Messages in this thread | | | Subject | Bug in the RTM_SETLINK kernel API for setting MAC address | Date | Mon, 07 Aug 2006 20:40:24 -0700 | From | Pavlin Radoslavov <> |
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It appears there is a bug in the RTM_SETLINK kernel API for setting the MAC address on an interface.
E.g., below is the relevant sample code for setting the Ethernet MAC address payload that works on 2.6.17.
/* Add the MAC address as an attribute */ struct sockaddr_storage ss_mac; struct sockaddr* sa_mac_p = (struct sockaddr *)&ss_mac; size_t sa_mac_len = 0; memset(&ss_mac, 0, sizeof(ss_mac)); sa_mac_p->sa_family = ARPHRD_ETHER; sa_mac_len = sizeof(sa_mac_p->sa_family) + ETH_ALEN;
memcpy(sa_mac_p->sa_data, ðer_addr, ETH_ALEN); rta_len = RTA_LENGTH(sa_mac_len); rtattr = IFLA_RTA(ifinfomsg); rtattr->rta_type = IFLA_ADDRESS; /* * XXX * rtattr->rta_len = rta_len; */ rtattr->rta_len = RTA_LENGTH(ETH_ALEN); memcpy(RTA_DATA(rtattr), sa_mac_p, sa_mac_len); nlh->nlmsg_len = NLMSG_ALIGN(nlh->nlmsg_len) + rta_len;
if (ns.sendto(buffer, nlh->nlmsg_len, 0, (struct sockaddr *)&snl, sizeof(snl)) != (ssize_t)nlh->nlmsg_len) { /* ERROR */ }
Note that the payload with the MAC address has to be "struct sockaddr" (or equivalent) and the length of that payload is the equivalent of "sizeof(sa_family) + mac_address_size".
However, the rta_len of the corresponding message MUST be set to "mac_address_size" rather than the real payload size which is "sizeof(sa_family) + mac_address_size". I believe this is incorrect, and rta_len is suppose to be set to the real payload size.
The particular problematic code in the kernel that checks for the payload size is inside net/core/rtnetlink.c, do_setlink():
... if (ida[IFLA_ADDRESS - 1]) { ... if (ida[IFLA_ADDRESS - 1]->rta_len != RTA_LENGTH(dev->addr_len)) goto out; err = dev->set_mac_address(dev, RTA_DATA(ida[IFLA_ADDRESS - 1])); ...
Where dev->set_mac_address() (typically/always?) expects to see a second argument of type "struct sockaddr".
Thanks, Pavlin
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