Linux Firewall not block dhcp requests

Steve Clark sclark at netwolves.com
Tue Aug 14 18:06:43 UTC 2012


Hello,

Can someone tell me how DHCP is seeing packets that according to my firewall log are being dropped?
Does DHCP read the packets before they get to the firewall like tcpdump does?

Chain fDROPnLOG (1 references)
  pkts bytes target     prot opt in     out source               destination
   143 16366 LOG        all  --  *      * 0.0.0.0/0            0.0.0.0/0           limit: avg 30/min burst 5 LOG flags 0 level 7 prefix `fw (fDROPnLOG) '
   143 16366 DROP       all  --  *      * 0.0.0.0/0            0.0.0.0/0

Aug 14 13:55:58 kernel: fw (fDROPnLOG) IN=eth0 OUT= MAC=ff:ff:ff:ff:ff:ff:5c:26:0a:73:b2:6a:08:00 SRC=10.254.207.66 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=128 ID=24427 PROTO=UDP SPT=68 DPT=67 LEN=308


tcpdump on eth0
13:55:58.667982 IP (tos 0x0, ttl 128, id 24427, offset 0, flags [none], proto UDP (17), length 328)
     10.254.207.66.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request from 5c:26:0a:73:b2:6a, length 300, xid 0xc5a1ea3f, Flags [Broadcast] (0x8000)
           Client-IP 10.254.207.66
           Client-Ethernet-Address 5c:26:0a:73:b2:6a
           Vendor-rfc1048 Extensions
             Magic Cookie 0x63825363
             DHCP-Message Option 53, length 1: Inform
             Client-ID Option 61, length 7: ether 5c:26:0a:73:b2:6a
             Hostname Option 12, length 12: "7pdawson0412"
             Vendor-Class Option 60, length 8: "MSFT 5.0"
             Parameter-Request Option 55, length 13:
               Subnet-Mask, Domain-Name, Default-Gateway, Domain-Name-Server
               Netbios-Name-Server, Netbios-Node, Netbios-Scope, Router-Discovery
               Static-Route, Classless-Static-Route, Classless-Static-Route-Microsoft, Vendor-Option
               Option 252
13:55:58.668418 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 328)
     10.254.207.65.67 > 10.254.207.66.68: [bad udp cksum ffd6!] BOOTP/DHCP, Reply, length 300, xid 0xc5a1ea3f, Flags [Broadcast] (0x8000)
           Client-IP 10.254.207.66
           Client-Ethernet-Address 5c:26:0a:73:b2:6a
           Vendor-rfc1048 Extensions
             Magic Cookie 0x63825363
             DHCP-Message Option 53, length 1: ACK
             Server-ID Option 54, length 4: 10.254.23.1
             Subnet-Mask Option 1, length 4: 255.255.255.192
             Default-Gateway Option 3, length 4: 10.254.207.65
             Domain-Name-Server Option 6, length 8: 172.16.11.180,172.16.11.181




-- 
Stephen Clark
*NetWolves*
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at netwolves.com
http://www.netwolves.com


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