arp error : /etc/ethers cannot have self MAC<->IP?
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From: Rahul on 6 Aug 2010 19:05 I was trying to specify a static arp list for servers in a cluster by using arp -f /etc/ethers cat /etc/ethers 00:07:43:06:8A:AD 192.168.0.1 00:07:43:06:8A:AA 192.168.0.2 00:07:43:06:8A:B0 192.168.0.3 00:07:43:06:8A:AB 192.168.0.4 00:07:43:06:8A:AC 192.168.0.5 00:07:43:06:88:38 192.168.0.6 [snip] I thought I could copy the same /etc/ethers on all servers but it seems that if the (MAC,IP) of the server itself is in the file then arp throws an error: SIOCSARP: Invalid argument arp: cannot set entry on line 1 of etherfile /opt/sbin/ethers ! Then if I comment out the offending line things work fine. e.g. on 192.168.0.1 I must remove line1 of /etc/ethers. Is this behaviour to be expected? Or is there something else at work here? -- Rahul
From: Pascal Hambourg on 6 Aug 2010 20:47 Hello, Rahul a �crit : > I was trying to specify a static arp list for servers in a cluster by using > > arp -f /etc/ethers > > cat /etc/ethers > 00:07:43:06:8A:AD 192.168.0.1 > 00:07:43:06:8A:AA 192.168.0.2 > 00:07:43:06:8A:B0 192.168.0.3 > 00:07:43:06:8A:AB 192.168.0.4 > 00:07:43:06:8A:AC 192.168.0.5 > 00:07:43:06:88:38 192.168.0.6 > [snip] > > I thought I could copy the same /etc/ethers on all servers but it seems > that if the (MAC,IP) of the server itself is in the file then arp throws an > error: > > SIOCSARP: Invalid argument > arp: cannot set entry on line 1 of etherfile /opt/sbin/ethers ! /opt/sbin/ethers ? > Then if I comment out the offending line things work fine. > > e.g. on 192.168.0.1 I must remove line1 of /etc/ethers. > > Is this behaviour to be expected? Or is there something else at work here? man 8 arp says : -i If, --device If Select an interface. [...] When setting a permanent or temp ARP entry this interface will be associated with the entry; if this option is not used, the kernel will guess based on the routing table. Read the last part. The host's own address is routed on the loopback interface lo, and trying to create an ARP entry on this interface leads to the error. I guess you can avoid this by explicitly specifying the interface in the arp command with the -i option.
From: habibielwa7id on 8 Aug 2010 03:11 On Aug 7, 2:05 am, Rahul <nos...(a)nospam.invalid> wrote: > I was trying to specify a static arp list for servers in a cluster by using > > arp -f /etc/ethers > > cat /etc/ethers > 00:07:43:06:8A:AD 192.168.0.1 > 00:07:43:06:8A:AA 192.168.0.2 > 00:07:43:06:8A:B0 192.168.0.3 > 00:07:43:06:8A:AB 192.168.0.4 > 00:07:43:06:8A:AC 192.168.0.5 > 00:07:43:06:88:38 192.168.0.6 > [snip] > > I thought I could copy the same /etc/ethers on all servers but it seems > that if the (MAC,IP) of the server itself is in the file then arp throws an > error: > > SIOCSARP: Invalid argument > arp: cannot set entry on line 1 of etherfile /opt/sbin/ethers ! > > Then if I comment out the offending line things work fine. > > e.g. on 192.168.0.1 I must remove line1 of /etc/ethers. > > Is this behaviour to be expected? Or is there something else at work here? > > -- > Rahul You just reversed the order, The ip addresses should be mentioned first like, 192.168.0.1 00:07:43:06:8A:AD
From: Pascal Hambourg on 8 Aug 2010 07:07 habibielwa7id a �crit : > On Aug 7, 2:05 am, Rahul <nos...(a)nospam.invalid> wrote: >> >> arp -f /etc/ethers >> >> cat /etc/ethers >> 00:07:43:06:8A:AD 192.168.0.1 >> 00:07:43:06:8A:AA 192.168.0.2 [...] > You just reversed the order, The ip addresses should be mentioned > first like, > 192.168.0.1 00:07:43:06:8A:AD man 8 arp says : As a special case for compatibility the order of the hostname and the hardware address can be exchanged.
From: habibielwa7id on 9 Aug 2010 04:14
On Aug 8, 2:07 pm, Pascal Hambourg <boite-a-s...(a)plouf.fr.eu.org> wrote: > habibielwa7id a écrit : > > > On Aug 7, 2:05 am, Rahul <nos...(a)nospam.invalid> wrote: > > >> arp -f /etc/ethers > > >> cat /etc/ethers > >> 00:07:43:06:8A:AD 192.168.0.1 > >> 00:07:43:06:8A:AA 192.168.0.2 > [...] > > You just reversed the order, The ip addresses should be mentioned > > first like, > > 192.168.0.1 00:07:43:06:8A:AD > > man 8 arp says : > > As a special case for compatibility the order of the hostname and the > hardware address can be exchanged. -Try it and you will find it will not work but in the order I mentioned, First the ip address then the mac address, This is what I found but on RedHat systems. A note for Rahul, The system will not add those static entries at startup automatically, So add command arp -f on rc.local for example, Also you should check after restarting the system if the static entries are working properly or not by issuing command arp -n for example, You will find an added M besides the static entries like, 172.21.0.3 ether 00:19:5b:71:10:3c C eth0 172.21.0.55 ether 11:22:22:22:22:22 CM eth0 In this case the first entry is dynamically assigned and the second entry is static. -I use static ARP entries on not secure networks to add extra security as it fights the man in the middle attacks that use ARP spoofing, And it also blocks blocking using ARP poisoning that may be caused by using dump applications like NetCut or some similar methods, The author didn't ask about that but I talked my be some bodies can benefit from that information any way specially if there networks are not secure. Regards, |