources that report this here:

http://louwrentius.com/why-filtering-dhcp-traffic-is-not-always-possible-with-iptables.html

https://serverfault.com/questions/297383/unable-to-block-dhcp-and-other-traffic-on-input-chain